THE UNIVERSITY OF TEXAS PUBLICATION NUMBER 5605 MARCH 1, 1956 Basement Rocks of Texas and Southeast New Mexico PETER T. FLA WN BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS, AUSTIN JOHN T. LONSDALE, Director Publications of The University of Texas COMMI'ITEE ON PUBLICATIONS L. u. HANKE C. T. McCORMICK D.L.CLARK H.Y.McCowN R. F. DAWSON A. MOFFIT J.R.D.EDDY c. P. OLIVER J. T. LONSDALE J. R. STOCKTON S. A. MAcCoRKLE F. H. WARDLAW ADMINISTRATIVE PUBLICATIONS AND GENERAL RULES w. B. SHIPP C.H. EAos J. G. AseBURNE F. H. GINASCOL c. E. LANKFORD The University publishes bulletins twice a month, so numbered that the first two digits of the number show the year of issue and the last two the position in the yearly series. (For example, No. 5601 is the first publication of the year 1956.) These bulletins comprise the official publications of the University, publications on humanistic and scientific subjects, and bulletins issued from time to time by various divisons of the University. The follow­ing bureaus and divisions distribute publications issued by them; communi­cations concerning publications in these fields should be addressed to The University of Texas, Austin, Texas, care of the bureau or division issuing the publication: Bureau of Business Research, Bureau of Economic Geology, Bureau of Engineering Research, Bureau of Industrial Chemistry, Bureau of Public School Service, and Division of Extension. Communications con­cerning all other publications of the University should be addressed to Uni­versity Publications, The University of Texas, Austin. Additional copies of this publication may be secured from the Bureau of Economic Geology, The University of Texas, Austin 12, Texas THE UNIVERSITY OF TEXAS PUBLICATION NUMBER 5605 MARCH I, 1956 -·­ Basement Rocks of Texas and Southeast New Mexico PETER T. FLAWN BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS, AUSTIN JOHN T. LONSDALE, Director The benefits of education and of useful knowledge, generally diffused through a community, are essential to the preserva­tion of a free government. SAM HOUSTON Cultivated mind is the guardian genius of Democracy, and while guided and controlled by virtue, the noblest attribute of man. It is the only dictator that freemen acknowledge, and the only security which freemen desire. MIRABEAU B. LAMAR PUBLISHED BY THE UNIVERSITY TWICE A MONTH. ENTERED AS SECOND-CLASS MATTER ON MARCH I2, I9I3, AT THE POST OFFICE AT AUSTIN, TEXAS, UNDER THE ACT OF AUGUST 24, I9I2 Contents PAGE Abstract 7 Introduction . 11 Purpose and scope of the investigation . 11 Previous work .. 11 Acknowledgments 12 Collection, treatment, and presentation of data . 14 Petrographic methods 15 Petrographic nomenclature 15 Evaluation of petrographic data . 19 Resolution of petrographic data 20 Discussion of structural nomenclature and concepts . 20 Lithology and structure of the Precambrian basement 23 General remarks . 23 Texas craton . 25 Definition ........................... ......... . 25 Llano uplift . . ............................................................ 26 Petrographic character of the subsurface Texas craton 27 Age of the Texas craton and relation to other Precambrian terranes 30 Paleozoic and younger structures of the Texas craton . 31 Van Horn mobile belt . 32 Definition 32 Age and possible subsurface extent of the Van Horn mobile belt 34 Paleozoic and younger structures of the Van Horn mobile belt . 35 Red River mobile belt .. ... ................... . . ................. . 36 Definition 36 Petrographic character of the Red River belt 36 Relation of the Red River belt to other Precambrian terranes and the Texas craton . 36 Age and mobility of the Red River belt 38 Paleozoic structures of the Red River mobile belt . 38 Fisher metasedimentary terrane . 39 Definition 39 Petrographic character of the Fisher terrane . 39 Relation of the Fisher terrane to the Texas craton and Red River mobile belt . 40 Panhandle volcanic terrane . 41 Definition 41 Petrographic character of the volcanic terrane . 41 Relation of the Panhandle volcanic terrane to other Precambrian rocks . 42 Age of the volcanic rocks . 43 Paleozoic structures of the volcanic terrane . 43 Swisher gabbroic terrane . . . ....................... 43 Definition and structural character .............................................................................. ..... . 43 Petrographic character of the gabbroic terrane . 44 Contact-metasedimentary rocks in the gabbroic terrane ...................... ................................. 45 Age of the gabbroic rocks . 46 Emplacement and structural history of the gabbroic rocks . 46 Wichita igneous province . . ............ -········· 46 General remarks and definition . 46 Arbuckle Mountains . 47 Wichita Mountains 47 Comparison of Wichita Mountains gabbro with gabbro of the Swisher terrane . 48 Subsurface Wichita igneous province .......................... ...... . 48 Correlation of Precambrian rocks in the Amarillo, Wichita, and Arbuckle uplifts . 50 Age of the Wichita igneous province and relation to other Precambrian rocks . 50 Paleozoic structures of the Wichita igneous province .......................................... .... . 51 The basement surface . . ............................. 53 Configuration of the surface . 53 PAGE Weathering of the basement surface 53 Paleozoic sedimentary rocks resting on the basement surface 54 General remarks . 54 Cambro·Ordovician rocks . 54 Siluro-Devonian rocks .......................... . 55 Mississippian rocks .................... 55 Permo·Pennsylvanian rocks . 55 Faults or fault zones in basement rocks . 56 The Ouachita foldbelt . 58 History of the problem .... .................................................................. . 58 Igneous and metamorphic exotic boulders in the Haymond boulder bed of the Marathon area .................... 59 Metamorphic rocks in Boquillas Canyon 59 Subsurface Ouachita foldbelt . 59 Petrographic character and correlation problems 59 Igneous rocks in the Ouachita foldbelt in Medina County .... 60 The Ouachita foldbelt as basement 60 Summary 61 Younger igneous rocks . 62 Analysis of gravity data 63 General remarks 63 Gravity anomalies of the Texas craton 63 Gravity anomalies of the Van Hom mobile belt . ... ...... ............ ......... 66 Gravity anomalies of the Red River mobile belt ... . .............. ..................... 66 Gravity anomalies of the Fisher metasedimentary terrane . ................... . 66 Gravity anomalies of the Panhandle volcanic terrane and the Swisher gabbroic terrane 67 Gravity anomalies of the Wichita igneous province ...................... 67 Summary . ............................ 67 Precambrian history of the Texas-southeast New Mexico area 68 Summary 68 Subdivision of Precambrian time 69 Growth of the North American continent . 69 Bibliography ........................... . 72 Appendix I-Glossary of petrographic nomenclature ..... 115 Textural and fabric terms ... . . ...................... 115 Rock nomenclature 116 Appondix II-Petrographic descriptions ........................ 119 Part 1, Texas ............................ ....................... ........................... 119 Part 2, Southeast New Mexico ....... 206 Appendix Ill-Magnetic susceptibility measurements on well cores and cuttings of Precambrian rocks in west Texas and southeast New Mexico ..................................................... 233 Abstract ..................... . .................................. 233 Introduction 233 Measured values of magnetic susceptibility ........ . 235 Bibliography ................... 238 Table l. Measured magnetic susceptibility values ..................... 240 Table 2. Key to sample numbers in Table 1 .......... . ·················. 241 Index ....... . ................................................. · ·························· 251 Illustrations FICURES- PACE 1. Index map of basement terranes and provinces . 24 2. Index map of regional structural features . 33 PLATES­ !. Basement rocks of Texas and southeast New Mexico ................ .In pocket II. Schematic cross sections of Texas basement rocks ... . ......In pocket III. Basement rocks in the Central Basin Platform (A), Fort Stockton high (B), and Muenster arch ( C). Enlarged from Plate I ................................................. .In pocket PHOTOMICROCRAPHS­ JV. Rocks from the Texas craton .................... ......... . 244 V. Rocks from the Red River mobile belt . . ............................ . ... 245 VI. Rocks from the Red River and Van Hom mobile belts ................... . 246 VII. Rocks from the Fisher metasedimentary terrane . ....................................... 247 VIII. Rocks from the Panhandle volcanic terrane ........................... ········ ... .. ........... 248 IX. Rocks from the Swisher gabbroic terrane . 249 X. Rocks from the Wichita igneous province ...... 250 Tables TABLES- PACE I. Data on Texas basement wells (Part I) and New Mexico basement wells (Part 2) ........74, 102 2. Nomenclature for common igneous rocks . ................................... ..................................... 17 3. Nomenclature for metamorphosed argillaceous rocks . 19 4. Age determinations in the Llano uplift ... .................. ..... . ................ ..... 27 5. Composition of the Texas craton . 28 6. Rock types of the Red River mobile belt . 36 7. Rock types of the Fisher metasedimentary terrane . .................. .... ........ 39 8. Rock types of the Panhandle volcanic terrane ... ................. .. ... 41 9. Rock types of the Swisher terrane . 44 10. Rock types of the Wichita igneous province 49 11. West Texas and southeast New Mexico wells that penetrate post-Cambrian igneous rocks ... 64-65 12. Tentative correlation of Precambrian rocks and structural events in Texas, soulhern Oklahoma, and southeast New Mexico . . ....................................... Facing 68 Basement Rocks of Texas and Southeast New Mexico PETER T. FLAWN ABSTRACT /ntroduction.-This publication de­scribes the geology of the Precambrian basement rocks of Texas and southeast New Mexico as determined by a study of well cores and cuttings generously fur­nished by oil companies operating in the area. Data gathered in the subsurface study are integrated with published data on exposed basement rocks. Table 1 lists 760 wells that penetrate basement in the area of study. Following a discussion of previous work, individual contributions to the project are acknowledged and methods of collection, treatment, and presentation of data are explained. The section is con­cluded by a discussion of petrographic and structural nomenclature and concepts. Litlwlogy and structure of the Precam­brian basement.-Seven major lithologic and lithologic-structural divisions can be recognized in the Precambrian rocks of the Texas-southeast New Mexico area. They are named as follows: (1) Texas craton, (2) Van Horn mobile belt, (3) Red River mobile belt, (4) Fisher meta­sedimentary terrane, (5) Panhandle vol­canic terrane, (6) Swish~r gabbroic ter­rane, and (7) Wichita igneous province (Pl. I). Texas craton.-The most fundamental basement element is the Texas craton. It is a northwesterly elongated stable or era· tonic mass composed mostly of plutonic ig­neous rocks of granitic or granodioritic composition; locally there are patches of dioritic and gabbroic rocks and of meta­morphosed sedimentary and igneous rocks. Cataclastically altered rocks are developed (1) in a linear zone in southwestern Roose­velt County, New Mexico, (2) along the eastern edge of the Central Basin Platform in Andrews and Ector counties, and (3) along the southern margin of the Fort Stockton high in Pecos County. Petro­graphically, the Texas craton is distin­guished by its great volume of granitic rocks. On the sou th east the craton is de­1 imited by the younger Ouachita foldbelt and Balcones fault zone which have de­veloped peripherally to it; on the north and west it passes out of the area of study and is partly concealed by the Panhandle volcanic terrane and complicated by younger intrusions of the Wichita igneous province ; in the far southwest it is bor­dered by the Van Horn mobile belt which eastwardly, through unknown geologic relationships, gives way to the younger Ouachita foldbelt; on the north and north­east the picture is vague, apparently the northern boundary is the Red River mobile belt. To the far west in New Mexico, meta­sedimentary rocks of varied grade and vol­canic rocks indicate the approximate po­sition of the cratonic margin, but poor control makes it difficult to establish a logical picture. Part of the Texas craton is exposed in the Llano uplift of central Texas where ancient metasedimentary rocks are in­truded by granites of several ages. Evi­dence from this exposure and from gravity maps indicates that the southeastward part of the craton has a greater volume of meta­sedimentary rocks than the northwestern part, which is apparently composed al­most entirely of plutonic rocks. Meta­morphism increases from northwest to southeast within the uplift. The gravity picture in the area of the Llano uplift is characterized by irregular or amoeboid anomalies that reflect the complex of gran­ite and intruded metamorphic rocks; this picture changes northward and westward Bureau of Economic Geology, The University of Texas to a more regular pattern, although per­haps this is in part due to the masking effects of the thickening sedimentary cover. A lead-uranium age determination on uraninite from a young intrusive peg­matite phase of the granite exposed in the eastern Llano uplift gives an age of 1,100 million years (Holmes, 1931, pp. 330--334, 351) ; this is checked by a magnetite­helium age determination on the Iron Mountain magnetite mass which has been dated as 1,050 millior. years (Hurley and Goodman, 1943, p. 321). Recent zircon age determinations on the granites of the western and central Llano uplift range from 874 to 942 million years. Zircon age determinations on granite cored in wells in Schleicher and Pecos counties give 1,000 and 910 million years respectively. Van Horn mobile belt.-The concept of a Precambrian mobile belt southwest of the craton in west Texas is based chiefly on the Precambrian rocks that crop out in Culberson and Hudspeth counties in the Van Horn area. The Van Horn rocks have been described in detail by King and Flawn ( 1953) . They consist of a metamorphosed sedimentary section, largely quartzofeld­spathic, about 20,000 feet thick intruded by rhyolite and diorite, also metamor­phosed, and thrust northward over a thick sequence of limestone, volcanic rocks, and sandstone that has suffered extreme de­formation along a linear zone that is probably congruent with the cratonic mar­gin. The nature of the original sediments, their thickness, and the character of the deformation, all suggest a deformed geo· syncline or a mobile belt. Stratigraphi­cally, the culminating orogeny in this area is post-Hazel formation and pre-Van Horn sandstone, or very probably late Pre­cambrian. Red River mobile belt.-The Red River mobile belt is an east-west-trending sub­surface belt of metasedimentary, meta­igneous, and igneous rocks which can be traced from Cooke County west to Floyd and Crosby counties and probably extends farther in both directions. Many of the basement wells in this area are localized in rather closely spaced groups on the struc­turally high Muenster and Electra elements of the Red River uplift-a late Paleozoic structural feature probably controlled by the older late Precambrian mobile belt­and boundaries are difficult to fix. From Denton and Cooke counties west to Foard County the belt is marked by a predominance of metasedimentary types ranging from medium to high metamor· phic grade in the east, where intrusions in the mobile belt are abundant, to lower metamorphic grade in the western part of the area. The western limit pre­sents a problem. Probably the belt con­tinues westward into Cottle and Motley counties and terminates in northern Crosby County, because metasedimentary rocks have been encountered in that area along the strike of the mobile belt. The uncertainty arises because another meta­sedimentary belt, the Fisher metasedi­mentary terrane, trends about north-south in the same area and is separated from the metasedimentary rocks of the Red River belt in Motley and Cottle counties by a narrow band of granite. If the course of the Red River mobile belt is projected still farther westward it corresponds with the trend of the younger Matador structures. The Precambrian rocks of the mobile belt as projected lie beneath the rhyolite flows of the Panhandle volcanic terrane. Fisher metasedimentary terrane.-The Fisher metasedimentary terrane is named for Fisher County, Texas, where, in the subsurface, it is well developed. Meta­sedimentary rocks have been encountered beneath Cambrian and younger strata in a wide arcuate area extending from northeast Nolan County north to Dickens County and including parts of Scurry, Jones, Stonewall, Fisher, and Kent coun­ties. On the south in Nolan and Fisher counties these rocks are mostly recrystal­lized quartzofeldspathic sediments and biotite schist, although a metamorphosed dolomite in Scurry County and a rhyo­lite in Jones County are probably part of the same terrane_ Northward in Dickens Basement Rocks, Texas-New Mexico County the rocks are lower metamorphic grade and consist mostly of weakly meta­morphosed arkose and phyllite very simi­lar to rocks adjacent on the north in the Red River mobile belt. Some evidence indicates that the metasedimentary rocks of the Fisher terrane are a northward sub­surface extension of the metasedimentary rocks exposed in the Llano uplift. Panhandle volcanic terrane.-In the Panhandle and south plains of Texas and parts of Roosevelt, Lea, and Chaves coun­ties of New Mexico, an extensive area is underlain by volcanic rocks. The rocks are composed chiefly of undeformed and unmetamorphosed flows of rhyolite por· phyry, with less amounts of rhyolite tuff and rhyodacite, latite, and andesite flows. The extrusive nature of the bulk of these rocks is indicated by associated tuffs, flowage structures, spherulites, and relict perlitic and crystallitic structures, al­though it is not unreasonable to expect shallow intrusives in association with such an extensive lava terrane. Locally associ­ated with the volcanic rocks are dolomite and argillaceous siltstone. Seemingly these sedimentary rocks are intercalated in and rest on the volcanic sequence and represent sedimentary accumulations dur­ing quiescent periods in the volcanic cycle and thereafter. Swisher gabbroic terrane.-The base­ment rock in northeastern Roosevelt County, New Mexico, and Castro, north­ern Lamb, Swisher, northern Hale, west· ern Floyd, Briscoe, and western Donley counties, Texas, is mostly gabbro and diabase with subordinate related diorite. These mafic rocks comprise the Precam­brian surface in the greater part of the area that lies between two separated parts of the Panhandle volcanic terrane, and in some wells they are overlain by Cam­brian or Ordovician rocks. Similar gab­hro and diabase in the form of sills have been penetrated in the surrounding vol­canic terrane. In some wells these gabbro sills in the volcanic sequence can be seen to have intruded and metamorphosed the sedimentary rocks intercalated with and overlying the volcanic rocks. Likewise, wells in the gabbroic terrane proper that did not encounter volcanic rocks also penetrate contact metamorphosed sedi­mentary rocks. The gabbro terrane does not form a con­spicuous high on the gravity map, and probably it is composed of a series of sheet-like intrusions instead of a deep­rooted mass. The fact that some wells in the volcanic terrane penetrate inter­layered gabbroic rocks and contact meta· morphosed sedimentary rocks before en· countering rhyolite porphyry, and some wells in the gabbroic terrane penetrate similar gabbroic-contact metasedimentary sequences without encountering volcanic rocks, suggests that perhaps this gab­broic terrane is, at least in some areas, no more than a thin skin overlying the vol­canic rocks which continue beneath. Wichita igneous province.-Zircon age determinations on one of the youngest granite series in the Wichita Mountains o{ southwestern Oklahoma show a late Precambrian age of 670 million years (Larsen et al., 1949). These Wichita Mountain igneous rocks and igneous rocks in the buried Amarillo Mountains of the Texas Panhandle are grouped together as the Wichita igneous province. The Wichita Mountains consist of a complex of granite and gab bro intrusions; in the buried Amarillo Mountains the rocks are mostly granite. In both areas the granites are commonly micrographic. Age relations.-The oldest unit is the Texas craton whose granites are dated at about 1,000 million years in the Llano uplift. The flanking Van Horn and Red River belts are younger features probably of late Precambrian age. Rocks of the Panhandle volcanic terrane, Swisher gab­broic terrane, and Wichita igneous prov­ince are the youngest Precambrian rocks in the area and may be part of the same igneous cyde. Age of the Fisher terrane is uncertain; it may be part of the craton and composed of relatively ancient rocks, or it may be a younger metasedimentary belt developed within the craton. Bureau of Economic Geology, The University of Texas Conclusion.-Concluding sections deal (1) with the configuration of the base­ment surface, Precambrian relief, the his­tory of Paleozoic sedimentation on the basement surface, and basement fault zones; (2) problems of the Ouachita fold­belt; (3) post-Precambrian igneous rocks encountered in wells; and (4) reflection of basement divisions in the regional gravity picture. Major lithologic-structural divisions of the basement in general agree well with regional gravity trends. The margin of the craton and the Van Horn and Red River belts are clearly shown; rootless strati­form terranes are not separately reflected. Gravity anomalies associated with the major Paleozoic structures cut across the older Precambrian trends. The final section is concerned with the Precambrian history of the region, cor­relation, subdivisions of Precambrian time, and theories of continental origin. It is suggested that the Texas craton was an independent continental nucleus dur­ing middle Precambrian time and subse­quently became a part of the larger Paleozoic North American continent by growth through development of flanking orogens during late Precambrian time. Appendixes.-Appendix I is a glossary of petrographic terms used in this paper; Appendix II presents abbreviated petro­graphic reports on all well samples studied in the course of the project and shows where the thin-sectioned material can be located; Appendix III is a report on magnetic susceptibility work done during the study, the results of which were largely negative. INTRODUCTION Purpose and scope of the investiga­tion.-An investigation of basement rocks in Texas and southeast New Mexico was begun in the spring of 1951, following completion of a study of exposed Pre­cambrian rocks in the Van Horn area of west Texas. Originally the project was intended to be phase three of a three-part program of study of Precambrian rocks of west Texas consisting of (1) investi­gation of Precambrian rocks of the Van Horn area, (2) investigation of Precam­brian rocks in the Franklin Mountains, and (3) investigation of Precambrian rocks encountered in wells in the west Texas-southeast New Mexico area. The results of the first phase of the project have been recently published (King and Fl awn, 1953) ; the second phase of the project was temporarily postponed be­cause of extensive military activity in the Franklin Mountains; the third phase of the project was expanded to include all of Texas where subsurface Precambrian rocks have been penetrated by wells; the results are presented herein. In the area of this study the term base­ment has been used to refer both to rocks whose Precambrian age is demonstrated by overlying lower Paleozoic sedimentary rocks (Panhandle, southeast New Mexico, west Texas, north-central Texas) and to ig­neous and metamorphic rocks whose age cannot be determined by stratigraphic methods (south-central and southwest Texas) . In the latter areas crystalline rocks are overlain by Mesozoic rocks. Thus, in an unrestricted sense the term basement refers to those igneous and metamorphic rocks on which younger sedimentary sequences are laid. In the Panhandle, southeast New Mexico, west Texas, and north-central Texas, base­ment is synonymous with Precambrian; in south-central and southwest Texas base­ment rocks are probably composed of metamorphosed Paleozoic rocks. Because there is a possibility that the latter are locally thrust over unmetamorphosed Paleozoic rocks, it may be that they can­not properly be considered as basement and that in the Texas area the term should be restricted to Precambrian rocks. The study of Precambrian basement rocks in Texas and southeast New Mexico was initiated after conference with a number of major oil companies in hopes that a comprehensive investigation of these rocks would aid in structural in­terpretations and would be of value in geophysical prospecting in this . region, which is so important in oil production. The area studied includes central, north, and west Texas and nine counties in south­eastern New Mexico. The writer under­took this study with the belief that it would be worthwhile to assemble and integrate data on basement rocks even if they were insufficient to draw pertinent geologic conclusions. For it is only in this way that information can be pre­served for future workers who might some day approach the same problem with twice this number of wells for control. It has been especially gratifying and fas­cinating to see major geologic trends de­velop and to integrate these trends to compose a logical picture of the hidden foundation below the much-studied sedi­mentary sections. Many of the conclusions drawn in this paper are tentative and ad­mittedly based on scanty evidence; no doubt they will have to be revised or even abandoned as more wells penetrate the ba~ement. Previous work.-No modern published work is devoted to the general problems of basement rocks in this area. Infor­mation on basement rocks from wells drilled in Texas before 1932 was com­piled by Sellards (1933, pp. 44-53, 127­110). Patton (1945a, 1945b) published descriptions of basement rocks from several wells in west Texas. Roth (1949) briefly discussed the basement in his paper on the paleogeology of the Texas Bureau of Economic Geology, The University of Texas Panhandle and included a map showing basement lithology. Moss ( 1936) dis· cussed the major features of the area in his general description of the buried Pre­cambrian surface in the United States. Basement rocks are mentioned in papers on various oil fields. A number of authors have made refer­ence to the Precambrian structures in cer­tain areas of Texas and the control that they have exerted on later structures. At an early date Ver Wiebe (1930) dis­cussed Precambrian trends in north Texas and the Panhandle. Moss (1936, p. 949) mentioned that the west-northwest trend of the Wichita geosyncline was inherited from the Precambrian, and this is in accord with the concept of the subsur­face Precambrian Red River mobile belt developed in this paper. Rettger (1932, pp. 486--490) outlined his concept of the "grain of Texas" as interpreted from magnetic anomaly maps and the struc­tures shown in exposed Precambrian rocks. Cheney and Goss (1952, p. 2237) discussed two main structural trends in the rocks of the Llano uplift; a main northwest trend and an intersecting north to north-northeast trend. Adams (1954) has given the name "Texas Peninsula" to a mid-Paleozoic positive feature whose axis more or less coincides with the axis of the Texas craton and which seems to be the result of an upwarping of the craton along its axis; he called the struc­ture the Texas arch. Cheney and Goss (1952, pp. 2262) referred earlier to the southeast part of this feature as the Con­cho arch and called the northwest-trend­ing basement nose the Concho platform. It is interesting to note how geologists in years past, working with relatively few subsurface data, were able to develop cogent hypotheses which have strength­ened with time. Moss' recognition of the Precambrian ancestor of the later Wichita ~tructure illustrates this, and his pen­etratin"0 anahsis deserves recognition. The only ;revious petrographic studies of subsurface basement rocks of particular areas known to the writer are (1) the work of Landes (1927) and Walters (1946) on Kansas subsurface Precam· brian rocks and (2) May and Hewitt's report (1948, pp. 129-158) on the base· ment complex of the Sacramento and San Joaquin valleys of California. The writer has published an abstract (Flawn, 1953b) and a progress report (1954) on the Texas basement problem. The bulk of the information on the basement is unpublished data in the files of various oil companies. Many com· panies maintain a basement contour map, and some have kept a file of petrographic descriptions of basement rocks. Mr. Rob­ert Roth, of the Wichita Falls office of the Humble Oil & Refining Company, has a very complete collection of polished core sections and thin sections of base· ment rocks encountered in wells in the Panhandle and north-central Texas area. The U. S. Geological Survey has con­siderable data on basement rocks in its Washington, D. C., and Roswell, New Mexico offices. Without exception these unpublished data were made available to the writer, who has leaned heavily upon them. Acknowledgments.-This project would have been impossible without the co­operation of individuals, companies, and government agencies concerned with the production of petroleum. The foundation of the study is the collection of all avail­able basement cores, cuttings, and thin sections, and this would not have been possible without the wholehearted co· operation of the individuals and organi­zations operating in Texas and southeast New Mexico. The following organizations and indi­viduals contributed materially to the suc· cess of this research. They are listed alphabetically by organization; the indi­viduals concerned are listed under the organization they represented at the time of their contribution to the project; Amerada Petroleum Corporation B. H. Harlton B. W. Kleihege J. A. Veeder Basement Rocks, Texas-New Mexico Anderson-Prichard Oil Corporation K. C. Anderson L. T. Teir The Atlantic Refining Company H. L. Cobb R. T. Cox H. M. Looney H.J. Morgan, Jr. S. L. Smith Brown Geophysical Company Hart Brown Cities Service Oil Company P. J. Beaver R. S. Hunt L. E. Patterson, Jr. Continental Oil Company H. H. Bybee A. E. Kersey G. W. Marshall F. L. Stead E. G. Stevenson C. D. Vertrees Cosden Petroleum Corporation J. S. Kelly ~·rost Geophysical Corporation C.H. Frost J.C. Rollins General Crude Oil Company J. C. Barker Gulf Oil Corporation A. W. Doshier R. T. Hazzard J. D. Moody Y. B. Newsom R. V. Hollingsworth Honolulu Oil Corporation D. E. Caussey H. E. Davis A. D. Slover Humble Oil & Refining Company W. E. Cox W. E. Dougherty A.H. Hedden V. C. Maley P.H. Mas•on Robert Roth R. D. Woods E. Russell Lloyd and John Hills Magnolia Petroleum Company J.E. Bucher D. E. Feray Jane Ferrell J. C. Freeman G. R. Gaige J. H. Halsey R. E. Murphy Jo•eph Neely J. T. Rouse J. R. Sandidge R. E. Wills Riley G. Maxwell Mid-Continent Petroleum Corporation C. E. Long, Jr. New Mexico Bureau of Mines and Mineral Resources R. A. Bieberman The Ohio Oil Company D. W. Franklin D. D. Heninger Phillips Petroleum Company R. J. Adams Addison Young Placid Oil Company J. R. Arnold The Pure Oil Company J. W. Luckett Seaboard Oil Company of Delaware R. E. Goodwin G. H. Sherrill B. D. Thomas Shell Oil Company J. A. Champion Ru.sell Ford J. E. Galley R. C. Spivey r;, H. Thompson J. L. Wilson Sinclair Oil & Gas Company L. L. Harden W. D. McEachin H. A. Merrill Skelly Oil Company L. S. Ditzel!, Jr. Allen Ehlers W.R. Kendall Slick-Moorman Oil Company R. V. West Standard Di! Company of Texas J.E. Aiams Thomas Beard R.H. Griffin J. H. Nich,1lson W. J. Witt, Jr. Stanolind Oil & Gas Company N. T. Brasher H. S. Edwards .T. C. Edens August Goldstein, Jr. T. L. Ingram S.S. Oriel E. L. Reed J.M. Reed J. B. Souther Sun Oil Compan~· J.M. Alcorn P.H. Horn R.H. Ley J.C. Monk L.A. Williams Sunray Oil Corporation Clarence Symes Superior Oil Company Fay Coil J. J. Maucini The Texa• Company Harvard Giddings R.H. Martin Bureau of Economic Geology, The University of Texas Tide Water Associated Oil Company R. C. Zethraus Union Oil Company of California S. C. Giesey The University of Texas V. E. Barnes R. K. Deford J. T. Lonsdale U. S. Geological Survey P. B. King Charles Milton T. F. Stipp The writer was assisted in the magnetic susceptibility work by W. R. Varnell, a graduate student in physics at The Uni­ versity of Texas and formerly a member of the staff of The Texas Company. His careful work on this phase of the prob­ lem is gratefully acknowledged. Special thanks are due John T. Lonsdale, Philip B. King, Raymond D. Woods, and Peter H. Masson for critically reading the manu­ script. Collection, treatment, and presentation of data.-The first task was compilation of a comprehensive list of all wells pene­trating basement in the area of study. This was achieved by combing the literature, particularly the annual review issue of the Bulletin of the American Association of Petroleum Geologists, by consulting the oil pages of back issues of newspapers, in particular the San Angelo Standard­Times, and by utilizing the files of a number of oil companies. Mr. T. F. Stipp, of the U. S. Geological Survey's Roswell, New Mexico, office, deserves special men­tion by reason of his generous contri­bution of tabulations of basement wells which he has kept up to date over a period of years. A preliminary list of basement wells was mimeographed and circulated with a request for corrections and additions. Following compilation of the master list, the writer visited district offices, di­vision offices, and research laboratories of a number of oil companies and service companies, discussed the project with geologists and administrative officials, and acquainted them of his need for base· ment cores and cuttings. The response was positive and immediate, and without exception these organizations were willing to devote the time and effort needed to obtain the necessary material. Needless to say, it was not possible to obtain samples from all the basement wells in the area. Records and samples from old shallow wells, mostly drilled before 1932, in the Panhandle, around the Llano uplift, and in north Texas are difficult to locate and, indeed, in many cases no longer exist. The amount of material available ranged from sizable cores through small core chips and cuttings to thin sections only. The only record of the basement encountered in some wells is preserved in the thin­section collections of certain oil com· panies. The Midland office of the Shell Oil Company, the Stanolind Oil & Gas Com­pany's research laboratory at Tulsa, the Humble Oil & Refining Company's Hous­ton research laboratory and Wichita Falls office, and the Midland office of the Hon­olulu Oil Corporation were kind enough to loan their collections of thin sections and thus made available data on many wells for which no sample material was located. Information on individual basement wells is presented in Table 1, which was compiled from the files of a number of oil companies. Some of the information received was conflicting, and although every effort was made to resolve the con­fli ct, it could not always be done. Where there were differences in the basic well information, such as location, completion date, elevation, or total depth, information in the records of the Texas Railroad Com­mission was considered final. Where dif­ferences were due to geologic interpre­tation in matters of depth to top of base­ment or age of formation overlying base­ment, the solution was not as easy, even with the fullest assistance of company geologists. Petrographic descriptions of thin sec· tions from each well studied are presented in abbreviated form in Appendix II. Base­ment wells, elevation of basement, for­mations resting on basement, basement rock types, and major geologic divisions Basement Rocks, Texas.New Mexico of the basement, are shown on Plate I. In areas where control from basement wells is adequate, contours are drawn on top of the basement and are intended to outline the major topographic and structural fea· lures of the known basement. They are not extended into areas where the base· ment has not been penetrated, and where some other so·called basement maps show contours extrapolated downward from known sedimentary horizons. Plate II shows schematic cross·sections of the base· ment along lines located on Plate I; Plate III is composed of enlargements of the Central Basin Platform, Fort Stockton high, and Muenster up.lift areas from Plate I. Probably more than 800 wells (ex· eluding Ouachita foldbelt wells) have penetrated basement rocks in Texas and southeast New Mexico; data for 760 of these are listed in Table L Many new basement wells have been drilled since the preparation of this manuscript. Cores, cuttings, or thin sections were obtained for 488 basement wells; 251 thin sections from 201 cored basement wells and 485 thin sections of basement cuttings from cored and uncored wells were studied. Pertinent specimens were photomicro· graphed (Pis. IV to X). At first glance it would appear that it was possible to col· lect material for only a small percentage of known basement wells ; however, a distribution factor must be considered. A large number of wells are crowded closely together in fields. For example, there are more than 200 basement wells in the Brunson area, 23 basement wells in the Keystone field, 9 basement wells in the Embar field. If enough basement ma­terial from various field wells was avail· able to give a representative picture of the basement in that locality, no special effort was made to get complete collec­tions ; on the other hand, very special efforts were made to obtain basement samples from the sparsely distributed wildcat wells between areas of more com· plete coverage. At the outset of the project, magnetic susceptibility and specific gravity de­terminations were made on suitable ma­terial received. After about 100 such de­terminations were made, the practice was discontinued for the following reasons: (1) of the total basement material re­ceived (cores, cuttings, and thin sections) satisfactory magnetic susceptibility de­terminations were restricted to core samples-only a few samples of cuttings were usable; (2) wide variations in sus­ceptibility of samples of similar rock types and in samples from the same well show that these measurements have little geologic significance unless sampling can hf. closely controlled, and in this study we are forced to deal with random well samples. The results of the susceptibility work are in Appendix III. An abstract of these results has been published (Flawn, 1953c, pp. 55-58). The mag­netic susceptibility research was made possible by the cooperation of Frost Geophysical Corporation and Frost Air­borne Surveys, Inc., of Tulsa, Oklahoma. A magnetic susceptibility bridge designed and built by this organization was shipped to Austin for the work. In assimilating and processing the data in this study, the writer was concerned with the petrography, structure, and topography of the basement, and the re· gional gravity picture--elements of all of these topics are included in the paper. Unfortunately no regional magnetic coverage was available for the study. PETROGRAPHIC METHODS Petrographic nomenclature.-Through·· out this report an attempt is made to use a standard and simplified petrographic­terminology. It is anticipated that this; publication will be of interest to pe­troleum geologists not familiar with all of the textural terms used by petrog­raphers, and a short glossary of petro­graphic terms is included to simplify reading (Appendix I). Bureau of Economic Geology, The University of Texas Rocks of unusual mineral composition which have been given geographic petro­graphic names are here reduced to com­mon petrographic names and their un­usual mineral composition is indicated by appropriate prefixes. Leeuwfonteinite, a biotite-hornblende syenite whose prin­cipal feldspar is anorthoclase, can be cited as an extreme example of a rock laboring beneath a geographic name that completely conceals its nature. In this report this rock would be called a biotite­hornblende -anorthoclase syenile. This term may seem more unwieldy than leeuwf onteinite, but it is not as unwieldy as a petrographic dictionary. Some may point out that syenite itself is a geographic name, but from a practical point of view, the mineralogy of syenite is well known by most geologists. The igneous rock nomenclature used in this paper is shown in Table 2, which should be used in conjunction with the glossary (Appendix I). This system com­bines elements of both the Niggli and Johannsen classifications (Niggli, 1931, pp. 296-364; Johannsen, 1939, pp. 141­162) . In the writer's opinion, most of the igneous rocks can, with appropriate mineral modifiers, be named with these common terms. Much of the confusion that exists to­day in the use of these common terms arises from a disagreement over the treat­ment of the mineral albite. Albite, the sodic end-member of the plagioclase series, is chemically and physically an alkali feldspar which, in petrogenic anal­ysis, is associated with potassium feld­spars. Mineralogically, however, albite occurs perthitically intergrown with po· tassium feldspar and as a separate min­eral constituent. It seems desirable in classification to recognize the separately­occurring albite as plagioclase. For ex· ample, the feldspar of classic granite is composed of predominant potassium feld­spar and subordinate oligoclase or ande­sine plagioclase; commonly the potassium feldspar is a perthite and contains albite. If, on the other hand, the oligoclase or andesine plagioclase is the predominant feldspar, the rock is a granodiorite. If the accompanying plagioclase is albite instead of oligoclase or andesine, there are two alternatives: (1) lump the al bite with potassium feldspar and call both rocks granite, perhaps distinguishing the one as albite granite, or (2) consider the rock in which albite predominates as a ~pecial variety of granodiorite and desig­nate it albite granodiorite. (The albite intergrown with potassium feldspar in the form of perthite or microperthite is con· sidered, for classification purposes, an inseparable part of the alkali feldspar.) The writer, in classification, has adopted the latter system. In organizing myriad petrographic data so as to fit nature's continuous systems into pigeonholes, this has proven superior in expressing sig­nificant differences in mineral compo­sition. The same problem, of course, arises with the quartz-free analogs of granite and granodiorite-syenite and diorite. If albite is separated from the potassium feldspars, an albite-rich syenite is an albite diorite (see Table 2). More­over, gradational varieties such as al­bite-oligoclase present less of a problem if albite is grouped with the plagioclase. Many of the rocks called micrographic granite in this paper are called grano­phyres by other petrographers. In this paper micrographic granite refers to granitic rocks distinguished by a more or less regu Jar intergrowth of quartz and alkali feldspar too fine-grained for recog­nition without magnification. For its coarser counterpart the name graphic granite has long been employed. The term granophyre has suffered a varied and con· flicting usage through the years, but the modern tendency is to use it to include granitic rocks showing regular or cunei­form intergrowth and/ or irregular inter· growth and/ or myrmekitic intergrowth of quartz and alkali feldspar (see glossary, Appendix I) . The term has become a sort of catch-all name for granitic rocks with a microscopic quartz-alkali feldspar in­tergrowth. Because this paper is pri­ TABLE 2. Nomenclature for common igneow rocks. Plagioclase* < 5% of tota I feldspar Fine-groi1ted and/or glossy equivalent Plagioclase> 5 < 50% of total feldspar Fine-groined ond/(;)r glossy equival~nl Plagioclase > 50 < 95% of total feldspar; alkali feldspar*>5% Fine-gro1Md and/or glossy ~quivo/11nl Plagioclase > 95% of total feldspar; alkali feldspar< 5% Fine-groined and/or glossy equivalent Quartz > 5% < 50% of the felsic minerals Plogioclose < An 50 Plogioclose > An 50 Al kal i-gra nite (normally< Y4 ferro­mognesion minerals) Rhyolile Granite (normally < 'f4 ferro­mognesion minerals) Rhyolite Gronodiorite (oligoclose or ~ndesine­others nomed) (normolly < 1/4 ferro­mognesion minerals) Rhyodocite Quartz diorite (oligoclose or ondesine­others nomed)+ (normally 'le-~ ferro­mognesion minerals) Docile Granogabbro (normally 114 -•12 ferro­mognesion mi nero Is) Rhyobosolt Quartz gabbro (normol ly '.4 -112 ferro­mognesion minerals Ouortz basalt Quartz< 5% of the felsic minerals Plogioclose > An 50 Al kali-syenite (normally < \14 ferro­mognesion m1nerols) Plogioclose < An 50 Trocflyte Syenite1 (normally Al -~ ferro­mognesion minerals) Trochyte srcenodiorite (oligoc ose or ondesine­others nomed)+ (normally !f4-!1z ferro­mognesion minerals) Trocflyondesite Diorite (oligoclose or ondesine­others nomed)* (normally \14-'k ferro­mognes io n m1nerols) Andesite Syenogabbro (normally 3/5-S/5 ferro­mognesion minerals) T rochybosolt Gabbro (normally ~1e-s1e ferro­mognesion minerals) BosoIt l::o e "' ;'! "' ~ ::i;, 0 ;.;... " ·"' f ~ e iii:: ~ g· *Albite is grouped with the alkali feldspar where it occurs in perthitic intergrowths; it is grouped with plagioclase where it is a distinct mineral constituen t. t If the plagioclase is other than oligoclase or andesine, the variety must be prefixed to the rock name, that is, albite granodiorite. NOTE : Rock names are prefixed wi th micro where grain size of the rock is between 0.05 and 1.0 mm, that is, microgranite. Rock names are prefixed with leuco to express abnormally low ferromagnesian content and with mela to express abnormally high ferromagnesian mineral content. Fine-grained 1<3 mm: coarse-grained >3<5 mm; very coarse-grained >5 mm. ,_. -.:i marily for the practicing petroleum ge­ologist rather than the petrographer, the writer has used the term micrographic granite in a broad sense so as to avoid introduction of a rock name not familiar to many geologists; in the rocks studied cuneiform and irregular intergrowths predominate-myrmekitic intergrowths are rare. It is evident from the name micrographic granite that a type of granite is in question ; granophyre, on the other hand, does not have this ad­vantage. Rhyolite porphyry is a very common rock type in parts of the Panhandle, west Texas, and southeast New Mexico basement, and the use of the term de­serves special clarification. Rhyolite is defined herein as a glassy and/or micro­crystalline igneous rock that is chemically and, except for the possible presence of glass, mineralogically equivalent to gran­ite. Rhyolite porphyry may be an ex­trusive rock-a lava flow-or an in­trusive rock (generally intruded in a shal­low or near-surface environment) . The principal mineral constituents of rhyolite and rhyolite porphyry are potas­sium feldspar and quartz with subordinate sodic plagioclase. A common rock type encountered in the basement contains phenocrysts of albite and/or potassium feldspar in a groundmass that is mostly cryptocrystalline. No quartz phenocrysts are present, but local coarsenings in the groundmass show that an appreciable quartz content is concealed by crypto­crystallinity. Because these rocks are identical with other rocks in the same area in which quartz is present as pheno­crysts or in which a more coarsely crystal­line groundmass reveals the presence of quartz, they are included as rhyolite porphyry. Probably some of these rocks strictly speaking are trachyandesite or, if one is an advocate of the term monzonite, they are latites. In the same way albite and quartz phenocrysts occur in a ground­mass composed almost entirely of alkali feldspar and quartz. This rock is a rhyo­lite porphyry, but the same rock with a cryptocrystalline groundmass might, on the basis of the phenocrysts alone, be called dacite or rhyodacite. Metamorphic rock terminology is in poor repair and a general overhaul to put it on a descriptive and quantitative basis is becoming imperative. Some metamorphic rocks are named purely on the basis of mineralogy (amphibolite); others, such as hornfels, are named on the basis of fabric. The most common metamorphi<' rocks, such as slate, phyllite, schist, and gneiss, are named according to the nature o{ their foliation or metamorphic struc­tures and their mineralogy. In practice, in dealing with cuttings of fine-grained weakly metamorphosed argil­laceous rocks, it is difficult to classify the rocks on the basis of structure and min­eralogy. With the aid of some excellent suggestions from Dr. August Goldstein, Jr., of the Stanolind Oil & Gas Company's Research Laboratory, the writer outlined a classification of weakly metamorphosed argillaceous rocks based on degree of mineral reconstitution, grain size, and structure (Flawn, 1953a). This classifi­cation is used in this paper (Table 3) in a slightly modified form. The lower grain size limit for schist has been revised downward from 0.5 mm to 0.1 mm to pro­vide an overlap between the lower grain size .limit for schist and the upper limit of phyllite. The term phyllite is customarily applied to a very fine-grained foliate rock whose constituent minerals cannot be re· solved without magnification. In a mega· scopic view the rock has a characteristic silky sheen or luster caused by tiny mica or chlorite plates in parallel orientation. Most of the rocks called phyllite contain sericite, chlorite, or biotite, and the name carries a connotation of low metamorphic grade. However, conditions of metamorphism, in particular stress conditions, may be such as to produce foliate rocks containing higher grade minerals while inhibiting develop· ment of coarser grain size. There seems to be no general agreement on whether foJi. ate rocks containing such minerals as am­phibole or garnet but with a grain size less Basement Rocks, Texas-New Mexico than 0.5 mm should be called phyllite or fine-grained schist. The term meta-argillite is not entirely satisfactory; it is hardly euphonious and incorporates the questionable practice of prefixing an already metamorphic rock name with meta. Possibly terms such as low-rank argillite and high-rank argillite are preferable, but the use of the term meta-argillite is continued in this paper. In this paper a quartz sandstone which has been weakly metamorphosed so that the argillaceous and ferromagnesian in­tergranular material is reconstituted to micas and chlorite but in which the quartz has not recrystallized is named meta­sandstone. For the higher grade rock in which quartz has recrystallized, the term metaquartzite is in common usage. When dealing with the related rock-arkose­a nomenclature problem arises. If, by analogy with quartz sandstone, the weakly metamorphosed unrecrystallized rock is called meta-arkose, we are left without an appropriate term for the higher grade recrystallized arkose. Unfortunate) y in a previous publication the writer used meta­arkose for the higher grade recrystal­lized arkose rocks in the Van Horn area (King and Flawn, 1953). He is now of the opinion, however, that meta-arkose should be restricted to weakly metamor­phosed partly reconstituted and recrystal­lized arkose and that a new term be pro­posed for recrystallized arkose equivalent in metamorphic grade to metaquartzite. In this paper the term metarkosite' is applied to such rocks. Possibly the writer may be criticized for not dusting off some previously used name for this type of rock and redefining it. Granulite is used by Harker (1939, p. 24°6), although he notes that it is open to objection because of previous varied usage. In the author's opinion metarkosite is preferable to granulite because it ex­presses in some degree the nature of the rock in question. The estimated mineral composition of all rocks examined in thin section is in­cluded in Appendix II. Readers accus­tomed to a different classification of rocks than is used herein may make changes in rock names to satisfy themselves. For ex­ample, many of the rocks called grano­diorite in this classification might be quartz monzonite to geologists favoring a different system of nomenclature. Evaluation of petrographic data.­ Study of rocks from thin section alone is l This term is equivalent Lo the arko.sile of Crout (1932, p. 367) ; the writer prerers melar1co1ito to indicate more clearly the metamorphic nature of !h e rock. Arko.site 1eem1 more or leu the same type of ii:roup term a1 quartxiie, and for many yean it bas proven ncctinary to di1tingui1h metaquartzite from orthoquarlzile. TABLE 3. Nomenclature for metamorphosed argillaceous rocks. DEGHEF: OF RECONSTITUTI0:-1 WIT II Ol/T Cl.EAVAGF. OR PARTING WITH CLEAVAGE OR PARTING WITH FOLIATION ------ Unreconstituted Claystone Grain sir.et < 0.01 mm. Shale Grain size <0.01 mm Less than 50 percent Argillite Clay-slate reconstituted Grain size up to 0.05 mm Grain size up to 0.05 mm ~---------­ More than 50 percent Met a· a r gill it e Slate reconstituted Grain size up to 0.05 mm Grain size up to 0.05 mm Completely Hornfels Phyllite reconstituted Grain size shows Grain size up to 0.5 mm wide range Schist and gneiss* Grain size >O.I mm • Conditions of metamorphism may be such as to produce medium or high-grade metamorphic rocks with l!irain size less than 0.1 mm. Because of their increased metamorphic i:rade such rocks cannot be called phyllites; they should be termed very fine-grained schist, gneiss, hornfels, et cetera, depending on their metamorphic structure or texture. i Grain size ranges are not strictly limiting. Bureau of Economic Geology, The University of Texas not a satisfactory procedure because many of the characteristic structures and re­1ations to be seen in the field and in hand specimens cannot be observed under the microscope. But when studying rocks from wells the geologist must accept a limited method of study; it is not possible to ob­serve field relations and, unless a core is available, it is not possible to make an effective megascopic examination. In· tcrpretations of basement lithology in those many wells for which only cuttings are available are, of course, subject to a greater error than interpretations of base­ment rock in wells that have been cored. The presence of conglomerate or breccia, for example, may be completely un­suspected in wells from which cuttings only are available. Likewise, metamorphic features such as gneissic structure or slaty cleavage; igneous features such as in· trusive contacts or flowage structures; and sedimentary features such as cross-bed­ding or ripple marks are commonly un· recognizable from the study of cuttings alone. In dealing with cuttings the amount of information that can be gleaned de­ pends on the ratio of size of cuttings to grain size of rock. Fine cuttings of coarse­grained rocks may be composed only of individual constituent mineral fragments that give no clue as to the fabric of the rock, whereas large cuttings of fine­grained rocks are representative rock chips. Thus, without opportunity to ex­amine field relations or even cores, the petrographer is handicapped and may have difficulty in classifying rocks that are altered, or without diagnostic texture, or whose texture cannot be determined because of fineness of cuttings. It is un­derstandable that under such circum­stances petrographers may differ on a rock name. Resolution of petrographic data.­There is little value in hundred5 of petro­graphic descriptions of basement well cores or cuttings unless the material can be organized or grouped to yield a picture with geologic significance. Four major rock types can be distinguished among the basement rocks in the west Texas­southeast New Mexico area: metasedimen­tary rocks, meta-igneous rocks, volcanic rocks (tuffs and lava flows), and plu­tonic rocks. The last heading includes megascopically crystalline intrusive ig­neous rocks of various mineralogy with characteristically hy"pidiomorphic granu­lar fabric such as granites, granodiorites, syenites, quartz diorites, diorites, and gab­bros. With the present spacing of wells it is not possible to map the various sub­types of the plutonic terrane except where a distinctive family, such as gabbro-dia­base, is penetrated exclusively over a wide area. In the large view, moreover, it is doubtful if such subdivision would be of great geologic significance in a regional study of the basement. Rock classifications set up pigeonholes defined arbitrarily so that the feldspar of a granite, for example, is predominantly potassium feldspar while the feldspar of granodiorite is dominantly plagioclase. In many so­called homogeneous rock masses the relative percentage of these two feldspars varies from place to place so that both granite and granodiorite are present. The same variation commonly takes place in the quartz content so that a rock mass may range in composition from quartz syenite to granite, depending on whether it has 4 percent quartz or 8 percent quartz. The writer has examined core fragments of basement rock taken at close intervals that ranged from granodiorite to quartz diorite to diorite and, presumably, such changes in compos1t10n are common within that intrusive body. If it were possible to study such bodies closely it is certain that these changes would be of geologic significance, but when dealing with random well samples such a study is beyond the realm of possibility. D1scuss10N OF STRUCTURAL NoMEN· CLATURE AND CONCEPTS A great deal of the credit for the mod­ern concepts of the structure of the earth's crust belongs to Leopold Kober whose Basement Rocks, Texas-New Mexico classic work Der Bau der Erde ranks high in geological literature. Discussing stable versus mobile elements of the earth's crust he says (Kober, 1921, p. 21): Die alten erstarrten Tafeln wollen wir bier kurzweg auch als Kratogen bezeichnen. Die Oro· genetischen Zonen als Orogen. Dieser Begriff fiillt in gewissem Sinne mit dem der Geosyn· klinale zusammen. Dabei miissen wir festhalten, dass das Orogen die ausgepresste Geosynklinale ist, also eine Zone, die vie] schmiiler ist als die urspriingliche Geo· synklinale. Wir werden spiiter sehen, dass wir das Breitenverhiiltnis des Orogen zur Geosynklinale mit 1 :2-3 setzen diirlten, d.h. die Geosynklinale wird durch die grossen Gebirgsbildungen im Orogen auf 'h-'h der urspriinglichen Breite zu· sarnmengepresst. The ancient solidified crustal plates we will here, for brevity, refer to a~ Kratogen ; the oro· genie zones as Orogen. This concept coincides in a certain sense with that of the geosyncline. Here we must continue to bear in mind that the orogen is the compressed or squeezed out geosyncline, that is, a zone which is much nar­ rower than the original geosyncline. We will later see that we may estimate the ratio of widths of the orogen to the geosyncline at 1 :2-3, that means the geosyncline is pressed together through the major mountain building in the orogen to 'h to % of the original width. Stille (1936, p. 84) suggested that Kraton would be a better word form than Kober's Kratogen, and from this the English cra­ton stems. Kay (1951, pp. 4, 107) defines craton as a relatively immobile part of the earth, although large size is also in· herent in the term. Epeirogenic movement, tilting, and warping do not impugn the concept of the craton as a stable or im· mobile block. It is contraposed to the mobile belt (Kober's orogen)-a linear and commonly arcuate prism of rock that at one time in geologic history achieved considerable mobility through the action of orogenic forces. Mobile belts of the geologic past are recognized today as foldbelts, and the record of the mobility of the constituent rocks is read in their folded, thrust-faulted, metamorphosed, and intruded state. As pointed out by Kay (1951, p. 4), stability and mobility are transitory properties. The craton may grow as once m<>bile areas are welded to it, or it may be divided by new mobile belts forming within the once stable area. The question of how much mobility a craton can achieve without invalidating the principle of stability or how much mobility must be attained before a prism of rocks qualifies as a mobile belt is one o{ degree: Recognition of a once mobile zone by a study of well cores and cuttings is an in­ductive process. Four characteristics are common to the major mobile belts of the past that are exposed to view at the earth's surface: (1) a great thickness of sedi­mentary rocks, (2) more or less meta­morphism and advanced to extreme de­formation of the sedimentary rock prism, (3) the presence of intruded meta-igneous and igneous rocks, and (4) a linear or arcuate form. Of these four characteristics only the first-thickness of the sedimen· tary prism-is not susceptible to demon· stration by study of well samples. Oil companies are for the most part reluctant to drill thousands of 'feet of metamorphic rocks, and only the driller who contracted to "go to granite" is happy drilling a soft mica schist. The microscopic record of the most important feature of the once mobile belt-mobility of the rock prism­can be read in the metamorphic structures of the constituent rocks. The word terrane is used in this paper chiefly because of its lack of genetic im· plications. It was much used by recon­naissance geologists before the turn of the century when, in investigations of terra incognita, its noncommital nature made it particularly apt. It is especially appli­cable in subsurface studies of Precam· brian rocks, which are certainly an un­known territory . .The term may be defined as an area of similar rock type. The Precambrian igneous rocks of the Wichita Mountains and their subsurface extension in the buried Amarillo Moun­tains of the Texas Panhandle are grouped together as the Wichita igneous province because they appear to be related min­eralogically and temporally. Some geologists may be troubled by application of the term "craton" to an area where the basement has subsided Bureau of Economic Geology, The University of Texas deeply in a number of basins, preferring to consider a craton as an upwarped en­duringly· positive area in the strict sense of a "shield." However, subsidence and de· velopment of sedimentary basins within the stable area do not preclude the con­cept of the craton. Kay (1951, p. 4) says: The early Paleozoic craton of North America which had persisting influence on continental de­velopment and has close correlation with present structures is an hedreocraton ("steadfast cra­ton") (Kay, 1947), a term introduced to retain a constant reference in discussing prolonged his­tory. Although the hedreocraton was compara­tively stable, geosynclines or basins subsiding during deposition formed in many areas at sev­eral times. The volume of contained rocks is great, but relatively small compared to that of the rocks in the orthogeosynclines that surround the hedreocraton. King (1951, p. 3) refers to the con­tinental nucleus, as it exists today as the Central Stable Region: This is the Central Stable Region of North America, the nucleus of the continent, which has been only mildly deformed since the beginning of Paleozoic time. Within it, Pre-Cambrian base­ment rocks stand relatively high, forming a wide platform which to the south and west is more or less mantled by Paleozoic and Mesozoic sedi­ments, but to the north and east emerges to form the Laurentian Shield. In this account, the Central Stable Region will be treated under two headings-the Laurentian Shield, or emerged part of the Pre-Cambrian platform, and the Interior Lowlands, where the platform is mantled by Paleozoic and later sedi­ments. The visible structures of the shield are mainly those of Pre-Cambrian rocks, and those of the surrounding lowlands are mainly those of the Paleozoic and Mesozoic cover. Since the begin· ning of Paleozoic time, the shield has probably been somewhat less mobile than the adjacent low· lands, and it may neYer have been completely covered by sediments. However, the tectonic dif­ferences between the two parts of the stable re­region are those of degree rather than kind, and the boundary between them is determined by re­tention or removal of the sedimentary cover by accidents of Cenozoic and earlier erosion. In this paper we are trying to re­construct structural history throughout a long span of time, and because structural properties such as stability and mobility are transitory features as applied to a particular segment of the earth's crust, a word of nomenclatorial caution is in order. The features shown on Plate I, the Texas craton and various mobile belts, are features of Precambrian time. Their stability and mobility are properties they possessed during a span of time in that era. The mobility of structural zones such as the Van Horn mobile belt and the Red River mobile belt was destroyed by de­formation and intrusion in late Precam­brian orogenies. The hedreocraton of Kay (1951, p. 4) and the Central Stable Re­gion of King (1951, p. 3), on the other hand, are names applied to the expanded continental nucleus as it existed at the beginning of Paleozoic time. This ex­panded stable core includes both the sta­bilized mobile belts of earlier time and the older Precambrian stable areas as well. The Texas craton is a smaller Pre­cambrian craton within the larger Paleo­zoic craton; the Van Horn and Red River mobile belts are late Precambrian mobile belts but also, without mobility, are part of the Paleozoic craton. The term Texas craton may be properly applied to both an independent Precambrian stable mass and to a geographical division of the Paleozoic craton (or hedreocraton) be­cause of its uninterrupted billion-year history of stability. LITHOLOGY AND STRUCTURE OF THE PRECAMBRIAN BASEMENT General remarks.-In the area of Texas and southeast New Mexico that concerns this project, Precambrian rocks crop out in four areas: in west Texas in the Van Horn area, the Pump Station Hills, and the Hueco Mountains (King and Flawn, 1953) and in the Llano uplift of central Texas (Paige, 1910, 1912; Stenzel, 1932; Barnes, 1945; Keppel, 1940; and Goldich, l 941) . Northeast of the area of study Pre­ cambrian rocks crop out in the Wichita and Arbuckle Mountains of southern Oklahoma (Hoffman, 1930; Taff, 1904; Taylor, 1915; Uhl, 1932) ; west of the area of study Precambrian rocks are ex­ posed in the Franklin Mountains of Texas (Richardson, 1909) and the Organ Moun­ tains, San Andres Mountains, and Oscura Mountains of south-central New Mexico (Lindgren et al., 1910; Dunham, 1935). Much information that aids in the in­ terpretations of the subsurface Precam­ brian rocks encountered in the basement wells in the Texas-southeast New Mexico region can be gained from examination of reports on these several areas. The lithologies of the basement rocks as determined by thin section study were plotted on the base map to determine (1) if any lithologic divisions are recog­nizable, (2) if there is a correspondence between the lithologic divisions and the major structures in the area, (3) if there is correspondence between the lithologic divisions and regional gravity features. It was found that on the basis of the petrographic study seven major divisions can be distinguished in the basement in the Texas-southeast New Mexico area; that in three areas there is a coincidence of a lithologic division and a major struc­ture; and that the regional gravity pic­ture is strongly influenced by these same three lithologic-structural divisions. The ~even lithologic and lithologic-structural divisions are herein named (1) Texas era· ton, (2) Van Horn mobile belt, (3) Red River mobile belt, (4) Panhandle vol­ canic terrane, (5) Fishermetasedimentary terrane, ( 6) Swisher gabbroic terrane, and (7) Wichita igneous province (fig. 1). Names of (5) and (6) are proper names taken from Texas counties in which the particular terrane is well developed. The fundamental basement element is the Texas craton-a northwesterly elon­gated mass of plutonic igneous rocks with lesser amounts of meta-igneous rocks and metasedimentary rocks that appar­ent!y constitutes a stable cratonic mass. To the southwest the craton is bordered by the highly deformed metasedimentary and meta-igneous rocks of the Van Horn mobile belt which have been thrust north­ward against the craton. To the north­east, control is poor but there are meta· sedimentary and intrusive igneous rocks, metamorphosed and unmetamorphosed, that occur in a belt south of and more or less parallel to the younger Wichita structure and are essentially congruent with the Red River uplift; this belt is herein called the Red River mobile belt. In the Panhandle and south plains the plutonic rocks of the craton are overlain by wide stretches of essentially unde­formed and unmetamorphosed volcanic rocks, mostly rhyolite porphyry and rhyolite tuff, of the Panhandle volcanic terrane. Extending from Nolan County north through Fisher, Kent, and Dickens counties is a belt of low-grade metasedi­mentary rocks called the Fisher meta­sedimentary terrane2 whose relationship to the craton and to the Red River mobile helt is an enigma. The Swisher gabbroic terrane is located in the south plains in the area of Swisher, Briscoe, Bailey, Lamb, Hale, and Floyd counties. :These rocks, gabbros and diabases, occur as an irregular subsurface mass occupying part 2 Jn an earlier phase of the basement 1tudy, al the time of publication of the progresa report (Flawo, 1954), the available well dala indicated that the Fisher meta&edimentary zooo WH arcuate in shape and hence the name Fi1ber mela&edimentary arc was adopted. Additional well information 1how1 thi1 fea• lure to have an irregular 1hape. The Lerm are i1 abaodoned in fa,•or o{ the more noncommillal terrane. Bureau of Economic Geology, The University of Texas Fie. 1. lnuex map of basement terranes and provinces. of the structural low of the Palo Duro basin, as sills in the undeformed late Precambrian volcanic terrane, and as sills intruding and metamorphosing sedi­mentary rocks of probable late Precam­brian age. The Swisher gabbroic terrane io apparently a very late Precambrian feature; possibly it is a large lopolith lying on and intruding the rocks of the Panhandle volcanic terrane along the axis of a major sag or syncline. North of the Red River mobile belt younger Pre­cambrian intrusive igneous rocks crop out in the Wichita Mountains of south­ Basement Rocks, Texas-New Mexico ern Oklahoma. These rocks are apparently correlative with the subsurface rocks of the Amarillo uplift and they are here grouped together as the Wichita igneous province. Most of the main structural features familiar to the petroleum geologist in the Texas-southeast New Mexico area are late Paleozoic, Laramide, or Tertiary in age. From this study it appears that ma­jor younger structures in several areas are directly related to and controlled by Precambrian elements. First, there is a correspondence between the late Pre­cambrian Red River mobile belt and the later structures of the Red River and Matador uplifts. Possibly the Wichita Mountain structure immediately to the north in Oklahoma is also controlled by this presently ill-defined Precambrian mobile belt. Second, the southeast bulge of the craton in central Texas (marked by the Llano uplift and the Balcones fault zone) seems to have exerted a con· trol over tectonic activity since middle Precambrian time when, with cessation oi emplacement of granite batholiths which are in part exposed in the Llano uplift, it became a stable mass. In Paleo­zoic time it controlled the location of the Ouachita trough or geosyncline which is deflected around it, and it stood as a buttressing foreland against late Paleo­zoic orogcny during the collapse of the Ouachita geosyncline. The same zone of weakness, the margin of the craton, is re­sponsible for the younger Balcones fault zone and associated igneous activity. In the Van Horn area the line between the craton and the mobile belt or orogen af!ain appears to have influenced, if not controlled, younger structural trends be­cause the boundary between the more mobile Mexican overthrust province to the south and the stable predominantly basin-and-range structure to the north roughly coincides with the margin of the craton. TEXAS CRATON Definition.-The Texas craton needs definition both in space and in time. Geo­ graphically, it is a great northwesterly elongated, mostly subsurface, mass of essentially granitic Precambrian plutonic rocks which extends from central Texas into southeastern New Mexico. Its bound­aries are marked more precisely by younger geological features (Pi. I). To the southeast the Texas craton is now de­limited by the Ouachita foldbelt and the Balcones fault zone which have developed peripherally to it; to the north and west it passes out of the area of study and is partly concealed by the Panhandle vol­canic terrane; in the far southwest it is bordered by the Van Horn mobile belt which, eastwardly, through unknown geologic relationships, gives way to the Ouachita foldhelt; to the north and north­east the picture is vague because of poor well control. In north Texas the Red River mobile belt appears to border the craton. To the west, however, an area comprising parts of Hall, Childress, Donley, and Col­lingsworth counties is composed of rocks of cratonic type, mostly granodiorite, but lies north of the Red River belt, south of the upfaulted rocks of the Wichita igneous province, and east of the volcanic terrane. This is part of a much larger block which to the west constitutes the floor on which the rocks of the Panhandle \'olcanic terrane were deposited. On the map (Pl. I) this block is labeled "Texas craton? ," but there is some doubt in the writer's mind whether the term should in­clude this block between the Red River belt and the Wichita igneous province or whether it should be restricted to the block south of the Red River belt. Throughout this paper the term Texas craton is used in the more extensive, albeit less definitive, sense. The corollary inter­pretation is that the Red River belt, in its western part, penetrates the stable area. If this Hal 1-Childress-Donley-Col lingsworth County block (and its sub-volcanic ex­tension to the west) is not part of the Texas craton there are two possibilities: (I) it is part of another or independent massif or (2) it is part of the Wichita igneous province. The problem may be Bureau of Economic Geology, The University of Texas resolved when core samples sufficient for absolute age determinations are available. In time, the existence of the Texas craton dates back about 1 billion years ago when this part of the crust was stabi­lized by widespread granitic intrusion. It existed as an independent or discrete stable block throughout part of late Pre­cambrian time; by the end of Pre­cambrian time it had Jost its position as a separate nucleus and was a part of the larger North American craton. Llano uplift.-Part of the Texas craton is exposed in the Llano uplift of central Texas where ancient metasedimentary rocks are intruded by granites of several ages. The metasedimentary rocks are di­vided into two units: (1) Valley Spring gneiss -mostly light-colored pinkish quartzofeldspathic rock and (2) Pack­saddle schist-mostly dark-colored mica and amphibole schists, locally graphitic, and amphibolite, with subordinate marble (Paige, 1910, 1912; Barnes, 1945) . Very little detailed mapping and petrographic work have been done on these rocks. Stenzel (1932, 1935) recognized three different granites in the Llano area which, in order of decreasing age, he named Town Mountain granites, Oatman Creek granites, and Sixmile granites. Town Mountain granites make up the bulk of the batholithic intrusions in the area. A lead­uranium age determination on a urani­nite sample from the Baringer Hill peg­matite, which cuts the Town Mountain granite, gave an age of 1,100 million years (Holmes, 1931, pp. 330-334, 351) . The relationship of the pegmatite to the Oatman Creek and Sixmile granites is open to question; by field relations all are younger than the Town Mountain but they may be differentiates of the Town Mountain. A magnetite·helium age determination on the magnetite from Iron Mountain, Llano County, Texas, resulted in a figure of 1.050 million years (Hurley and Good­man, 1943, p. 321), a value that cor­responds closely to the Baringer Hill pegmatite age determination of 1,100 mil­ lion years. The geologic significance of the close correspondence of values is not clear. The Iron Mountain magnetite mass is within the Valley Spring gneiss, one of the metasedimentary units in the Llano uplift which is intruded by the Town Mountain granite and which has not been adequately studied. The Valley Spring gneiss is in general very low in iron, and doubtless the Iron Mountain mass is a concentration of iron brought about by some later process. V. E. Barnes (personal communication, 1953) says that he has considered the iron concentration to be the result of processes operating in con­junction with the metamorphism of the Valley Spring gneiss because of the con­cordant structure of the iron mass and because it is cut by pegmatite and aplite dikes. According to this reasoning the magnetite body should be older than the Baringer Hill pegmatite, but the cross­cutting dikes in the Iron Mountain mass are far from any granite mass and their age and parentage are uncertain. An­other possible explanation for the ap­parently anomalous youthfulness of the Iron Mountain mass (assuming no an­alytical errors) is that the magnetite re­crystallized during the general period of batholithic intrusion of which the Bar­inger Hill pegmatite was a part. This hypothesis is somewhat weakened in that tl1e only igneous bodies cropping out near the magnetite mass are the cross­cutting pegmatites and aplites, which do not seem to have affected the iron mass to any great degree. Recent zircon age determinations on the granites of the western and central Llano uplift range from 874 to 942 million years, and al­though these determinations are some­what younger than the 1,100 million years figure given by Holmes (1931, pp. 330­334, 351), there is reasonably good agree­ment considering (1) complex geologic relationships involving several periods of granitic intrusion and (2) the limit of accuracy of this relatively new technique of age determination (± 10 percent) 3 (Table 4) . Basement Rocks, Texas-New Mexico TABLE 4. Age determinations in the Llano uplift. MILLION YEARS 1. Uraninite from Baringer Hill (Holmes, 1931, pp. 330-334, 351)... 1,100 2. Magnetite from Iron Mountain (Hurley and Goodman, 1943, p. 321) 1,050 a. Zircons from Town Mountain and Oatman Creek granites in western and central Llano unlift (unpub­ lished determinations by E. S. Lar­ sen, Jr., U.S. Geological Survey, through 1he courtesy of R. M. Hutchinson, Kansas State College, personal communication, 1953) ..... 874 to 942 4. Zircons from the Big Branch gneiss in the Blowout quadrangle, western Blanco County (through the cour­tesy of H. W. Jaffe, U.S. Geologi­ cal Survey, personal communica· tion, 1955).. ... . . . .. . . . ..............910 to 970 5. Zircons from the llanite dike 9 miles north of Llano (through the courtesy of H. W. Jaffe, U.S. Geological Survey, personal com­munication, 1955) .. 850 On the basis of these age determinations the batholithic granite exposed in the Llano uplift has an age of about 1,000 million years and the intruded sedimen­tary rocks are still more ancient. The gravity picture in the area of the Llano uplift is characterized by irregular or amoeboid anomalies that reflect the complex of granite and intruded meta­morphic rocks; this picture changes north­ward and westward to a more regular pat­tern, although perhaps this is in part due to the masking effects of the thicken­ing sedimentary cover. Barnes (personal communication, 1954) has observed a general increase in de­gree of metamorphism and deformation from northwest to southeast within the Precambrian rocks of the Llano uplift. The Big Branch and Red Mountain gneis­ses, meta-igneous units showing prominent llowage and sheared structures, are pres­ent to the southeast but have no counter­part to the northwest. Sericitic and chlo­ritic members of the Packsaddle schist in the northwest are replaced by amphi­bolites in the southeast. Garnetiferous rocks are found in the southeastern part of the uplift but are not reported from 1 Advance informalion on new zircon age delerminalion1 by E. S. Lanen, Jr., of the U. S. Ceological Survey, lhrougb the courle1y of R. M. Hutcbinaon, Kan1a1 State College (letter, December 2, 1953). the northwest area. The Valley Spring gneiss changes from an almost friable rock in the northwest to a very hard siliceous rock in the southeast. Barnes cautions, however, that our lack of knowl­edge of these rocks precludes long dis­tance correlations and, in the case of the Packsaddle schist for example, the ap­parently low-grade rocks called Pack­saddle schist in the northwestern part of the area may he a separate and younger unit not equivalent to the southeastern Packsaddle schist. Thus the generalization of an increase in metamorphism and de­formation from northwest to southeast must be made with reservations. If the generalization stands, however, it is evi­dence from the rocks themselves to sup­port the concept that the southeast bulge of the craton was the foreland mass against which the forces of the late Paleo­zoic orogeny, and perhaps similar late Precambrian forces, were concentrated. Petrographic character of the subsur­face Texas craton.-Inspection of Plate I shows that in the vast area from central Texas northwestward into southeast New Mexico most basement wells penetrate igneous rocks---essentially plutonic rocks. There are minor occurrences of meta­morphic rocks of sedimentary origin that apparently have been invaded by these plutonic rocks and, locally, the plutonic rocks are gneissic either through cata­clastic metamorphism or primary llowage. But the great volume of plutonic rock is composed of granite and granodiorite with subordinate quartz diorite, diorite, syenite, gabbro, and diabase. Petrographi­cally the Texas craton is distinguished by its great volume of granite and grano­diorite; vast granitic terranes exposed in shield or cratonic areas in various parts of the world may be comparable, but modern close scrutiny of the exposed shields reveals a complexity of detail that cannot be observed in a subsurface study. Some idea of the great preponderance of granite and granodiorite in the craton can be gained from Table 5. These figures are, of course, an approximation based on Bureau of Economic Geology, The University of Texas the almost 300 wells penetrating the cra­tonic area. The erratic distribution of wells in the area of study results in certain statistical inaccuracies. The high con­centration of granite and granodiorite wells in the Brunson and Winkler areas of the Central Basin Platform is partly re­sponsible for the high granite-granodiorite percentages. Close .spacing of wells reach­ing basement in these local areas leads to overemphasis of the importance of the rock type of these areas in the over-all picture. However, even if the Brunson and Winkler areas are discounted, granite­granodiorite is still the predominant rock type of the craton where known elsewhere. Because of the meager control in large areas and the large number of variables not susceptible to evaluation, no attempt is made to develop a statistically accurate picture of the composition of the craton by weighing the rock type with respect to a calculated area. TABLE 5. Composition of the Texas craton. PERCENT Granite .............. ................ 50 Granodiorite .... 20 Q!1a~tz diorite 8 D1ont~ .. . .............. . 5 Syenite and syenodiorite 4 Gab bro and diabase . . ..... .. .... ........... 3 Meta-igneous rocks (mostly granite, granodiorite and quartz diorite gneiss, in that order) 7 Metasedimentary rocks 3 100 Well control is not complete enough to provide a basis for subdivision of plu­tonic rocks into their various families. Control is best in the Brunson area of eastern Lea County and in northern Pecos County, both areas on the Central Basin Platform. In the Brunson area there is a rather monotonous repetition of granite and granodiorite with minor quartz sye­nite -all relatively fine-grained rocks with a large number falling into the microgranite and microgranodiorite range. There are exceptions; in the Conti­nental Oil Company No. 1 Warren A-29 there is an olivine gabbro and in the Sin­clair No. 1 Barton there is an olivine­augite syenite. Both these rocks are dis­tinctly abnormal in this area and may represent later intrusions. The augite-an­desine microgranite porphyry encountered in the Magnolia No. 1 Shaw-Federal in southeastern Chaves County also belongs to this anomalous clan. The notable feature of the basement on the Fort Stockton high in Pecos County is that all the major families of igneous rocks are represented from granite through gabbro but without any apparent syste­matic distribution. Granite and grano­diorite are the most common types. Some generalizations are in order on the petrographic character of the granite and granodiorite that comprise the bulk of the craton. These rocks are rich in alkali elements and poor in iron and magnesium. The potassium feldspar is mostly microcline microperthite and microperthite. The plagioclase ranges from albite to oligoclase and the more calcic andesine variety is uncommon. Ferromagnesian minerals seldom exceed 5 percent of the rock by volume; the mafic varietal minerals are mostly biotite and its alteration product chlorite. If albite is grouped with the alkali feldspar in classification many of the rocks here called albite granodiorite be­come granite. If any chemical-mineralogi­cal qualifiers can be applied to the plu­tonic rocks of the Texas craton as a group, these rocks are leuco-alkali­granites and granodiorites. There are, of course, exceptions which, because they are conspicuous, prove the rule. Metasedimentary rocks have been pene­trated on the craton in the following areas: (1) the Embar field in Ector County (2) the Keystone field in Winkler County (3) the Brunson area of Lea County (4) the southern part of the Fort Stockton high in Pecos County . (5) north and west of the exposed Precam­brian rocks of the Llano uplift. In addition, along the western margin of the area, metasedimentary rocks appar­ently associated with volcanic rocks occur Basement Rocks, Texas-New Mexico in southwestern Chaves County and north­western Eddy County. The relationship of these rocks to the craton is not clear; probably they are younger rocks and part of another as yet unrecognized province. In the Embar area of Ector County the common plutonic rock type is quartz microdiorite, but biotite schist was pene· trated in the Phillips No. 15 Embar, meta· quartzite was encountered in the Phillips No. 23 Embar, and the Texas No. 6 Cow· den went into either a granite or an ar· kose gneiss. Although the geologic re­lationships of these rocks cannot be de· termined from the several random well samples available, it is reasonable to sup· pose that the metasedimentary rocks have been invaded by the unmetamorphosed quartz diorite. This is the common re­lationship in shield areas where older and commonly fragmentary metasedimentary terranes are engulfed in a sea of younger plutonic rocks or perhaps exist as un­granitized remnants in a great mass of granitized rocks. The metamorphic rocks are thoroughly rocrystallized, and al­though there is a lack of indicator min­erals they are apparently of medium metamorphic grade. In the Keystone area of Winkler County the plutonic basement rock shows little variation from well to well and is granite and granodiorite, lo­cally showing gneissic structure. In one well, however, the Phillips No. 5 Walton, the basement rock is a hornblende-biotite­albite-quartz schist. Again, we can only speculate on the geologic relationships. Thin sections were examined from more tl1an 80 basement wells from the Brunson area of Lea County in southeast New Mexico and all save one are composed of plutonic rocks, the great majority of granite and granodiorite composition. The lone exception is the Gulf No. 1 Amanda in which fragments of micro· granite and biotite-quartz-oligoclase schist occur together in cuttings from the 7.332-foot interval. In the Fort Stockton high area of Pecos County, wells have penetrated a variety of plutonic rocks showing varied degrees of alteration. However, the principal rock type again is granite and granodiorite. The most south· erly basement well on this high, Stanolind No. 1 Hinyard Cattle Company, appar­ently penetrates at least 300 feet of meta­morphic rocks including metaquartzite, metarkosite, and amphibolite. The state­ment is qualified because thin section coverage is spotty. North and west of the Llano uplift in Kimble, Mason, McCulloch, Lampasas, and Coleman counties wells have en· countered amphibolite, graphitic biotite schist, hornblende schist, arkose gneiss, scapolite-diopside gneiss, and diopside marble. In metamorphic grade and in mineralogy these rocks are not dissimilar to the rocks which make up the Pack­saddle schist and Valley Spring gneiss in the exposed Precambrian rocks of the Llano uplift. In view of their geographic association with the uplift it is reasonable to consider them as part of the Packsad­dle-Valley Spring metasedimentary ter­rane which was invaded by granites about 1,000 million years ago. On the western margin of the area of study in southern Chaves, southeastern Lincoln, and northwestern Eddy counties, New Mexico, rocks of probable metasedi­mentary origin occur in a number of wells but control does not permit estab­lishment of a definite trend. These rocks include metaquartzite in the Humble No. I Pearson in Eddy County and the Conti­nental No. 1 Lankford in Chaves County; metarkosite in the Stanolind No. 1 Pi­cacho Unit in Lincoln County; a chlorite phyllite of possibly meta-igneous origin in the Magnolia No. 1 Burro Hills Unit in Eddy County; and a rock of indeterminate nature in the Magnolia No. 1 Black Hills Unit in Chaves County. This rock consists of angular quartz grains and biotite flakes in an essentially cryptocrystalline ground­mass and possibly is a variety of argillite. An isolated wildcat in the northwest cor­ner of Debaca County (South Basin No. 1 Good) penetrates epidote-biotite-horn­blende schist. These rocks probably are not part of the Texas craton. In certain basement areas the plutonic rocks of the craton show varied degrees of alteration to gneiss; this is predomi­nantly a cataclastic alteration with gneissic structures produced by crushing and shear­ing. Locally the gneissic structure is appar­ently a primary flowage phenomenon, and locally it appears to be the result of recry­stallization of the parent rock under condi­tions of regional metamorphism. This lat­ter type cannot with certainty be dis­tinguished from the more ancient and in­vaded rocks of the craton. The principal areas of development of metamorphosed plutonic rocks are ( 1) in a more or less linear northwest-trending zone in south­western Roosevelt County, (2) in the southern part of the Fort Stockton high in Pecos County, and (3) in an irregular area in western Andrews County, north­western Ector County ( Embar area) , and northeastern Winkler County (Keystone area). In the Roosevelt County area my­ lonitized and cataclastically altered plu­ tonic rocks occur along a zone about 50 miles long. Diorite gneiss in northwestern Crosby County and arkose or granite gneiss in Motley County are apparently re­ lated to the Red River mobile belt, although it is not clear whether they are metamor­ phic rocks of the mobile belt or rocks of the bordering craton metamorphosed during the active period of the mobile belt. On Plate I they are included within the mobile belt. Metamorphic rocks of indeterminate origin (metasedimentary or meta-igneous) are found in Humble No. 1 Bolt in Kim­ ble County (amphibolite) and in Su­ perior No. 1 McDowell in Runnels County (anthophyllite?-albite hornfels). Except in the southeastern part of the craton in the area of the Llano uplift, metamorphic rocks have been encountered in relatively few of the basement wells of the Texas craton. It seems likely that if metamorphic rocks were as common in the western part of the craton as they are in the Llano uplift area they would have been more frequently penetrated by base­ ment wells. The distinctive amoeboid gravity pattern that characterizes the meta­sedimentary and igneous complex of the Llano uplift area is not repeated to the west. It seems clear, therefore, that the southeastern part of the craton contains a greater volume of metasedimentary rocks than the northwestern part which is ap­parently almost entirely composed of plutonic rocks. The recently completed Shell and Sin­clair No. 1 Purcell in Williamson County is the most southeasterly basement well in the area and is probably close to the southeast edge of the Texas craton. The well is reported to have passed through about 4,000 feet of Pennsylvanian black shales of Ouachita facies? into under­lying Ellenburger and Cambrian strata of foreland facies before encountering Pre­cambrian granite gneiss. Apparently this well is near or just within the margin of the old Ouachita trough where early Paleozoic foreland sedimentary rocks lie on Precambrian rocks of the Texas craton. Age of the Texas craton and relation to other Precambrian terranes.-lf the ex­posed and dated batholithic granites of the Llano uplift are correlative with the great mass of concealed granites and granodiorites to the west, the age of the Texas craton is middle Precambrian or older. Such a correlation is, of course, tenuous and must be regarded with res­ervations, but there are two factors which tend to substantiate the correlation and the age. The granites exposed in the Llano uplift are petrographically similar to those that are concealed to the northwest. This is worthy of mention but very little weight can be given to correlation on this basis alone. Much more significance lies in the relationships of geological features whose Precambrian age can be demon­strated and which are younger than the craton. In the Panhandle, unmetamor­phosed and essentially undeformed lava flows lie upon the plutonic rocks of the craton and are overlain by Cambra-Ordo­vician sedimentary rocks. These lava flows are late Precambrian rocks and the rocks upon which they lie must be older. The Basement Rocks, Texas-New Mexico craton was the foreland buttress against which the late Precambrian orogeny in the Van Horn area was spent. This orogeny can he stratigraphically dated as late Pre­cambrian, and it is logical to presume that the foreland of that mobile zone has a greater antiquity. In short, this great gra· nitic block behaved as a stable unit in late Precambrian time. Very recently two additional zircon age determinations became available through the courtesy of H. W. Jaffe, of the U. S. Geological Survey (personal communica­tion, 1955). The age of the granite cored in the Atlantic Refining Company No. 1 Roberts in Schleicher County was found to be 1,000 million years; granite cored in the Phillips Petroleum Company No. 1-C Puckett in Pecos County yielded an age of 910 million years. These age determina· lions support correlation of dated granites exposed in the Llano uplift with similar buried granites to the west. During the waning of Precambrian time much tectonic and igneous activity ap· parently took place on and around the Texas craton. Much of late Precambrian history can only he surmised, hut it is known that the southwestern margin was under powerful compressive forces as­sociated with the deformation of the Van Horn mobile belt and that in the north· western part igneous extrusions were building up a great pile of tuffs and flows. To the northeast and north was a long belt of sedimentary rocks, the Red River mobile belt, which apparently was meta­morphosed in late Precambrian time. Paleozoic and younger structures of the Texas craton.-With th~ beginning of Paleozoic time a much more complete rec­ord of the tectonic history of the craton can be read in the distribution and nature of the younger sedimentary rocks. Cheney and Goss (1952) summarized the post· Precambrian tectonic history of the south· eastern part of the craton. Their contours on the top of the Precambrian (Cheney and Goss, 1952, fig. 9) show the Llano uplift as the exposed part of the northwest elongated Concho arch which they he­lieved is controlled by or at least parallels a Precambrian orogenic trend. They· indi· cated that the Concho arch extends north­west to the Texas Panhandle hut has sub­sided beneath the Permian basin (1952, p. 2262) and cited evidence to indicate its repeated upwarping between early Ordo· vician and late Pennsylvanian time. Sub­sequent tilting to the west accentuated the Llano uplift, Bend axis, and Concho plat· form. The Concho arch is thus the result of a warping of the old stable mass-the Texas craton-and the controlling "Pre· cambrian orogenic trend" is the axis of the craton. Cheney and Goss also suggested that the San Marcos arch is an extension of the Concho arch-Llano uplift structural axis. This is not easily reconciled with the theory of the Texas craton as developed in this study. If the concept of a stable Texas craton is accepted, the San Marcos arch theory of Cheney and Goss means that a northwest-southeast structural axis within the craton and controlled by a more fundamental Precambrian axis extends beyond the margin of the craton into the flanking Ouachita foldbelt-an entirely different structural province. The nature of the San Marcos arch is not thoroughly understood and its existence as a struc­tural feature is doubted by Weaver (1951). .T. E. Adams (1954) has called atten· lion to the existence of a feature he has called the Texas Peninsula. The axis of this feature extends from New Mexico southeastward through the Llano uplift. Analysis of the sedimentary section shows that at the close of Ordovican time an arch (the Texas arch) developed along this axis which effectively separated the west Texas basin from the Oklahoma basin, and its continued rise is shown by the pinch-out of the Siluro·Devonian sec· tion against it on both the northeast and southeast sides. At the end of Devonian time this feature ceased to rise and it is covered by Mississippian strata. The Texas Peninsula and the Concho arch are apparently related features. The axis of this backbone corresponds with that of Bureau of Economic Geology, The University of Texas the Texas craton described in this paper and results from mid-Paleozoic arching of the craton along its axis; the two fea­tures are congruent, and the Paleozoic arch is a later structure controlled by the axis of the craton. Westward, the craton was depressed during Paleozoic time to form the west Texas basin (King, 1951, p. 58). This basin was fragmented by the northwest· trending features of the late Pennsyl­vanian orogeny which formed the Diablo Platform, Central Basin Platform, Mid­land basin, and Delaware basin (fig. 2) . Possibly these structures have an ultimate control in previously established basement trends because their general northwest strike is characteristic of the craton in its shallower and exposed portions to the east (Cheney and Goss, 1952, p. 2262) and parallels the long axis of the craton. ,The influence of the southern bulge or cusp of the Texas craton on subsequent geologic events is strikingly shown on the geologic map of the State (Darton et al., 1937), where the outcrop belts of Upper Cretaceous and younger formations sweep around it in broad curves. This outcrop pattern is the direct result of post-Cre· taceous movements on the Balcones fault zone and erosion but the fundamental con· trol is the southeast bulge of the craton. This was the foreland mass against which the forces of the late Paleozoic orogeny were dissipated, and there may have been similar late Precambrian forces acting at the same point. The concept of the buttress nature of the Llano uplift is not new and has been discussed for many years. It was concisely slated by Van der Gracht (1931, p. 1051): In south-central Texas the Ouachita Front swings around the foreland buttress of the Llano· Burnet uplift, a massif of pre-Cambrian basement rocks, showing old pre-Carboniferous northwest· southeast axes, a trend which is maintained in the Concho Divide farther northwest. and (p. 1052) : ... This important massif, which acted as a sufficiently resistant buttress to deflect the entire course of the Ouachita chains, seems clearly to indicate that a much more primary cause existed in the underlying floor. The· Llano Uplift is not caused by the pressure of the Ouachita chains against the Concho axis, since it was not de· formed along Ouachita trends, as the open foot· hills folds, and as the Arbuckles were affected by the Wichita push on the Hunton arch. All the Ouachita onslaught may have done to the Llano· Burnet massif was, possibly, an accentuation of the uplift with some north-northeast faulting. One point in the second quotation de­serves clarification. While it is clear that the Ouachita foldbelt was squeezed against and around the southern bulge of the craton or, in Van der Gracht's terms, the Llano-Burnet massif, this buttress did not so much deflect the course of the Ouachita chains as it controlled the site of the Ouachita geosyncline. Van der Gracht (1931, p. 1054) noted the "striking parallelism" of the Balcones fault zone to the Ouachita trend and sug­gested that position of the fault zone was controlled by a still-active Ouachita Mountain front. Earlier, Udden (1919) linked the Balcones fault zone to the Oua­chita foldbelt, although he did not recog· nize it as such. Foley (1926, pp. 1267­1268) said that the Balcones zone was caused by a downsinking of the Coastal Plain around the Llano uplift and that the Mexia fault zone was a later adjust­ment due to the Balcones faulting. Sel­lards (1935, pp. 61-66) related the Bal­cones fault zone to the "Llanoria geo­syncline ... which acted as a hinge be­tween the rigid foreland region and the subsiding landmass." Moss (1936, p. 948) likewise suggested that the Balcones and Mexia fault zones are over the orig­inal zone of weakness that allowed the Ouachita geosyncline to form and that the geosynclinal belt was the hinge for the Gulf Coastal Plain subsidence. All these authors inferred a basement control for the Balcones structure but none of them explicitly stated that the funda­ mental control for all these features is the margin of the craton, which is suggested from this basement study. VAN HORN MOBILE BELT Definition.-The name Van Hom mobile belt is applied to a deformed and Basement Rocks, Texas-New Mexico Fie. 2. Index map of regional structural features. metamorphosed prism of metasedimentary and meta-igneous rocks which lies south­west of the craton in far west Texas. Its subsurface extent is largely unknown but exposures in the Van Horn area of Cul­berson and Hudspeth counties give some idea of its lithology and structure. The exposed Precambrian rocks have been described in detail by King and Flawn ( 1953), In brief, they consist of a meta­morphosed sedimentary section, largely quartzofeldspathic, about 20,000 feet thick intruded by rhyolite and diorite, also metamorphosed, and thrust north­ward over a thick sequence of limestone, volcanic rocks, and sandstone that has undergone extreme deformation. The na­ture of the original sediments, their thick­ness, and the character of the deformation, all suggest a deformed and collapsed geo­syncline or a mobile belt. Age and possible subsurface extent of the Van Hom mobile belt.-The age of the culminating orogeny in this area can be stratigraphically dated as late Precam­brian, although there is evidence of pre­vious orogenic periods (King and Flawn, 1953, p. 129). The youngest Precambrian unit involved in the orogeny is the Hazel sandstone which, under the scarp of the Sierra Diablo north of the locus of maxi­mum deformation along the trace of the Streeruwitz thrust fault, is undeformed and unmetamorphosed. The Hazel sandstone is overlain by the undeformed and unmeta­morphosed Van Horn sandstone of Pre­cambrian (?) age which in turn is uncon­formably overlain by lower Ordovician strata (King and Flawn, 1953). There is insufficient well control to make it possible to trace the Van Horn mobile belt in the subsurface, and were it not for the Van Horn exposures, its existence would probably not be suspected. To the west in the exposures of the Franklin Mountains the Lanoria quartzite may be equivalent to part of the metasedi­mentary sequence in the Van Horn area (King and Flawn, 1953, p. 130) but this correlation is tenuous at best. Scattered basement wells between the Van Horn area and the Franklin Moun­tains have penetrated rhyolite, quartz diorite, and granite; rhyolite is exposed in the Pump Station Hills and gran­ite crops out in the Hueco Mountains. Cor­relation between the Pump Station Hills rhyolite and that which intrudes metasedi­mentary rocks in the Van Horn area is suggested but is of little aid in delimiting the boundaries of the Van Horn mobile belt. To the northeast there is a gap of about 200 miles between the outcrop of rocks of the Van Horn mobile belt and the basement wells penetrating the craton on the Central Basin Platform in southern Lea County. '.\ew Mexico. Probably the funda­ mental boundary between the craton and the Van Horn mobile belt is close to the axis of maximum deformation of the mo­bile belt marked by the trace of the Steeru­witz overthrust fault, because northward the late Precambrian Hazel sandstone, ex­tremely contorted along this deformational axis, flattens out as though it were a fore­land deposit on the craton surface. To the east in central Culberson County the Hum­ble No. 1-B Reynolds Cattle Company well bottomed in what is called Bliss sandstone. Thin sections of cuttings from the interval 5,370 to 5,380 feet show that among the fragments of sedimentary rock are some fragments of what appear to be recrystal­lized arkosic rock very similar to Precam­brian metarkosite that crops out in the Wylie Mountains about 20 miles to the west. Little reliance can be placed on the unsatisfactory sample available, but it is possible that the basement terrane in this area is that of the Van Horn mobile belt. Eastward from this well there is a span without basement control to the basement wells in the craton on the Fort Stockton high. South of the Van Horn area in north­western Presidio County two wells pene­trate basement rocks. The Hunt No. 1 Pre­sidio Trust encountered metarkosite be­neath Permian strata. The rock is similar to that which crops out in the Van Horn area, except that it shows more indications of introduced feldspathic material; there is little doubt of its Precambrian age. About 20 miles southeast of this well the Welch No. 1 Espy has penetrated granite beneath Ordovician strata. There is no lithologic basis on which to distinguish this :ock from the granites of the craton, but its geographic position suggests that this rock may be an intrusion into the rocks of the Van Horn mobile belt. The abundant granite pegmatites in the south­ernmost exposures of the Van Horn mobile belt in the Van Horn Mountains, about 40 miles to the northwest, and the evidence of granitization in the rock from the Presidio Trust well, 20 miles to the northwest strongly suggest igneous activity in thi~ part of the mobile belt. Thus, although we Basement Rocks, Texas-New Mexico are unable to delimit the Van Horn mobile belt, its existence is suspected over a con­siderable area (PL I) . Paleozoic and younger structures of the Van Horn mobile belt.-The later (post­Precambrian) structural history of this part of Texas has been complex and the extent to which the major structure of the old mobile belt influenced younger struc­tures is problematical. Regarding the pres­ent-day structure of the area, King (1935, p. 222) writes: The northern part of trans-Pecos Texas, north of the Texas and Pacific Railway, is a part of the Basin and Range province. The mountains here are broad blocks of flat-lying or gently tilted Pa­leozoic rocks which rest on a pre-Cambrian base­ ment.... South of the Texas and Pacific Railway, nor­mal faulting has a conspicuous effect on the to­pography in only a few areas. The region has been one of greater mobility than that farther north, and the sedimentary and volcanic rocks which form the mountains have been tilted, flexed, and in places strongly folded by post­Mesozoic movements older than the last faultin g. ... The southern part of trans-Pecos Texas, in which these features are displayed, is most close­ly allied to the mountains and highlands of north­eastern Mexico, such as the Sierra Madre Orien­tal, and it forms their northern end. This mobility is that achieved by the north­west-trending Mesozoic geosyncline of northern Mexico and was not inherited from the late Precambrian Van Horn mo­bile belt which was consolidated by orog­eny long before. Baker (1935, pp. 211-212) says: .. . the mountainous area of Trans-Pecos Texas is separable structurally into two well-marked divisions. The dividing zone between the two, first noted by R. T. Hill, and called the Texas Lineament by F. L. Ransome, extends from Point Conception on the Pacific Coast of southern Cali­fornia to Cape San Roque, the easternmost point of South America, the zone determining the north­eastern coast line of South America and passing through the island of Cuba . ... It is probably the greatest single structural line of the Western Hemisphere. Baker says that north of this lineament is the Trans-Pecos rift valley province and south of it is the northeastern Mexican Cordilleran or Overthrust province. The inter-continental aspects of this structural line noted by Baker are beyond the province of this study, but it is interest­ing to note that the boundary between two great structural provinces of Mesozoic and Cenozoic age corresponds to the local boundary between the old stable cratonic nucleus and the late Precambrian Van Horn mobile belt as established in this pa­per. The Mesozoic geosyncline of northern Mexico apparently formed on the hinter side of the late Precambrian mobile belt. The Precambrian rocks themselves are exposed through block faulting of an old positive domical structure often referred to as the Van Horn dome (King and Flawn, 1953, p. 132). This structure included part of the Van Horn mobile belt and part of the stable area or craton to the north so that it is apparently not affected by the boundary between the two provinces to which such a great structural . role is ascribed in the preceding paragraph. Ap­parently final compressive movements con­nected with the last stages of the late Pre­cambrian orogeny were able to incorpo­rate both foreland and hinterland elements into a single broad area of high-standing Precambrian rocks which endured until it was broken by Cenozoic faulting. The fea­ture evidently formed in late Precambrian or early Paleozoic time because the Paleo­zoic sediments which were deposited on it are thin or have been partly or wholly stripped off by erosion. Possibly there was more than one period of uplift. The line which separates basement rocks overlain by Cambro-Ordovician rocks and basement rocks overlain by Permian rocks corresponds roughly to the boundary be­tween the Van Horn mobile belt and the craton to the north . . This raises the ques­tion whether the southern part of what has been called the Van Horn dome is not ac­tually a regional uplift of the older Pre­cambrian mobile belt? A Paleozoic high area corresponding to the old mobile belt would have greater geographic extent than that ascribed to the Van Horn dome (Baker, 1935, pp. 182-185) and would certainly affect oil exploration in the area. Although it was the site of great tectonic and igneous activity in late Precambrian time, the Van Horn mobile belt has not been recognized as an important prove­nance for early Paleozoic sedimentation. The early Paleozoic elastic sediments in the Ouachita geosyncline where it is ex­posed in the Marathon area are believed to have a southerly source while the Van Horn mobile belt, passive after its culmi­nating orogeny, assumed a foreland role (King, 1937, pp. 22, 44). RED RIVER MOBILE BELT Definition.-The name Red River mo· bile belt is applied to a roughly east· west-trending belt of metasedimentary and meta-igneous rocks which can be traced from Cooke and Denton counties on the east to Floyd and Crosby counties on the west, and possibly extends considerably farther in both directions. It is wholly a subsurface feature, and poor well control in the eastern part of the area, where base­ment wells are localized in closely spaced groups on the structurally high Muenster and Electra elements of the Paleozoic Red River uplift, makes it impossible to fix boundaries. To the west, boundaries are established but because of the confluence of other Precambrian terranes in this area their validity is in question (Pl. I) . Petrographic character of the Red River belt .-From Denton and Cooke co unties west to Foard County the belt is marked by a predominance of metased imentary types including biotite schist, hornblende schist, garnetiferous schists, metamor­phosed conglomerate, metaquartzite, met· arkosite, and meta-arkose. In one well, Hollandsworth No, 31 Fette in Cooke County, a long basement core penetrated metaquartzite and sillimanite-biotite schist, the highest grade metasedimentary rock encountered in this study. Thus the belt is characterized by low, medium, and high· grade metasedimentary rocks. On the east end of the zone on the Muenster arch in Denton and Cooke counties, the rocks are medium-to high-grade rocks, commonly containing amphibole and garnet of meta­morphic origin, and have been intruded by granite, granodiorite, syenodiorite, and diorite. Their advanced metamorphic grade is the result of a higher degree of deformation and the intrusion of the igne­ous rocks. In the central part of the belt in Wichita, Archer, and Clay counties the metasedimentary rocks are low grade and co mposed predominantly (on the basis of scattered samples) of meta-arkose and metagraywacke. To the west in Foard and Wilbarger counties metamorphic grade in­creases somewhat, and the predominant rock type is hiotite schist, although lower grade rocks are also present. The rocks encountered in 56 wells that penetrate rocks of the Red River mobile belt for which samples are available are grouped by lithologic type in Table 6. Among the metasedimentary rocks which constitute about 56 percent of the total as calculated from available data, the high proportion of metagraywacke and meta· arkose and their more highly metamor­phosed equivalents is significant. The gray­wacke and impure arkose type of sedi· mentary rocks characterize rapid geo­synclinal sedimentation in orogenic belts; graywacke in particular indicates a syn­orogenic sedimentary environment. TADLE 6. Rock types of the Red River mobile belt. PERCENT Metasedimentary rocks . 56 (meta-arkose, metarkosite, meta­ graywacke, and metaconglomerate, 34%; phyllite, metaquartzite, horn· blende schist, garnetiferous schist, mica schist, 22%) Meta-igneous rocks . . 11 (may include some highly meta­ morphosed paragneisses whose sed­ imentary origin is difficult to prove) Igneous rocks 33 (granite, quartz diorite, syenite, diorite, syenodiorite, gabbro, rhy­ olite) 100 The rocks of this terrane are so diverse that generalizations about modal composi­tion are impractical. For petrographic de­scriptions, refer to Appendix II. Relation of the Red River belt to other Precambrian terranes and the Texas cra­ton.-The western limit of the Red River mobile belt presents a decided problem be­cause of confluence of a number of other Basement Rocks, Texas-New Mexico Precambrian units. Probably the Red River belt continues westward from Foard County into Cottle and Motley counties be­cause metasedimentary rocks, mostly meta­arkose and sericite phyllite, with subordi­nate higher grade arkose gneiss and met­arkosite, have been encountered in that area along the strike of the mobile belt. The uncertainty arises from another metasedi­mentary belt, the Fisher metasedimentary· terrane which trends about north-south in the same area (Pl. I). The north end of the Fisher metasedimentary terrane is sep· arated from the metasedimentary rocks in Cottle and Motley counties by a band of granite about 20 miles wide. The character of the metasedimentary rocks in the Fisher terrane and the Red River belt are so simi· lar that a relationship between the Fisher terrane and the Red River belt should be considered. Problems of the Fisher terrane are discussed in a separate section. Be· tween the low-grade metamorphic rocks of the Red River belt in Cottle and Motley counties and the blanketing volcanic rocks of the Panhandle terrane in Lubbock and Hale counties, several wells have pene· trated basement rocks which indicate that the Red River belt continues westward and is covered by the lava flows of the volcanic terrane. The only recognized metasedimen­tary rocks in this area are a muscovite schist from the Sinclair No. 1 Massie in central Floyd County and a metaconglom­erate composed of igneous pebbles in the Humble No. 1 Montgomery in northern Crosby County. The schist in the Massie well is a higher grade metamorphic rock than those in the Red River belt in Cottle and Motley counties. In northwest Crosby County there is a diorite gneiss in the Humble No. 1 Irvin. Because of its posi­tion it is reasonable to assume that the metamorphism of this rock was associated with the tectonic activity of the Red River mobile belt, but it is not certain whether this rock is part of the flanking craton or a metamorphosed younger intrusive into the mobile belt. To the north the granite or arkose gneiss in the Amerada No. 1 Bir­ney presents the same problem as the diorite gneiss in the Humble No. 1 Irvin in Crosby County. Farther south in Mot· ley County the Humble No. 1-D Matador encountered rhyolite porphyry, although there is no petrographic evidence to indi­cate whether it is a flow or intrusive rock. It is too far east to belong to the Pan· handle terrane so that it might best be considered as a flow among the weakly metamorphosed sedimentary rocks of the Red River belt (the low-grade meta­morphism that has affected the rocks in this area would have had little noticeable effect on a pure quartz-alkali feldspar rock such as rhyolite porphyry). The presence of granodiorite and quartz diorite within the Red River trend in this area further complicates interpretation. The western end of the Red River mobile belt is of particular interest be­cause it penetrates the stable area as a long finger, and probably a linear zone in the craton was mobilized during this late Precambrian activity. To the north Precambrian igneous rocks are brought to the surface in the Wichita Mountains by late Paleozoic uplift and extend northwestward into the Panhandle in the buried Amarillo Mountains. Pos· sibly these igneous rocks, mostly micro· graphic granite with some gabbro, are late Precambrian intrusives into the mobile zone. The Sztykgold No. 1 Charles in Mon­tague County penetrated more than 1,000 feet of basement rock consisting of al­ternating granite and diabase (see Table 1)-perhaps this sequence is related to the granite-gabbro sequence exposed in the Wichita Mountains. The relation of the Red River mobile belt to the Texas craton is not definitely known because there is a large area south of the belt where control is lacking. Pre­sumably the craton extends north of the widely spaced control points in Shackel­ford and Comanche counties and the Red River belt is marginal to it. Between the granite penetrated in the Gallagher and Lawson No. 1 Terry in Comanche County and the metasedimentary rocks of the Red Bureau of Economic Geology, The University of Texas River belt in Denton County, the Gar­land-Anthony No. 1 Hammons in Parker County penetrated biotite schist. Basement wells in this area are too sparse for other than rank speculation on the significance of this well. However, if this well has en­countered rocks of the Red River belt instead of a metasedimentary part of the Texas cralon, it means that ( 1) the south­ern boundary of the Red River belt on the east should be moved farther south or (2) the eastern end of the Red River belt turns southward and arcs around the Texas craton. Age and mobility of the Red River belt. --No direct evidence is available on the age of deformation of the Red River mobile belt. Tenuous reasoning suggests that it may have formed in late Pre­cambrian time. The zircon age determi­nation in the Wichita Mountains (670 million years, Larsen et al., 1949, p. 27) is late Precambrian, but the relationship of the Wichita Mountain rocks to the Red River mobile belt is in question; if the Wichita Mountain granites are syno­rogenic and constitute a core of the mo­bile belt, the late Precambrian age of the belt is well supported; if the Wichita Mountain rocks are a post-orogenic in­trusion we know that the belt was mobile in an earlier period. The Red River belt appears to be marginal to the Texas craton ; according to popular geosynclinal theory and the geologic record, younger mobile zones commonly flank older stable elements. The alternative hypothesis is that the Red River belt is an older ter­rane, possibly part of the craton. This idea, however, is weakened by the long uninterrupted course of the belt. Metasedi­mentary terranes known to be part of the craton, such as in the Llano uplift area, are fragmented by bathoiithic intrusion. The record of mobility of the Red River belt that can be read in the petrographic study of its constitutent rocks is varied. Again, our conclusions are based on scanty data and vulnerable reasoning. If a rock in thin section shows microfold­ing, microfaulting, contortion, slippage along S planes, and other evidence of penetrative movement, the rock experi­enced strong deformation and internal movement. Here again application of these data is limited by our sample. We cannot on the basis of a single core chip slate that the metasedimentary belt achieved more or less mobility in a cer­tain segment because of the possibility that our sample records only a local mo­bility in, for example, the rocks of a thrust plate. However, we can apply the lessons learned in the study of regionally metamorphosed terranes exposed in vari­ous parts of the world; in general, re­gionally metamorphosed rocks character­ize orogenic belts and higher grade meta­morphic rocks mark the parts of the belt that achieved greater mobility; com­monly increased mobility and igneous intrusion are associated. The rocks of the Red River element encountered to the east are medium to high-grade meta­morphic rocks accompanied by intruded igneous material; to the west (Pl. I) the rocks show predominantly low-grade metamorphism with sporadic areas of medium-grade metamorphic rocks. Mo­bility of this belt was probably greater to the east and relatively slight to the west. Paleozoic structures of the Red River mobile belt.-Near the end of Cambrian time or at the beginning of Ordovician time a trough formed within the stable area (the hedreocraton of Paleozoic time) and extended from southwest Oklahoma into the Texas Panhandle. This trough has been called the Wichita geosyncline and its history is concisely described by King (1951, pp. 145-148). This trough or geo­syncline was deformed in Pennsylvanian time, and the resulting chain of the Wichita system extends northwestward from the Criner Hills through the Wichita Mountains into the Amarillo uplift of the Texas Panhandle; to the south another part of the system forms the Red River and Matador uplifts which are completely subsurface features. Here is a Paleozoic orogenic belt which is apparently located Basement Rocks, Texas-New Mexico within the confines of the Paleozoic era· ton, and which is at least in part coinci· dent with a Precambrian mobile zone-­the Red River mobile belt. The Paleozoic structures directly associated with the Red River mobile belt are the Red River uplift, including the Muenster and Electra elements, the Matador uplift, and possibly the Wichita and Amarillo uplifts. It is, of course, these later uplifts that have brought the metasediments of the older belt into prominence, but from the west end of the belt where control is adequate more or less to delimit its boundaries, it seems that the younger structures tend to reflect the trend of the older structure. The Matador uplift, which extends westward into the area of the Texas era· ton along the trend of the Red River uplift, supports the idea that the Pre· cambrian Red River mobile belt extends westward, possibly penetrating the Texas craton (Pl. I), although the far western parts of the Matador uplift may reflect only a zone of weakness and not actually a Precambrian mobilized zone. Basement wells along the Matador uplift penetrate rocks of the Panhandle volcanic terrane rather than metasedimentary rocks of the Red River belt, so that we cannot expect to find direct petrographic evidence of the continuation of the Red River belt to the west. The fact, however, that the Mata· dor structures are along the projected trend of the Red River belt is very sug· gestive. The anomaly of the late Paleozoic tee· tonic units of the Wichita system which diverge so sharply from the main late Paleozoic trend of the Ouachita foldbelt, may perhaps be explained by renewed activity on the Red River mobile belt, the Paleozoic stresses being deflected along trends that originated during much earlier deformation. FISHER METASEDIMENTARY TERRANE Definition.-The name Fisher meta· sedimentary terrane is applied to a rather irregular area of basement surface in west-central Texas, extending from east­ern Nolan and Taylor counties north to Dickens County and including parts of Scurry, Jones, Stonewall, Fisher, and Kent counties. The eastern boundary of the terrane cannot be fixed because of the paucity of basement wells in the area; there is some scanty evidence to indicate that the metasedimentary terrane contin­ues south into Runnels and Coleman coun­ties (Pl. I). Petrographic character of the Fisher terrane.-To the south in Nolan and Fisher counties the Precambrian rocks are mostly metarkosite, arkose gneiss, and biotite schist, although a meta­morphosed dolomite in the Humble No. 1 Nachlinger in Scurry County and a rhyolite in the Hunter & Hunter No. 1 Steele in Jones County are probably part of the same terrane. Northward in Dickens County the rocks are of lower meta· morphic grade and consist mostly of meta-arkose and sericite phyllite very similar to those rocks immediately to the north in the Red River mobile belt. The amounts of various rock types in the Fisher terrane are given in Table 7. These figures are rough approximations based on 23 rock types encountered in 19 wells. TABLE 7. Rock types of the Fisher metasedi­ mentary terrane. PERCENT Metasedimentary rocks . 83 (metamorphosed arkosic rocks­ meta-arkose, metarkosite, arkose gneiss, and minor metaquartzite­ 52%; mica schist, phy!lite, and meta·argillite, 27%; metadolomite, 4%) Volcanic and meta-igneous rocks... 17 (granite gneiss, microsyenite gneiss, rhyolite porphyry flows) 100 There are too few wells in the Fisher ter­rane to provide a sound basis for the tabulations in Table 7. The metadolomite, for example, was encountered in only one well. However, the preponderance of arkosic rocks of varying metamorphic grade shown in the table is probably characteristic of the belt and indicates sedimentary accumulations from a pre· dominantly granitic source. The arkosic rocks show varied percent· ages of quartz, alkali feldspar, and pla· gioclase, any of which may predominate. The common alkali feldspar is micro· cline; plagioclase ranges from oligoclase· andesine to albite. Quartz and feldspar are commonly accompanied by chlorite, sericite-muscovite, magnetite or ilmenite, leucoxene, calcite, pyrite, epidote, red iron oxide, sphene, apatite, and zircon. In more highly metamorphosed varieties biotite is present. These rocks are in part low-grade metamorphic rocks showing unrecrystallized quartz and feldspar and partly reconstituted intergranular ma· terial (sericite, chlorite, epidote) with an essentially elastic fabric, and in part medium-grade metamorphic rocks in which the quartz and feldspar have re­crystallized to form a mosaic or granular aggregate (granoblastic fabric) and the intergranular fraction has been reconsti· luted to plates of mica and chlorite. Where there is an orientation of the mica to impart a rude foliation the rocks are termed arkose gneiss. Grain size is for the most part within 0.05 to 0.2 mm. Biotite schist and biotite phyllite are the most common representatives of the foliate rocks. These rocks are composed of quartz, plagioclase, and oriented bio· tite plates. Chlorite, epidote, magnetite or ilmenite, pyrite, calcite, apatite, and zircon are commonly present in minor quantities. The fabric is lepidoblastic to cataclastic; grain size ranges from 0.05 to 0.2 mm. Other rock types listed in Table 7 are mostly individual samples and generali· zations are meaningless. For petrographic description, refer to Table 1 and Ap· pendix II. Relation of the Fisher terrane to the Texas cralon and the Red River mobile belt.-The relation of the Fisher meta. sedimentary terrane to the Texas craton and to the Red River mobile belt to the north is a major problem. The Fisher terrane seems to lie on or within the era· ton; its limits, particularly to the south, are vague because of poor well coatrol. To the north the rocks are low grade, show only the beginnings of reconsti· tution, and do not appear to have attained great mobility; to the south metamorphic grade increases and schists and gneisses are present. The highly sheared dolomite in the Humble No. 1 Nachlinger well suggests mobility. Three hypotheses must be considered: (1) the Fisher terrane is composed of late Precambrian metasedi­mentary rocks in a basin within the craton, (2) the Fisher terrane is composed of more ancient metasedimentary rocks in· truded by the granites of the craton, and (3) the Fisher terrane is related to the Red River mobile belt. The low-grade rocks of the northern part of the terrane do not appear to have been extensively invaded by granite and tend to support the first hypothesis. To the south, how· ever, the metasedimentary rocks resemble the exposed metasedimentary rocks in­vaded by granite in the Llano uplift. Moreover, metasedimentary rocks en· countered in wells north and west of the Llano uplift in Menard and McCulloch counties suggest a possible connection be· tween the Fisher terrane and the Pack· saddle-Valley Spring metasedimentary sequence of the Llano uplift. A meta· morphic rock penetrated in the Superior No. 1 McDowell in Runnels County be­tween the southernmost limit of the Fisher terrane as now recognized and the north­ernmost extension of the Llano uplift meta· sedimentary rocks is a crystalloblastic anthophyllite? -albite rock unlike the com· mon types in either terrane and is not a very convincing link between them. The only evidence to indicate a re­lationship to the Red River belt is the similarity of the rocks in the north part of the Fisher terrane in Dickens County to rocks of the Red River belt in Cottle and Motley counties. Such evidence is far from conclusive and certainly the trends of the two belts are widely diver­gent (Pl. I). Basement Rocks, Texas-New Mexico It has been demonstrated many times in the study of metamorphosed terranes that excessively deformed and metamorphosed rocks in one area are correlative with more or less flat-lying weakly metamorphosed rocks in another area. The weakly meta· morphosed rocks of the northern part of the Fisher terrane do not preclude the hy· pothesis of equivalence with the Llano up· lift metasedimentary rocks. In the earlier phase of the work (Flawn, 1954) the writer favored the hypothesis of a late Pre­cambrian intracratonic basin for the Fisher temme, but additional well data from the southern parts of the terrane make it an open question and no definite age can be assigned to these rocks. That they are in· deed Precambrian is demonstrated by over. lying Cambrian strata in the south part of the area. Solution of the problem awaits additional basement well control in the Coleman -Taylor-Runnels-Concho County area. PANHANDLE VOLCANIC TERRANE Definition.-Volcanic rocks constitute basement in three separated areas in the Panhandle and south plains of Texas and part of eastern New Mexico. The southern­most area of volcanic rocks underlies parts or all of Bailey, Lamb, Hale, Lubbock, Hockley, Cochran, Yoakum, Terry, and Lynn counties in Texas and extends west­ward into northern Lea County and south­ern Roosevelt County. A small area of base· ment volcanic rocks in central Chaves County, New Mexico, is also included in this terrane. Farther north volcanic rocks form the basement in parts of Parmer, Castro, Swisher, Deaf Smith, Handal!, Armstrong, and Donley counties in Texas and extend wcslwarcl into· Curry County, New Mexico. Volcanic rocks of this ter­rane also extencl northward to the lop of the Panhandle and are encountered in Old­ham, Potter. Carson, Gray, Hartley, Dal· lam, ancl Sherman counties, Texas. To these separatccl areas of the basement sur­face the name Panhandle volcanic terrane is applied (Pl. I). Petrographic character of the volcanic terrane.-The rocks of the volcanic tcrrane are composed chiefly of undeformed and unmetamorphosed flows of rhyolite por­phyry and associated rhyolite tuff, but rhyodacite, trachyte, trachyandesite, and andesite also have been encountered. The only recognizably metamorphosed vol­canic rock in this sequence is the metatuff in the Stanolind No. 1 Fuller in Quay County, New Mexico. The extrusive nature of the bulk of these rocks is indicated by associated luffs, flow structures, spheru· lites, and relict perlitic and crystallitic structures, although it is not unreasonable to expect shallow intrusives in association with such an extensive lava terrane. Considering the diversified suite of rocks that commonly results from a vol­canic cycle, the rocks of the Panhandle vol­canic terrane show striking uniformity over a wide area (Table 8). The figures in Table 8 are approximated on the basis of about 90 wells penetrating volcanic rocks. TABLE 8. Rock types of the Panhandle volcanic terrane. PER CF.NT Rhyolite porphyry . 68 Rhyolite 6 Rhyolitc tuff . ..... ...... ..... .. 10 Trachyte porphyry . s Trachyte tuff . 3 Andesite 4 Andesite tufT 1 Rhyodacite tufT . 1 Trachyandesite .. 1 Basalt ........ ............. ..... 1 100 About half of the volcanic rocks ( exclud­ing tuffs) show flow and/or microspheru­litic structures which in the light of asso­ciated tuffs indicates an extrusive origin. The "average rhyolite porphyry" in the area consists of about 65 to 90 percent quartz-alkali feldspar groundmass which may be microgranular, micrographic, mi­crospherulitic, or cryptocrystalline, or some combination thereof. Although quartz makes up a substantial part of the groundmass of these rocks, it is not every­where present as phenocrysts. Albite com­monly forms the bulk of the phenocrysts; Bureau of Economic Geology, The University of Texas microperthite phenocrysts are generally subordinate to albite or absent completely. Red iron oxide, magnetite or ilmenite, chlorite, leucoxene, calcite, apatite, and zircon are the common accessory minerals; muscovite or sericite, sphene, biotite, am· phibole, epidote, pyrite, rutile, anhydrite, fluorite, and tourmaline are less commonly present. Grain size of the groundmass ranges from cryptocrystalline to 0.1 mm but generally falls below 0.05 mm; pheno­crysts rarely exceed 3 mm and for the most part range between 0.5 and 2 mm. Tuffs are distinguished by the presence of rock fragments and relict vitroclastic fabric. For descriptions of subordinate rock types, refer to Plate I and Appendix IL A well in the volcanic terrane in north­ ern Lea County, the Amerada No. 1 State BTA, penetrated basement rock that has been variously called chert, novaculite, and rhyolite by petrographers who have ex­ amined it. In the writer's opinion it is a silicified rhyolite (halleflinta) ; it shows micrographic structure and is geographi­ cally within the area of the volcanic ter­ rane. The Phillips No. 6 URB on the northern edge of the volcanic terrane in southwest­ ern Gray County is of special economic in­ terest. Between about 3,040 and 3,175 feet this well penetrated a section of brecciated volcanic rocks, trachyte and rhyolite por­ phyry, brecciated albite diorite, with hard rhyo.lite tuff at the bottom and was com­ pleted as a 104-barrel per day producer from the 3,150 to 3,175 foot zone. The writer believes the interval from 3,040 to 3,175 feet is part of the Panhandle volcanic terrane intruded by diorite sills and not a detrital "arkose" or "wash" resting on the basement. The excessive fracturing and brecciation shown in thin sections of the cuttings from this interval indicates high porosity and was probably the cause of the entrapment of the oil. Relation of the Panhandle volcanic ter­rane to other Precambrian rocks.-With­ in the area of the volcanic terrane several wells have ( 1) penetrated diabase or gab­bro and then encountered rhyolite por· phyry ;• t•l (2) penetrated rhyolite por­phyry, then gabbro, and bottomed in rhyo­lite porphyry; «"> (3) penetrated gabbro and contact metamorphosed sedimentary rocks and bottomed in rhyolite por· phyry;«•l (4) bottomed in diabase or gabbro.• pene­trated rhyolite porphyry, contact meta­morphosed sedimentary rock, and bot­ tomed in rhyolite porphyry. The rocks in these wells explain the rela­tion of the Panhandle volcanic terrane to the Swisher gabbroic terrane which lies be­tween the two separated parts of the vol· canic terrane. The gabbroic rocks intrude the volcanic rocks in a number of sills and are related to the main gabbro body of the Swisher terrane which is apparently a great lopolith lying on top of and occupy­ing a synclinal depression in the volcanic terrane which extends beneath. Prior to the intrusion of the gabbro, late Precambrian sedimentary rocks apparently rested on the lava surface and were locally intercalated with the lavas. Where the gabbroic rocks intruded and metamorphosed these sedi· ments they were preserved; elsewhere they were removed in pre-Ellenburger or pre­Mississippian time. The nature of these al­tered sedimentary rocks is discussed in the section on the Swisher terrane. In Cochran County the Shell No. 1 Pitt­ man passed from Ellenburger into granodi­ orite. Apparently this well marks an inlier where the younger volcanic rocks have been stripped away so that plutonic rocks lie directly beneath the Paleozoic rocks. Farther west in Chaves County, New Mex­ ico, in an isolated part of the volcanic ter­ rane, the Honolulu No. 1 McConkey Estate penetrated an anomalous sequence of rhyo­ lite porphy'ry with granite and microgran­ odiorite between. There is no ready ex· planation for this sequence unless there has been an error in labeling the samples and the rhyolite porphyry rests on the granite. 4 (•) Humble No. l Hyslop, Deaf Smith County; Humble No. I Hobgood, Hockley County. (b) Colorado lnteratate No. 25-A Bivin1, Poller Count1. (e) Hunt No. %Ritchie, Bri1coe County. (d) Humble No. l Campbell. Hockley County; Coaden No. l Barker, Cochran County. (e) Hauie Hunt Tru1t No , 1 Helm1, Arm1trong County. Basement Rocks, Texas-New Mexico Considering the extrusive nature of the volcanic rocks it is apparent that the floor on which they were deposited was com· posed of older rocks. It is suggested that the rhyolites were extruded, at least in part, on plutonic rocks of the craton. The volcanic rocks of the Panhandle terrane probably overlap and are younger than the metasedimentary rocks of the Red River belt; apparently the rocks of the Red River belt extend westward beneath the volcanic rocks along the line marked by the Mata· dor structures. About 200 miles to the southwest, rhyo· lite extrusives are found in the Franklin Mountains and metamorphosed rhyolite is intrusive in the Van Horn mobile belt. Rhyolite whose geologic relationships are concealed crops out at Pump Station Hills in Hudspeth County and has been encoun· tered in the American Land No. 1 Rose­borough in Hudspeth County and in the Hunt & Turner No. 1 McMillan in south­ern Otero County, New Mexico. The asso­ciation of these rhyolites with micro­graphic granites or granophyres suggests they may be largely intrusive. King and Flawn ( 1953, pp. 125-131) discussed the relationships of the west Texas rhyolites and attempted a tentative correlation which here is expanded in Table 12. Rhyolite por­phyry is also present in the Wichita Moun­tains of southwest Oklahoma, but accord­ing to Gerald Chase (personal communi­cation, 1953) these rhyolites are part of an intrusive granite series and are not lavas. Geologic conditions in Precambrian time such as to permit the formation of a terrane of more or less uniform rhyolitic lava over an area about 200 miles long and 150 miles wide must have been quite simi· Jar to those responsible for the volcanic activity of Tertiary time which built up great lava accumulations to the southwest. It is further interesting to note that al­though the Precambrian lavas do not con· tain the alkaline suite of amphiboles and pyroxenes that distinguish the Tertiary lavas, the persistence of the sodic plagio­clase and the high potassium feldspar con· tent show an alkaline affinity. Age of the volcanic rocks.-A late Pre· cambrian age for the volcanic rocks is in· dicated by their essentially undeformed and unmetamorphosed state, because they seem to overlie rocks of the Red River belt and plutonic rocks of the craton, and be­cause they are in some wells overlain by Cambrian or Ordovician strata. In general, the rhyolite is overlapped by Cambrian or Ordovician strata to the south and by younger Mississippian, Pennsylvanian, or Permian beds to the north (Pl. I; Table 1). Paleozoic structures of the volcanic ter· rane.-Rocks of the volcanic terrane have been involved in Paleozoic tectonic move· ments along the Matador trend and the Amarillo uplift and have been downwarped in the Plainview or Palo Duro basin5 where gabbroic rocks of the Swisher terrane oc­cupy part of the structural low of the basin. SWISHER GABBROIC TERRANE Definition and stmctural character.­ The basement rock in northeastern Roosevelt County, New Mexico, and Castro, northern Parmer, northern Lamb, Swisher, northern Hale, western Floyd, Briscoe, and western Donley counties, Texas, is mostly gabbro and diabase and is named the Swisher gabbroic terrane. These mafic rocks comprise the Precambrian surface in the greater part of the area that lies between two sep· arated parts of the Panhandle volcanic terrane and, because several wells in the volcanic terrane (p. 42) have pene· trated a gabbroic sequence overlying the volcanic rocks, it appears that the gabbro terrane proper is a great lopolith that oc­cupies a sag or syncline in which the vol­canic terrane has been downwarped. Gab­broic rocks have also been penetrated in wells in the volcanic terrane that do not bottom in volcanic rocks (p. 42). Two such wells, together with a third that penetrates gabbro and bottoms in rhyolite, occur along an east-west linear ~ Tolten (1954) discuue1 the several names applied to tbit buin and condudet lhal Palo Duro basin ba, priority and i1 t:ireferable . zone m northern Hockley and Cochran counties.• The overlying sedimentary rocks are displaced by a major east­west fault of the Matador trend immedi­ately north of the line connecting these wells. Rhyolite porphyry overlain by Per· mian rocks forms the upthrown block to the north while the gabbroic rocks, over­lain by Cambrian strata, are present as a narrow band on the downthrown side. These gabbroic rocks are probably a rem­nant of the once overlying lopolith pre­served on the downthrown side of the fault zone. Movement on the Matador fault zone took place in late Paleozoic (pre-Permian) time, but earlier Paleozoic or Precambrian movements are indicated by preservation of Cambra-Ordovician strata that rest di· rectly on basement in the immediate area (Pl. I) and by removal of the gabbroic rocks on the upthrown side of the fault zont>. The gabbroic rocks might have been a talus on the downthrown side of the fault, but this is not supported by the na­ ture of the alteration (pp. 137, 167-169) and the uncontaminated state of the gab· hroic material (only cuttings are arnilable for study). Even this intrrprPtation re­ quires Prerambrian movement on the l\Iatador fault zom•. l'<'l.rOf,raphic character of lhl' (!.abhroic lerrane.-The approximate relative abun­ dance of various types of gabbroic rocks in the Swisher terrane is shown in Table 9. These figures are based on 44 rock types encountered in 25 wells. Gabbroic rocks in this area tend to be leuco-varieties, with the ratio of the plagioclase to ferromagne­ sian minerals higher than is normal. The plagioclase of the gabbroic rocks shows an indistinct zonation and is com· monly altered in varving degree to sericite or seri cite and epidote-zoisite. In some the alteration is restricted to the more calcic cores while the more sodic rims or mantles arc clear. In some it is clear that the highly sodic nature of the plagioclase is the result of alteration, the calcic plagioclase break­ ing down to a mixture of sericite, epidote­ 'Humble i\'o. 1 Ho~ood, Hocklf'y County; Humble No. l C:ampbell, Hockley Counly; Cosden No. l Buker, Cochran County , So Table 1. TABLE 9. Rock types of the Swisher terrane. PERCENT Gabbro .. . ...... ........ . 33 (mostly leuco-gabbro, commonly with sodic plagioclase) Olivine gab bro . . . . .. ... . . ........... . 33 (more than 50% leuco·olivine gab· bro; predominantly normal calcic plagioclase) Diabase .... . ... 12 (leuco-and sodic varieties are subordinate) Olivine diabase ................................. . 6 (about one-half are leuco-varieties) Diorite ........ . ................ . 7 (more than one-half show sodic plagioclase) Basalt 5 Olivine syenogabbro 2 Iron ore . 2 100 zoisite, and albite (this type of alteration has been called saussuritization and its product saussurite-gabbro). In others, however, the sodic plagioclase is not asso­ciated with alteration products and it seems to be pyrogenic and the result of a selec­tive metasomatic process, perhaps deuteric. In rocks that show plagioclase with calcic cores and albite rims the albite evidently crystallized in the magmatic stage. Al­though the sodic plagioclase is conspicuous in these gabbroic rocks, it is from a quan­titative vi·ew subordinate to labradorite which constitutes the plagioclase of most of the rocks of this terrane. On the map (Pl. I) the gabbroic rocks characterized by sodic plagioclase show some local grouping but no over-all trend or pattern. In some wells the sodic varieties are in close association with normal gab­bros. Two main groups of wells have pene­trated sodic gabbros (although not exclu­sively sodic gabbros): (1) Three wells within the Panhandle volcanic terrane in Cochran and Hockley counties penetrated gabbroic rocks with sodic plagioclase, all in an advanced stage of alteration to chlo­rite and sericite; and probably the sodic plagioclase is the result of secondary proc­esses (a deeper and less altered gabbro in one of these wells shows normal calcic plagioclase). (2) Four wells in northeast Briscoe and southeast Armstrong counties penetrate normal and sodic gabbroic rocks interlayered with contact·metasedimentary Basement Rocks, Texas-New Mexico rocks; the sodic nature of the plagioclase in some of the gabbro is apparently due to secondary processes attendant on altera· tion but it could not be in others where albite mantles calcic cores. Possibly an in­terchange of material between sedimentary rocks, their pore solutions, and the magma during intrusion and contact metamor· phism caused the end-phase of the magma to become enriched in soda. The pyroxene of the rocks of the Swisher 1errane ranges from colorless augite to deeply tinted lavender to brown augite; nrthopyroxenes are rare hut in one sample hypersthene accompanies augite. In gen· eral the augite occurs as discrete grains between plagioclase laths or as a mantle on olivine, but in a few diabases it occurs as large continuous host grains. Olivine is commonly partly altered to iddingsite or brownish chlorite. Magnetite and/or ilme­nite is commonly present in discrete scat­tered grains more or less overgrown by red·brown biotite, but locally it shows plu­mose structures. Chlorite and to a lesser degree sericite are ubiquitous as secondary alteration products. Secondary green am­phibole and primary green-brown am· phibole are widely distributed in small amounts. Apatite and sphene are the com· mon accessory minerals. Minor quantities of leucoxene, epidote, alkali feldspar, py· rite, calcite, serpentine, talc, rutile, quartz, and nontronite are also present. Grain size ranges from 0.05 mm in ba­salt and fine-grained diabase upward through the microgabbros to 1 or 2 cm in very coarse gabbros. Fabric is hypidio­morphic granular in gabhro and micro­gabbro; ophitic to subophitic in diabase; and porphyritic-microgranular in basalt. In two wells within the area of the gab­bro terrane, Anderson-Prichard No. 1 Get­tys in Lamb County and Sunray No. 1 Kimbrough in Parmer County, basement rocks penetrated are albite syenodiorite and micrographic granite, respectively. This apparently anomalous occurrence within the gabbro terrane is perhaps best explained by the hypothesis that these rocks are differentiation products of the gahbroic magma. The assoc1al!on is not uncommon in exposed gabhro complexes. None of the conspicuous differentiation types found in the Wichita Mountains ( anorthosites, no rites, troctolites) have been observed in Swisher terrane rocks, but locally the leuco-gabbros approach anorthosite and in one well a zone high in magnetite-ilmenite occurred within the gahbro section (Colorado Interstate No. 25-A Bivins, Potter County). The writer has also included in this terrane some out· lying occurrences of apparently related diorite (Pl. I) . Contact metasedimentary rocks in the gabbroic terrane.-ln some wells in the gabbro terrane proper, and in wells pene· trating gahbro and bottoming in rhyolite, the gabbroic rocks are associated with sedi­mentary rocks which have been metamor­phosed by them. These wells are El Paso Natural Gas No. 1 West Texas Mortgage· Loan, Bailey County; Lion No. 1 Bridwell, Bailey County; Hunt No. 1 Ritchie, Briscoe County; Hunt No. 2 Ritchie, Briscoe County; Sun No. 1 Haberer, Cas· tro County; Sun No. 1 Herring, Castro County; Hunt No. 5 Ritchie, Donley County; Placid No. 1 Kelly, Donley County (Table 1). In addition, the Hassie Hunt Trust No. 1 Helms in Armstrong County penetrated serpentinized dolomite between rhyolite layers without encoun· tering gahhro. The sedimentary rocks are carbonate rocks and argillaceous or arkosic siltstones. The carbonate rocks, whatever their origi­nal character, are now dolomites which show three overlapping stages of altera· tion: (1) individual grain boundaries lose their sharpness and there is incipient de­velopment of serpentine and/or talc; (2) dolomite remnants are completely en· veloped by talc and/or serpentine; (3) tre· molite and diopside form and the rock is a diopside · tremolite · dolomite hornfels. The elastic rocks, which were originally calcareous argillaceous siltstones, were more resistant to metamorphism than the carbonate rocks, and the effect of meta· morphism is indicated mainly by a finely Bureau of Economic Geology, The University of Texas fibrous mineral, probably an amphibole, which penetrates the other mineral con­stituents. In some slides there is recrystal­lized chert and some slides contain small porphyroblastic muscovite grains, but these are rare. For the most part the silt­sized quartz and feldspar grains have not recrystallized. The fabrics are of a static or hornfels type, without preferred directional orientation of mineral constituents. Most of these weakly metamorphosed elastic rocks may be classified as argillites or meta-argillites \Flawn, 1953a). They ap­pear to be the metamorphosed remnant. of what once may have been an extensive sedimentarv terrane lying on and partly intercalated with the volcanic rocks. Ex­cept where they were intruded by gabbro and involved in the general downwarp of the Palo Duro basin, they were removed by pre-Ellenburger erosion. Age of the gabbroic rocks.-The ga~­ broic rocks are younger than the volcamc rocks as they intrude both them and overlying sedimentary rocks and meta­morphose the latter. They are ~recambrian and not Paleozoic, as Cambnan or Ordo­vician strata overlie them in the more southerly wells (Bailey, Cochran, and Hocklev counties). Farther north the gab­broic ;ocks, like the associated volcanic rocks, are overlapped by Mississippian, Pennsvlvanian, and Permian strata. The late P;ecambrian age is also supported by correlation with the rocks in the Wichita Mountains \Table 12). Emplacement and structural history of the gabbroic rocks.-The sills and _sheets of gabbro and diabase include medium to coarse-grained varieties and must have had a more deep-seated environment of emplacement than the host lavas ~nd over­ lying sedimentary rocks. From this we :an deduce the following late Precambrian structural history: ( 1) Surficial deposits of rhyolite lava and tufI laid down on rocks of the craton and Red River mobile belt. (2) Subsidence and accumulation of sedimen­tary rocks-carbonates and siltstones. (3) Continued subsidence with intrusion of gabbroic rocks in the deeper parts of the Pre­cambrian basin. (4) Pre-Ellenburger? removal of the greater part of the Precambrian sedimentary cover to form a basement surface of rhyolite lava and gabbro including contact-metamorphosed rem­nants of sedimentary rocks. Gabbroic rocks of the Swisher terrane occupy part of what is now the structural low of the Palo Duro basin. Either the accumulated mass of gabbro in and on the lavas of the Panhandle volcanic terrane was sufficient to cause crustal subsidence and initiated the downwarp of the basin in late Precambrian time, or the emplace­ment of the gabbro lopolith was controlled by the axis of a broad synclinal downwarp caused by more fundamental tectonic forces. Gravity measurements in this area do not show a conspicuous gravity-high and tend to confirm the interpretation of the Swisher terrane as a relatively thin stratiform body of gabbro without suffi­cient mass to cause crustal subsidence. Because Mississippian strata now rest on basement rocks over a large part of the Palo Duro basin, the concept of a mid­Paleozoic high area transecting this basin and exposing basement rocks in pre­Mississippian time is supported. Cambro­Ordovician rocks rest on basement only on what were the flanks of the former high area. The structural history of this general basin area is then (1) downwarping and basin formation in late Precambrian time (east-west trend?) following extrusion of the lavas, (2) mid-Paleozoic uplift along a northwest-southeast trend to form a high backbone, and (3) late Paleozoic sub­sidence along a more or less east-west trend to form the basin as it is known to­day. WICHITA IGNEOUS PROVINCE General remarks and definition.­ North of the Red River in southern Okla­homa, and beyond the boundary select~d for the subsurface study, late Paleozoic uplift has brought Precambrian rocks to view in the Wichita and Arbuckle Moun­tains. The name Wichita igneous province is applieu co me late ;>,·ecambrian intru­sive igneous rocks which crop out in the Ba.sement Rocks, Texas-New Mexico Wichita Mountains and extend north· westward in the subsurface to form a large part of the Amarillo Mountains. Whether the Precambrian rocks of the Arbuckle Mountains belong to this prov· ince or an older igneous cycle is not yet known. No complete study is available on the Precambrian rocks of the Arbuckle Mountains, and the principal references on the area are still the reconnaissance and partial studies of Taff (1904), Taylor (1915), and Uhl (1932). The Precam· brian rocks of the Wichita Mountains have been reported on by Hoffman (1930). Subsequently, Gerald Chase of the Oklahoma Geological Survey has been studying the Precambrian rocks; he has amassed a wealth of carefully or· ganized field, petrographic, and chemical information but a paper has not yet been published. The writer profited greatly by discussions and field conferences with Chase and from examination of the large collection of thin sections in the files of the Oklahoma Geological Survey. Arbuckle Mountains.-Taff (1904) re· ports that the principal rock type in the Arbuckle Mountains is biotite granite with associated phases of quartz monzo· nite and dikes of basic rock, aplite, and granite porphyry; in addition there are masses of aporhyolite and granite por· phyry. Taylor (1915) and Uhl (1932) describe two apparently related granites, the Tishomingo and the Troy granites, and a number of subordinate plutonic types and dike rocks. Uhl (1932, pp. 34­46) also describes extrusive rocks, mainly rhyolite porphyry, from the Timbered Hills area. These descriptions do not suggest any distinctive rock types that would be convincing in correlating the Arbuckle Precambrian rocks with those to the northwest in the Wichita and Arna· rillo uplifts, but it is interesting to note that the Troy granite, like the Mount Scott granite in the Wichita Mountains and many samples from the buried Arna· rillo ridge, is micrographic (micropeg· matite of Uhl, 1932, p. 11). The rhyolite porphyry from the Timbered Hills merits comparison with the vast terrane of late Precambrian rhyolite porphyry flows in subsurface to the west. Although not men· tioned by previous geologists, metasedi· mentary rocks are present in the Arbuckle Mountains but their nature and extent are not known (Chase, personal communi· cation, 1953) . One sample examined by the writer is a biotite-hornblende schist and indicates a regionally metamorphosed tcrrane. Wichita Mountains.-The oldest Pre· cambrian rock and the only metasedi­mentary rock in the Precambrian ex· posures of the Wichita Mountains is the Meers quartzite which occurs as xenoliths in the Mount Sheridan gabbro,7 the oldest igneous unit in the area. Samples of the Meers quartzite show varied de­grees of alteration, but the typical altered rock consists of grains of finely rutilated quartz closely set in a sponge of albite· oligoclase and both penetrated by sillima· nite needles. Sillimanite ranges from a trace to about 20 percent of the rock. Lo· cally the rock contains quartz that occurs as round grains with the appearance of original detrital constituents. This poses a problem because the sillimanite indi· cates an advanced metamorphic grade; in contact metamorphic processes quartz recrystallizes at lower temperature than that necessary for the formation of sil· limanite. The round quartz grains are de­ceptive; the writer believes that their ap· parent "elastic" nature is due to close packing in a sponge of plagioclase of near! y equal relief and that they are actually round inclusions in a feldspar host. Metamorphism of the rock, although not everywhere equal, is in general ad· vanced. The most significant feature of the quartzite is that it does not appear to have Leen regionally metamorphosed to any great degree before intrusion of the gabbro, indicating the gabbro intruded a sedimentary terrane. Gerald Chase (personal communi­cation, 1953) has distinguished many dif· ferent igneous phases in seven major 'i' Nome is lhat proposed by Gerald Chase, manuscript, 19.)3. Bureau of Economic Geology, The University of Texas igneous rock series but the major di­visions are an older differentiated gab­broic sequence comprising about 6 per­cent of the exposed area and a younger granite and granite porphyry sequence. Age determinations on the youngest granite series show an age of 670 million years; the younger igneous rocks of the area are thus dated as late Precambrian (Larsen et al., 1949, p. 27). The gubbroic rocks of the Wichita Mountains are characterized by a dial­lage8 pyroxene. The olivine is very fresh without pronounced development of alter­ation products. Red-brown biotite, a prominent constitutent of many gabbros, is virtually absent. The Wichita rocks are a differentiation series and the more common gabbro is associated with troc­tolite, norite, anorthosite, and magnetite­ilmenite concentrations. Although gabbro outcrops in the Wichita Mountains are very conspicuous they comprise only a small part of the total area of Precam, hrian exposures. The granite series in the Wichita Moun­tains consists mostly of alkali granites with a very low ferromagnesian mineral content. There is little plagioclase in these rocks other than that which is per­thiticall y intergrown with potassium feld­spar (Chase, personal communication, 1953). The micrographic granite (Mount Scott granite of Chase) is the most ex­tensive unit in the area. Rhyolite porphyry forms limited out­crops in the Wichita Mountains. This rhyolite is intrusive and part of the younger granite porphyry series (Chase, personal communication, 1953 ) ; it is probably not equivalent to that of the Panhandle volcanic terrane farther west. Comparison of Wichit,a Mountains gab­bro with gabbro of the Swisher terrane.­ Differences betwen the Wichita Mountain gabbroic rocks and the gabbroic rocks of the subsurface Swisher gabbroic terrane farther west are summed up as follows: 1 The term di.lla~e UMd in this paper re!eu to a do~ely qace~ parallel ~uled 1lruclure aAd doea not c•nnote a particu­!ar mineral 1peeie1.. ( 1) !he purple-tinted titaniferous pyroxene of the Swisher terrane is markedly different from the col~rless diallage pyroxene of the Wichita Mountains. (2) Olivine in Swisher terrane rocks is typi­cal.ly .altered to iddingsite or chlorite while the W1ch1ta Mountain olivine is characteristicall) fresh . .<3) R~d-brown biotite, a prominent accessory mineral m the Swisher terrane rocks is rare in Wichita Mountain gabbros. ' (4) Products of magmatic differentiation com­mon in the Wichita Mo·Jntains (anorthosite troc­ tolite, norite, and magnetite·ilmenite conc~ntra· lions) have not been encountered in the rocks of the Swish~r t_erran~, with the possible exception of magnet1te-1lmemte concentrations in the Colo­rado Interstate No. 25-A Bivins in Potter County. In the writer's opinion the mineralogic differences in these rocks do not pre­clude the possibility that they are prod­ucts of the same igneous cycle acting over a wide area. August Goldstein, Jr., and H. D. Wenland (personal communi­cation, 1954) report that they have studied subsurface gabbroic rocks en­countered in wells in the Wichita Moun­tain area which contain tinted pyroxene and altered olivine, and are in many ways similar to gabbroic rocks of the Swisher terrane farther west. Although no correlation is warranted it is interesting to note that diallage pyroxene occurs in svenodiorite in the Continental No. 1 Berr-y in Cooke County and in leuco-microgabbro in the Barkley­Meadows No. 14-A Stephens in Wilbarger County; both of these wells penetrate in­trusions in the Red River mobile belt iust south of the Wichita Mountain rocks. The only other occurrence of a diallage pyroxene in the area studied is in gabbro encountered in the Standard of Texas No. 1 Heard-Federal in Lincoln County, New Mexico. Subsurface Wichita igneous province. -The Amarillo Mountains consist of a basement ridge extending northwestward from Oklahoma through Wheeler, Gray, Carson, and Potter counties, Texas, and turning northward into Moore and Sher­man counties, Texas. To the west in Old­ham County another basement high (Bravo dome) seems to he the north­western limit of a series of basement Basement Rocks, Texas-New Mexico peaks south of the main Amarillo ridge in Childress, Hall, Donley, Armstrong, and Oldham counties. In the basement terrane of the Pan· handle there are two main rock types: the lavas and the luffs of the Panhandle ~·olcanic terrane and the intrusive igneous rocks, mostly granite (commonly micro­graphic) with subordinate gabbro, of the Wichita igneous province. In the eastern part of the Amarillo trend in Collings­worth and Wheeler counties the ridge is composed entirely of intrusive rocks of the Wichita igneous province; northwest­ward in Gray and Carson counties the ridge proper is composed of Wichita ig­neous province rocks but volcanic rocks of the Panhandle terrane comprise the south side of the ridge; farther northwest in Potter County most of the Amarillo ridge is composed of volcanic rocks and the intrusive Wichita province rocks are restricted to the north side; at the north­western extremity of the buried ridge in Hartley County, Wichita igneous prov­ince rocks again form the topographic high. North of the Amarillo Mountains in Dallam, Sherman, Moore, Hutchinson, Roberts, and Hemphill counties basement wells are very widely spaced and bound­aries cannot be drawn. A well in south­east Sherman County penetrated rhyolite porphyry an_d suggests that rocks of the volcanic terrane are also present north of the buried ridge; in northwest Sherman Countv a well encountered a metamor­phosed arkosic rock which may represent a remnant of the terrane invaded by the rocks of the Wichita igneous province (Pl. I). In Oldham County south of and separated from the Amarillo ridge proper a basement high, the Bravo dome, is com­posed almost entirely of micrographic granite. This granite high is surrounded bv volcanic rocks of the Panhandle ter­r~ne. Plate I shows that there is no defi­nite correspondence between rock type and topography of the basement-the late Paleozoic structures of the Wichita system have uplifted both the rocks of the Wichita igneous province and the Pan­ handle volcanic terrane together. In gen­eral, the granites of the Wichita igneous province seem to correspond only to topo­graphic and structural highs whereas the volcanic rocks occur in both high and low areas. A rough approximation of the amounts of various types of rock in the subsurface Wichita igneous province is shown in Table 10. These figures are based on ap­proximately 40 basement wells for which samples are available. TABLE 10. Rock types of the Wichita igneous province. PERCENT Granite ............................................. . 56 (about 30% is micrographic gran­ ite from the Oldham County Bravo dome) Diabase 14 Granodiorite 12 Quartz diorite 7 Gab bro 7 Diorite ..... 2 Rhyolite porphyry ................... . 2 100 Granites predominate and are in gen­eral characterized by low plagioclase content and low ferromagnesium mineral content. The micrographic granites in the Oldham County area are practically de­void of plagiocl ase. The alkali feldspar is commonly microperthite or, to a lesser degree, microcline microperthite. Biotite. chlorite, magnetite or ilmenite, calcite, apatite, zircon, sphene, and fluorite are present in minor amounts. Three well~ (Holt No. 3 Bailey, Gray County; Hum­ble No. 1-E Matador, Oldham County; and Smith No. 2 Farren, Wheeler County) encountered granite containing alkali amphibole or pyroxene. This indi­cates an alkaline tendency in harmony with the alkali-rich granites in the Wichita Mountains and the associated riebeckitic pegmatite phases. Grain size of the granitic rocks ranges from 0.5 to 8 mm; fabric is hypidiomorphic granular to micrographic with definite areal group­ing of the micrographic rocks in the Old­ham County area. Bureau of Economic Geology, The University of Texas Correlation of Precambrian rocks in the Amarillo, Wichita, and Arbuckle up­ lifts.-Correlation of the Precambrian rocks exposed in the Wichita Mountains with the subsurface Precambrian rocks in the Amarillo Mountains to the northwest on the same late Paleozoic structure is probable but not proved. There is no ab­solute petrographic evidence for the cor­relation other than a generally similar granite which commonly shows micro­graphic texture (granophyre) and an al­kaline tendency. Granites of the Texas craton are only sporadically micro· graphic. The granite in the Humble No. 1-E Matador in Oldham County is re· ported to contain a distinctive alkali am­phibole (riebeckite) similar to that occur­ring in pegmatitic phases of a late granite series in the Wichita Mountains (Robert Roth, personal communication, 1953.) Alkali amphibole and pyroxene also oc­cur in granite encountered in the Holt No. 3 Bailey (Gray County) and the Smith No. 2 Farren (Wheeler County.) The oc­currence of these alkaline ferromagnesium minerals in outcrop and subsurface sup­ports the correlation. Moreover, there is good well control from the Wichita Mountains into the Texas Panhandle and there seems to be no major change in basement rock type. The relation between the Precambrian rocks of the Wichita and Arbuckle Mountains has recently been critically examined by W. B. Hamilton of the U.S. Geological Survey (manuscript, 1953). He points out that the Wichita Mountain granites occur as an alkaline to sub-alka­line complex of sheets and funnels, whereas the Arbuckle granites are coarse­textured calc-alkaline rocks typical of a batholithic complex. He concludes that the Wichita Mountain rocks, in view of the typical granophyre-gabhro association, are probably part of a lopolith emplaced in a static or tensional tectonic environ­ment while the Arbuckle rocks are batho­lith type, much older, and probably in­trusive into a mobile belt. The dissimi­larity of the Precambrian rocks of the two areas suggests to Hamilton that the Paleozoic trend is unrelated to a basement structure. If Hamilton is correct, the batholithic granites of the Arbuckle Mountains might be part of a stable or foreland area north of the Red River mobile belt and associated Wichita ig­neous province, as was suggested by Van der Gracht (1931, p. 1007.) One other possibility occurs to the writer: the north­ern limit of the Red River mobile belt is not established and it may be that the batholithic Arbuckle granites are part of a deep synorogenic intrusion into the Red River belt. The older synorogenic Ar­buckle granites and the younger granites of the Wichita system might be of differ­ent ages but still be related to the same basement structure, the Red River mobile belt. Age of the Wichita igneous province and relation to other Precambrian rocks. -An age determination of 670 million years on a younger granite of the Wichita Mountains (Larsen et al., 1949, p. 27) shows that the granitic rocks of the Wichita Mountains are considerably younger than the granites of the Texas craton as exposed in the Llano uplift (1 ,000 million years). In this study the Precambrian rocks of the Wichita Mountains and their subsur­face continuation into the Texas Pan­handle are grouped together as the Wichita igneous province; they are a late Precambrian complex of granites, com­monly micrographic, and gabbroic rocks. If these rocks are post-orogenic intrusions cmplaced in a static or tensional environ­ment, as suggested by Hamilton, they must, in view of their very late Precam­Lrian age, be considered with the Swisher gabbroic terrane and the Panhandle vol­vanic terrane as parts of one great late Precambrian igneous cycle. If the rhyolite eruptions, the gabbro intrusions, and the granite intrusions are related phenomena, a certain age sequence may be worked out by some tenuous correlations. The gab­broic rocks of the Swisher terrane intrude Basement Rocks, Texas.New Mexico and are younger than the rhyolite por­phyry of the Panhandle volcanic terrane. The rhyolite porphyry of the volcanic terrane is younger than the metasedi­mentary rocks of the Red River mobile belt which pass beneath them (PL I). The gabbro and associated differentiated rocks of the Wichita Mountains rocks are older than the granites of the Wichita Mountains-Amarillo Mountains trend. If the sequence is matched by equating the gabbros of the Swisher terrane with the gabbros of the Wichita Mountains, the rhyolite porphyry lava·tuff series is older than the gabbro·granite series. Moreover, the rocks of the volcanic terrane are sur· ficial deposits while the gabbro that in· trudes them is commonly a medium to coarse·grained rock. Therefore a period of subsidence is indicated between the rhyolite eruptions and the gabbro·granite intrusive period. Whether or not the granite ever intruded the rhyolite cannot be definitely established because the granite may be a more deep-seated rock brought into juxtaposition with the rhyo· lite flows by Paleozoic tectonic move· ments. The subsidence may well have been the first phase in the development of the Paleozoic basin of the area. The sheet· like nature of the gabbro and granite in­trusions in the Wichita Mountains and the sheetlike nature of the gabbro of the Swisher terrane have been discussed; as suggested by Hamilton these may all be parts of a great lopolith intruded at the close of Precambrian time. Whether the Arbuckle Mountains are a part of the Wichita igneous province or older does not affect the concept of the Wichita igneous province. The general parallelism and geographic coincidence of the Red River mobile belt, Wichita igneous province, and late Paleo­zoic uplift suggests all are controlled by a major tectonic feature of the basement. Although divergences of structural trend are recognized, they emphasize rather than invalidate the parallelism. In dealing with great prisms of rocks of varied physical properties involved in tectonic adventures over a long period of time, drawing·board parallelism should not be expected. Eastward, the subsurface rocks of the Red River mobile belt are directly soutl). of the Wichita igneous province exposures in the Wichita Mountains, and rocks similar to those in the Wichita Mountains have been penetrated in the Red River trend (Sztykgold No. 1 Charles, Montague County) . Here, the inference that the Wichita igneous rocks intrude the Red River mobile belt seems well founded. To the west, however, the trends of the Red River belt and the Wichita igneous province diverge as if split by the block of plutonic rocks tentatively con­sidered to be part of the Texas craton (PL I). The more southerly' Red River trend, composed of metasedimentary, meta·igneous, and igneous rocks, is litho­logically quite different from the unmeta­morphosed igneous rocks of the Wichita province. Only one well, Lubbock Machine & Supply No. 1 Alexander in Collingsworth County, penetrated meta· sedimentary rocks along the Wichita province trend. This divergence of trend weakens the theory that the Wichita ig­neous province is intrusive into the Red River belt. Even with more well control the problems will not be easily solved be­cause of the concealing blanket of vol­canic rocks of the Panhandle terrane. Final settlement of these problems will be materially aided by detailed study of the Arbuckle Mountain igneous rocks and by publication of the work on the Wichita Mountains. The writer inclines toward Hamilton's suggestion that the Precam· brian rocks of the Wichita Mountains are post·orogenic intrusions, perhaps part of a lopolith. A tentative correlation of igneous-structural events in various parts of the area of study is attempted in Table 12. Paleozoic structures of the Wichita igneous province.-During late Paleozoic time the Precambrian rocks of the Wichita igneous province were raised in the Wichita and Amarillo uplifts. These up· lifts are related in time and trend to the Red River-Matador uplifts, the Criner Hills uplift, and the Arbuckle uplift. They are all elements of the Wichita system (Van Der Gracht, 1931, p. 999; King, 1951, pp. 147·148.) The general trend of the Paleozoic uplifts is west to north­west; in the broad view they are sub· parallel but in detail, projected axes of individual uplifts are diverging or inter­secting. There is a parallelism of Paleozoic and Precambrian trends in this area. The known trend of the metasedimentary rocks of the Red River mobile belt para!· leis the late Paleozoic Red River­Matador uplifts. Where the granites of the Wichita igneous province have been separated from the volcanic rocks in the Panhandle, the granites fall into north­westerly elongated areas whose axes parallel the Paleozoic axes of the Ama­rillo-Wichita Mountains uplifts. Al­though no direct relationship between a Precambrian trend and the Arbuckle or Criner Hills uplifts has been established, there is a strong indication that the entire Wichita system has a Precambrian an­cestor. Late Paleozoic tectonic forces ap­plied tQ an incompletely stabilized Pre­cambrian orogenic zone composed of relatively incompetent metasedimentary prisms, large and small intrusive masses, and established planes of weakness re­sulted in a series of subparallel and en echelon folds, faults, and uplifts. THE BASEMENT SURFACE Configuration of the .mrface.-The to· pography of the basement surface is par· tially shown by 500-foot structure contours on Plate I. These contours are based on wells actually penetrating basement and no attempt is made to utilize estimated basement elevations from wells bottoming in Ordovician or Cambrian rocks. In the deep basins and in the basin-and-range country of far west Texas where basement wells are lacking, no basement contours are shown. Most of the major near surface struc­ tural features in the Texas and southeast New Mexico area are reflected in the con­ figuration of the basement surface. The major late Paleozoic uplifts elevated the basement surface several thousands of feet on the Central Basin Platform, the Ama­ rillo uplift, the Matador uplift, and the Red River uplift (fig. 2) . Until 1953 the de­·pressed basement surface in the deep ba­sins of west Texas and southeast New Mex­ico had not been encountered by the drill. However, recently the Richardson & Bass No. 1 Cobb-Federal in Eddy County in the Delaware basin encountered granite at 16,396 feet (-12,881) . There are still no basement wells in the Midland basin proper. Basement rocks lie at the surface in the Llano uplift of central Texas, the Van Horn area, Pump Station Hills, Hueco Mountains, and Franklin Mountains in west Texas, and in the Wichita and Ar­buckle Mountains of southern Oklahoma. West of the Llano uplift the basement surface slopes more or less evenly toward the Midland basin. North of the Llano up­lift there is only a slight basement expres­sion of the late Paleozoic north-south fea­ture which Cheney and Goss call the Bend axis (Cheney and Goss, 1952, fig. 9; Sel­lards, 1933, pp. 91-93). Northwest of the Llano uplift there is a prominent north­west-trending nose which corresponds to the Concho Platform of Cheney and Goss ( 1952, fig. 9) and whose axis parallels the more extensive mid-Paleozoic Concho or Texas arch. North of the uplifted Matador structures (basement elevation -3,500 to -4,000 feet) the basement surface dips rather sharply into the Palo Duro basin (about -7,000 feet) and then rises more or less evenly toward the Amarillo uplift and the Oldham County high. The Amarillo uplift proper is marked by an abrupt topographic discontinuity. The basement surface on higher parts of the Amarillo uplift attains elevations in excess of +1,000 feet. The regional configuration of the base­ment surface is due mostly to Paleozoic and younger structural movements and is well known; detailed information on the topography developed in Precambrian time and during subsequent emergences of the basement surface is sparse. V. E. Barnes (personal communication, 1953) has determined that about 800 feet of re­lief existed on the Precambrian surface in central Texas at the time the lowest Cam­brian rocks were deposited. The rounded granite domes on this surface are very simi­lar to those known in the Missouri lead dis­trict. Recent exploration of the so-called Cambrian trend from Coke County north to Cottle County indicates that hills or peaks of hard granite or arkose gneiss pro­trude above a metasedimentary surface of mica schist and phyllite. In the Van Horn area the surface on which the late Pre­cambrian? Van Horn sandstone was de­posited was a hilly, deeply eroded terrain; the surface of Precambrian rocks on which Permian rocks were deposited in this same area was a rolling one with perhaps 300 feet of relief. Weathering of the basement surface.­ It would be interesting to study the de­gree of weathering of the basement rocks where they are overlain by Paleozoic rocks of different ages. Unfortunately, however, the writer had little control over the inter­val of the basement samples contributed to the project. For such an investigation it Bureau of Economic Geology, The University of Texas would be necessary to study the entire basement suite of samples from the top of the basement to total depth or to fresh rock. Only for a few wells was such a suite of samples available, and the study of the alteration characteristics of the basement rocks was not attempted. PALEOZOIC SEDIMENTARY ROCKS RESTING ON THE BASEMENT SURFACE General remarks.-ln the Texas and southeast New Mexico area, basement rocks are in direct sedimentary contact with strata representing all Paleozoic pe­riods. The formation now resting on the basement is either ( 1) the original cover deposited from a Paleozoic sea lapping on the exposed basement surface or ( 2) a deposit by an overlapping or overstepping Paleozoic sea on an uncovered basement surface following tectonic dislocation and stripping of older Paleozoic formations from the uplift by erosion. Boundaries be­tween formations of different ages which overlie the basement are shown on Plate I. This is an areal geologic map of the un­derside of a surface. Analysis of the rela­tionships of the overlying formations to the basement rocks and to each other yields a broad picture of the paleogeog­raphy of this area during Paleozoic time. Throughout this study of basement rocks the writer has relied on geologists practicing in the various districts for basic well information, including identification of the formation resting on basement. In some areas there is divided opinion on whether a formation is Permian or Penn­ sylvanian, Silurian or Devonian, or Cam­ brian or Ordovician; in order to abstain from this controversy, which is beyond the bounds of the study, and because the regional picture can be satisfactorily de­ veloped by using a combined nomencla· lure, the terms Permo-Pennsylvanian, Si­ luro-Devonian, and Cambro-Ordovician are herein employed. Although not satis­ factory to the paleogeographer, these com­ bined-time terms provide a general index of basement positive and negative areas during the Paleozoic and are as good as the data permit. Cambra-Ordovician rocks. -Cambro· Ordovician rocks lie on the basement in a great arc extending from southern Okla· homa and north Texas through central Texas into west Texas and southeast New Mexico. These ·strata lap on to a basement high that was present in Paleozoic time in northern New Mexico and Colorado; a nose of this positive feature extended southeastward into central Texas. To the southeast a thin Cambro-Ordovician sec· tion is present over this feature which sub· sequently became positive in Silurian and Devonian time; northwestward, in the area of Dickens County, Cambro-Ordo­vician strata pinch out and Mississippian rocks lie directly on basement (Pl. I). The designation Cambro-Ordovician does not include rocks of the same age throughout the area of study. To the east of a north-south line through western Schleicher County, a line which approxi­mately delimits the Eastern Platform, Cam­bro-Ordovician rocks include the Upper Cambrian Riley and Wilberns formations as well as the Ellenburger group. To the west in the Midland and Delaware basins Wilberns and Riley equivalents cannot be positively identified, and possibly no Cam­brian rocks are present. Facies changes in the Cambrian rocks of central Texas sup· port the concept of a limited Cambrian basin. One should bear in mind, therefore, that although the designation Cambro-Or­dovician in central and north Texas in­cludes Upper Cambrian rocks, the same designation in the western part of the area refers to a unit in which the older rocks present in the east are missing and which may" not include any Cambrian rocks. Within the area where Cambro-Ordo­vician rocks rest on the basement, tectonic movements have produced local highs in Sutton, Schleicher, and Coke counties where older Paleozoic beds have been re· moved and Pennsylvanian rocks rest di· rectly on basement rocks. Likewise on major late Paleozoic uplifts, such as the Basement Rocks, Texas-New Mexico Fort Stockton high, Central Basin Plat­form, Red River uplift (including Muen­ster and Electra elements) , and Matador uplift, older Paleozoic strata were removed by erosion following uplift and the highs are capped by overstepping late Pennsy). vanian or Permian rocks. The Matador structures in Floyd, Motley, and Hale counties transect the northwest-trending arch, and it is not certain whether Cam· bro-Ordovician rocks were ever deposited in that area. In the northern part of the Texas Pan· handle Cambro·Ordovician rocks are pres­ent in the Anadarko basin on the north flank of the Amarillo uplift; presumably, by analogy with the Wichita Mountains of Oklahoma, Cambro-Ordovician rocks were removed by erosion from the uplifted basement of the Amarillo Mountains prior to the overstepping late Pennsylvanian and Permian seas. Much of what is called the Palo Duro basin was an upwarped part of the middle Paleozoic backbone and in this area Mis· sissippian rocks lie directly on basement. However, in the northwestern and south· western parts of this basin, on what used to be the flanks of the old positive area, Cambra-Ordovician rocks rest on base· ment. The Cambra-Ordovician contact with the basement is lapped over by Mis· sissippian rocks. Siluro-Devonian rocks.-Except for a local high on the east flank of the Central Basin Platform in Gaines County, Siluro· Devonian rocks in sedimentary contact with basement rocks are found only in southeast New Mexico. Siluro·Devonian rocks rest on the old basement surface in Chaves and Roosevelt counties, New Mex­ico, as part of a progressive Paleozoic overlap which includes Cambro-Ordovi· cian, Siluro-Devonian, Mississippian, and Pennsylvanian strata. In addition, Siluro· Devonian rocks rest on basement in ex· treme southwest Chaves and northwest Eddy counties. In this ill-defined area Siluro-Devonian rocks lap over the Cam­bro-Ordovician-basement boundary and lie directly on the basement on an iso· lated early Paleozoic high area. Mississippian rocks. -Mississippian rocks rest directly on basement in Chaves County, New Mexico, and in the southern Panhandle of Texas in an elongated area that corresponds to the Texas Peninsula. The extent to which Ordovician strata covered this backbone is unknown, and possibly some Ordovician rocks were re· moved from it prior to Mississippian deposition. The area was positive during Silurian and Devonian time. Rocks of this age show an off-lapping relationship and are restricted in Texas to smaller pockets in the larger Ordovician basins. Mississip­pian rocks lap over the Cambro-Ordovi· cian-basement contact. Farther west in Chaves County, New Mexico, the Siluro­Devonian sea was more extensive and overlapped the limit of Cambro-Ordo­vician deposition. Within the area of Mississippian-base· ment contact are local areas where Penn· sylvanian and Permian rocks rest directly on the Precambrian surface. These highs are for the most part structural uplifts along the Matador trend where Mississip· pian rocks have been stripped off by ero· sion. Permo-Pennsylvanian rocks. -Except for the Debaca County, New Mexico, area where Pennsylvanian rocks rest on the basement as a result of progressive over· lap on a northwestern positive mass, Penn· sylvanian and Permian rocks are in con· tact with basement rocks where late Paleo­ zoic uplift resulted in stripping off of older Paleozoic rocks to expose the base· ment. Late Pennsylvanian and Permian rocks rest on uplifted basement in the following areas: ( 1) southwest of Van Horn in Hudspeth, Culberson, and Presi· dio counties, (2) Fort Stockton high, (3) Central Basin Platform, (4) Matador up· lifts, ( 5) Red River uplift, ( 6) Amarillo uplift of the Texas Panhandle, and (7) Otero, southwest Chaves, and Lincoln counties, New Mexico. In the Van Horn area the area of uplift, as defined by the boundary between Cambra-Ordovician Bureau of Economic Geology, The University of Texas rocks on basement and Permian rocks on basement, approximately corresponds to the boundary between the Van Horn mo· bile belt and the Texas craton. In addition to these areas where the Pennsylvanian·Permian contact with base· ment is clearly the result of uplift, there is a northwesterly.elongate area in eastern Roosevelt County, New Mexico, and west· em Cochran and Bailey counties, Texas, where Pennsylvanian and Permian rocks lie directly on basement. This area is on strike with the Matador trend to the east but the area of contact is not lineated parallel to that trend. The controlling structure here is a northwest·trending fault in southwest Roosevelt County, New Mexico. The basement rocks along this fault show extensive cataclastic alteration and mylonitization. The upthrown side on the northeast is capped by a Cambro· Ordovician remnant which indicates ( 1) that the area was at one time covered by Cambro·Ordovician rocks which are in ex· tensive contact with the basement on the downthrown side of the fault and (2) post· Ordovician and pre.Silurian uplift did not altogether effect a removal of the Cambro· Ordovician rocks. Northwest of the Cam· bro·Ordovician rocks on the upthrown side of the fault Siluro·Devonian and Mis· sissippian rocks rest directly on the base· ment, although they appear in part to have been stripped off by post-Mississippian (probably Pennsylvanian} uplift. Permo· Pennsylvanian rocks blanketed the area and rest on the basement surface uncov· ered by the post·Mississippian uplift. Con· tacts of basement terranes, Cambro·Ordo· vician strata, Siluro-Devonian strata, and Mississippian strata are displaced by the northwest-trending fault (Pl. I). FAULTS OR FAULT ZONES JN Ihsn1E:->T Horn:s Three major basement faults or fault zones and a number of smaller ba~··ment faults are shown on the map, Plate I. Study of the O\'erlying sP no ready explanation for these phenomena. Grai·ity anomalit>s of the Van llorn mo­bile bclt:-Onh· generalized graYity data are arnilable t~ the writer for the area of the \'an Horn mobile belt . A northwest­trending minimum o\·er the area reflects the met:isedimentary prism of the mobile belt: the area immediately northeast and east is marked by a series of northwest-trending maxima in Brewster, Jeff Davis, Culberson, and Hudspeth counties that perhaps delineate the margin of the craton. Most of these features reflect structures in the sedimentary rocks above the base­ment, rather than Precambrian structures, although the former may, in turn, have been influenced by Precambrian structure. The series of northwest-trending maxima in Brewster, Jeff Davis, Culberson, and Hudspeth counties, for example, probably extends southwest of what was the limit of the craton of late Precambrian time. By Mesozoic time the craton had been en­larged by the welding-on of the Ouachita foldbelt which, like the older craton, acted as a stable foreland for the Mesozoic Mexican geosyncline. Gravity anommies of the Red River mo­bile belt.-Although dwarfed by the prom· inent maximum of the Wichita system, a series of east-west elongated positive and negative areas extends, with interruptions, from Montague County west into Roose­velt County, New Mexico, and coincides with the trend of the Precambrian Red RiYer mobile belt and the younger Mata­dor structures. As in the Central Basin Platform problem, the elevations of the basement rocks in the Matador structures and the density contrast between the base­ment rocks in the uplifts and the flanking sedimentary rocks are insufficient to ac­count for the magnitude of the anomalies along the trend. Apparently these east­west elongated anomalies are a reflection of the prism of metasedimentary and in­trusive rocks that constitute the Red River belt; as interpreted, the anomalies tend to support the extension of the Red River belt beneath volcanic rocks of the Pan­handle volcanic terrane. Gravity anomal,ies of the Fisher meta­srdiml'n/a.ry terrane.-No obvious corre­>pondenee exists between the metasedi­mentary rorks of the Fisher terrane as delimited and the gravity trends of the same area. The roughly north-south-trend­ing Fisher belt is almost at right angles to the northeast-trending negative belt that Basement Rocks, Texas-New Mexico extends across the entire craton. The meta­sedimentary rocks encountered in Fisher County (the Abilene Minimum of Logue, 1954, pp. 134-135 and map) fall into one of these minima; the northern part of the Fisher belt corresponds to a vague mini­mum that exists as a saddle between two maxima. A possible explanation is that the Fisher metasedimentary rocks are rela­tively rootless while the northeast-trending negative belt is the result of deep-seated crustal phenomena. Gravity anomalies of the Panhandle vol­canic terrane and the Swisher gabbroic terrane.-The Panhandle volcanic terrane is an essentially stratiform mass of lavas and luffs which conceals underlying base­ment rocks so that interpretation of grav­ity anomalies is uncertain at best. Because there is no conspicuous gravity maximum marking the subsurface extent of the dense gabbroic rocks of the Swisher terrane, evi­dence from drill records that they are a stratiform rootless pluton or series of plu­tons is confirmed. The general area under­lain by gabbroic rocks is marked by grav­ity minima; but as the gabbroic rocks co· incide with the structural low of the Palo Duro basin, the minima are probably re­lated to subcrustal adjustments in connec­tion with the formation of the basin. Gravity anomalies of the Wichita igne­ous province.-By far the most conspicu­ous feature of the gravity picture of Texas and Oklahoma is the series of maxima which coincides with the Muenster, Wich­ita, and Amarillo uplifts; these maxima begin rather abruptly in northwest Collin County, Texas, and extend northwestward through Oklahoma to Potter County in the Texas Panhandle. An offset to these north-west-trending maxima is indicated in Deaf Smith, Oldham, and Hartley counties but it cannot he evaluated because of lack of coverage on the Matador Land & Cattle Company property in this area. The maxi­mum of +35 mg on the Wichita trend in Kiowa and Greer counties of Oklahoma, is contrasted to a -81 mg minimum to the south in Wilbarger County, Texas. Again the amount of basement uplift and the density contrast between granite and flanking sedimentary rocks is inadequate to account for the magnitude of the anom­aly. Either there is a greater amount of gahbroic rocks in the Precambrian rocks of the Wichita-Amarillo trend than is ap· parent from examination of surface ex­posures and drilling records or the answer lies in the distribution of sialic and simatic material in the suhcrust. SUMMARY Major lithologic-structural divisions of the basement in general agree well with regional gravity trends. The margin of the craton and the two principal Precam­brian mobile belts are well expressed, whereas rootless stratiform terranes are not separately reflected. The regional grav­ity picture is controlled largely by base­ment phenomena, hut in some areas it is difficult to separate the effects of older Precambrian structural trends from younger Paleozoic features. The north­westerly elongated maxima that mark the Muenster, Wichita, and Amarillo uplifts cut across other gravity trends and are a reflection of a Paleozoic structure, not­withstanding a more or less coincident Precambrian ancestral belt of tectonic and igneous activity. PRECAMBRIAN HISTORY OF THE TEXAS-SOUTHEAST NEW MEXICO AREA SUMMARY The Precambrian history of the Texas­southeast New Mexico area is summarized in Table 12. The oldest known rocks in the area are exposed in the Llano uplift of central Texas and constitute part of the Texas craton. Our observations on the age of these rocks are confined to their rela­tively latter-day history when, about 1 bil­lion years <1go, a sequence of metasedi­mentary and meta-igneous rocks was ex­tensively invaded by granite. However, the existence of the craton or stable area probably dates from that time. The granite and granodiorite that are exposed in t~e Llano uplift and have been encountered rn many wells to the north and west effec­tively consolidated this part of the con­tinent into a relatively immobile block in middle Precambrian time. Although the dated granites of the Llano uplift cannot be positively correlated with the great ex­panse of granite and granodiorite that ex­tends beneath the sedimentary cover to the west, the mineralogy of the granitic rocks and evidence that this great granitic block as a unit influenced later Precam­brian developments in the north and west suggest the correlation. The correlation is further substantiated by new zircon age de­terminations on granites encountered in wells west of the Llano uplift (p. 31 \. The concept of the Texas craton as a stable area formed in middle Precambrian time is based not upon its great volume of granites, although these are suggestive­and dated as about 1,000 'million years old in the Llano uplift area-but upon its re­lationship to late Precambrian rocks. The behavior of the Van Horn mobile belt in· dicates a stable mass to the north in late Precambrian time; the great expanse of late Precambrian lava flows in the north­ern part of the area seems to have had a stable floor; and the probably late Pre­cambrian Red River mobile belt, although its relations are obscure, is at least geo­ graphically marginal to the central gra­nitic terrane. The pattern of late Precam­brian igneous and tectonic events around the granitic terrane tends to confirm its stable role during late Precambrian time. In late Precambrian time there was ex­tensive tectonic activity and associated ig­neous activity along the margins of the craton, both to the north in the area of the Red River mobile belt and to the far west in the Van Horn area. In these areas great prisms of sedimentary rocks were deformed and metamorphosed; in the Van Horn area where the rocks are ex­posed the thrust came from the south and the geosyncline was squeezed against the craton; the nature of the orogenic activity in the Red River belt is unknown. The metasedimentary rocks encountered in southern Chaves and Eddy counties of New Mexico probably also mark the mar­gin of the craton in late Precambrian time. Following the orogenic activity in north­central Texas, southern Oklahoma, and west Texas, or perhaps in part contempo­raneous with it, floods of rhyolitic lava were erupted on the surface of the craton in the Texas Panhandle and eastern New Mexico. A profusion of rhyolitic rocks, both intrusive and extrusive, was also em­placed in the Van Horn-El Paso area. Rhyolite that has been cataclastically metamorphosed intrudes the older meta­sedimentary rocks of the Carrizo Moun­tain group in the Van Horn area; un­metamorphosed rhyolite crops out in the Pump Station Hills in Hudspeth County and in the Franklin Mountains; rhyolite has been encountered in two wells in the same area ( p. 43) . The last major period of igneous ac­tivity in the Texas-Oklahoma area con­sisted of intrusion of the gabbro lopolith of the Swisher terrane in the southern Panhandle and the possibly correlative gabbro-granite intrusions of the Wichita igneous province which are partly ex­posed in the Wichita Mountains. Basement Rock3, Texas-New Mexico SUBDIVISION OF PRECAMBRIAN TIME There are great differences in the state of our knowledge of Precambrian rocks in their separated exposures on the North American continent. In some areas where there have been thorough geologic studies, notably the Lake Superior region, a formal time scale has been established; in many other areas the terms early, middle, and late are used in a relative sense. Thus, the early Precambrian rocks in one area may be equivalent to middle or late Precam· brian rocks in other areas. In addition to the determinations of relative ages of rocks in outcrop by con· ventional geologic methods, there is a growing body of data on the absolute ages of rocks by various methods of radio­active age determination. One day these data will be sufficient to assign definite spans of time to the early, middle, and late divisions so that Precambrian rocks in separated areas can be fitted into an over­all scheme. The oldest dated rocks are in the neighborhood of 2,500 million years (some allegedly older rocks from South Africa are still in question), and the youngest known Precambrian rocks range between 600 and 700 million years. As· suming that the beginning of Precambrian time dates from the oldest known rocks and not from the theoretical age of the formation of the crust, Precambrian time includes a span from ±500 million years to 2,500+ million years, but there has yet been no attempt by a responsible commit­tee to give numerical significance to the early, middle, and late divisions. In this paper the term middle Precam· brian is applied to the granites of the Llano uplift whose ages are about 1,000 million years, and late Precambrian is ap­plied to the 670 million-year old granite in the Wichita Mountains. It is likely that the late Precambrian designation will stand but possibly in future years, in a general Precambrian time classification, an age of 1,000 million years will be con· sidered late Precambrian. GROWTH OF THE NORTH AMERICAN CONTINENT Any regional study of ancient Precam· brian rocks encounters fundamental geo· logic problems on the composition of the ancient earth and the origin of continents. In the Texas and southeast New Mexico area we have evidence of a craton of con· siderable antiquity, though not of such great age as the interior part of the Cana­dian shield. Theories that explain the ori­gin of the North American continent by accretion and growth around the Cana· dian shield as a single nucleus are hard put to explain the origin of this ancient Texas stable area hundreds of miles to the south of Precambrian rocks of the same age in the Canadian shield. (Compare radioactive age determinations; Holmes, 1931, pp. 321-351; Hurley, 1950; Col­lins et al., 1954.) A valuable discussion of ideas on con­tinental growth has been included by Kay (1951) in his memoir on North American geosynclines. He states that if continents have grown by accretion from a central cratonic nucleus, the age of batholithic intrusions should be progressively less from the center outward and that there is, in fact, such a progression in the rocks of the Canadian shield area. He states fur­ther that the presence of very old intru­sions in areas such as the Black Hills, central Colorado, Great Bear Lake, and central Texas indicates that the problem is more complex and that there may have been several cratonic nuclei of the sort suggested by J. T. Wilson (1949, p. 180). Evidence presented herein supports the idea of continental growth through coa­lescence of cratonic nuclei; the craton of central and west Texas qualifies as such a nucleus. The presence of peripheral mobile belts crowded against this nucleus is in ac­cord with modern theory, but there are insufficient data to determine the extent to which these mobile belts have been rigidi­fied by batholithic intrusion. The Red River late Precambrian mobile belt seems, for example, to have remained a zone of Bureau of Economic Geology, The University of Texas weakrn'ss throughout much of later grolo­gic time. It strongly influenced the Paleo­zoic orogenic devt'lopment in the area and, to a limited extent, regained a degree of mobility although without Paleozoic meta­morphism and intrusion. The 12.000 or more feel of Ordovician, Silurian, Devo­nian, and Mississippian in the Wichita trough are geosynclinal and their accumu­lation was followed by folding, thrust­faulting, and uplift in Pennsylvanian time. This northwest-trending Paleozoic orog­enic belt which penetrates the enlarged Paleozoic craton and coexisted with the marginal Ouachita geosyncline may have been controlled by a not entirely stabilized Precambrian orogen and occupied the site of a fundamental "continental joint"; probably the general crustal instability at­tending the development and deformation of the marginal Ouachita geosyncline con­tributed to the development of the Wichita system. The Precambrian Van Horn mobile belt lies between the craton and the exposure of the Ouachita foldbelt in the Mara­than uplift, but the relationship of the two orogens is uncertain. Granite of the craton was encountered in southern Pecos County in the deep Puckett wells drilled by Phillips Petroleum Company about 20 miles north of the Ouachita front. There seems little possibility that the Van Horn mobile belt occupies this narrow zone. More probably the Van Horn mobile belt is limited in its eastward extent and was welded to the craton in late Precambrian time to form a more or less stable south­western cusp or bulge of the craton in Paleozoic time. This stable addition to the craton caused the Ouachita geosyncline to depart from the broad arc it formed around the Llano buttress and again swing southwest, as indicated by the Marathon-Solitario trend. The Van Horn mobile belt, then, seems to have achieved greater post-Precambrian stability than its northern counterpart, the Red River belt. Possibly this was due to more extensive batholithic intrusion in its hinter reaches. The pegmatites in the southern Van Horn Mountains, the granitized rock in the Hunt No. 1 Presidio Trust, and the gran­ite in the Welch No. 1 Espy in Presidio County suggest an extensive granite ter­rane in the southern part of the Van Horn belt. According to the multiple-nucleus the­ory of continental origin and growth, the Texas craton was one of several nuclei ap­parently considerably younger than the nucleus or nuclei that constituted the south-central parts of the Canadian shield. Probably the Texas continental nucleus was separated from the main continental nucleus of the Canadian shield through­out early and middle Precambrian time. These continental nuclei continued to grow by dynamic tectonic processes and concomitant batholitic intrusion; mobile belts formed peripherally to the nuclei and were welded to them by tectonic and igne­ous activity. In late Precambrian time mo­bile belts studied in this project were in existence in what is now southwest and north-central Texas; how many similar belts were incorporated into the Texas craton in earlier Precambrian time is un­known. By the end of the Precambrian time or at least by the beginning of Cam­brian time, the North American continent had been welded into a stable mass from Texas to northern Canada, and new mo­bile belts had begun to form in the areas of what is now the Appalachian-Ouachita trend and the Cordilleran trend (Kay, 1951, pp. 7-15). This stable continental mass that existed at the beginning of Cam­brian time is the hedreocraton of Kay (1951, p. 4), and the Texas craton dis­cussed herein is the southwestern part of this feature and was added to it at some period in the latter part of Precambrian time. To some geologists the picture of a stable continental mass at the beginning of Cambrian time consisting of coalesced continental nuclei is somewhat marred by the presence of the late Paleozoic orogenic belt, the Wichita system, trending north­west and west within the so-called stable region. King (1951, p. 4) has commented Basement Rocks, Texas-New Mexico on the occurrence of orogenic belts within the stable region: The broad tendency has, ... been for the stable region to be extended outward by growth and final immobilization of the surrounding orogenic belts, but during this process the stable region has from time to time been modified, and even temporarily mobilized. The edges have served as forelands of the adjacent orogenic belts; during growth and deformation of these belts the edges have been downwarped, faulted and moderately folded. Some branches of the orogenic belts have also penetrated and fragmented the southwestern corner of the stable region, thereby isolating from the main body such outlying bastions as the Colorado Plateau. Possibly the analysis of the control of the Paleozoic Wichita system by an in· completely stabilized Precambrian orogen can be extended to the continent as a whole. 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Amer., vol. 43, pp. 143-144. ----(1935) Pre-Cambrian structural con­ditions in the Llano region, in The Geology of Texas, Vol. II, Structural and economic ge­ology: Univ. Texas Bull. 3401, Jan. l, 1934, pp. 74-79. STILLE, HANS (1936) Wege und Ergebnisse der geologisch-tektonischen Forschung: Festschrift der Kaiser-Wilhelm Gesellschaft zur Forderung der Wissenschaften, Band 2, pp. 77-97. TAFF, J. A. (1904) Preliminary report on the ge­ology of the Arbuckle and Wichita Mountains in Indian Territory and Oklahoma: U.S. Geo!. Survey Prof. Paper 31, 97 pp. TAYLOR, C. H. (1915) Granites of Oklahoma: Oklahoma Geo!. Surv. Bull. 20, 108 pp. TOTTEN, R. B. (1954) Palo Duro basin, Texas: Bull. Amer. Assoc. Petr. Geo!., vol. 38, pp. 2049-2051. UooEN, J. A. (1919) Observations on two deep borings near the Balcones faults : Bull. Amer. Assoc. Petr. Geo!., vol. 3, pp. 124-131. UHL, B. F. (1932) Igneous rocks of the Arbuckle Mountains: Master's thesis, Univ. Oklahoma, 54 pp. VAN DER GRACHT, w. A. J. M. VAN WATF.R­SCHOOT (1931) Permo-Carboniferous orogeny in south-central United States: Bull. Amer. A"oc. Petr. Geo]., vol. 15, pp. 991-1057. VER Wrnnr, W. A. (1930) Ancestral Rocky Moun· tains: Bull. Amer. Assoc. Petr. Geol., vol. 14, pp. 765-788. WALTERS, R. F. (1946) Buried pre-Cambrian hills in northeastern Barton County, central Kansas: Bull. Amer. Assoc. Petr. Geo!., vol. 30, pp. 600:..110. WEAVER, PAUL (1951) Is there a pre-Cretaceous San Marcos arch? (abst.): Amer. Assoc. Petr. Geo!., Program, 1951 Regional Meeting, Aus­tin, Texas. WILSON, J. T. (1949) The origin of continents and pre-Cambrian history: Royal Soc. Canada, Trans., vol. 43, sec. 4, pp. 157-184. 74 Bureau of Economic Geology, The Univer$ity of Texa$ TABLE I PART 1-DATA ON TEXAS BASEMENT WELLS Numbers affixed to county names are location numbers on the map, Plate I. Rock names and sequences shown in this table are commonly condensed on Plate I. Page numbers refer to petrographic reports on individual wells in Appendix II. DF, derrick YEAR IJF TOP OF COUNTY OPERATOR and FARM LOCATION Sec·Block·Survey COM· ELEVA­PLETED TION TOTAL DEPTH BASEMENT Depth Elev. Andrews-I Gulf #9-E University "Z" Andrews-2 Humble #3 Lineberry Andrews--3 Humble #I Pinson Andrews-4 Humble # l Scarborough Andrews-5 Phillips #38 University Andrews-6 Phillips #45 University Andrews-7 Phillips # 50 University Andrews-8 Phillips # 57 University Andrews-9 Phillips #58 University Andrews--10 Phillips #60 University Andrews-11 Phillips #61 University Andrews---I2 Phillips # 5-M University Andrews--I3 Phillips #lO·M University Andrews-I4 Phillips #I6-M University Andrews-IS Phillips #I8-M University Andrews-I6 Phillips #I9-M University Andrews-17 Phillips #20-M University Andrews-I8 Shell #1 Cox Andrews--I 9 Shell #1 Nelson Andrews-20 Shell #I·A Nelson Andrews-2I Shell #1-E Scarborough Andrews-22 Shell and Texas #I Collins Andrews-23 Sinclair-Prairie #I Grisham- Hunter Andrews-24 Stanolind #I McCrea Andrews-25 Stanolind # I Sims Andrews--26 Stano! ind #I Stiles Andrews--27 Stanolind #3-AE University Andrews-28 Stanolind #4-AE University Andrews-29 Stanolind #5-PP University Andrews-30 Stanolind #6-PP University Archer-I Phillips #I Bullington Armstrong-I Hassie Hunt Tr. #I J.L. Cattle Co. Armstrong-2 Hassie Hunt Tr. #I Helms 42-I3-UL 8-A40-PSL I3-A40-PSL 7-A4(}-PSL 3(}-IC}-UL 19-1(}-UL 29-1(}-UL 29-10-UL 29-IC}-UL 29-IO-UL 20-10-UL 3I-I3-UL 3I-I3-UL 31-13-UL 31-I3-UL 3I-I3-UL 3I-I3-UL 5-A31-PSL 8-A40-PSL 3-A40-PSL ll-A3I-PSL 4-A40-PSL I4-73-PSL 24-A39-PSL I6-A39-PSL 8-A38-PSL 31-13-UL 31-13-UL 3I-13-UL 3I-13-UL M. Doyle I25-G5-B&P 2-2-H&GN 49 3299 51 3334 47 3335 44 3423 43 3255 43 3256 44 3247 44 3239 44 3242 44 3249 45 3247 48 3269 48 3247 49 3262 49 3279 49 327I 50 3262 44 3437 46 3335 51 3342 46 3385 48 3350 44 3242 47 3380 47 3385 50 3414 48 3262 48 3268 49 3261 50 3249 44 I032 51 2774 52 3I74 llllO 10668 10861 109'29 8005 8015 7857 8045 79'26 7867 7902 10828 10790 I0826 10980 11011 10957 ll061 I0606 10335 9714 I0380 11322 I0479 I0960 11500 10601 I0500 I0687 10893 79I5 7012 6574 ll078 -7779 10665 -733I 10830 -7495 10905 -7482 7974 -47I9 8004 -4748 7846 -4599 8034 -4795 7877 -4635 7839 -4590 7860 -46I3 10724 -7455 10714 -7440 10807 -7545 10945 -7666 1099I -7720 10930 -7668 llOSO -76I3 10585 -7250 10314 -6972 9703 -63I8 10376 -7026 113I5 -8073 10472 -7092 10876 -7491 ll475 -806I 10590 -7328 10498 -7320 10685 -7424 10855 -7606 79I3 -688I 6930 -4181 6070 -2896 Armstron g-3 Hunt #4 Ritchie 122-G6-A&G 52 2691 7072 6810 -4ll9 Armstrong-4 Annstronl!-S Placid #1 Matheson Standard of Texas #1-A Palm 173-B3-H&GN 14I-B4-H&GN 49 52 3195 3509 4675 6140 4673 6115 -1478 -2606 Armstrong-6 Stanol ind #1 Corbin 275-B4-H&GN 43 3383 6120 6118 -2735 75 Basement Rocks, Texas-New Mexico floor; b'32, well completed before 1932, exact date unknown; ni, no information; nsl, no samples located; -X, affixed to volcanic rock name means that microstructures and fabric indicate an extrusive origin. The absence of -X, however, does not necessarily indicate an intrusive origin. See Table 11 (pp. 64-65) for wells numbered with P (e.g., Brewster-IP). ACl·: OF FORMATION ON J.ITllOJ.OCY OF P RECAMBRIA N MATEHJAL BASEMEST BASEMENT INTERVAL PROVINCE STUOll:D PACE Cambra-Ord granite 11100-05 Texas craton cuttings 119 Cam bro-Ord granite 10665-68 Texas craton cuttings 120 microgahbro 1066;...68 cuttings 120 Cam bro-Ord mil'rogranite 10855-60 Texas craton cuttings 120 Cam bro-Ord !!ranite 10910-26 Texas craton cuttings 120 albite diorite 10926-29 core 120 Cambro-Ord mic.:rop;ranite 8000-05 Texas craton cuttings 120 Cambrian nsl Cambrian J!ranite gneiss 7854-57 Texas craton core 120 Permian microgranite 8030-35 Texas craton cuttings 120 Cambra-Ord ~ranite 7922-26 Texas craton core 121 Cambra-Ord n~I Cambra-Ord nsl Cambra-Ord quartz diorite 10820-25 Texas craton cuttings 121 Cambro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord microgranite 11030-40 Texas craton cuttings 121 granite 11057~1 core 121 Cam bro-Ord lew·o-diabase 10600-06 Texas craton cuttings 121 Cambro·Ord syenite 10330-35 Texas craton cuttings 121 Cambra-Ord leuco-albiie-quartz diorite 9711-14 Texas craton core 121 leuco-albite diorite 9711-14 core 121 Cambra-Ord syenite 10370-80 Texas craton cuttings 122 Cam bro-Ord granite 11.11 5-22 Texas ('Taton cuttings 122 Ordovician syenite 1047;,..79 Texas craton core 122 Ordovician syenite 10871-83 Texas craton core 122 syenite 10924-42 core 123 syenite 10942-60 core 123 Cam bro-Ord Jeuco·microdiorite 11500 Texas rraton core 123 Cambra-Ord leuco-quartz microdiorite 10590-95 Texas craton cuttings 123 Cam bro-Ord granite 10490-500 Texas craton cuttings 123 Cambra-Ord nsl Camliro-Ord quartz diorite gneiss 1088;...93 Texas craton cuttings 123 Cam bro-Ord meta p;raywacke 7913-15 Red Ri,·er mobile belt core 123 Mississippian rhyolite tufT 69.~0-70 Panhandle volcanic terrane cuttings 124 rhyolite porphyry-X 6970-7000 cuttings 124 Mississippian rhyolite-X 6070-6180 Panhandle volcanic terrane cuttings 124 rhyolite porphyry-X 6070-6180 cuttings 124 rhyolite tufT? 6070-6180 cuttings 124 snpentinized dolomite 6180--0280 .~ cuttings 124 trarhyte porphyry-X 6~00-6570 ~ c uttings 1::?~ rhyoliie tufT 6300-6S70 cuttings 125 tremolite-talc hornfels 6080-6180 "' ruttin~s 124 rhyolite porphyry 6080-6180 " c·uttinµ:~ 124 snpentinized dolomite 6180-6280 ·'§ cuttings 124 ~ilt:->tonr 6180-6280 Cf) cuuinµ:s 124 Mh•sissippian lr1wo-microgabbro 6810-7070 Swisher gabbroic terrane cutt in g:s 125microgabbro 6810-7070 cuttings 12S Pennsylvanian rhyolite porphyry-X 4650 Panhandle volcanic lerrane cuttings 125Pen n!\ylvanian rhyoli te porphyry-X 614{)-41 Panhandle volcanic terrane core 125Pennsyl,·anian rhyolile porphyry-X? 6118-19% Panhandle volcanic terrane core 125 76 Bureau of Economic Geology, The University of Texas YEAR DF TOP OF LOCATION COM­ ELEVA­ TOTAL BASEMENT COUNTY OPERATOR ond FARM Sec-Block-Survey PLETED TION DEPTH Deplb Elev. Bailey-! El Paso Nat. Gas #1 W. Tex. Mtge. Loan 5&-A-MB&B 45 3945 9127 8710 -4765 Bailey-2 Lion #1 Bridwell 78-A-MB&B 52 3892 8953 8810 -4918 Bailey-3 Phillips # 1-A Stevens 24-B-MB&B 51 4025 8244 8130 -4105 Bailey-4 Shell # 1 Nichols Lab 13-Lge 212­Crosby CSL 51 3810 9051 8988 -5178 Borden-! Sinclair # 1 Bryan 40-32-ELRR 49 2961 10804 10789 -7843 Briscoe-I Amerada #1 Hamilton 41-3-T&P 51 2474 8775 8742 -6268 Briscoe-2 Hunt #1 Ritchie 68--R. F. Stevenson 51 2492 7900 7475 -4983 Briscoe-3 Hunt #2 Ritchie 54-G&-ELRR 51 2597 7764 7130 -4533 Briscoe-4 Hunt #10 Ritchie 74-G6-A&G 52 2411 8165 7910 -5499 Briscoe-5 :Midstates # 1 Hickok & Reynolds 39-A -C. L. Craig 51 2362 8299 8085 -5723 Briscoe-6 Standard of Texas # 1 Owens 142-MlO-D&SE 51 3295 8393 8340 -5045 Brown-I McDonald & Campbell #1 Smith G. A. Parker 1433 3434 3395 -1962 Carson-I Cities Service # 1 Whittemore 14-7-I&GN 52 3416 3.~26 3312 +104 Carson-2 Dunnigan (Cabot) #1 Ellis 127-7 -I&GN 49 3369 5365 5315 -1946 Carson-3 Empire Gas & Fuel #1 Lane 72-4-I&GN b'32 3188 2980 2550 +638 Carson-4 Phillips # 1 Ardis 3-B4-H&GN 53 3417 6257 6185 -276R ea..on-5 Reiger # 1 Forster 93-7-I&GN 34 3324 3327 3326? -2? Carson-6 Shamrock # 1 Thompson 15-7 -I&GN b'32 3383 3404 3045 +338 77 Basement Rocks, Texas-New Mexico AGE OF FORMATION ON LITHOLOGY OF PRECAMBRIAN MATF.RIAL BASEMENT BASEMENT INTERVAL PROVINCE STUDIED PAGE Cambrian gab bro 8710-20 Swisher gabbroic terrane cuttings 126 diabase 8720-90 cuttings 126 serpentinized dolomite 8790-8800 cuttings 126 gabbro 8800-8950 cuttings 126 leuco-olivine gabbro 8960-9050 cuttings 126 metasiltstone 9050-90 cuttings 126 olivine gabbro 9090-9100 cuttings 127 Cambro-Ord leuco-gabbro 8810-8950 Swisher gabbroic terrane cuttings 127 leuco-olivine gabbro 8951-53 core 127 Permian? rhyolite porphyry-X 8190-8200 Panhandle volcanic terrane cuttings 127 Cam bro-Ord rhyolite 9030-40 Panhandle volcanic terrane cuttings 12B rhyolite porphyry 9030-40 cuttings 128 Cam bro-Ord nsl Mississippian quartz diorite 8775 Texas craton cuttings 12& Mississippian leuco-gabbro 747!>--7590 Swisher gabbroic terrane cuttings 128 dolomite-talc rock 7590-7635 cuttings 129 meta-arkose 7590-7635 cuttings 129 meta-arkose 763!>--7730 ~ cuttings 129 tremolite-dolomite ·; hornfels 7730-7745 -10135 siltstone 1006!>-10135 serpentinite 10114-10128 altered ferromagnesian­rirh rock 10128-10133 olivine gabbro 1013!>-10500 diahase 10135-10500 leuro-olivine gabbro 10447-10459 ~ahbro 10460-10500 rhyolite porphyry-X? 7380-7384 Swisher gabbroic terrane Swisher ga bbroic terrane? Panhandle volcanic terrane? Swisher gabbroic terrane Wichita igneous province? Panhandle vokanic terrane cuttings cuttings cuttings core cuttings cuttings cuttings cuttings cuttings cuttings cuttin gs core cuttings cuttings core core core 134 134 134 134 134 134 134 135 135 135 135 135 135 135 135 136 136 outlier? Camhrinn Camhrian C.ambrian f.amhrian Cambrian Cambrian phyllitic metagraywacke 3273 metn-arkose 2183-2184% nsl J?ranodiorite 2560--2565 meta,:?:raywacke 2935 granite 4250-4255 Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt cuttings core cuttings core cuttings 136 137 137 137 137 C.amhro-Ord leuc-o-albite gabbro 8660-8610 5,.,.·isher gabbroic terrane cuttings 137 Permian rhyolite porphyry-X? rhyolite porphyry-X? l 077 !>-I 0788 10788 Panhandle volcanic terrane cuttings core 137 137 Permian Permian Cam bro-Ord Cam bro-Ord Cam bro-Ord rhyolite porphyry-X nsl granodiorite rhyolite-X? rhyolite porphyry 7393-7394 11490-11494 12620-12630 12678 Panhandle volcani c terrane Texas era ton ? Panhandle volcanic terrane Panhandle volcanic terrane core cuttings cuttings core 138 138 138 138 Cam bro-Ord rhvol ite rhyolite porphyry rhyolite porphyry 10770 10820-10839 10839-10840 Panhandle volcan ic terrane cuttings cuttings cuttings J.'l8 138 138 Cam bro-Ord rhyolite luff 1120!>-11214 Panhandle volcanic terrane core 139 Pennsylvanian granodiorite 571(>...5728 Texas craton core 139 Penn~yl vanian Pe:nnsylvanian C.ambro-Ord Cam bro-Ord Cam bro-Ord microgranite microgranite i,?ranodiorite granodiorite nsl nsl 5630-5635 5670-5675 5517-5525 6708-6709 Texas craton Texas craton Texas craton cuttings cuttings core core 1.39 139 139 1.39 YEAR DF TOP OF LOCATION COM­ ELEVA· TOTAL BASEMENT COUNTY OPERATOR and FARM Sec·Block·Survtiy PLETED TION DEPTH DEPTH ELEV. Coleman-I Chandler (Killam) #I Gill -5-Wm. York 54 I428 4395+ 3230 -I802 Coleman-2 Naylor #I Stone Collingsworth-I Bridgeport #I-E Hughes Collingsworth-2 Continental #I McDowell 73­-Childress 78-11-H&GN I07-22-H&GN 53 46 26 2I29 2052 2346 5348 4I50 2830 5338 4I20 2760 -3209 -2068 -414 Collingsworth-3 Lubbock Machine & Supply # l Alexander 4-16-H&GN 54 2200 4100 4570 -1900 Collingsworth-4 Obrien & Cline # l Fairbanks Collingsworth-5 Superior of I.al. #1 Brown Colling•worth-6 Union #I Glenn 23-23-H&GN 7S-22-H&GN 87-11-H&GN 49 51 47 2551 2343 2092 2087 5710 4183 2076 5645 4180 +475 -3302 -2086 Comanche--! Davis #I Hanson Sam Bowers 48 1183 4040 4036 -2853 Comanche--2 Cooke--! Cooke--2 Gallagher & Lawson #I Terry Continental #I Berry Continental # I Whaley N. H. Kuykendall J.M. Culp W. C. Winters 38 52 52 I334 1008 1082 5259 6043 4282 5257 5738 4243 -3923 -4730 -3161 Cooke--3 Cooke--4 Duffy et al. # l Bailey-English Gull Prod. #I Donald E. Faris J . Guffey b'32 b'32 877 906 2273 3I35 2273 2900 -1396 -1994 Cooke--5 Hollandsworth # 31 Fette Hrs. of Cyrus Underwood 51 926 5196 4986 -4060 Cooke--6 Hollandsworth #I Thomason T. Bell 49 745 2391 2385 -1614 Cooke--7 Hollandsworth #2 Thomason T. Bell 49 745 2467 2461 -1716 Cooke-8 Cookc-9 Cooke--10 Cooke--11 Kadane & Sons #I Coursey Maguire #IF. S. Bundy McElreath & Suggett #1 Whaley Muenster #I Yosten W. F. Shaw W. Phelps SA&MG G. Ivy 45 48 b'32 b'32 I052 790 916 1059 2962 2106 2340 3790 2962 2085 23I2 2750 -I908 -1295 -1396 -1691 Cooke-12 Phillips #1-CT Atcheson J. Davis 47 960 2271 2258 -1298 Cooke-13 Phillips #3 Atcheson J. Davis 47 970 2176 2166 -1196 Cooke--14 Phillips .#3 Dangle E. Langford 44 1062 2519 25I7 -1443 Cooke--15 Phillips # l Fielder E. Langford 44 1027 2940 2926 -1899 Cooke-16 Phillips # I Reitar J.M. Culp 44 1070 .3218 3204 -2134 Cooke--I7 Phillips #2-A Reitar E. Langford 45 1087 3256 3230 -2143 Cooke--18 Texas #1-C Hutson SPRR 45 961 4240 4130 -3169 Cottle-I Anderson-Prichard # l Lynch 66-1-J. Poitevent 46 200I 5834 5825 -3824 Cottle--2 General Crude #13-1 Swenson 13-B -J. H. Stephens 54 2066 5724 4810 -2744 Cottle-3 General Crude #33-I Swenson 33-B -J. H. Stephens 54 2I37 5460 5410 -3273 Cottle-4 llumblc #1-J Matador 15-E -Matador Land & Cattle Co. 51 1748 8087 8055 -6307 Cottle-5 Jones & Stasney #I Wiley 6-B -J. H. Stephens 52 2038 5245. 4740 -2702 Cottle-6 Merry Bros. & Perini #I Pursell 40-B -J. H. Stephens 33 20I2 4740 4650 -2638 Cottle--7 Ramsey #I Lynch 66­-J. Poitevent 40 2020 5694 5670 -3650 Cottle-8 SeahoardandShamrock #ITapper 6­ -AB&M 48 1992 6656 6635 -4618 Cottle-9 Signal #I Swenson 28-B -J. H. Stephens 51 2087 560I 5585 -3498 1 Robert Roth (pcrtonal communication, 1954) reporlt garneliferou1 acbl1Li no thin section studied. 2 Personal communicalioo, Robert Roth, 1953. No thin section studied. I Lithology determined too late for inclusion on map, Plate J. 81 Basement Rocks, Texas-New Mexico ACE OF FORMATION ON LITHOLOGY OF PRECAMBRIAN MATERIAL BASEM ENT BASEMENT INTERVAL PROVINCE STUDIED PAGE Cambrian granodiorite gneiss biotite schist 3445-3450 3515-3520 Texas craton cuttings cuttings 139 139 scapolite·diopside gneiss 3575-3580 cuttings 140 scapolite-diopside gneiss 3765-3770 scapolite-diopside gneiss 3940-3945 diopside-calcite gneiss 4045-4050 diopside marble 4195-4200 calcite·scapolite·diopside cuttings cuttings cuttings cuttings 140 140 140 140 gneiss 4285-4290 scapolite-diopside gneiss 4390-4395 cuttings cuttings 140 140 Cambrian granite 5338-5348 Texas craton core 140 Pennsylvanian nsl Permian nsl Cambrian garnetiferous schistl ni granite2 ? Wichita igneous province? Cambrian Cambrian quartz diorite nsl 5685-5710 Wichita igneous province? cuttings 141 Cambrian nsl Cambrian Pennsylvanian granite syenodiorite 5255-5257 6032­6033 Texas craton Red River mobile belt cuttings core 141 141 Pennsylvanian biotite-quartz-albite gneiss 4277-4287 Red River mobile belt core 141 Pennsylvanian? nsl Pennsylvanian? metaquartzite hornblende-oligoclase 3010-3015 Red River mobile belt cuttings 142 Cam bro-Ord schist metaquartzite 3010-3015 ±5000 Red River mobile belt cuttings core 142 142 sillimanite-biotite ni schist nsl ±5105 core 142 ni nsl ni Pennsylvanian hornblende schist nsl 2960-2962 Red River mobile belt core 142 Pennsylvanian? Cambrian Pennsylvanian Cam bro-Ord? Cambro-Ord Pennsy)vanian Pennsylvanian Pennsylvanian Pennsylvanian biotite-garnet schist horn hlende schist biotite schist hornblende schist metagraywacke metaquartzite granite granite granite l!ranite p:ranite granite2 2312-2340 3160 3165 3578-3652 2263-2271? 2165-2170 2517-2519 2517-2519 2935-2940 3216 3253-3256 4240 Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt Red River mobile belt core cuttings cuttings cuttings core cuttings cuttings cuttings cuttings core core 142 143 143 143 143 14.3 143 143 144 144 144 Cambra-Ord?, Mississippian? Pennsylvanian Mississippian lenro-albite-quartz mirrodiorite mic-rogranodiorite quartz diorite scririte phyllite metarkosite3 5810-5820 5820-5830 5410-5423 5il9­5722 5449-5452 Texas craton Red River mobile belt Red River mobile belt cuttings cuttings core core core 144 144 144 144 145 Cambrian Pennsylvanian Pennsylvanian Cam bro-Ord Cambrian Cambrian sericite phyllite meta·arkose meta-arkose and diabase meta·arkose meta·arkose metarkosite microgranodiorite meta·arkose nsl 8082-8087 4850-5000 5000-5005 500S­524S 4659-4740 4659-4740 5670-5680 6655 Red River mobile belt Red River mobile belt Red River mobile belt Texas craton Hed River mobile belt core cuttings cuttings cuttings cuttings cuttings cuttings core 145 145 145 146 146 146 146 146 YEAR DF TOP OF LOCATION COM­ ELEVA­ TOTAL BASEMENT COUNTY OPERATOR ond FARM SM-Bloek-Su"ey PLETED TION DEPTH Dopth Elo•. Crane-I Crane-2 Atlantic #2-A University Loffiand # 3 Tubbs 33-31-UL 9-B27-PSL 48 37 2553 2540 11645 7168 11623 7115 -9070 -4575 Crockett-I Amerada #1-D Shannon Lot 2-Lge 3-Archer CSL 48 2881 78.15 7825 -4944 Crosby-I Continental #I Swenson L&C 75-2 -H&GN 48 2454 84J:l 8385 -5931 Crosby-:? Crosby-3 Deep Rock # 2 Morgan Jones Gulf #I Martin 11 -1-WCRR -5 -A. D. Meyer 52 50 2659 2519 8771 8435 8765 8420 -6106 -5901 Crosby-4 Humble # I Irvin 889-C3-ELRR 53 3I65 9947 9922 -6757 Crosby-5 Crosby--6 Humble #1 Montgomery Ohio # 1 Morgan Jones 21­-B&B 140-2-H&GN 47 48 3108 2579 9707 8638 9700 8630 -6593 -6051 Dallam-I Pure # 1 Federal Land Bank 17-7 -FDW Subd 42 4335 6863 6860 -2522 Dallam-2 Pure # I Sneed Heirs 13-18­Cap Synd 44 4013 6779 6768 -2755 Dall am-3 Texas #1 Cap. Freehold Land Trust 10-7 -Cap Synd 45 4563 6169 6110 -1547 Deaf Smith-I Honolulu #I Ponder Dea f Smith-2 Humble # I Hyslop Lge 436-St Cap Lands 18-Lge 418-St 49 399 I l019I 9980 -5989 Cap Lands 45 4448 7805 7750 -3302 Denton-I Hunt #I Forester W. Nelson 42 749 2520 2290 -1541 Denton-2 Hun t #1 Martin J. Parks 44 809 2978 2976 -2167 Denton-3 Hunt #2 .Martin J. Stewart 49 747 2136 2119 -1365 Den ton-4 Jen kins. Kelsey, Jones & Eubanks #1 Waide T. Carpenter b'32 797 1913 I870 -1691 Denton-5 Stanolind #I Dunn J. Parks 44 813 3416 3408 -2595 Denton--6 Texas #I Yeatts W. J. Hendrix b'32 77 1 2026 2013 -1242 Di ckens-I Humble #3 Matador 2-J -W. Jackson 48 2404 7737 7645 -5241 Dickens-2 Humble #2-F Matador 27-AS-J. S. Callaway 52 2196 7542 7455 -5259 Dickens-3 Humble #1-G llfatador 8-C-C. U. Connellee 47 2732 8246 8232 -5500 Di ckens-4 Livermore # Bird 288­1 -H&GN 45 2S60 8390 8322 -5828 Di ckens-5 :'llagnol ia # I Wiley 30f>-I -H&GN 48 2307 7716 7697 -5390 Dickens--6 Natl. Petr. Assoc. #I Blackwell f>-C-C. U. Connell ee 50 2722 8387 8370 -6548 Dickens-7 Dirkens-8 Norsworthy #I Burleson Plarid #I Emery f>-WC-J. C. Kell er 262-1 -H&GN so 52 2027 2284 7511 7852 7480 7798 -5453 -5568 Dickens-9 Placid #I Goess 3­ -AB&l\f 51 2744 8474 8442 -5698 Dickens-IO Placid # I Huges 18'­I -H&GN so 2.~11 79~0 7936 -5625 Dickens--II Placid #I Swenson 226-I -H&GN 51 2253 7796 7737 -5484 Dirkens--I2 Union Cal. # I Elliott 177-1-H&GN 49 2309 8117 8115 -5806 Dickens-I3 Woodward # I Williamson 189­I-H&GN 47 2263 7787 7781 -5518 Donley- I Doswell #I :'llrMurtry 40-C3-GC&SF 50 2763 5375 5245 -2482 Donl ey-2 Honolulu # I Ozier 5f>-C6-GC&SF 48 2848 5893 5850 -3002 Don ley-3 Humble # I Roach f>-C4-TTRR 49 2955 5265 5230 -2275 Donley-4 Hu nt #5 Ritchie 108-GS-J. Myers ni 2560 6503 6460 -3900 Donley-5 Placid #I Kelly 47­G7-A&G 51 2520 7070? 6955 -4435 Donley--6 Shamrock #I Adair If>... A -J. G. Adair 43 2566 5358 5345 -2779 Donley-7 Stanol ind #I Broome 46-20-H&GN 44 2445 6756 6748 -4303 4 lnform•tioo from Stanolind Oil l Cas Company'• 1ample 101. No 1hin 1ttlinn 1h1died. 83 Basement Rocks, Texas-New Mexico AGl::Or FORM A TIO:'< ON UTHOLOGYOF PHECAMBRIAN MATF:Rl:\I. BASF.MF.N T BASEMENT INTERVAL PROV INCE STUDIED PAGE Cam bro-Ord metagranodiorite 11642-11645 Texas naton core 146 Cambrian granite 7120-7160 Texas craton cuttings 147 Cam bro-Ord mil'rogranite 7825-7835 Texas craton cuttings 147 Cambro-Ord nsl Cambro-0rd gran ite'! 8760-8770 Texas craton cuttings 147 Pennsylvanian nsl Camhrian diorite gneiss 9947 Texas craton?, Red River mobile belt·? core 147 Mississippian Jnt'taconglomerate 9700-9707 River mol>ile l>elt? core 147 Mississippian granite 8635-8638 Texas craton cuttings 148 Cam bro-Ord nsl Cam bro-Ord granite 6775-6779 Wichita igneous province? cuttings 148 Cam bro-Ord rhyolite porphyry 6168-6169 Panhandle volcanic terrane core 148 Mississippian nsl Pennsylvanian dial>ase 7750-7800 Panhandle volcanic terrane cuttings 149 rhyolite porphyry-X? 7803-7805 core 149 Pennsylvanian nsl Pennsylvanian microdiorite Red River mobile belt core 149 Pennsylvanian nsl Pennsylvanian? garnet-biotite-quartz­ oligoclase gneiss 1882 Red River mobile belt core 149 Pennsylvanian hornh1ende schist ?4 3414-15 Red River mobile belt core Pennsylvanian? hornblende-andesine gneis.• (amphibolite) 2013 Red River mol>ile belt core 149 Mississippian quartz diorite 77.%-7737 Texas craton core 149 micrographic granite 7735-7737 core 149 Mississippian meta·arkose and Mississippian me ta·argillite granite 7735-7742 8230-8240 Fisher metasedimentary terrane Texas craton core cuttings 150 150 quartzose granite 8246-8248 core 150 Mississippian quartzose granite 8390 Texas craton core 150 Mississippian hiotile phyllite metaquartzite 7700-7716 7710-7716 Fisht'r metasedimentary terrane cuttin gs cuttings 150 150 Mississippian metarkosite grani te 7710-7716 8370-8380 Texas rraton cuttings cuttings 150 151 alhite syenodiorite 8380-8.~84 cHttin~s 151 Cam bro-Ord nsl Mississippian nsl Mississippian nsl Mississippian nsl Mississippian nsI Cam bro-Ord !t'U<'o·al hite·quartz xas aaton? Wichita igneous cuttings 151 Cambrian granite ? province? Panhandle volcanic terrane core 152 granodiorite 5890-5893 core 152 Cambrian rn irrogranite porphyry 5265 Trxas era ton 't core 152 Cam bro-Ord Mississippian Cambrian Cam brian rhyolite porphyry sanlh· '32 1600 660 304 +1296 Gillespie-2 Lewis # I Koll N. side Fredericksburg b'32 1725 418 168 +1557 Gillespie-3 Thou;and Islands # 1 Hayden Surv. 144 b'32 Hl50 1505 11112 +6611 Glasscock-I Shell # 1 Clarke 5-32-T4S-T&P 46 2663 10970 10964 -8301 Gray-1 Ama-Gray # l i\lathers Trust 63-25-H&GN 48 2511 2385 2385 +236 Gray-2 Bock-Anderson # 1 Beavers 12-1-B -2-H&Gl'i" b'25 3144 3800 3305 -161 Gray-3 Danciger # 1 Bradford 123-B -2-H&GN b'32 3091 2741 2670 +421 Gray-4 Gerber # l Stubbs 9-3-B&B 33 2777 3100 2885 -108 Gray-5 Graham et al. (Taconian Oil) #1 Sullirnn 1.16-3 -l&GN b'32 3273 2990 2900 +373 Gray--0 Graywash #1-A Carpenter 22-25-H&GN 49 2622 2305 ni ni Gray-7 Gray wash # 1-B Carpenter 23-25-H&GN 49 2438 2696 2571? -33'! Gray-8 Hiekman # 1 Brown 15-30-H&GN 48 2713 2718 2264 +449 Gray-9 Holt #3 Bailey 58-52-H&GN 52 2596 2801 2413 +1s3 Gray-10 l\lagnolia #.J, Latham 153-3-l&GN b'32 3276 3434 2875 +401 Gray-11 Operators' Oil # 1 Bowers 93-B -2-H&GN b'32 3038 3090 2800 +238 Gray-12 Phillips # 1 Bralley 7-C2-CCSD&RGNG 51 .~058 3114 3027 +35 Gray-13 Phillips #2 Keahey 220-B2-H&GN 54 3275 ni 2616 +659 Gray-14 Phillips #6 URB 189-B2-H&GN 54 3279 3175 3040 +239 Gray-15 Phillips #1 Worley 129-B2-H&GN 50 3247 2833 2825 +422 Gray-16 Shamrock # l McCracken 31-25-H&GN 52 2886 2602 2525? +361? Gray-17 Shamrock # 1 Taylor 6-2 -H&GN 30 2839 3039 ni ni Gray-18 Sidwell #I Bowers 92-93-B2-H&GN 49 3100 3293 3250? -150? Gray-19 Skelly #1 Heitholt 153-3 -IGN b'31 3275 2860 2835 +440 Gray-20 Smith #1-E Johnson 189-E-D&P 44 2847 2861 2860? -13? Gra,·-21 Texas #15-A Chapman 8-26-H&GN 48 2555 2342 2340? +215 '! Grai·-22 Texas #6-B l\lcLarty-Lester 10-1-ACH&B 35 2727 3038 2400 +327 Gray-23 Warner # l lllorse 68-25-H&GN 47 2562 2830 ni ni Hale--1 Amerada # 1 Kurfees 6-N -H&OB 44 3308 10250 10225 -6917 Hale-2 Honolulu o.nd Sinclair# 1Clements 19-D7-ELRR 51 3349 10162 10000 -6651 Hale-3 Humble and Stanolind #1 Byrd 17-K-T&P 41 3251 6760 6750 -3494 Hale--! Standard of Texas #1 Keliehor Lg:e 3-Callnhan CSL 51 3280 10895 10050 -6770 Hale--5 Stanolind #2 Fisher 5-CL-ELRR 47 3314 8394 8042 -4728 Hale--0 Stanolind # 1 Hegi 7-5 -C. F. Stevenson 47 3264 9974 9275 -6011 Hall-1 Amerada #I Hughes 101-SS-D&P 51 2095 8121 8100 -6005 Hall-2 Humble #1 \loss 119-1-SPRR 42 1987 4886 4885 -2898 Hall-3 Hum!.!.• :!:!: l Weaver 51-1-SPRR 40 1950 4840 4820 -2870 87 Basement Rocks, Texas-New Mexico AGE OF FORMATION ON LITHOLOGY OF PRECAMBRIAN MATERIAL BASEMENT BASEMENT INTERVAL PROVINCE STU Ill ED PAGE Cretaceous nsl ni nsl Cambrian granite 1446 Texas craton cuttings 159 Cambrian granite 10969 Texas craton ruttings 159 Permian nsl Permian granodiorite 3200--3205 Wichita igneous province cuttings 159 analcite diabase 3200-3205 cuttings 159 ni nsl ni nsl ni nsl ni nsl Permian nsl Permian nsl Permian granite 2290-2721 Wichita igneous province cuttings 160 ni granite 2991-2995 Wichita igneous province cuttings 160 granite 3418-3431 cuttings 160 ni nsl Permian nsl ni rhyolite porphyry 2618-2628 Panhandle volcanic terrane cuttings 160 Permian volcanic rocks and albite diorite 3043--3110 Panhandle volcanic terrane cuttings 160 rhyolite luff 3175 cuttings 161 rhyolite porphyry 3175 cuttings 161 Permian granite 2824-2833 Wichita igneous province cuttings 161 Permian leuco-gabbro 2520-2590 Wichita igneous province cuttings 161 Permian diabase 2410-2550 Wichita igneous province cuttings 161 Permian granite 3253-3293 Wichita igneous province cuttings 161 ni granite 2935-2948 Wichita igneous province cuttings 161 granite 2984-2990 cuttings ]bl Permian Ienco-quartz diorite? Wichita igneous province cuttings 162 Permian nsl Permian nsl Permian nsl Mississippian olivine gabbro !0245-!0250 Swisher gabbroic terrane core 162 leuco·olivine syenogabbro !0245-10250 core 162 Mississippian Ienco-olivine gabbro !0060-!0070 Swisher gabbroic terrane cuttings 162 olivine gabbro 1014-0-10150 cuttinp;s 162 Mi~sissippian granodiorite 6750-6760 Texas craton cuttings 162 Mississippian granite 10080-10090 Texas craton? cuttings 163 leuco-albite-quartz diorite 10080-10090 cuttings 161 alhite µ:ranodiorite 10140-10170 cuttings 163 leuco-albite diabase 10140-10170 cuttings 163 leuco-albite diabase 10220-10300 C"uttings 163 albite granodiorite 10220-10300 ruttings 16.'l alhite ~ranodiorite !0300-10400 cuttings 163 albite granodiorite 10400-10500 C"uttings 163 alhite J!Tanodiorite 10500-10600 cuttings 163 albite p:ranodiorite 10600-10780 cuttings 163 Mississippian rhyolite flow-breccia 8045-8050 Panhandle volcanic terrane cuttings 163 rhyolite ffow-breccia 8048 core 164 rhyolite tu ff 8300-8305 <'ore 164 rhyolite tuff-breccia 8370-8380 cuttings 164 rhyolite porphyry & tuff 8370-8.390 cuttings 164 Mississippian rhyolite porphyry 9974-9974% Panhandle volcanic terrane eore 164 Cambrian granodiorite 8108 Texas craton cuttings 165 Cambrian nsl Cambrian albite granodiorite 4820-4840 Texas craton <'ore 165 granodiorite gneiss 4839-484-0 cuttings 165 YEAR DF TOP OF LOCATION COM­ ELEVA­ TOTAL BASEMENT COUNTY OPERATOR and FARM Sec-Block-Survey PLETED TION DEPTH Deplb El.,., Hartley-I Hartley-2 Hartley-3 Hartley-4 Bridwell #I Houghton Bridwell #I-A Houghton Bridwell #2-A Houghton Holmes & Heck #I Coots Lge 202­St Cap Lds 5I Lge 202-St Cap Lds 52 Lab I4-Lge 2IO-JJ Subd 52 58­-XR b'29 3899 393I 3873 4I8I 4464 4454 4106 2990 4444? -545? 4395 --464 4095 -222 2965 +I216 Hartley-5 Humble #2 Shelton 63--Rio Bravo Subd b'27 3869 3076 2977 +892 Hartley-6 Kerr-McGee #I Berneta 29-2I-St Cap Lds 52 3823 605I 6034 -2211 Hartley-7 Hartley-8 Hartley-9• Hartley-10 Hartley-11 Kerr-McGee #2 Berneta Kerr-McGee #3 Berneta Kerr-McGee #I Shelton Phillips #I-GG Bivins Pure #I Lankford 30-2I-St Cap Lds I9-2 I-St Cap Lds 3­-Bravo Subd 28-2I-Cap Sch Lds I48--48-H&TC 52 52 54 53 49 3720 3718 ni 3828 3934 6626 588I 4I95 6I78 7853 6I35 5860 ni 6117 7845 -2433 -2I42 ni -2289 -3911 Hartley--I2 Hartley-I3 Hartley-14 Shamrock #I Dammier Sinclair-Prairie #I Bivins Est. Stanolind #I Beck 7-LE-G&M 5-26--ELRR 164--15-CS 54 53 46 4023 3664 4256 6113 5926 5959 60i0 5842 59I5 -2047 -2178 -I659 Hartley-15 Texas Gulf #I Matador 53--22-Cap Synd 52 3702 4801 4796 -1094 Hockley-I Big Chief #I DeLoache Lab 72-Lge 7-Reeves CSL 50 3632 11452 11420 -7788 Hockley-2 Honolulu #1-A Lockett 2-1-PSL 53 3464 12214 122IO -8764 Hockley-3 Hockley-4 Hockley-5 Honolulu and Signal #1-24 Elwood Est. Honolulu and Sunray #I Moore Humble #I Campbell Lab 24--Lge 5-Wil­barger CSL 48 4--0-PSL 52 16--A-R. M. Thompson 46 3405 3345 3440 11632 11305 11730 11605 11295 11600? -8200 -7950 -8160? Hockley-6 Humble #I Hobgood Lab 10-Lge 693-St Cap Lds 50 3470 10179 9060 -5590 Hudspeth-I Amer. Land #I Roseborough 7-21-Tws 6--PSL 29 4799 1786 1600? +3I99? Hudspeth-2 California #I Theison I9-E-UL 30 5109 4850 4732 +377 Hudspetb-3 General Crude #I Merrill-Voyles 8-69-T&P 52 3874 4792 4782 -908 Hutchinson-I H. T. McGee #I Smith-Capers IO-Y -M&C b'32 2908 3I75 31:15 -227 Jones-I Hunter & Hunter #I Steele Robt. Smith 46 I680 5095 5074 -3394 Kent-I Kent-2 Chapman & Mcfarlin #26 Cogdell 716--97-H&TC General Crude #I P. Jones 169­G-W&NWRR 50 48 2262 2036 7983 7589 7970 7400? -5708 -5634? Kent-3 General Crude #82-1 Jones 82-G-W&NWRR 54 2150 7685 7654 -5504 Kent-4 Kent-5 Humble #I4 Spires Superior #8 Wood "I94" 719­97-H&TC 194--G-W&NWRR 50 50 2435 2I65 8292 79IO 8247 7805 -5812 -5640 Kimble-I Kimble-2 Humble # I Bolt Phillips #1 Spiller I34-­IO­ -Mary Tolliver -W. A. Choice 48 45 2025 2227 4I71 4264 328I 4229 -I256 -2002 King-I Kin g-2 King-3 King-4 Kin g­5 Continental #I Martin Con tinen tal #2 Martin Helmerich & Payne #I Gillespie Humble #4 Bateman Humbl e #43 Bateman 167­F -H&TC 173­-H&TC 113-F-H&TC 101-A-J. B. Rector 114--A-J. B. Rector 50 50 53 44 48 1842 I982 I796 I740 173I 720 I 7100 6803 6388 663 I 6950? -5 I08? 7060 -5078 6770 -4974 638I -4641 6626 -4895 King-6 King-7 Humble #70 Bateman Humbl e #I Ross 118-A-J. B. Rector 27­-S. L Graves 5I 48 1781 1772 6315 6601 62\i4 6592 -4513 -4820 11 Dala received loo late for inclusion on map, Plate I. 89 Basement Rocks, Texas-New-Mexico ACE OF FORMATION ON LITHOLOCY OF PRECAMBRIAN MATERIAL BASEMENT BASEMENT INTERVAL PROVINCE STUDIED PACE Permian rhyolite porphyry-X 4450-4460 Panhandle volcanic terrane cuttings 165 Permian Permian Permian rhyolite porphyry-X rhyolite porphyry nsl 4430-4458 4-080-4106 Panhandle volcanic terrane Panhandle volcanic terrane cuttings cuttings 165 165 ni albite granodorite 2990 Wichita igneous province cuttings 165 albite granodiorite 3048 cuttings 166 Pennsylvanian Pennsylvanian Pennsylvanian ni nsl rhyolite porphyry-X? granite rhyolite porphyry 5878-5881 3892 Panhandle volcanic terrane Wichita igneous province? Panhandle volcanic terrane cuttings core core 166 166 166 Pennsylvanian Cambro-Ord microgranite porphyry granite 6130-6176 7848 Wichita igneous province? Wichita igneous province? cuttings core 166 166 Pennsylvanian nsl Pennsylvanian nsl Pennsyhanian rhyolite porphyry 5955-5959 Panhandle volcanic terrane core 166 Pennsylvanian rhyolite porphyry 4800-4802 Panhandle volcanic terrane core 166 Cam bro-Ord Cambra-Ord rhyolite porphyry micrographic granite 11445-11450 12214 Panhandle volcanic terrane Panhandle volcanic terrane cuttings cuttings 167 167 Cambra-Ord Cambrian rhyolite porphyry-X rhyolite porphyry-X 11630-11632 11304-11305 Panhandle volcanic terrane Panhandle volcanic terrane cuttings core 167 167 Cambrian albite diabase albite diabase albite diabase albite diabase 11621 11690-11700 11710--11720 11720 Swisher gabbroic terrane? cuttings cuttings cuttings core 167 167 167 168 Cambrian albite microdiorite gab bro gab bro gab bro quartz-epidote rock 9060--9250 9250--9265 9465-9490 9595­9917 Panhandle volcanic terrane cuttings cuttings cuttings cuttings core 168 168 168 168 168 feldspar-quartz-epidote rock 9919 core 168 albite andesite 9924 core 168 rhyolite porphyry-X 10174 core 169 rhyolite porphyry-X 10175 core 169 rhyolite porphyry-X 10177 core 169 Permian Ordovician Ordovician rhyolite porphyry-X rhyolite porphyry rhyolite porphyry micrographic granite nsl 10179 1600-1610 1625-1786 4844-4848 core cuttings cuttings ruttin~s 169 169 169 169 ni nsl Cambra-Ord rhyolite? metarkosite? 5090-5093 Fisher metasedimentary terrane cuttings 170 Cambro-Ord nsl Cambro-Ord Cambro-Ord Cambro-Ord Cam bro-Ord rhyolite porphyry granite granite granite 7580­7590 7669-7684 8260-8270 789o-7910 Fisher metasedimen tary terrane Texas craton Texas craton Texas craton cuttings core cuttings cuttings 170 170 170 170 Cambrian Cambrian biotite amphibolite microgranite 4167-4170 4255-4260 Texas craton Texas craton core cuttings 171 171 Cambro-Ord Cambro-Ord granite nsl 6980--7200 Texas craton cuttings 171 Cambrian nsl Cambrian granite 6388 Texas craton core 171 Cambrian nsl Cambrian nsl Cambrian granite 6600--6601 Texas craton core 172 Bureau of Economic Geology, The University of Texas 90 YEAR DF TOP OF l.OCATION COM· ELEVA­ TOTAL BASEMENT COUNTY OPERATOR and FARM Sec-Block-Survey PLETED TION DEPTH DEPTH ELEV. King-8 King-9 King-10 King-11 King-12 Ohio #1 Burnett Ohio # 1 Pitchfork L&C Ohio #2·A Ross Superior # 1 Pitchfork L&C Superior # 2 Pitchfork L&C J. Gates Lgc 1-Somerville CSL 24-­-R. B. Masterson 171-A96-J. H. Gibson 176-A96-J . H. Gibson 47 45 45 46 46 1850 1940 1769 2017 2024 6438 6974 6474 7145 7045 6435 -4585 6970? -5030? 6462 -4703 7145 -51 28 7040 -5016 King-13 Tidewater and Seaboard # 1 Pitchfork L&C 143-A65-BS&F 50 2029 7035 7025 -4996 Knox-1 Seaboard #1 Big Four Ranch DL&C 53 1650 7040 7026 -5376 Knox-2 Benedum & Trees # 1 Big Four Ranch 32-4-D&WRR 53 1646 7188 7122 -5476 Lamb-1 Anderson-Prichard # 1 Ge ttys 59-1 -Halsell Subd 52 3719 9315 9280 -5561 Lamb-2 Honolulu # 1 Halsell Lab 19-Lge 219­Castro CSL 47 3795 9138 9070 -5275 Lamb-3 Humble # 1 Jackson 19­T -R. M. Lamb-4 San Juan # 1 Jones Thompson 16­687­St Cap Lds 44 53 3461 3465 7191 9000 7185 8960 -3724 -5495 Lamb-5 Seaboard #1 Jackson Lab 15-Lge 680-St Ca p Lds 47 .17 15 9.127 9257 -5542 Lamb-6 Stanolind # 1 Hopping 25­F-T. A. Thompson 42 3561 9624 9600 -6039 Lampasas-1 Texoleum Trust #1 White P. T. Hill b'32 1250 3000 3000 -1750 Lampasas-2 Western Lampasas Oil # 1 Whittenburg 38-229-John Boyd b'32 1450 4180 3580 -2130 Lubbock-1 Lubbock­2 Amerada # 1 Stribling Bank line # 1-A Elliott 9-D7-ELRR 33­JS-ELRR 48 51 3240 3270 10679 11406 10650 11395 -7410 -8125 Lubbock-3 Honolulu # 1 Rhoades 9-E-GC&SF 47 3216 10471 10450 -7234 Lul>bock-4 Humble # 1 Farris 29-P -ELR R 51 3350 11783 11776 -8426 Lubbock-5 Lubbock-6 Magnolia # 1 Johnson P hill ips # 1 Kary 88­C -D&WRR 25-07-ELRR 45 49 3166 3317 10178 11450 10169 11430 -7053 -8113 Lynn-1 Lynn-2 Honolulu # 1 King P hillips #1-A Bartley 424--21-HE& WT 1372-1 -ELRR 47 45 3216 2981 10756 9909 10746 9900 -7530 -6919 Mason-1 Cochran & Steward # 1 Brandenburger M. Patton b'32 1700 1900 1065 + 635 McCulloch-1 McCulloch-2 Burfo rd & Bri mm #1 Cawyer Haby & All ison, Haby well D. Mechels 138­-C. Volmar b'32 b'32 1422 1935 2130 1920 2100 1900 -678 +35 McCull och-3 P rai ri e # 1 Zelle H&TC b'32 1498 3516 3309 -1811 McCu ll och-4 McCulloch-5 Sterrett # 1 Scoggin Thomas # 1 Craig 911­-J. Henk 1351­-C. Usner 50 b'32 1705 1755 3010 3666 2650 3473 -945 -1718 McCulloch-6 Thomas # 1 White Fisher & Miller h'32 1750 3406 2982 -1232 Menard-1 Menard-2 American RepuMics # 1 Bradford Deep Rock # 1 Bevans 29­-J. V. Massey 31-A-GH&SA 47 52 2043 2218 2745 5147 2680 5120 -637 -2902 Menard-3 P hill ips # 1 Meta 501--J. W. Bradfo rd 45 2335 3939 3863 -1528 Menard-4 Sheffield and Dakota-Texas #1 Rudder 7­ -TTRR 51 2103 2247 2210 -7 Mi lls-1 Mi ller # 1 Savoy E. M. Pease 46 1518 4230 4225 -2707 Mills-2 Venture # 1 Harrison & Slayden T. Carroll b'32 1271 3268 3268± -1997± Mitchell-1 Humble #1 Pratt 28-25-T&P 47 2292 8131 8115 --0423 Mi tchell-2 Sun #2 Elwood 25-16-SPRR 48 2158 8659 8464 -6303 Mon tague-1 Montague-2 Boyd #3 Maddox Bridwell #1 Bouldi n C. W. Thompson E. Votaw b'32 b'32 864 795 2273 3024 2262 2683 -1398 -1888 91 Basement Rocks, Texas-New Mexico AGE OF FORMATION ON l.ITHOLOGY OF PRf.CAMRRIAN MATERIAi . BASEME:\'T BASEMENT INTERVAL PROVI NCE STUDIED l'AGE Cambrian microgranite 6430-6438 Texas craton cuttings 172 Cambrian Cambrian microgranite granite 6897 1> 177 S499 7074 6823 6165 5460 702S -2803 -2360 -IS99 --4058 Oldham- IS Superior and Lazard #1-312 '.\latador Lge 312-St Cap Lds S4 3795 6965 6963 -3168 Parker-I Garland-.-\nthony #I Hammons J. Johnson 52 871 7798 7660 -6783 Parmer-I Gulf # 1-.\ Keliehor 5--Brown Subd-Gregg CSL S3 3996 9628 9S62 -5566 Parmer-2 Stano I ind #I Jarrell 19-B-St Cap Lds ...... 41 77 8162 8160 -3983 Parmer-3 Sunray #I Kimbrough 23­-Doud & Feeler 48 3985 9423 88~S -4870 Parmer-4 U.S. Smelt. & Rig. #1-A Osborn er 5-RIN-R3E-Cap Synd Subd S2 4165 9720 9700? -S535? Pecos-I Aldrich # I St. Nat"! Bank 2;,-140-T&STL 46 2491 4748 4730 -2239 Pecos­ 2 Anderson-Prichard #I Boren -1--11(}-Pink-Phelps 41 2469 4880 4803 -2339 Pecos-3 Pecos-4 Pecos-S Anderson-Prichard # 2 Boren Anderson-Prichard #2 Masterson Anderson-Prichard # 1-A -1--11(}-Pink·Phelps 10-1--1(}-H&GN 2-1--140-T&STL 41 39 43 2471 2430 25S3 4858 4740 4565 4780 4519 4560 -2309 -2089 -2008 '.\lasterson Pecos-6 Pecos-7 Pecos-8 Burk Royalty #2 Shearer Byrd-Frost #I Giesecke Childress Royalty #I '.\lasterson 107-1(}-H&GN 5-1--11-H&GN 10-1--1(}-H&GN 43 45 39 2435 2S93 2430 4740 4677 .',60S 4740 46SS 450S -2305 -2062 -2075 Pecos-9 Gulf # I Gan·in 6(}-11-H&GN 43 2Sl6 4537 4Sl8 -2002 Pecos-10 Gulf #I '.\ l illar 39-11-H&G"i 41 2516 4538 4532 -1956 Pecos-11 Gulf #2 '.\lillar 43-11-H&GN 42 2534 449-l 4464 -1930 Pecos-12 Gulf #3 l\lillar 43-11-H&GN 42 2543 4-!06 43<;2 -1849 Pecos-13 Gulf #I o·sullivan 38-11-H&GN H 2485 .o.643 4630 -2145 Pecos-14 Pecos-IS Helmerich & Payne # l Barnes Humble # I B. F. Smith 13(}-1(}-H&GN 12-14!;-T&STL 45 44 2400 2605 4687 Sl75 4680 Sl49 -2280 -2544 Pecos­ 16 Humble #2 St. Nat"! Bank El Paso 1-1.-.1-T&STL 45 2478 4838 4796 -2318 Pecos-17 Humble # 1-L University !;-18-UL 48 2S23 56-IO 5294 -2771 Pecos-18 Humble # I Wilson 1-145-T&STL 44 26SI S238 S23S -2584 Pecos-19 Los Nietos #1-B University 2(}-26-UL 49 2677 S649 S220 -2543 Pecos-20 \lcCandless #1-10 Atlantic 10-141-T&STL 44 2520 4878 4877 -2357 Pecos­ 21 l\lcCandless # IOI Atlantic 101-IJ-H&GN 45 2397 4103 3099 -702 Pecos-22 l\lcCandless #I Turney 19-141-T&STL 43 2S ll 4986 49fs0 -2469 Pecos-23 Pecos-24 McCandless #I University l\lagnolia #3 Fromme 2(}-26-UL 106-1(}-H&GN 43 43 2629 U S7 SS l 4 4697 S507 4663 -2878 -2206 Pecos-2S Pecos-26 Pecos-27 l\lagnolia #2-96 Powell-State Midcontinent #I Shearer Olson & l\lcCandless # l Crockett 96-10-H&GN 107-1(}-H&GN 5-110-TCRR 43 43 41 2-!08 2424 2.-.01 4700 4726 4S26 468S 4658 4S2-l -2277 -2234 -2063 Pecos-28 O'Neill # I Tyrrell Trust 602­ -P.H. Fall Sl 2S69 S394 5370 -2801 Pecos-29 Pan American #1-4 MacDer 26-1-14-T &STL 48 2756 5297 5255 -2499 Pecos-30 Phillips # I Pascoe 114-11-H&GN 4-l 2448 464S 4580 -2132 Pecos-31 Phillips # 1-A Puckett " B" 44-101-TCRR 53 3325 1653S 16SIO -13185 Pecos-32 Phillips # 1-C P uckett 42-101-TCRR S3 3330 14930 14903 -11513 Pecos-33 Phillips # 1-D Puckett 26-101-TCR R 53 3321 14320 14210 -10889 Pecos-34 Peros­3S Peros-36 Pecos-37 Shell p ,\(;F Pennsyl\'anian Pennsylvanian Pennsylvanian Pennsyl,·anian micrographic granite micrographic granite granite micrographic granite 6892-6897 6167-6173 5992-5998 7074 \Vichita igneous province \\'ichita igneous province Wichita igneous province Wichita igneous province cort" core core co re 18-1 18-1 185 185 Pennsylranian nsl Cambrian biotite-hornblende schist 7660± core 185 Mississippian Pennsyl,·anian Missis.sippian? oli•·ine gabbro olivine gabbro rhyolite porphyry-X micrographic granite 9578-958.J. 9627-9628 8160-8161'h 8870-8880 Swisher gabbroic terrane Panhandle volcanic terrane Texas era ton ? core core core cuttings 185 185 185 186 Mississippian rhyolite porphyry-X 9700-9710 Panhandle volcanic terrane cuttings 186 Cam bro-Ord Cambro-Ord Cam bro-Ord Permian nsl nsl granodiorite gneiss diorite 4858 4570 Texas craton Texas craton core core 186 186 Cambrian Cam bro-Ord Permian Permian Cam bro-Ord Permian Cambrian Cambrian Cambrian Permian? Permian Cam bro-Ord Permian Permian Permian Permian Cambro-Ord? Permian Permian Cam brian Cam bro-Ord Permian granite nsl nsl gab bro granodiorite granite microgranite granite Jeuco·quartz microgabbro nsl diorite nsl granodiorite gneiss granite gneiss microgranite microgranite microgranite nsl granite gneiss microgranite syenite microsyenite nsl nsl 4560-4564 4600-4605 4525-4530 4536-4538 4489-4493 43%-4404 4630-4635 5167-5168 5630-5640 5235 5550-5560 5615-5620 4980-4986 5513 4667-4682 4682-4697 Texas craton Texas craton Texas craton Texas craton Texas craton Texas craton Texas craton Texas era ton Texas craton Texas craton Texas c.:raton Texas craton Texas uaton Texas craton Texas craton cuttings cuttings cuttings core cuttings cuttings cuttings cuttings core core cuttings cuttings cuttings cuttings core core core 186 186 187 187 187 187 187 187 187 188 188 188 188 188 188 188 188 Cambrian Permian Permian Cambro-Ord Cambrian Cambrian Cambrian Permian Cambro-Ord Permian Permian Cambro-Ord Permian Perm ian Permian nsl nsl granite gneiss mkrodiorite granite granite granite microgranite nsl microgranite microgranite rhyolite porphyry quartz diorite metarkosite mt"tarkosite biotite amphibolite metaquartzite nsl albite granodiorite 5290-5295 4630-4640 16510-16525 14923 14309-14320 5200-5204 5350-5360 5280-5313 5280-5313 5745 6473-647311. 6487-6489 6721-6724 6721-6724 4860-4864 Texas craton Texas craton Texas craton Texas c.:raton Texas craton Texas craton Texas craton Texas craton Texas craton Texas craton Texas craton cuttings cuttings cuttings core core cuttings cuttin gs cuttings cuttings cuttings core core core core cuttings 189 189 189 189 189 189 189 190 190 190 190 190 190 190 190 96 Bureau of Economic Geology, The University of Texas YEAR DF TOP OF Sec· Block-Survey PLETED TION DEPTH Depth Eln. COUNTY OPERATOR and FARM LOCATION COM­ ELEVA­ TOTAL BASEMENT Pecos-42 Superior and McCandless # l -2B-Pink-Phelps 43 2466 4648 4620 -2154 Crockett-State Pecos-43 Union of California #1-C Heiner 589-105-GC&SF 45 2557 6162 6125 -3568 Potter-1 Amarillo Oil & Gas #3 Masterson 102-018-D&P b'32 3434 3082 2698 +757 Potter-2 Amarillo Oil &Gas # 5 Masterson 31-3-Gunter & Munson b'32 3279 2230 2205 +1014 Potter-3 Canadian River #4-B Masterson 103--018-D&PRR 42 3331 I952 I940 +1391 Potter-4 Colo. Interstate #25-A Bivins 2--0I8-D&PRR 39 3669 3030 2460 +I209 Potter-5 Colo. Interstate #4I-B Masterson 84--3-Gunter & Munson 53 3356 2496 2400? +944? Potter---0 Emerald #I Masterson 82-3-Gunter & Munson b'20 3433 2130 2045 +I465 Potter­ 7 Greater Amarillo Oil #I 20-3 -Gunter Masterson & Munson b'32 3423 2595 2045 +1378 Potter--8 Prairie #I Bivins 42-M-20-Gunter & Munson b'32 3238 3485 2525 +m Potter-9 Ranch Creek #I Masterson --018-D&PRR b'32 3397 2480? 2480 +917 Potter-10 Ranch Creek #I Masterson 2-B-11-ELRR b'32 3434 2675 2200 +I234 Potter­ 11 Sinclair #2 Bivins 28--0I8-D&PRR 49 35I9 2908 2863 +656 Potter-I2 Sinclair-Prairie #I Bush 23-6-BS&F 39 3424 6I6I 5IOO? -I676? Potter-I3 Standard of Texas #I Bush I2-20F-ELRR 52 3510 6847 6824 -3314 Presidio-I Hunt #I Presidio Trust 99-3-D&PRR 53 3858 8Ill 8005 -4I47 Presidio--2 Welch #I Espy 110-4 -H&TC 52 4746 7837 7773 -3027 Randall-1 Placid #I Greeley 53-I -TTRR 5I 3759 8244 8200 -4441 Reagan-I Big Lake # I3-C University 25­9-UL 46 2675 9854 9853 -7175 Roberts-I Phillips # 1 Jenkie 38-2-GH&H 48 3167 11737 11719 -8552 Runnels-I Superior #I McDowell 80-T&NORR 48 I856 6307 6200 -4344 San Saba-I San Saba-2 Cayce # l llloore Newman #I Weldon C. Hernandez 5­-H&TC b'32 49 I250 1563 I659 3022 1655 3020 -405 -1457 Schleicher-I Schleicher-2 Schleicher-3 Schleicher-4 Atlantic #I Roberts Humble #1 Spencer Humble #I Stanford Phillips #1 Callan I75-A-HE&WT 176-A-HE&WT 196-A-HE&WT 311­-J. F. Wilhelm 53 53 53 44 2405 2374 2447 2306 7751 6978 9032 6065 7745 6897 9017 5955 -5340 -4526 -6570 -3649 Scurry­1 Scurry-2 Scurry-3 Humble #1 Nachlinger Magnolia #I-F McDonnell Est. Stanolind #1 Jordan I46-3 -H&TC 34I-97-H&TC 579-97-H&TC 53 50 49 2419 2512 2779 8271 8546 8922 8060 8544 8905 -5641 -6032 -6I26 Scurry--4 Sun and Ohio #I Helms 633-97-H&TC 49 2I22 7524 7520 -5398 Shackleford- I Honolulu #I Pool 35­-UL 49 I339 5276 5273 -3934 Sherman-I Sherman-2 Humble #I lllorris l.T.1.0. #I Bryan 79-lT-T&NO 369-IT-T&NO 54 47 3740 3640 6IOO 705I 6085 5114 -2345 -I476 Basement Rocks, Texa.s-New Mexico 97 AGE OF FORMATION 01' BASEMENT LITHOLOGY OF BASEMEST INTERVAL PRECAMBRIAN PROV INCE MATERIAL STUili ED PAGE Cam bro-Ord nsl Cam bro-Ord microdiorite 6145-6150 Texas craton cuttings 190 granite 6145-6150 cuttings 190 mirrodiorite 6155-6160 cuttings 190 mirrogabbro 6155-6160 cuttings 191 ni trachyte porphyry-X 2750 Panhandle Yolcanic terrane core 191 leuco-diabase 2765 core 191 ni Jrranite 2200 Wichita i~neous province cuttings 191 Penn!;y )\"anian rhyolite porphyry 195-1--1952 Panhandle ,·olcanic terrane core 191 PennsyJ,·anian rhyolite tufT rhyolite porphyry 1952 2460-2467 Panhandle vokanic terrane core cuttings 191 191 leuco·olivine µ:a bbro 2546-2552 cuttings 192 )puc·o-olh·in e diabase 2610-2660 cuttings 192 diabas.e and iron ore 2767-2776 cuttings 192 diabase and iron ore 2875-2883 cuttings 192 oJi,·ine diabase ha>a lt porphyry 2956-2962 3002-3010 cuttings cuttings 192 192 rhyolite porphyry 3013-3030 cuttings 193 PennsyJ,·an ian mirrogranite 2419-2424 W'irhita igneous province cuttings 193 ni ni rhrnlite tufT rhyolite porphyry trarhyte tufT trachyte porphyry 2065± 2125-2130 2777 2777 Panhandle volcanic terrane Panhandle \"olcanic terrane cuttings cutt ings cuttin p:s cuttings 193 193 193 193 ni ni oli\'ine diahase trarhyte tufT 2585-2595 2480 '\Vic-hita igneous province Panhandle vokanic terrane core cuttin~s 193 193 ni trachyte porphyry nsl 2480 cuttings 193 Pennsylvanian nsl Pennsrhanian Prnnsyl\"anian rhyolile porphyry rhyolitr porphyry-X? rhyolite porphyry-X rhyolite porphyry-X 5~00-5700 5900-6161 6158-6161 6843-6848 Panhandle \"olranic terrane PanhanJle volcanic terrane cuttings cuttings core core 193 194 194 194 Permian Cambrian? granitized metarkosi te ~ranite 8110 7830 \'an Horn mobile belt Van Horn mobile belt core core 194 195 Pennsylvanian rhyolite porphyry-X 8240-8244 Panhandle volcanic terrane cuttinp:s 195 Cambrian nsl Camhro-Or TION DEPTH De pth Ele,. , Winkler-3 A. G. Carter # 6-E Walton l-B3-PSL 44 2965 9780 9710 -6805 Winkler---4 A.G. Carter #7-E Walton l-B3-PSL 44 2962 9914 9695 -6731 Winkler-5 A. G. Carter and Pure #8-E Walton 2-B3-PSL 45 2964 9724 9913 -6951 Winkler-6 Gulf #46-E Keystone 6-B2-PSL 44 2971 10005 9990 -7019 Winkler-7 Winkler-8 Winkler-9 Winkler-10 Winkler­11 Winkler­12 Winkler-13 Winkler-14 Winkler­15 Winkler-16 Winkler-17 Winkler-18 Winkler-19 Winkler-20 Winkler-21 Winkler-22 Winkler-23 Gulf # 50-E Keystone Gulf #51-E Keystone Gulf #59-E Keystone Gulf #62-E Keystone Gull #65-E Keystone Gulf #68-E Keystone Gulf #69-E Keystone Gulf #70-E Keystone Gulf #73-E Keystone Gulf #75-E Keystone Gulf #93-E Keystone Gulf # 133-E Keystone Phillips #2 Bashara Phillips #5 Bashara Phillips #4 Walton Phillips #5 Walton Richardson &Bass # 1-E 6-B2-PSL 10-B3-PSL 6-B2-PSL 10-B3-PSL 6-B2-PSL 6-B2-PSL 10-B3-PSL 6-B2-PSL 6-B2-PSL 6-B2-PSL 10-B3-PSL 10-B3-PSL 21-77-PSL 21-77-PSL 2-B3-PSL 2-B3-PSL 45 45 45 45 45 45 46 46 47 46 46 48 45 44? 45 46 2966 2951 2970 2963 2962 2970 2955 2961 2975 2964 2951 2946 2963 2976 2964 2960 10090 9700 9920 9660 9909 9969 9871 9749 9842 9810 9663 9711 9951 10122 9714 9740 10085 9676 9910 9640 9900 9950 9650 9744 9830 9700 9661 9666 9890 10114 9685 9627 -7119 -6725 -6940 -6677 -6938 -6980 -6695 -6783 -6855 -6736 -6710 -6720 -6927 -7138 -6721 -6667 McCutcheon 15-B2-PSL 45 2962 9761 9760 -6798 Winkler-24 Richardson &Bass # l KC Stock Co. 25-B2-PSL 45 2940 10673 10620 -7680 Winkler-25 Richardson &Bass #10-E Walton l-B3-PSL 44 2957 9858 9838 -6881 Winkler-26 Richardson &Bass #15-E Walton l-B3-PSL 45 2967 9695 9690 -6723 Winkler-27 Richardson &Bass # 23-E Walton l-B3-PSL 45 2962 9771 9760 -6798 Winkler-28 Richardson &Bass # 31-E Walton 2-B3-PSL 46 2958 9791 9650 -6692 Winkler-29 Richardson &Bass # 32-E Walton l-B3-PSL 47 2957 9622 96!0 -6653 Winkler-30 Richardson &Bass #42-E Walton 2-B3-PSL 46? 2962 9745 9685 -6723 Winkler-31 Sinclair #6­A Walton 20-77-PSL 45 2964 9958 9940 -6976 Yoakum- I Continen ta! # l Rodgers 106-D -J. H. Gibson 51 3865 13016 12985 -9120 Yoakum-2 Fikes &Murchison #17-C Elliott 832-D-J. H. Gibson 49 3623 11210 11189 -7566 Yoakum-3 Stanolind #I Argo 98-D-J. H. Gibson 52 3809 13131 13125 -9316 Basement Rocks, Texas-New Mexico 101 ACE OF FORMATI0:-1 O:"\ BASEME'.\'.T LITHOLOGY OF BASEMENT INTERVAL PRF.CAMBRIAX PROVINCE MATF.HIAI. STUDIED PACE Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Carn bro-Ord granite 99%-10000 Texas craton cuttings 203 Cambrian Cambrian granite granite nsl 10000-10005 10080-10090 Texas craton cuttings cuttings 203 203 Cam bro-Ord nsl Cam bro-Ord Cam bro-Ord granite nsl 9650-9660 Texas craton cuttings 203 Cam bro-Ord nsl Cam bro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord granite granite granodiorite granite nsl 9865-9871 9744-974S 9830-9840 9800-9810 Texas craton Texas craton Texas craton Texas craton cuttings cuttings cuttings cuttings 203 203 203 203 Cam bro-Ord nsl Cam bro-Ord granodiorite gneiss 9915 Texas craton core 204 Cam bro-Ord nsl Cambrian Cam bro-Ord granite biotite schist 9705-9710 9685-9690 Texas craton Texas craton cuttings cuttings 204 204 Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord Ordovician granodiorite nsl 9857-9858 Texas craton core 204 Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cambro-Ord nsl Cambro-Ord microgranodiorite 9955-9965 Texas craton cuttings 204 Ordovician trachyandesite tufl' rhyolite porphyry 13015 13015 Panhandle volcanic terrane core core 204 204 Ordovician Ordovician rhyolite tull' micrographic granite nsl 13016 11200-11205 Texas craton core cuttings 205 205 102 Bureau of Economic Geology, The University of Texas PART 2-DATA ON NEW MEXICO BASEMENT WELL~ YEAR l>F TOP OF LOCATION COM· EI.EVA· TOTAL BASEMENT COUNTY OPERATOR and FARM Sec-Dlo'"k-Surv~y PLETED TION DEPTH Depth ElcY. Chaves-1 Amerada #l·RA State 22-8S-32E 50 4373 11621 11580 -7207 Chaves·-2 Arkansas # 1 Manning 14-15S-17E 26 5455 3400 3245 +2210 Chaves-3 Barnsdall #l·A State 23-8S­32E 49 4459 12040 11994 -7535 Chaves-4 Black # 1 Shildneck 24-16S-20E 48 4365 6996 6655 -2290 Chaves--5 Buffalo # .3 Comanche Unit 26-11S-26E 49 3669 6175 6173 -2504 Chaves­6 Continental # 1 Lankford 2-14S-26E 47 3409 8099 7980 -4571 Chaves-7 DeKalb #1 Lewis 13-10S-25E 46 3721 5650 5526 -1805 Chaves-8 Chaves-9 DeKalb and Magnolia #1 White Franklin, Ashton & Fair #1 13-9S-28E 51 3888 7463 744ll -3560 Orchard Park 22-12S-28E 51 3563 5828 5320 -2242 Chaves-10 Gulf #1 Jennings 5-SS-30E 50 4055 8326 8293 -4238 Chaves-11 Gulf # 1 State-Chaves U 10-18S-16E 52 6490 3147 2960 +3530 Chaves-12 Honolulu # l Hinkle-Federal 24-11S-27E 50 3745 7315 7305 -3560 Chaves-13 Honolulu # l Levick-State 16-10S-27E 50 3861 7215 7198 -3337 Chaves---14 Honolulu #1 McConkey Est. 10­9S-26E 51 3843 6371 5950? -2107? Chaves-15 Honolulu # 1 Texas-State 13-11S-27E 50 3763 6933 6928 -3165 Chaves-16 Humble #1 Gorman-Federal 30-15S-22E 48 4168 5849 5750 -1583 Chaves-17 Humble #1-N State (seeTable 10) 35-14S-17E 44 3615 4014 3350 +265 Chaves-18 Humble #1-U State 10-12S-27E 48 3672 7851 7835 -4163 Chaves-19 Humble #1-Y State 33-11S-27E 50 3708 7430 7411 -3703 Chaves-20 Magnolia #1 Black Hills Unit 31-17S-20E 46 4902 6085 5900? -998 Chaves-21 Magnolia #1-B O'Brien l-9S-28E 50 3949 7666 7656 -3707 Chaves-·-22 Magnolia #I Shaw-Federal 6-13S-31E 53 4031 12072 12067 -80.% Chaves--23 Magnolia #l·Z State 36-7S-29E 50 4186 8731 8723 -4537 Chaves-24 Magnolia # 1 Turney-Federal 23-14S-22E 48 4075 5342 5305 -1230 Chaves-25 Olson # l Noble Trust 18-4S-27E 50 3874 8034 6556 -2682 Chaves--26 Richfield # 1 Comanche Unit 13-11S-26E 47 369 6129 6120 -2511 Chaves-27 Richfield # 1 Coll 18-11S-27E 45 3673 6630 617 -2944 Chaves-28 Richfield #I Mullis 21-15S-29E 47 3809 12153 12148 -8.139 Chaves-29 Richfield #l·A Trigg 35-14S-27E 48 3528 9993 9970 -6442 Chaves-30 Richfield #1-3 White 6-12S-29E 47 3710 9058 9040 -5330 Chaves-31 Sanders #1 Sanders (Saunders?) 25-5S-24E 50 3872 5355 4900'! -1028? Chaves-32 Spartan # 1-25 State 25-5S-29E 50 4339 8911 8903 -4564 Chavcs-33 Sun #1 Pinion 19-19S-!7E 51 6544 1911 1800? +4744? Chaves-34 Chaves-35 Sun #2 Pinion Union of California and DeKalb 20-19S-!7E 52 6314 1659 1645 +4669 #I State 27-11S-27E 49 3791 7582 7566 -3775 Curry-I Union Prod. #1 Jones 18-5N-37E 53 4239 8180 8124 -3885 103 Basement Rocks, Texas-New Mexico AGE or FOHMATION O'.\' LITHOLOGY OF PRF.CAMBR!AN MATERIAL RASFME'.\'T BASEMENT INTERVAL PROVINCE STUDIED PAGE Cam bro-Ord rhyolite and ihyolite porphyry 11580-11621 Panhandle volcanic terrane cuttings 206 Permian nsl Mississippian basalt 12034-12040 Panhandle volcanic terrane? core 206 diabase ? cuttings 206 Ordovician Ordovician granite albite granodiorite diabase nsl 6850-6860 6860-6870 6860-6870 Texas craton cuttings cuttings cutt'.ngs 206 206 206 Cam bro-Ord metaquartzite 8040-8050 cuttings 207 Permian metaquartzite granodiorite 8075-8095 5635-5638 Texas craton cuttings core 207 207 Cambro-Ord granite 7458-7463 Texas craton core 207 Permian albite granodiorite granite 5350-5810 5814-5827 Texas craton cuttings core 207 208 Cambro-Ord granite 8300 Texas craton core 208 granite 8319 core 208 Cambrian tufT? 3100± core 208 Devonian granite 7310-7315 Texas craton core 209 al bite-quartz microdiorite 7310-7315 core 209 Silurian-Dev Devonian albite granodiorite rhyolite porphyry microgranodiorite p:ranite rhyolite porphyry 7210-7250 5970-5980 6340-6350 6350-6360 6364-6371 Texas craton Panhandle volranic terrane cuttings cuttings cuttings cuttings core 209 209 209 209 209 Cam bro-Ord nsl Cambro-Ord Permian? p;ranite diabase 5848-5849 3476, .lo00-03, Texas craton (Tertiary?) core core 209 210 3804-09 metaquartzite 3835 core 210 diabase 3939 core 210 Cambro-Ord Cam bro-Ord Silurian Camhro-Ord granite granite argillite? 1tranod iorite 7847-7851 742.5-74~0 5930-5940 7665-7666 Texas craton Texas c1aton ? Texas craton cuttings cuttings cuttings core 210 211 211 211 Ordovician micro~ranite porphyry 12070 Texas craton core 211 Camhro-Ord Permian? granodiorite granodiorite gneiss 8728 5321-5324 Texas craton Texas era ton? cuttings core 2]] 211 epidote-chlorite­ oligoclase gneiss 5321-5324 core 2ll Mississippian sericite phyllite 7630-7660 ? core 212 s~ricite phyllite 8030 core 212 Cambro-Ord rhyolite porphyry-X? 6128 Panhandle volcanic terrane core 212 Ordovician-Dev nsl Cam bro-Ord granodiorite 12143-12153 Texas craton core 212 Cam bro-Ord Cam bro-Ord alhite diorite syenodiorite granodiorile quartz microdiorite 9980-9990 9980-9990 9980-999.~ 9046-9047 Texas craton Texas craton cuttings cuttings cuttings core 212 212 212 213 Mississippian sheared rhyolite Cam bro-Ord porphyry nsl 4940-5290 Panhandle vol canic terrane cuttings 213 Permian albite andesite porphyry-X 1850 Panhandle volcanic terrane core 213 Permian nsl Cambro-Ord granite 7575-7580 Texas craton Pennsylvanian nsl cuttings 213 104 Bureau. of Economic Geology, The University of Texas YEAR DF TOP OF l.OCATIO:< COM· El.EVA· TOTAL BASEMENT COC:on C.it.i('"$ SerYh-e ::± ;).~ State 4-225­3/E 15-21S-37E 46 SI 3447 3447 762S 8034 7614 7997 -4167 -4550 Lea­27 Cities Sen·ice #I Burger B-28 28-20S-37E 52 3560 9379 93i3 -5813 Lea-28 Continental .:!t5 Burger A-19 19-20S-38E so 3546 9731 9720 --0174 Lea-29 Continental ='3 Elliott . .\·15 1&-22S-37E 47 3382 7813 7812 --4430 Lea-30 Continental .:ti Elliott B-15 l&-22S-37E 47 3384 7353 7351 -3967 Lea-31 Continental .:t2 Elliott B-15 15-22S-37E 49 3372 7513 7SJO -4138 Lea-32 Continental .:t5-. .\ Elliott l&-22S-37E 48 3310 7700 761.10 --4310 Lea-33 Continental .:!tJ-E Hawk B-3 3-21S-37E 51 3465 7975 7958 -4493 Lea-3-l Continental =~-E Hawk B-3 3-21S-37E 51 3474 8021 8005 -4531 Lea-35 Con tinental .:t3-E Ha"·k B-3 3-21S-37E 51 3.JSO 8191 8170 -4690 Lea-36 Continental ::t-1-E Hawk B-3 3-2JS-37E 51 3-132 8070 8060 -4628 105 Basement Rocks, Texas-New Mexico ACE OF FORMATION 0:-1 LITHOLOGY OF PRECAMBRIAN MATERIAi. BASEMENT BASEMENT INTERVAL PROVINCE STUJ>IEJ> PACK Pennsylvanian? arkose (not basement) 7140-7149 ? cuttings Note: Arkose contains numerous frag· ments of rhyolite suggesting under· lying volcanic terrane Pennsylvanian Pennsylvanian granite granite 6720--0730 6467 Texas rraton? Texas craton cuttings core 214 214 Pennsylvanian biotite-hornblende schist 4774-4779 ? core 214 Pennsylvanian nsl Pennsylvanian leuco-diorite quartz-diorite quartz-diorite 608(}-6090 6080--0090 6168--0174 Texas craton cuttings cuttings cuttings 214 214 215 Cam bro-Ord Cam bro-Ord diorite metaquartzite 1076(}-10765 8243-a248 Texas era ton? ? cuttings core 215 215 Ordovician metarkosi te 8243-a248 chlorite-epidote phyllite 1129(}-11310 chlorite phyllite 11300-11310 metabasalt? 11310-113121h core cuttings cuttings core 215 215 216 216 Cam bro-Ord granite 16459 Texas craton core 216 Silurian-Dev microgranite 9886-98871h Texas craton? core 216 Cambro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord Ordovician granodiorite granite granite granite nsl 7803 7687 7634 7410 Texas craton Texas craton Texas craton Texas craton cuttings cuttings cuttings cuttings 216 216 216 216 Cam bro-Ord microgranodiorite Cam bro-Ord Cam bro-Ord porphyry microgranodiorite nsl 7928 7844 Texas craton Texas craton cuttings cuttings 217 217 Cam bro-Ord Cam bro-Ord microgranite microgranite albite micrograno­ 10211 10214 Texas craton cuttings cuttings 217 217 Cam bro-Ord diorite silicified rhyolite 11006 11716 Texas craton Panhandle volcanic terrane cuttings core 217 217 Cam bro-Ord Cam bro-Ord Cam bro-Ord silicified rhyolite microgranite porphyry microgranite nsl 11754-11755 10250 Texas craton Texas craton core core cuttings 217 218 218 Cam bro-Ord Cam bro-Ord Cambrn-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord alhite microgranodiorite 7858 microgranodiorite 7868 microgranodiorite 7906 microgranite 8113 microgranite 7519 microgranite 8016 microgranite 7661 nsl Texas craton Texas craton Texas craton Texas craton Texas craton Texas craton Texas craton cuttings cutting• cuttings cuttings cuttings cuttings cuttings 218 218 218 218 218 218 218 Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord Cam bro-Ord microgranodiorite microdiorite 8030-8034 9373-9376 Texas craton Texas craton cuttings core 219 219 Cam bro-Ord granite (stringer) nsl 9373-9379 core 219 Cam bro-Ord nsl Cambro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cambro-Ord nsl Cam bro-Ord nsl Cambro-Ord nsl Bureau of Economic Geology, The University of Texas 106 YEAR OF TOP OF l.OCATION COM · ELEVA· TOTAL BASEMENT COU:>TY OPERATOR and FARM S<"c:-Rlo<"k-Survey PI.ETEn TION DEPTH Dcp:h Elev, Lea-37 Con tinental #l·E Hawk B-10 l0-21S-37E 51 .~4:l4 7950 7945 -4511 Lea-38 Continental #2-E Hawk B-10 l0­21S-37E 51 :l450 7981 79;·0 -4520 Lea-39 Continental #3-E Hawk B-10 l0--21S-37E 51 3427 7728 7728 -4301 Lea-40 Continental #5-E Hawk B-10 10-21S-37E 52 3462 8079 8075 -4613 Lea-41 Continental #l·E Lockhart A-27 27-21S-37E 49 3431 7782? 7780? -4349? Lea-42 Continental #2-E Lockhart A-27 27-21S-37E 50 3-l32 7770 77(.7 -4335 Lea-43 Continental # 3-E Lockhart A-27 27­21S-37E 50 3418 7652 7585 -4167 Lea-44 Continental #4-E Lockhart A-27 27­21S-37E 51 3417 7541 7530 -4113 Lea-45 Continental #1-S Lockhart A-27 27-21S-37E 50 3432 7866 78o7 -4425 Lea-46 Continental #5 Lockhart B-11 ll-21S-37E 52 3469 7831 7825 -4356 Lea-47 Continental #3-E Lockhart B-11 ll-21S-37E 51 3426 7658 7640 -4214 Lea-48 Continental #4-E Lockhart B-11 ll-2l S-37E 52 3462 78ll 77R3 -4321 Lea-49 Continental #6-E Lockhart B-11 ll-21S-37E 52 3473 8065 8196 -4723 Lea-50 Continental #1 Lockhart B-12 12-21S-37E 52 3480 8268 8254 -4774 Lea-51 Continental #4 Lockhart B-12 12-21S-37E 53 3467 8206 8196 -4729 Lea-52 Continental #1-A Lockhart B-13 13­21S-37E 53 3427 7600 75-20 -3793 Lea-53 Continental #1 Nolan ll-21S-37E 50 3423 7523 7480 -4057 Lea-54 Lea-55 Lea-56 Continental #2 Skaggs B-23 Conti nental # 5 Skaggs B-23 Continental #1-E Wan tz 23-20S-.17E 23­20S-37E 21-21S-37E 44 47 49 3540 3539 3440 10465 10231 8304 10463 10155 8304 -6890 --0616 -4864 Lea­ 57 Continental #1 Warren A-29 29-20S-38E 49 3538 9391 9360 -5822 Lea-58 Continental #1 Wa rren B-27 27-20S-38E 51 3552 9392 9330 -5778 Lea­ 59 Continental #2 Warren B-28 28-20S-38E 50 3549 9072 8970 -5421 Lea-60 Continental #2 Warren B-29 29-20S 38E 49 3548 9852 9830 --0282 Lea---01 Gull # 1 Amanda 25-22S-37E 47 3317 73.% 7330 -4013 Lea---02 Gulf #8 J. N. Carson 28­21S-37E 48 3451 8005 8000 -4549 Lea-63 Gulf #5-A J. N. Carson 28-21S-37E 48 3444 7910 7881 -4437 Lea---04 Gulf #7-A J. N. Carson 33-21S­37E 48 3459 7644 7643 -4184 Lea-oS Gull #9-A J. N. Carson 28-21S-37E 49 3455 8073 8072 -4617 Lea-66 Gull #6-C J. N. Carson 28-21S-37E 49 3446 7500 7493 -4047 Lea---07 Gulf #8-C J. N. Carson 28­21S-.17E 49 3435 7743 7726 -4291 Lea---08 Gulf #4-A Cole-State 16­22S-37E 47 3411 7651 7644 -4233 Lea---09 Gull #5 Eubank 22­21S­37E 50 3424 7756 7755 -4331 Lea-70 Gulf #6 Eubank 22­21S-37E 50 3425 7686 7686 -4261 Lea-71 Gulf # 5-F Graham-State 36-19S-36E 48 3586 9822 9819 -6233 Lea-72 Gull #7 King 28-21S-37E 48 3447 8063 8050 -4603 Lea-73 Lea-74 Lea-75 Lea-76 Lea-77 Lea-78 Lea­79 Lea-80 Lea-81 Lea-Sia Lea-82 Lea-83 Lea-84 Gull #10 King Gulf # 12 King Gulf #15 King Gulf #21 King Gul f #6 LaMunyon Gull #16-E LaMunyon Gulf #8-E Leonard Gulf # 10-E Leonard Gulf #2 Stitcher Humble #10 Greenwood Humble #11 Greenwood Humble # 12 Greenwood Humble # 1 Keinath-Federal 28-21S­37E 28-21S-37E 28-21S-37E 28­21S-37E 28-23S-37E 27-23S-.17E 2-21S-.17E 2-21S-37E 4-22S-37E 9-22S-37E 9-22S-37E 9-22S-37E 8-21S-38E 48 49 49 50 48 52 52 52 46 47 47 47 45 3456 3441 3461 3440 3294 3283 3487 3483 344-0 3428 3427 3424 3582 804-0 7975 8146 7935 10218 10165 7926 8168 7980 7711 7501 8090 9954 8030 7970 8135 7925 102(15 10120 792(1 8165 7979 7700 7495 8080 9890 -4574 -4529 -4674 -4485 --0911 --0837 -44.~3 -4682 -45.~9 -4272 -4068 -46S6 -6308 Lea-85 Lea-86 Lea-87 Lea­88 Lea-89 Lea-90 Lea-91 Lea-92 Lea-93 Lea-9.i Lea­95 Humble #3-V State Humble #5.V State Humble #6-V State Humble #8-V State Humble #9-V State Lion #l Wylie Magnolia #9 Brunson-Argo ll1a1?nolia # 10 Brunson-Argo Ma1?nolia # 11 Brunson-Argo llfa!!nolia # 12 Brunson-Argo llia~nolia # 16 Brunson-Argo 10-21S-37E 10­21S-37E I0-21S-37E I0--21S­37E 10-2l S-37E 5-23S-37E 9-22S-37E 9-22S-37E 9-22S-37E 9-22S-37E 10--22S-37E 51 51 51 52 52 46 46 46 46 46 48 3463 :1470 3464 34S2 3462 :l3RS 3436 3433 3426 3432 3425 7673 R.'!96 7717 7573 824-0 RS l 9 7881 7901 7644 7471 7454 7665 824-0 7700 751[) 8200 8518 7830 7895 7643 7476 7438 -4202 -4470 -4246 -4058 -4738 -5133 -4384 -4462 -4217 -4044 -4013 Ba4ement Rocks, Texa4-New Mexico AGE OF FORMATION OJ\ LITHOLOGY OF PRECAMBRIAN MATERIAL OASEMEl'iT BASEMENT INTERVAL PROVINCE STUPIED PAGE Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord albite microgranodiorite 7791 Texas craton core 219 Cambrian nsl Cam bro-Ord nsl Cam bro-Ord nsl Permian nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord mirrogranite 8058-8065 Texas craton cuttings 219 Cam bro-Ord nsl Cam bro-Ord µranite 8202 Texas craton cuttings 219 microdiorite 8202 cuttings 219 Permian syenite 7514-7539 Texas craton cuttings 220 syenite 7585-7590 cuttings 220 Ordovician nsl Ordovician nsl Ordovician nsl Cam bro-Ord nsl Cam bro-Ord granodiorite 9361-9391 Texas craton core 220 olivine gabbro 9371-9372 core 220 Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord granodiori te 9850-9852 Texas craton core 220 Permian biotite schist 7332 Texas craton cuttin~s 220 micro!!ranite 7332 cuttings 220 Cam bro-Ord nsl Camhro-Ord granite 7881 Texas craton core 221 Camhro-Ord nsl Camhro-Ord nsl Cam bro-Ord nsl Cam bro-Ord nsl Cam bro-Ord micro~~ranite 7649 Texas craton cuttings 221 Cam bro-Ord nsl Cam bro-Ord nsl \..am bro-Ord Cam bro-Ord albite microgranodiorite 9820 p;ranite 8051-8060 Texas craton Texas craton core core 221 221 micro!!ranite 8060-8063 core 221 Camhro-Ord n~l Camhro-0rd nsl Camhro-Ord nsl Camhro-Ord nsl \,amhro-Ord nsl Camhro-Ord nsl Camhro-Ord nsl Camhro-Ord nsl Camhro-Ord microAranite 7980 Texas craton core 221 Cam bro-Ord microgranite 7710 Texas craton cuttings 222 Camhro-Ord microgranite 7495-7500 Texas craton cuttings 222 Cam bro-Ord nsl Cam bro-Ord i:ranodiorite 99.'il-9954 Texas craton core 221 microgranite 99.'il-9954 cuttini:s 222 Cam bro-Ord granite 7665-7670 Texas craton cuttings 222 Cam bro-Ord Cam bro-Ord (!: Tanite granite 8.~95-8399 7705 Texas craton Texas craton cuttings core 222 222 Cam bro-Ord Cam bro-Ord granite granite 7560-7565 8235-8240 Texas craton Texas craton cuttings cuttings 222 222 Cam bro-Ord nsl Cambrian nsl Cam bro-Ord nsl Cam bro-Ord nsl Ordovician nsl Cam bro-Ord nsl 108 Bureau of Economic Geology, The University of Texas YEAR f)f TOP OF LOCATION COM-El.EVA-TOTAL BASEMENT COl"'iTY OPERATOR and FARM ~C"C'·Hl(IC'.k-Survey ru:nn TIO~ UEl'TH Depth Eln. Lea-96 Lea-97 Lea-98 Lea­99 Lea-100 Lea-101 Lea-102 Lea-103 ;'\lagnolia #9 E. 0. Carson lllagnolia #10 E. 0 . Carson !llagnolia # 13 E. 0. Carson :Magnolia #HE. 0. Carson lllagnolia # 17 E. 0. Carson lllagnol ia #2 J. N. Carson i\Iagnolia #4 Corrigan lllagnolia #7 Corrigan 33-21S-37E 33-2 JS-37E 33-21S-37E 33-21S-37E 28-21S-37E 33-21S-37E 33-2 1S-37E 33-21S-37E 47 48 47 47 48 47 47 47 .1469 MM 3461 3471 3461 3462 3541 3449 8172 8216 7591 8220 8156 7270 7662 7446 8171 8214 7590 8215 8152 7460 7644 7443 -4702 -4750 -4129 -4744 -4691 -3998 -4193 -3994 Lea-104 lllagnolia # 1 Laura :l!ay 35-22S-37E 49 3313 8834 8834 -5521 Lea-105 l\laf!nolia #6 :llarshall 34-21S-37E 48 3457 7477 7473 -4016 Lea-106 Lea-107 Lea-108 McAlester Fuel .#1-C Jl!cC!ure lliid-Conlinent # 1 Lyn ch l\loran #2 Owen 14-15S-37E l-22S-37E 14-21S-37E 51 47 50 3803 3364 3442 13983 7236 7614 13716 7080 7534 -9913 -3716 -4092 Lea-109 I.ea-llO Ohio # 1 Jl!uncy Ohio #3 :lluncy 24-22S-37E 24-22S-37E 46 49 3331 3328 7298 7447 7292 7447 -3961 -4ll9 Lea-lll Ohio #7-B :llarshall 27-2JS-37E 50 3430 7774 7771 -4341 Lea-ll2 Ohio #9-B l\larshall 27­21S­37E 51 3425 7591 7580 -4155 Lea­ ll3 Ohio #9 Viarlick 15-21S-37E 51 3424 7503 7490 -4066 Lea-ll4 Ohio #5-C Warlick 15-21S-37E 50 3427 7827 7775 -4348 Lea-115 Ohio #6-C Warlick 15-21S-37E 50 3430 7847 7846 -4116 Lea­ ll6 Ohio #8-C Warlick 15-21S-37E 51 3426 7627 7615 -4189 Lea-ll7 Lea-ll8 Olson and At lantic # 1 Langlie O'Neill # 1-.\ State ll-25S-37E 36-28S-38E 43 52 3133 3576 9592 9724 9555 9710 --0422 -6134 Lea-ll9 Penrose # 4 Elliott B-9 9-22S-37E 46 3435 7971 7691 -4526 Lea-120 Penrose #3 Hinton 12-22S-37E 45 3344 7387 7381 -4037 Lea-121 Lea-122 Lea-123 Lea-124 Penrose # 1 Penrose-Federal Penrose # 1 Rogers Penrose #4 Walden Penrose #5 Walden 9-22S-37E 7-22S-38E 15-22S-37E 15-22S-37E 45 46 48 48 3429 3347 3412 3414 8370 7742 7819 7669 8351 7678 7805 7669 -4922 -4331 -4393 -4255 Lea-125 Lea-126 Lea-127 Lea-128 Lea­129 Lea-130 Lea-131 Lea-132 Lea-133 Lea­134 Lea-135 Lea-136 Lea­137 Lea-138 Lea-139 Lea-1.J-0 Lea­141 Lea­142 Lea-143 Lea-144 Lea­145 Lea­146 Lea-147 Lea-148 Lea-1 ~9 l.ea­l fi O Lea­151 Lea­152 l.ra-J5:l Lea-154 Lea­155 Lea-156 Lea-157 Lea-158 Lea-159 Lea­160 Lea-161 Penrose #3-A Walden Phillips # 1 Shipp Phillips # 1 Sims Phillips #2 Sims Rowan &Penrose #5 Cary Rowan &Penrose #I Elliott A-15 Rowan & Penrose #I Elliott B-15 Rowan & Penrose #.3 Walden Rowan & Penrose #6 Walden Samedan # 2 Parks Shell # 5 Aqro-Herring Shell #6 Argo-Herring Shell #7 Ar!!o-Herring Shell #8 Ari:o-Herring Shell #9 Arf!o·Herrin g Shell #4-.\ Arf!o-Herring Shell #6-A .\rp:o-Herring Shrll #8-A Ar!!o-Herring Shell #JO-A .\rgo-Herring Shell # 1 Carter Shell #I Chesher Shell #-i LiYinp:ston Shrl l #3 Rinewalt Shell #4 Rinewa lt Shell :ft 3 State Shell # 4 State Shell # ~ State Shrll #6 Stale Slwll :it7 State Shell #8 Sta te Shrll ::±9 Turner Shell :;t Li Sta te Shell :ft I Taylor Glrnn Sl1rll ;!:3 Tavlor Glenn Shrll ;!:4 Turner Shell :ft II Turner Shell #H Turner 15-22S-37E 20-18S-37E 24-22S-37E 24-22S-37E 22-22S-37E 15-22S-37E 15-22S-37E 15-22S-37E 15-22S-37E 14-22S-37E 15-2 1S-37E 15-2l S-37E 15-2lS-37E 22-21S-37E 15-21S-37E 22-2lS-37E 22-2lS-37E 22-2l S-37E 22-21S­37E 32-17S-39E 12­21S-37E 3-21S-37E 4-22S-37E 4-22S-37E 2-21S-37E 15-21S-37E 2-21S-37E 2-21S-37E 2-21S-37E 2-21S-37E 22-21S-37E 2-21S-37E 3-21S-37E &--21S-37E 22-21S-37E 22-21S-37E 22-21S-37E 47 49 45 47 48 48 46 46 47 47 50 50 51 51 51 50 50 51 51 50 52 52 47 47 50 51 51 51 51 51 50 52 48 52 49 50 51 3409 3747 3333 3334 3374 3392 3416 3415 3407 3345 3429 3428 3457 3435 3445 3426 3428 3435 3437 3646 3465 3429 3469 3462 3467 3431 3483 3488 3473 3498 3423 3502 3484 3493 3436 3420 3423 7568 12626 7377 7305 8086 7690 7365 7581 7788 7324 8091 7908 8193 8188 8189 7810 7907 8188 8130 l iUJ44 7695 8167 7986 7957 7906 7567 7956 8207 7854 8156 7951 8147 8590 8224 7890 7782 7758 7528 12590 73:l3 7295 8070 7689 7325 7570 7787 7322 7975 7897 8185 8185 8186 7760 7897 8185 8120 l ; o~ LITHOLOG Y OF PRF.CAMRRIAN MATEHIAL BASEMENT BASEMENT INTERVAL PRO\il'.\CI·'. STUIHEU PAGE Cambro-Ord nsl Cam bro-Ord nsl Cambro-Ord nsl Cam bro-Ord nsl Cam bro-Ord Cam bro-Ord albite granodiorite nsl Texas craton core 222 Cam bro-Ord nsl ni ns l Cam bro-Ord nsl Permian nsl Ordovician nsl Permian nsl Permian nsl Permian nsl Cam bro-Ord nsl Cam bro-Ord nsl Cambro-Ord nsl Cambro-Ord nsl Cambro-Ord nsl Cam bro-Ord nsl Permia n ? nsl Cam bro-Ord micro~ranite 9584-9592 Texas craton core 223 Cam bro-Ord nsl Cambro-Ord nsl Permian Cambro-Ord microgranite nsl 7387 Texas craton cuttings 223 Penni an nsl Cam bro-Ord nsl Cam bro-Ord nsl Cambro-Ord nsl Cam bro-Ord Pennian gran ite nsl 12620-12625 Texas craton cuttings 223 Penni an nsl Cam bro-Ord nsl Cam bro-Ord nsl Pennian nsl Cam bro-Ord nsl Cam bro-Ord nsl Permian nsl Cam bro-Ord Cambro-Ord granite nsl 7980-8089 Texas craton cuttings 223 Cambro-Ord Cam bro-Ord izranite nsl 8185-8190 Texas craton cuttings 223 Camhro-Ord Cam bro-Ord Cambro-Ord Cambro-Ord p:ranite granite granite nsl 8185-8189 7760-7800 7885-7907 Texas rraton Texas craton Texas craton cuttin µ:s cuttin~s cuttings 22.> 22.1 223 Cam bro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord {!ranitc granite granodiorite granite nsl 8125-8132 14020-14040 76.30­7665 8150-8167 Texas era.ton Texas craton Texas craton Texas craton ruttinizs rnttings cuttings cutting:s 224 224 224 22-l Cam bro-Ord nsl Cam bro-Ord Cam bro-Ord Cam bro-Ord Cam bro-Ord Cambro-Ord microgranodiorite granite mkrogran odiorite micro~ranite nsl 7900-7905 7550-7567 7955-7956 8205-8209 Texas (Taton Texas craton Texas craton Texas craton cuttinf!S cuttings core cuttings 224 224 22+ 224 Cam bro-Ord Cambro-Ord granodiorite nsl 8132-8150 Texas craton cuttinµs 22+ Cambro-Ord nsl Cambro-Ord Cam bro-Ord Cambro-Ord Cambro-Ord Carnbro-Ord granite mirrogranite microgranite granite granite 8575-8590 8225-8228 7890-7896 7770-7783 7725-7755 Texas craton Texas rraton Texas craton Texas craton Texas craton ruttin~s ruttinµ:s ruttin~s cuttings cuttings 225 225 225 225 225 llO Bureau of Economic Geology, The University of Texas Basement Rocks, Texas-New Mexico lll YEAR DF TOP OF COU~TY OPERATOR ond FARM LOCATION ~·Block·Survoy COM· PLF.TED ELEVA· TION TOTAL DEPTH BASEMENT Depth Ele.. Lea-162 Shell #15 Turner 22-21S-37E 51 3416 7472 7430 -4014 Lea-163 Shell # 16 Turner 22­215-37£ 52 3427 7864 7850 -4423 Lea-164 Sinclair # 1 Barton 23­215-37£ 52 3407 7787 7786 -4379 Lea-165 Sinclair #4 Brunson 4-22S-37E 46 3436 7883 7880 --4444 Lea-lr>6 Sinclair # 5 Brunson 4-22S-37E 46 3450 7846 7840 -4390 Lea-167 Sinclair #8 Brunson 3-22S-37E 48 3436 7445 7440 -4004 Lea-168 Sinclair # 1 Hill 26-21S-37E 48 3386 7642 7610 -4224 Lea-169 Sinclair #2 State 367 36-21S-37E 47 3376 7685 7640 -4264 l.ea-170 Lca-171 Lea-172 Lea-173 Lea-174 Lea-175 Lea­176 Skelly #3·A Baker Skelly #5-B Baker Skelly #6·B Baker Skelly #14-B Baker Skelly # l Sticher Stanolind #2 Corrigan Stanolind #l·A Eva Owen 26-22S-37E 10-22S-37E 10-22S-37E 10-22S-37 E 4-22S-37E 33-21S-37E 3-22S-37E 45 46 47 50 47 47 47 3324 3417 3421 3418 3449 3449 3432 8154 7322 7582 7590 8053 7452 7356 8151 7312 7579 7509 8047 7445 7330 -4827 -3895 -4158 -4171 -4598 -3996 -3898 Lea-177 Stanolind # l W. H. Jones 19­19S­39E 45 3581 10596 10570 -6989 Lea-178 Stanolind #4 South llfattix 15-24S-37E 51 3266 10270 10260 -6944 Lea­ 179 Stanolind # 5 South Mattix 22-24S-37E 51 3247 11150 11148 -7901 Lea-180 Stanolind #6 South Mattix 15-24S-37 E 52 3279 10544 10525 -7246 Lea­181 Stanolind #l·T State-Andrews 32-22S-38E 47 3398 8063 7470 -4072 I.ea-182 Stanolind #l·U State 2-20S-38E 49 3!'80 10015 10005 -6425 Lea-IR3 Stanolind #11-X State 4-19S-38E 52 3639 8160 8050 -4411 Lea-184 Lea-185 Lea­186 Texas # 1 Blinebry Texas #4 Blinebry Texas #2 Lockhart 19-22S-38E 20-22S-38E 18-22S-37E 45 46 45 3369 3397 3354 7517 8377 7597 7435 -4066 83W -4923 7560 -4206 Lea-187 Tidewater #l·E Brunson 4-22S-37E 46 3441 7600 7592 -4151 Lea-188 Tidewater # 3-S State 15-21S-37E 50 34E8 7629 7614 -4156 Lea-189 Tidewater #4·S State 15-21S-37E 51 3459 7896 7858 -4399 Lea-190 Tidewater #5-S State 15-21S-37E 51 34EB 8148 8128 -4670 Lea-191 Tidewater #7-S State 15-215-37£ 52 3459 8145 8141 -4682 Lincoln-I Lincoln-2 National Exploration #1 Piracho Standard of Texas #1 Heard­ 21-l!S-18E 24 5031 2199 1670 +4361 Federal 33­6S-9E 51 5892 8050 7750 -1858 Linroln­ 3 Stanolind # 1 Picacho Unit 10-12S-18E 45 5958 2843 2425 +3533 Otero-I Hunt & Turner #1 Mc:\lillan 5-26S-16E 43 4260 2176 2170 +2090 Otero-2 Standard of Texas #1 Scarp Unit 18-21S-18E 48 5340 2664 2580 +2760 Quay-1 Quay-2 Penrose #1 Pippins Stanolind #1 Fuller 35-12N-34E 25-8N-20E 52 43 4063 4459 6128 6747 5290 6730 -1225 -2271 Roo>•velt­1 Austral #I Saddler 29-4S-32E 52 4424 8154 8126 -3702 Roosevelt-2 Roosevelt-3 Goldston #1-A Lambirth·State Magnolia # 1 Brown 36-5S-32E 6-7S-34E 51 51 4436 4347 8297 9067 8038 7100 -3602 -2753 Roosevelt-4 Mai:nolia #1 A. K. Smith ll-7S-33E 48 4382 10015 9965 -5583 Roosevelt-5 lllid-Continent # 1 Strickland 9-4S-35E 51 4280 7608 7480 -3600 Roo•evelt-6 Shell # 1 Harwood 27-7S-35E 39 4199 7957 7945 -3746 Roosevelt-7 Shell # 1 Saunders 5-8S-37E 48 4072 8679 8640 -4568 Roosevelt-a Signal # l Bell-Federal 33-3S-33E 53 4310 7996 7990 -3680 Roosevelt-9 Southern Union # 1 Lucas 5-2N-30E 46 4249 7155 7140 -2891 AGE OF FORMATION ON LITHOl.OGY OF PRECAMBRIAN MATEiUAL BASEMEl'iT BASEMENT INTERVAL PltOVINCE STUUIED PAGE Cam bro-Ord granite 7430-7470 Texas craton cuttings 225 Cam bro-Ord nsl Permian olivine-augite syenite 7786 Texas craton core 225 Cam bro-Ord microgranodiorite 7880-7883 Texas craton cuttings 225 Cam bro-Ord microgranodiorite 7840-7845 Texas craton cuttings 225 Cam bro-Ord nsl Permian nsl Permian granite gneiss 7642-7646 Texas craton core 226 granite 7680 cuttings 226 Ordovician nsl Permian Cam bro.Ord microgranodiorite nsl 7315-7322 Texas crnton cuttings 226 Cam bro-Ord nsl Cam bro-Ord Cambro-Ord microp:ranodiorite nsl 8052-8053 Texas craton cuttings 226 Permian nsl Cam bro-Ord Cam bro-Ord microgranite nsl 10570-10580 Texas craton cuttings 226 Cambrian nsl Cam bro-Ord nsl Permian nsl Cnmbro-Ord nsl Permian granite 8150 Texas craton core 226 Permian Permian Permian Cam bro-Ord microgranite granite gran ite nsl 7510-7515 8370-8375 7590-7595 Texas craton Texas craton Texas craton cuttings cuttings cuttings 226 226 227 Cambro-Ord nsl Cambro·Ord nsl Cam bro-Ord nsl Cambro·Ord nsl Permian nsl Pennsylvanian olivine gabbro olivine gabbro 7800-7870 8050 ? cuttings core 227 227 Permian metarkosite metarkosite 2685-2690 2740-2750 ? cuttings cuttings 227 227 Permian rhyolite porphyry 2175 core 227 micrographic granite 2175 core 228 Cambro·Ord gabbro gabbro syenogabbro 2592-2610 2630-2660 2655-2660 ? cuttings cuttings core 228 22B 228 Pennsylvanian? nsl Pennsylvanian Silurian rhyolite metntuf! amphibolite 6746-6747 8130-8156 Panhandle volcanic terrane ? core core 229 229 Silurian-Dev amphibolite 8287 core 229 Cam bro-Ord homblende-quartz-albite gneiss 7110 Texas craton? core 229 hornblende-quartz-albite gneiss 7200 core 229 granodiorite gneiss 7215 core 229 Cam bro-Ord Permo-Penn Permian Pennsylvanian Devonian Pennsylvanian leuco-albite syenodiorite 9067 rhyolite porphyry-X 10000-10016 diabase 7508-7513 albite diabase "bottom hole" rhyolite 7935-7955 rhyolite 7950-7955 rhyolite 8640-8679 rhyolite porphyry 8650 leuco-olivine gabbro 7990-7996 diabase 7140-7155 Panhandle volcanic terrane Swisher gabbroic terrane Panhandle volcanic terrane Panhandle volcanic terrane Swisher gabbroic terrane ? core cu ttings core cuttings cuttings cuttings cu ttings core core cutti~i;• 230 230 230 230 230 2.30 2.31 2.31 231 231 112 Bureau of Economic Geology, The University of Texas COt;NTY Roosevelt-IO OPERATOR and FARM Spartan # 1-36 State l.OCATION Sec·Block-Survey 36--4S-31E YEAR COM-I'LET ED 51 DF ELEVA­TION 4516 TOTAL DEPTH 7263 TOP OF BASEMENT Depth Ele•. 7120 -2604 Roosevelt-11 Roosevelt-12 Tidewater # 1 Grady Best Tidewater # 1 Boone 27-2S-29E 7-5S-30E 51 51 4384 4412 7277 8475 7090 8410 -2706 -3998 Union-I Quaker State #I Zurick 2-21N-34E 36 4625 2925 2925± +1300 113 Basement Rocks, Texas-New Mexico ACE OF FORMATION ON LITHOLOGY OF BASEMENT BASEMENT Permo-Penn granite quartz-hornblende­ plagioclase gneiss quartz diorite gneiss quartz diorite gneiss Miss? Ord? mylonitized granite Silurian·Dev mylonitized syenite leuco-diorite Pcnnian? nsl INTERVAL 7140 7204 7210 7240-7242 126;,...1211 8450-M65 8460-8465 PRECAMBRIAN PROVINCE MATERIAL STUUIED PAGE Texas craton? core 231 Texas craton Texas craton core core core core cuttings cuttings 231 232 232 232 232 232 APPENDIX I GLOSSARY OF PETROGRAPHIC NOMENCLATURE This glossary is included as an aid to those not familiar with petrographic no­menclature and to clarify the usage of .ambiguous petrographic terms. The writer has leaned heavily on the excellent glossary of Johannsen (1939, pp. 163­288), but there are differences between .some definitions given by Johannsen and usage in this paper. As used by the writer, the term texture includes the elements of grain size, de­gree of crystallinity, and relationships of minerals and/or rock fragments to each other. For this third category, the term fabric is preferred, but many writers, in­cluding Johannsen, use texture as a synonym for fabric. Arguments for the simplified philosophy of rock nomencla­ture favored by the writer are presented on pages 16-19. TEXTURAL AND FABRIC TERMS 1. Anti-perthitic.-A term applied to plagio­clase that shows more or less regularly dis­tributed inclusions of potash feldspar all of whi ch have the same optical orientation. Cf. poikilitic. 2. Cataclastic.-A fabric resulting from crush­ing and shearing of constituent minerals which commonly are strained, deformed, and granulated. The ultimate result of cataclasis is mylonite. .3. Clastic.-A fabric of sedimentary rocks formed by the accumulation of solid mineral and rock particles. The mineral and/or rock fragments, which have been more or less modified by weathering and transporting agencies, are commonly held in a cement or matrix. 4. Cryplocrystalline.-A texture so fine that in­dividual grains cannot be resolved under high magnification 11 but whose crystalline nature is demonstrated by polarization effects. 5. Crystalloblastic.-A fabric of metamorphic rocks caused by simultaneous recrystalliza­ 11 ll i• difficult to auign • nand.er lo tJ.i1 mapificalion be· .C.UM 1pecial objeclivH aad dniu1 ca• increue re101ution. For practical purpotc1 ..high mqnification.. a1 u1ed in the dcfini· tion ii Jen than the greatest mag:ni.fication Lhat can be ebtaincd with tt.e 1landanl equipment of the pcilrographic microtcepe ('Sx objecti,.. + 10:1: ocular -450x) aad 1realer than tho c•mm•n intcrmedi•Le working Dlaglli6cation (lOx objective + lOz ocular -lOOz) . Of courM, the m•pificalion at which • minflral crain c•n fi.nt b. re10l•ecl it eon1ider•bly le.1 than that ritquired for working on it. lion of old mineral constituents and/or growth of new materials as a result of meta­morphism. Shapes of mineral grains are de­termined by the relative powers of the min­erals to assert their own crystal boundaries during growth in the solid state. A general term including diablastic, lepidoblastic, gra­noblastic, and porphyroblastic fabrics. 6. Diablastic.-A variety of crystalloblastic fab­ric produced by the intergrowth in the solid state of two or more mineral constituents which include and penetrate each other. It is synonymous with sieve fabric and hornfels fabric but distinguished from poikiloblastic fabric in which included minerals were in­ active in an active host mineral. 7. Fibrolamellar.-A fabric characterizing fi. brous and scaly aggregates of serpentine min­erals. 8. Foliated.-A general term for a metamorphic fabric that shows oriented planar elements. Foliation as used in this paper includes spe­cific planar structures such as schistosity, slaty cleavage, and gneissic structure. 9. Gneissic.-A metamorphic fabric, character­ized by parallel orientation of platy or linear minerals but without schistosity, a gneiss is a less perfect foliate rock than a schist be­ cause it commonly contains less dimension­ ally oriented micaceous and/or prismatic minerals-the bulk of the rock is commonly a recrystallized quartz.feldspar aggregate. Some gneisses show a layering expressed in grain size and/or mineral composition. 10. Granoblastic.-A variety of crystalloblastic fabric in which equan t minerals form a gran­ular aggregate or a mosaic. lOa. Granophyric.-A fabric of granitic igneous rocks in which quartz and alkali feldspar are intergrown more or less irregularly. Cf. mi­crographic; granophyre. 11. Hypidiomorphic granular.-A fabric in which some mineral constituents are idio­morphic, some are partly idiomorphic, and others are xenomorphic. This fabric is thought to characterize deep·seated plutonic rocks that crystallized from a magma, in which the early forming minerals developed their own crystal shapes. Recent investiga­tions of large volumes of granitic rocks al­legedly formed by metasomatic processes weakens the inference of molten magmatic origin that formerly was drawn from this particular fabric. Synonymous with hypauto· morphic granular and subhedral granular. 12. ldiomorphic.-A term referring to minerals that show their own characteristic crystal outlines. 13. Lepidoblastic.-A variety of crystalloblastic fabric in which oriented platy minerals im· part a pronounced foliation. 14. Metasomatic /abric.-A secondary fabric caused by alteration of the original mineral Bureau of Economic Geology, The University of Texas constituents of the rocks, most commonly by deuteric or hydrothermal agencies. The origi­nal minerals may exist as relics showing cor­rosion, embayment. and replacement by new minerals. This fabric is difficult to recognize if alteration is complete. 15. Mi crocrntallin e.-Crystalline rocks whose crystallinity can be determined only by mi­C'rosropic examination. ln this paper used mainly in the description of the groundmass of \"olcanic rocks. 16. i'1icrogranular.-.\ fine cryslalline aggregate wl.10se granularity is visible only under the m1rroscope. 17. . \ficrographic.-.J.. term applied in this paper to a more or less regular intergrowth of two minerals that can be discerned onlv bv micro­ scopic examination (grapldc is ·thC mega· scopic equirnlent of this term). :'\lost com­monly the term is applied to a quartz and potassium feldspar intergrowth. It is essen­tially synonymous with granophyn·c and mi­cropegmatitic but some petrographers prefer to reser\"e granophrric for less regular inler­growths between micrographic and myrme­ kitic. 18. i\/icrolitic.-J\. fabric of rock composed main· ly of fine mineral laths or rods of microscopic size called microlites. This has been used as a general term to include trachytU: where the microlites are in a parallel orientation, felt)' for a non-oriented aggregate of microlites, and terms gi\"en to describe various propor­ tions of microlites and glass. In this paper it is used as a synonym for felty. 19. llficrospherulitic.-A fabric of \"Olcanic rocks wh~rein the original glass crystallized (de­\"itrified) wholly or in part to radial crystal aggregates or spherulites of a size that can be recognized only under the microscope. 20. Mrrmekitic.-A term applied to Yennirular intergrowths of quartz and plagiodase feld­spar. Cf. micrographic. 21. Ophitic.-:\ hypidiomorphic granular fabric characteristic of diabase in which plap:io­rlase laths in radial or triangular pattern are included in large pyroxene indi'"iduals. Near­lv svnon\"IDOUS with dfobasic. 22. Perihiti~.-.\ term applied to potassium feld­ spar that conta.ins included sodic plap:ioclase as parallel \"einlets, oriented bleb<, or irregu­ lar palrht'~. 23. Pvikilitic.-A fabric characterized by spongy host minerals that contain inrlusion~ of other mineral~. ).!enerally not in a common lattice oril'ntation. 24-. Porplirritic.-.-\ fabric whirh sho,\·s larp:er <'l!·:--tal :-" or pllt.'nocryst.s of ont" or more min­erals in a finer crystaHine and/ or p:las.sy l! roundma::::. 25. Porph;·robla.nce of ~lass. mineralogically equiYalent to diorite (No. 5). 2 . .4rgi//i1e.-Structureless, weakly metamor­phosed, less than 50 percent reconstituted a.rg:illaceous rork up to 0.05 mm in grain size. 3. Basalt.-.\ glassy and/ or finely crystalline ip:neous rock, commonly porphyritic, that is chemically and, except for the possible pres­ enct' of l!lass, minera)ogically equiYalent to gabbro (No. 6) . -1. Dacite.-A glassy and/or finely crystalline ip;neous rock, commonly porphyritic, that is chemically and, except for the possible pres­ ence of glass, mineralogically equivalent to quartz diorite (No. 22). Basement Rocks, Texas-New Mexico 5. Diorite.-A megascopically granular igneous rock composed about equally of an interme­diate to calcic plagioclase and ferromagne­sian minerals, most commonly hornblende, with various accessory minerals (Table 2). 6. Gabbro.-A megascopically granular igneous rock composed about equally of calcic plagio­ dase and ferromagnesian minerals, common­ly olivine and pyroxene, with various acces­ sory minerals (Table 2). 7. Gneiss.-A megascopically crystalline meta­morphic rock characterized by an imperfect foliation (due t9 poor orientation or a pau­city of tabular or platy minerals) and layers or bands defined by mineral segrep;ation and/ or grain size. Orthogneiss is applied to gneisses formed by metamorphism of igneous rocks; paragneiss is applied to gneisses formed by metamorphism of sedimentary rocks; howeYer, the nature of the original rocks cannot always be recognized. Primary gneiss is applied lo gneiss formed by flowage of consolidating magma; this is an igneous rock and metamorphic processes are not in­volved in its formation. 8. Granite.-A megascopically granular plu­ tonic igneous rock 12 composed dominantly of potassium feldspar with subordinate sodic plagioclase, quartz, ferromagnesian minerals, and various accessory minerals (Table 2) . 9. Granodiorite.-A megascopically granular plutonic igneous rock composed dominantly of sodic plap;ioclase with subordinate potas­ sium feldspar, quartz, ferromagnesian min­ erals, and various accessory minerals (Table 2) . 9a. Granophyre.-A granitic igneous rock char­acterized by intergrown quartz and alkali feldspar. The term suffers from a history of varied usage, but the modern tendency is to apply it to rocks in which the quartz and alkali feldspar show a cuneiform (micro­ graphic), irregular, or myrmekitic inter­ j?rowth. Granophyre as a rock name is more inclusive than granophyric as a textural term. 10. Leuco.-A prefix for the common igneous rock types to denote a variety with an ab­normally low ferromagnesian mineral con­tent. 11. Mela.-A prefix for the common igneous rock types to denote a variety with an abnormally high ferromagnesian mineral content. 12. Meta.-A prefix to a rock name indicating that the mineral and/or chemical composi­tion of the rock has been modified by altera­tion by metamorphic processes excluding weathering but its original character is still discernible. 13. Meta-argillite.-Structureless, weakly meta­morphosed, more than 50 percent reconsti­tuted argillaceous rock up to 0.05 mm in grain size. 14. Meta·arkose.-A weakly metamorphosed ar­kose in which the quartz and feldspar have 12 t:'ndf'r (a\'Ofab(e l~mper8lure-presrnre Conditions granite un •ho occur u 5tocks. sills. dikes, elc .. but 1he great volume of tt:ranile is in balholithic masses. Ruuhs o{ r~cenl &ludi~s of tome luge granite masse1 indic•le that the lerm must be u­panded lo include rock of metuomatic origin. not recrystallized but the intergranular ma­terial has been reconstituted. 15. Metarkosite.-A metamorphic rock result­ing from the recrystallization of arkose by metamorphic processes. Equirnlent to meta· quartzite in metamorphic grade. Equals the arkosite of Grout which is unsatisfactory as a metamorphic rock term because of its use in sedimentary petrography for unmetamor­phosed rocks. 16. Metaquartzite.-A metamorphic rock result­ing from recrystallization of quartz sand­stone by metamorphic processes; the rock is composed predominantly of quartz and con­tains less than 25 percent feldspar. 17. Metasandstone.-A weakly metamorphosed sandstone in which the quartz has not re­crystallized but the intergranular material has been reconstituted. Orthoquartzite is a quartzose sedimentary rock with siliceous cement. Although sedimentary in origin this rock breaks across quartz grains and there­fore fulfills the fundamental definition of a quartzite; unfortunately this fundamental definition has been ignored by some sedimen­tary petrographers who apply the term to pure quartz sandstones. 18. Micro.-A prefix for the common igneous rock types to denote a fine-grained variety with a grain size between 0.05 and 1.0 mm. 19. Monzonite.-A megascopically granular ig­neous rock composed about equally of potas­sium feldspar and sodic or intermediate pla­gioclase with subordinate ferromagnesian minerals and various accessory minerals. Not used in this paper; cf. syenite and syenodio­ rite. 20. Phyllite.-Completely reconstituted (crystal­line) foliated low-grade metamorphic rock up to 0.5 mm in grain size. 21. Porphyry.-An igneous rock characterized by larger crystals, phenocrysts, in a finer grained groundmass; commonly the same minerals occur in two generations-as pheno~ crysts and in the groundmass. 22. Quartz diorite.-Similar to diorite but with an appreciable quartz content<> 5 percent) and, commonly, appreciable but subordinate potassium feldspar (Table 2). Synonymous with tonalite. 23. Quartz mon:onite.-A megascopically granu­lar igneous rock composed about equally of potassium feldspar and sodic or inter­mediate plagioclase with subordinate quartz and ferromagnesian minerals and various ac­cessorv minerals. This term is not used in this paper, althoul!h it is popular in the western part of the United States. The writer has attempted to use as few rock names as possible, and because the field of quartz monzonite is ornrlapped by the fields of g:ranite and granodiorite, it can be omitted. 24. Rhyodacite.-A glassy and/or finely crystal­ line igneous rock, commonly porphyritic, that is chemically and, except for the pos­sible presence of glass, mineralogically equiv­alent to granodiorite. 25. Rhrolite.-The extrusive equivalent of a granite. This is the most common definition, Bureau of Economic Geology, The University of Texas but the same rock also occurs as shallow in· trusives and the term should not by definition be limited to extrusive rocks. Perhaps a bet­ter definition is: Rhyolite is a glassy and/or finely crystalline igneous rock, commonly porphyritic, that is chemically and, except for the possible presence of glass, mineral· ogically equivalent to granite. 26. Schist.-Schistose metamorphic rock rang· ing from low to high metamorphic grade. Schist is commonly megascopically crystal· line, but in this paper 0.1 mm is established as the lower grain size limit for schist with medium to high-grade metamorphic min· erals; schistose rocks with low-grade meta· morphic minerals and a grain size less than 0.5 mm are classed as phyllite ( p. 18) . 27. Syenite.-A megascopically granular igneous rock composed dominantly of potassium feld· spar with subordinate sodic plagior.lase, fer­romagnesian minerals, and various accessory minerals (Table 2). 28. Syenodiorite.-A megascopically granular igneous rock composed dominantly of an intermediate plagioclase with subordinate potassium feldspar, ferromagnesian minerals, and various accessory minerals (Table 2). 29. Syenogabbro.-A megascopically granular igneous rock composed about equally of cal· cic plagioclase and ferromagnesian minerals, commonly pyroxene, but with appreciable potassium feldspar content. Various acces· sory minerals are present (Table 2). 30. Trachyandesite.-A glassy and/or finely crys­ talline igneous rock, commonly porphyritic, that is chemically and, except for the pos­sible presence of glass, mineralogically equivalent to syenodiorite. 3l. Trachyte.-A glassy and/ or finely crystalline igneous rock, commonly porphyritic, that is chemically and, except for the possible pres· ence of glass, mineralogically equivalent to syenite. APPENDIX II PETROGRAPHIC DESCRIPTIONS Evaluation of petrographic descrip­tions.-The following pages contain petrographic descriptions of thin sections of basement rocks, either cuttings or core chips, from wells listed in Table 1. They are arranged alphabetically by county and again alphabetically by operator under the county heading. Each individual report is headed by the name of the well, the nature of the sample (whether core chip or cuttings), the depth of the sample, and the ownership of the thin section studied. The list includes thin sections from the Gulf Oil Corporation, Honolulu Oil Corporation, Humble Oil & Refining Company, Shell Oil Company, Stanolind Oil & Gas Company, and Bureau of Eco­nomic Geology. The primary purpose of preparing and studying these thin sections was to classify the basement rock in the various wells so as to be able to map major rock types. The following abbreviated petrographic descriptions are presented to that end: minerals were identified, a mode was esti­mated, grain size was measured, and the texture or fabric was determined. All these elements are essential in classification of the rock. It was not considered feasible to make a very detailed petrographic study of each slide for the purpose of this proj· eel. Rosiwal modal analysis and precise determination of mineral species by im­mersion and other methods are of value in investigation of an exposed rock mass that can be freely sampled, but such re­finements are of little actual value when trying to interpret major geologic features with small random samples separated by many miles. The percentages of minerals given are estimated figures derived by examination of the entire slide under low magnification and of randomly selected portions of the slide under higher magnification. Thin sections of core chips of fine·grained rocks are the most satisfactory subjects for modal estimation under the well-known sampling theory which calls for size of sample to increase with grain or fragment size of the sampled material. Thus a mode estimated from a standard thin section of a fine-grained rock would be more likely rep­resentative of the rock than a mode esti­mated from a standard thin section of a coarse-grained rock. The least satisfactory thin sections for model estimation are those of fine cuttings of coarse-grained rocks; these yield a thin section of mineral frag­ments rather than rock fragments. It is im­possible, for example, to distinguish be­tween granite and granodiorite in such a thin section, because both rocks have sub· stantial amounts of potassium and plagio­clase feldspar, but because we are primar­ily interested in whether the rock is a vol· canic rock, a plutonic rock, or a meta· morphic rock, these sections of cuttings are very useful. PART I-TEXAS ANDREWS COUNTY Gulf #9-E Univer.;ity "Z" cuttings 11100-05 Bureau of Economic Geology Microcline microperthite (67%), quartz (15%), albite (15% ), biotite (1%), pyrite 0%), calcite (l% ) , chlorite (tr), apatite (tr). Plagioclase is partly sericitized and partly replaced by calcite; biotite is a green-brown variety. Grain size: 0.5 to 2 mm. Fabric: xenomorphic granular (poorly shown in a few fragments). Rock: granite. Bureau of Economic Geology, The University of Texas Humble #3 Lineberry cuttings 10665-68 Bureau of Economic Geology Microcline microperthite ( 47% ), oligoclase ( 40%), quartz (12%), magnetite or ilmenite 0 % ), chlorite (tr), zircon (tr), apatite (tr)_ Quartz and plagioclase locally show myrmekitic inter­growths. Grain size: 0.2 to 2 mm. Fabric: xenomorphic granular. Rock : sranite. Humble #3 Lineberry cuttings 10665-68 Bureau of Economic Geology Calcic plagioclase (66%), augite (25%), magnetite or ilmenite (8o/o), biotite 0 %), apatite (Ir). Biotite is red-brown; very poor slide shows only a few fragments. Grain size: 0.2 to 0.5 mm. Fabric: hypidiomorphic granular. Rock: microsabbro. Humble #1 Pinson cuttings 10855-60 Bureau of Economic Geology Microcline (65%), quartz (15o/o), albite (10%), biotite (8%), sphene (2o/o), myrmekite (tr), apatite (tr), zircon (tr). Biotite pleochroism is olive to very dark brown. Grain size: 0.2 to 0.4 mm. Fabric: xenomorphic granular. Rock: microgranite. Humble #1 Scarborough cuttings 10910-26 Shell Oil Co. Microcline (70%), quartz (20%) , albite (5o/o), biotite (4o/o), chlorite 0%), leucoxene (tr), apatite (tr), zircon (tr). Albite is partly kaolinized; biotite pleochroism is olive-brown to very dark brown-the biotite is partly altered to chlorite; quartz and feldspar show mutual em­bayment. Grain size: 0.1to3 mm. Fabric: xenomorphic granular. Rock : granite. Humble #l Scarborough core 10926-29 Stanolind Oil & Gas Co. Andesine (58%), hornblende (30%), biotite (5o/o), magnetite or ilmenite (4o/o), calcite (2%), pyrite 0 %), leucoxene (tr), chlorite (tr), apatite (tr). Hornblende grains show patchy uneven pleochroism and centers of grains commonly are altered to a fibrous, highly birefringent min­eral. Grain size: l mm. Fabric: hypidomorphic granular. Rock: microdiorite. Humble #l Scarborough core 10926-29 Bureau of Economic Geology Albite (69%), chlorite 120%), biotite (5o/o), ilmenite (3%), pyrite 0%), leucoxene 0%), calcite 0%), apatite (tr), quartz (tr). Chlorite occurs in large fine-granular masses; biotite is partly altered to chlorite and occurs in "nests" and scattered plates; calcite partly replaces plagioclase. Grain size: 2 mm. Fabric : hypidiomorphic granular. Rock: altered albite diorite. Phillips #38 University cuttings 8000-05 Bureau of Economic Geology Mirrocline microperthite, quartz, cakite, chlorite, zircon. Cuttings consist of only a few small grains so that estimates of mineral percentages are not practical. Grain size: 0.5 mm. Fabric: hypidiomorphic granular. Rock: microsranite. Phillips #50 University core 7854-57 Humble Oil & Rig. Co. Microcline microperthite (57o/o), quartz (20o/o), albite 05%), calcite (5o/o), biotite 0 %), leucoxene 0 % ) , fluorite (l% ) , muscovite (tr), zircon (tr) . Quartz shows severe strain; calcite occurs in dirty masses, apparently replacing a primary ferromagnesian mineral; biotite is present as bleached and altered remnants; general distribution of minerals is irregular-there are quartz-rich areas and microcline-rich areas. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granit e. Phillips #50 University core 7854-57 Stanolind Oil & Gas Co. l\licrocline microperthite (50% ), quartz (20%), oligoclase 06%), hornblende (8%) , fluorite (3o/o), biotite? (2%), calcite (1%), chlorite (tr), zircon (tr). Pale green hornblende is smeared out into lenses and layers; quartz shows severe strain and dimensional orientation; chlorite occurs in veinlets and within hornblende grains as result of alteration of hornblende; biotite? is very pale with only a faint pleochroism and may be a highly birefringent chlorite. Grain size : 0.2 to 2 mm. Fabric: gneissic to xenomorphic granular. Rock: sranite sneiss. Phillips # 57 University cuttings 8030-35 Bureau of Economic Geology Microline (56o/ol, oligoclase (20%), quartz (20o/o), biotite (4%), fluorite (tr), calcite (tr), apatite (tr) . zircon (tr) . l\licrocline is locally microperthitic; biotite pleochroism is yellow­brown lo very dark brown; quartz shows strain. Grain size: 0.3 to l mm. Fabric: :renomorphic granular. Rock: microgranite. Basement Rocks, Texas-New Mexico Phillips #58 University core 7922-26 Bureau of Economic Geology Microcline microperthite (40%), albite-oligoclase (20%), quartz (20%), hornblende (8%), biotite (5%), chlorite (4%), calcite (2%), leucoxene (1%), sphene (tr), sericite (tr), zircon (tr). Microcline microperthite ranges from almost pure microcline to microperthite that is almost all plagioclase (anti-perthite) ; hornblende is yellow-green and occurs in part in large poikilitic grains; it is partly replaced by calcite ; hiotite pleochroism is pale red-brown to very dark brown-it is partly altered to chlorite; sericite occurs in Yeinlets. Grain size: 0.5 mm to l cm. Fabric: hypidiomorphic granular. Rock: granite. (Photomicrograph, Pl. IV, A.) Phillips #5-M University cuttings 10820-25 Bureau of Economic Geology Andesine (56%), quartz (15%), hornblende (15%), biotite 00%), pyroxene? (2%), mag­netite or ilmenite (2%), pyrite (tr), epidote (tr), calcite (tr), orthopyroxene (tr), sphene (tr I. Plagioclase is partly sericitized and locally bent; hornblende, yellow-green to green pleochroism, is locally poikilitic; biotite peochroism is pale to red-brown. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: quartz diorite. Shell # l Cox cuttings 11030-40 Shell Oil Co. Quartz (60%), microcline (29%), albite (8%), biotite (2%), muscovite 0%), apatite (tr). Albite shows incipient al teration to sericite; biotite pleochroism is green-olive-brown to very dark brown. Slide consists of only a few small fragments, commonly of only a few grains each, and estimated percenta~es (above) are probably not representative of the rock; the high quartz content is not significant. Grain size: 0.1 to l mm. Fabric: hypidiomorphic granular. Rock : microgranite. Shell #l Cox core 11057-61 Stanolind Oil & Gas Co. Microcline microperthite (62%), albite-oligoclase (20%), quartz 05%), biotite (3%), sphene (tr), magnetite or ilmenite ( trl, leucoxene (tr), sericite (tr), fluorite (tr), apatite (tr), zircon (tr). Plagioclase is zoned; biotite is a green-brown variety; sphene is partly altered to leucoxene. Grain size: l to 5 mm. Fabric: hypidiomorphic granular. Rock: granite. Shell # l Nelson cuttings 10600-06 Shell Oil Co. Labradorite? (60%), augite (20%), magnetite or ilmenite (8%), biotite (5%), calcite (5%), sericite (2%). Labradorite is partly altered to sericite; augite is pale brown and probably titaniferous; biotite is partly bleached. Grain size: 0.1 to 0.4 mm. Fabric: subophitic. Rock: leuco-diabase. Shell # l Nelson cuttings 10606 Shell Oil Co. Labradorite? (73%), magnetite or ilmenite 00%), augite (8%), sericite (3%), biotite (2%), chlorite (2%) , calcite (2%). Augite is violet-brown and probably titaniferous; biotite is a red­brown variety, partly altered. Grain size: 0.1 to 0.3 mm. Fabric: subophitic'!-not many frag­ments in slide. Rock: altered leuco-diabase. Shell #l·A Nelson cuttings 10330-35 Bureau of Economic Geology J\licrocline microperthite (43%), oligoc]a,e (40%), magnetite or ilmenite (5%), altered fer­romagnesian-pyroxene? ( 5%), calcite ( 5%), biotite (2%), amphibole (tr), apatite (tr). Biotite is partly altered and "ery deeply colored ; pla~ioclase is partly replaced by calcite. Grain size: 0.5 to 2 mm. Fabric : hypidiomorphic granular. Rock: syenite. Shell #1-E Scarl>orough core 7911-14 middle Shell Oil Co. Albite (79%), quartz 00%), biotite (6%), ilmenite (3%) , alkali feldspar (1 %) , leucoxene (I%), sphene (tr), apatite (tr), zircon (tr). Quartz is interstitial to feldspar subhedrons; biotite pleochroism is pale to \'ery dark brown; ilmenite is surrounded by leucoxene; apatite occurs in prisms and round grains and zircon forms halos in biotite. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: leuco-albite-quartz diorite. Shell #1-E Scarborough core 7911-14 bottom Shell Oil Co. Albite (78%), biotite 00%), hornblende (8%), epidote (2%) , magnetite or ilmenite 0%), calcite (1%l, apatite (tr), zircon (tr). Biotite pleochroi>m is pale to very dark brown­commonly it is rimmed with magnetite or ilmenite; hornblende, pleochroism yellow-green to green, is mostly altered to a colorless highly birefringent mass ; epidote is probably from alteration of hornblende; apatite is in round grains and elongated prisms. Grain size: 0.2 to 2 mm. Fabric: hypidiomorphic granular. Rock: leuco-albite diorite. Bureau of Economic Geology, The University of Texas Shell and Texas # l Collins cuttings 10370-80 Bureau of Economic Geology Microcline microperthite (54%), oligoclase (40%), altered ferromagnesian mineral (3%), calcite (2'fo), magnetite or ilmenite (l'fo), apatite (tr), zircon (tr). Oligoclase is in part in a sub-micrographic intergrowth with microcline and superficially resembles quartz; oligoclase is partly replaced by calcite. Grain size : 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: syenitc. Sinclair-Prairie #1 Grisham-Hunter cuttings 11315-22 Bureau of Economic Geology Microcline microperthite (64%), quartz (25%), albite (5%), hornblende (2%), calcite (2% ), ilmenite (l% ) , leucoxene (l% ) , sphene (tr), muscovite (tr), zircon (tr). Quartz is strained, sutured, and granulated; hornblende pleochroism is yellow-green to dark green; sphene rims ilmenite. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular-locally cataclastic. Rock: granite. Sinclair-Prairie #1 Grisham-Hunter core 11321-22 Stanolind Oil & Gas Co. Microcline microperthite (71% ), quartz (15%), biotite (5%), oligoclase (5'fo), chlorite (l'fo), muscovite (170), calcite (1%), fluorite (l'fo), magnetite or ilmenite (tr), zircon (tr). Grain size: 2 to 5 mm. Fabric: xenomorphic granular. Rock : granite. Stanolind #1 McCrea core 10475-79 Stanolind Oil & Gas Co. Microcline microperthite (62%), oligoclase (30%), calcite (3%), chlorite (4%), leucoxene (l% ) , biotite (tr), muscovite (tr), pyrite (tr}, apatite (tr). Only a few grains of microcline show microperthitic structure; oligoclase commonly occurs as twinned patches in microcline; biotite, light brown to very dark brown pleochroism, is partly altered to chlori te; leucoxene, calcite, and some chlorite are associated together in fine masses. Grain size : l to 2 mm. Fabric: xenomorphic granular. Rock: syenite. Stanolind #1 Sims core 10871-83 Stanolind Oil & Gas Co. Microcline microperthite (80%), albite (15%), leucoxene (2%), chlorite (2%), ilmenite (1% ), calcite (tr), apatite (tr) . Microcline is only sparsely microperthitic; chlorite is finely fibrous; leucoxene surrounds ilmenite. Grain size: 0.5 to 2 mm. Fabric : xenomorphic granular (feldspars are mutually interpenetrating) . Rock: syenitc. Stanolind # l Sims core 10871-83 Stnnolind Oil & Gas Co. Microcline microperthite (73%), chlorite (20%), sericite (5%), magnetite or ilmenite (2'fo), leucoxene (tr), apatite (tr). Chlorite is replacing a mineral that was graphically intergrown with feldspar (amphibole?); apatite shows a core and an overgrowth. Grain size: 0.1 to 0.5 mm. Fabric: subgraphic. Rock : microsycnite. Stanolind #1 Sims core 10871-83 Stanolind Oil & Gas Co. Microcline (69%), chlorite (30%), sericite 0%l, ilmenite (tr), leucoxene (tr). Microcline is in a subgraphic intergrowth with chlorite; chlorite is in fine granular masses as result of alteration or replacement of an unknown primary mineral-an amphibole? Grain size : 0.5 mm. Fabric : micrographic. Rock: microsyenite. Stanolind # l Sims cuttings 10880-90 Shell Oil Co. Microcline microperthite (80%), al bite (20%), chlorite (tr), zircon (tr). An exceptional development of perthitic structure with veins and patches of albite extremely varied in size oc­cu rring throughout the potassium feldspar. Grain size: 0.1 to l mm. Fabric: xenomorphic granular. Rock: lcuco-microsycnite. Stanolind # l Sims cuttings 10890-10900 Shell Oil Co. Slide is composed of a few grains of microcline microperthite and albite. Stanolind #I Sims core 10902-13 Stanolind Oil & Gas Co. Chlorite (90%), leucoxene (10% ), magnetite or ilmenite (tr), calcite (tr). Nearly all rock is fi,~]y fibrous microcrystalline chlorite. Basement Rocks, Texas-New Mexico Stanolind #1 Sims core 10913-24 Stanolind Oil & Gas Co. Serpentinized? feldspathic mass (95%), red iron oxide (4%), leucoxene 0%), chlorite (tr), ilmenite (tr). Altered feldspathic groundmass shows vague, subgraphic, and myrmekitic structures; only identifiable mineral grains are relict albite grains; alteration mineral in groundmass is apparently serpentine. Grain size: less than 0.02 mm. Fabric: metasomatic­relict microgranular. Rock : altered feldspathic igneous rock. Stanolind #1 Sims core 10924-42 Stanolind Oil & Gas Co. Microcline microperthite (67%), chlorite (30% ), leucoxene (1%), magnetite or ilmenite (1%), red iron oxide (1 o/o), calcite (tr), sericite (tr), biotite (tr), apatite (tr). Chlorite replaces amphibole; apatite is zoned. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: syenite. Stanolind # l Sims core 10942-60 Stanolind Oil & Gas Co. Microcline (69%), chlorite (30o/o), sericite 0%), magnetite or ilmenite (tr), hematite (tr), red iron oxide (tr). Grain size: 0.5 to 3 mm. Fabric: subgraphic. Rock: altered sye11 ite. Stanolind # 1 Stiles cuttings 11500 Stanolind Oil & Gas Co. Andesine (76o/o), magnetite or ilmenite 05o/o), augite? (5%), amphibole (2o/o), biotite (1%), apatite (1 % ) . Plagioclase occurs in laths, some of which are zoned; biotite pleochroism is pale brown to red-brown; amphibole occurs in long colorless needles; augite? is present as altered relicts; apatite occurs in long needles. Grain size: 0.5 mm. Fabric: hypidiomorphic granular. Rock : lcuco-microdiorite. Stanolind #3-AE University cuttings 10590-95 Bureau of Economic Geology Oli~odase or andesine (60%), myrmekite (20%), hiotite 00%) , quartz (10%), apatite (tr), calcite (tr), zircon (tr). There is only one very small fragment of basement rock in slide. Grain si1.e: 0.2 to 0.5 mm. Fabric: xenomorphic granular. Rock: leuco-quartz m.icrodiorite. Stanolind #4-AE University cuttings 10490-500 Bureau of Economic Geology Micror.line microperthite (45%), al bite (25%), quartz 05% ), biotite 05%), apatite (tr) Plagioclase is partly kaolinized; biotite is a very dark-colored green-brown variety. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Stanolind #6-PP University cuttings 10885-93 Bureau of Economic Geology Oli~oclase-andesine (34%), quartz (30%), hornblende (20%), biotite 05%), sphene 0%), ilmenite (tr), epidote (tr), apatite (tr), calcite (tr). Hornblende pleochroism is yellow-green to ~recn, biotite pleoc-hroism is pale brown to dark red-brown; quartz shows very severe strain; sphcnc envelops ilmenite. Grain size: 0.5 mm. Fabric: ranges from hypidiomorphic ~ranular­catnc.last ic in some fra~mr.nts to g:neissic in others. Some fragments are nearly all quartz and feldspar (hypidiomorphic feldspar and crushed quartz) with little hornblende; other fragments are ~neissic with oriented biotitc, hornblende, quartz, and feldspar grains and greater content of ferromagnesian minerals. Rock: quartz diorite gn eiss. ARCHER COUNTY Phillips #I Bullington core 7913-15 Bureau of Economic Geology Al bite and microcline microperthite (56%), quartz (25%), chlorite (5%), biotite (5%), epidote (4o/o), sericite (3%), muscovite(!%), sphene 0%), ilmenite (tr), calcite (tr,), apatite (tr), rhyolite fragment (tr). Plagioclase occurs in little-modified subhedral grains, partly sericitized; microcline microperthitc is partly kaolinized ; there are about equal amounts of each feldspar in the rock; quartz occurs in poorly sized angular grains and angular fragments of quartz mosaic; epidote, chlorite, sericite, and ~reen biotite occur as mixed masses between the grains of quartz and feldspar and result from reconstitution of original argillaceous matrix; sericite also occurs as result of alteration of plagioclase; muscovite occurs in fairly fresh plate•, some of which are bent, and is a second-cycle mineral; ilmenite is surrounded by sphene. Grain size: 0.2 to 0.5 mm. Fabric : relict elastic-incipient crystalloblastic (quartz and feldspar not recrystallized-low-grade metasedimentary rock). Rock: metagraywacke. Bureau of Economic Geology, The C:11it-ersity of Texas :\R~ISTRONG COUNTY Hassie Hunt Trust #I J. L. Cattle Co. cuttings 6930-70 Humble Oil & Rig. Co. Quartz and alkali feldspar (67%), ralcite (15% ), chlorite (8% ), sericite (5%), magnetite or ilmenite-and lt:"m·oxerH" (5nite orrurs as scattered grains; leuroxene i:;. pre-sent along grain boundaries and as ca\"ity lininf!S. Grain size: 0.0:.:! to 0.2 mm. Fabric: relict pyroclastic'? Rock: altered and possibly reworked rhrolite 111fJ. Hassie Hunt Trust #I J. L. Cattle Co. cuttings 6970-7000 Humble Oil & Rfg. Co. Groundmass (75'/ol, albite (10'7o ), rork fragments (4%), chlorite (3%) , microperthite (3%), leucoxrne! maf!netite or ilmenite, and red-brown iron oxide l3%), calcite (~~),apatite ltd. :\lostly eryptocrystalline groundmass shows tlowage structure outlined by serieite fibers, locally it is microspherulitic with patrhes of spherulites separated by rracks filled with later quartz and feldspar; albite and microperthite O\'Cllr as phenocrysts in rounded and corroded subhedrons; chlorite ocn1rs as caYity fillings and fibers in the groundma$$; calcite replaces part of the !!round­mass and the al bite phenocrysts: microspherulitic and micrographic grains appear to be frag­ments of earlier fornwd roek. Grain size: groundmass cryptocrystalline; phenocrysts 0.5 to 2 mm. Fabric: porphyrilic. Rock: rhyolite porphyry. Hassie Hunt Trust # l Helms cuttings 6070-6180 Humble Oil & Rig. Co. (I) Alkali feldspar (50%), quartz (44%), magnetite or ilmenite and leucoxene (3%), amphi­bole'! (2o/ol, epidole (l<;i-.l, zircon (trl, apatite (trl. Feldspar is partly kaolinized; am phi bole?, apparently secondary, occurs in minute fibers, and identification is not certain. Grain size: 0.05 to 0.1 mm. Fabric: microgranular. Rock: rhrolite. (2) Groundmass (89%), al bite (10%), magnetite or ilmenite (I%l, apatite (tr). Quartz­alkali feldspar groundmass shows Aowage structure; al bite occurs as phenocrysts. Grain size: groundmass less than 0.02 mm; phenorrysts up lo o.~ mm. Fabric: porph)Tilic. Rock : rhrolite porphyry. (3) Groundmass (77%), quartz and feldspar (15%), rock fragments (5o/o), calcite (1%), leuroxene (I%), red-brown iron oxide (I'lo), zircon (Ir), chlorite (trl . Groundmass is microcrystalline to cryptocrystalline and composed mostly of alkali feldspar and minute fibers ol amphibole? in a fine mat; the amphibole? appears lo be secondary; partly kaolin­ized feldspar and quartz occur as broken and em bayed grains; rock fragments are rhyolite. Grain size: groundmass less than 0.01 mm; feldspar and quartz fragments 0.1 to 0.2 mm. Fabric: p)Toclastic? Rork: rh_rnlite tufJ? (4) Plagioclase, quartz, calcite, sphene, zircon, magnetite or ilmenite, chlorite, rock fragments. Grain size: 0.1lo0.2 mm. Fabric : elastic. Rock: arkose (debris from Yolcanic rocks). (5) Su.mmarr: Slide is composed of a variety of rocks (rhyolite, rhyolite tuff, rhyolite porphyry, and arkose) that have in common a rnlcanic origin either directly or indirectly. (I) Talc (77'7< l , amphibole-tremolite? (20%), carbonate (3%). Talc and amphibole are asso­ Hassie Hunt Trugi #I Helms cullings 6080-6180 Bureau of Economic Geology ciated in fibrous masses: carbonate occurs in scattered grains. Grain size: 0.02 to 0.2 mm. Fabric: crystalloblastic. Rock: tremolite-talc horn/els. (2) Groundmass (72%), quartz (7%), feldspar (15%), magnetite or ilmenite (5%), chlorite (I%), apatite (tr), zircon (tr). Quartz-alkali feldspar groundmass is microgranular to mirrographic; feldspar phenocrysts are heavily kaolinized; quartz is in round and embayed phenocrysls: chlorite is alter biotite. Grain size: groundmass 0.01 to 0.1 mm. Fabric: por­phyritic. Rock: rhrolite porphyry. Hassie Hunt Trust #1 Helms cuttings 6180-6280 Bureau of Economic Geology (I) Dolomite (90%), serpentine 00%), leucoxene (tr), chlorite? (Ir). Dolomite is partly altered to serpentine; chlorite? is a brownish variety. Grain size: 0.1 mm. Fabric: metaso­matic. Rock: serpentini:ed dolomite. (2) Fragments ol siltstone showing no signs ol metamorphism but similar in other respects to the metasiltstone associated with altered dolomite in other wells in the Swisher gabbroic terrane. Hassie Hunt Trust #I Helms cullings 6180-6280 Bureau of Economic Geology (I) Dolomite, serpentine, tourmaline, nontronite?, chlorite?. Dolomite is partly altered to serpentine; masses and vein lets of pale lo reddish brown tourmaline are present in one fragment; percentages of minerals are extremely variable in different fragments. Grain size: 0.01 to 0.2 mm. Fabric : metasomatic. Rock: serpentini:ed dolomite. Basement Rocks, Texas-New Mexico (2) Fragments of siltstone showing no signs of metamorphism but similar in other respects to metasiltstones associated with altered dolomites in other wells in the Swisher gabbroic terrane. Hassie Hunt Trust #1 Helms cuttings 6180-6280 Humble Oil & Rig. Co. Dolomite, serpentine, calcite, amphibole?. Fragments contain dolomite that shows a varying degree o[ sepentinization; some fragments are composed almost entirely o[ fibrolamellar masses of serpentine which locally contain small fibers of amphibole?; serpentine is cut by veinlets of calcite. Grain size: dolomite 0.01 mm. Fabric: metasomatic. Rock: serpentinized dolomite. Hassie Hunt Trust #1 Helms cuttings 6300-6570 Humble Oil & Rig. Co. 0) Groundmass (66%), albite (20%), epidote (5%), calcite (5%), red iron oxide (3%), leucoxene (lo/o), magnetite (tr}. Groundmass is obscured by red iron oxide stain but appears to be mostly composed of partly kaolinized alkali feldspar, locally it is microspheru· litic; albite occurs as phenocrysts; calcite replaces part of groundmass; epidote occurs in veinlets; magnetite is in scattered grains partly altered to red iron oxide. Grain size: ground· mass less than 0.02 mm; phenocrysts up to 2 mm. Fabric : porphyritic. Rock: trachyte porphyry. (2) Groundmass (43%), albite (30%), rock fragments (15%), microperthite (5%), red iron oxide (4o/o), leucoxene 0%), magnetite or ilmenite 0%), calcite 0%), apatite (tr). Groundmass shows a relict vitroclastic fabric preserved in a fine granular mass of alkali feld­spar and quartz; feldspar occurs in angular grains; rock fragments are rhyolite, in part microgranular but also showing flowage structures, micrographic structures, and micro· spherulitic structures. Grain size : groundmass less than 0.02 mm; feldspar and rock !rag· ments up to 2 mm. Fabric: relict vitroclastic. Rock: rhyolite tuff. Hunt #4 Ritchie cuttings 6810-7070 Humble Oil & Rig. Co. 0) Andesine-labradorite (62%), augite (25%), chlorite (4o/o), hornblende (3o/o), alkali feld­spar (3%), magnetite or ilmenite (3%), pyrite (tr), apatite (tr}. Amphibole, green-brown, is apparently derived from pyroxene; chlorite is an olive variety with variable birefringence and may be after olivine; alkali feldspar, finely micrographic, occurs between plagioclase laths; apatite occurs in long needles. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rork: leuco·microgabbro. (2) Plagioclase (52%), augite (4-0%), magnetite or ilmenite (8%). Plagioclase is in oriented elongate laths. Grain size: 0.1 to 0.2 mm. Fabric: hypidiomorphic granular. Rock : micro· gabbro. Placid #1 Matheson cuttings 4650 Humble Oil & Rig. Co. Groundmass (84%), albite (8%), microperthite (4%), chlorite (3%), magnetite or ilmenite 0%), apatite (tr), leucoxene (tr). Microgranular quartz-alkali feldspar groundmass shows flowage and local coarse areas; microperthite and partly sericitized albite are phenocrysts; chlorite is after biotite and contains flecks of leucoxene. Grain size: groundmass 0.01 mm; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Standard of Texas #1-A Palm core 614-0-41 Bureau of Economic Geology Groundmass (89%), microperthite (5%), quartz (5%), leucoxene 0%), sphene (tr), magne­tite or ilmenite (tr), apatite (tr), albite (tr), zircon (tr). Microcrystalline-cryptocrystalline· microspherulitic groundmass shows well-developed flowage structure and local coarsenings of quartz and sericitized feldspar; microperthite and albite occur as phenocrysts but albite is mostly limited to areas of coarsened groundmass; quartz phenocrysts are embayed, corroded, and fractured; magnetite or ilmenite is present as cavity fillings and in fine lines of grains parallel to and emphasizing the flowage structure; sphene is almost completely altered to leucoxene. Grain size: groundmass cryptocrystalline to 0.2 mm. Fabric: porphyritic. Rock: rhyolite por­phrry. (Photomicrograph, PL VIII, B.) Stanolind #1 Corbin core 611B-19'h Bureau of Economic Geology Groundmass (82%), albite-oligoclase (llo/o), microperthite (4%), magnetite or ilmenite 0%), biotite 0%), chlorite 0%), zircon (tr), red iron oxide (tr). Microcrystalline microgranular quartz-alkali feldspar groundmass contains coarser "eyes" and shows vague flowage structure defined by the more coarsely crystalline material; plagioclase and microperthite are present as phenocrysts; red-brown biotite is partly altered to chlorite. Grain size: groundmass less than 0.02 mm; phenocrysts up to 2 mm. Fabric : porphyritic. Rock: rhyolite porphyry. Bureau of Economic Geology, The University of Texas Stanolind #1 Corbin core 6118-19% Stanolind Oil & Gas Co. Groundmass (79%), microperthite (10%), albite C >%),magnetite or ilmenite 0%), red iron oxide (tr). l\ticrocrystalline microgranular groundmass is composed of quartz and alkali feld· spar; microperthite and albite occur as phenocrysts. r.rain size: groundmass less than 0.02 mm; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. BAILEY COUNTY El Paso Natural Gas #1 West cuttings 8710-20 Bureau of Economic Geology Texas Mortgage and Loan Plagioclase (43%), chlorite (25%), dolomite (25%), magnetite or ilmenite (5%), biotite (2%), apatite (tr). Plagioclase is almost completely sericitized; chlorite occurs as result of alteration of ierromagnesian minerals and in cross-cutting veinlets; dolomite occurs in veinlets cutting igneous rock and as porphyroblasts in the igneous rock along the vein walls-mostly it is associated \dth magnetite or ilmenite, iron oxide, and chlorite; magnetite or ilmenite occurs as scattered grains and in veinlets; biotite shows pale to very dark brown pleochroism and is partly altered to chlorite. Grain size: igneous rock 0.5 to 2 mm; vein material 0.2 to 0.5 mm. Fabric: metasomatic. Rock: altered igneous rock-probably a gabbro. El Paso Natural Gas #1 West cuttings 8720-90 Bureau of Economic Geology Texas Mortgage and Loan Labradorite (60%), chlorite (15%), augite 00%), carbonate (7%), magnetite or ilmenite (4%). biotite (3%), iddingsite? 0%), amphibole (tr), apatite (tr). Plagioclase is partly sericitized; chlorite is derived from alteration of augite and is associated with carbonate; augite shows alteration to chlorite and carbonate in some fragments; biotite is red-brown; am phi bole relicts occur in masses of chlorite. Grain size: 0.5 to 1 mm. Fabric: subophitic. Rock : diabase. El Paso Natural Gas #1 West cuttings 8790-8800 Bureau of Economic Geology Texas Mortgage and Loan 0) Fragment composed of serpentine contammg tiny corroded dolomite remnants. (2) Fragment composed of serpentine containing irregular separated masses of talc. (3) Fragment composed of dolomite with indistinct grain boundaries. ( 4) Fragment of dolomite partly altered to serpentine; the serpentine contains minute am­ phibole needles and pyrite cubes. Grain size: less than 0.05 mm. Fabric: metasomatic-most of slide is composed of fragments of 0) above. Rock: serpentinized dolomite. El Paso Natural Gas #1 West cuttings 8800-8950 Bureau of Economic Geology Texas Mortgage and Loan Plagioclase (54%), augite (25%), chlorite 00%), magnetite or ilmenite (6%1, biotite (3%), amphibole (2% l, calcite (tr), epidote (tr), apatite (tr). The plagioclase, probably originally calcic, is altered to scricite or sericite and chlorite; augite is predominantly a violet-brown· tinted variety but locally there are patches of colorless augite in optical continuity with the colored grains ; chlorite occurs as both an olive-drab and a green variety; biotite is red-brown; amphibole includes a green secondary am phi bole and relicts of green-brown primary mineral; apatite is in grains and needles. Grain size: 0.2 to 2 mm. Fabric: hypidiomorphic granular. Rock: gabbro. El Paso Natural Gas #1 West cuttings 8960-9050 Bureau of Economic Geology Texas J\fortgage and Loan Labradorite (65%1, augite 02%), magnetite or ilmenite (6%), olivine (5%), biotite (5%), iddin{!site''-chlorite·mica (3%), chlorite (2%), amphibole 0%), apatite 0 %). Zoned labrador· ite is in an ad\'anced stage of sericitization; olivine is partly altered to a mass of what seems to be iddingsite·mica·chlorite; both green-brown and red· brown biotite are present; amphibole is a secondary green variety. Grain size: 0.2 to 2 mm. Fabric: hypidiomorphic granular. Rock: leuco·olivine gabbro. El Paso Natural Gas #1 West cuttings 9050.90 Bureau of Economic Geology Texas Mortgage and Loan Quartz-feldspar-sericite·chlorite mass (71%), carbonate 05%), quartz 00%), feldspar (3%), leucoxene (!%),magnetite or ilmenite (tr), apatite (tr), zircon (tr). Fine matrix of quartz­feldspar.sericite-chlorite contains some fine crystalloblastic needles that may be amphibole; Basement Rocks, Texas-New Mexico carbonate occurs in patches with indistinct grain boundaries and as vein lets; feldspar and quartz are present as angular grains in the matrix; mineral percentages are highly variable in di£Terent fragments. Grain size: quartz and feldspar 0.1 mm; quartz-feldspar-sericite-chlorite matrix less than 0.02 and mostly less than 0.01 mm. Fabric: elastic-vague indications of beginning meta· morphism. Rock: calcareous sandy metasiltstone. El Paso Natural Gas #1 West cuttings 9090-9100 Bureau of Economic Geology Texas Mortgage and Loan Labradorite (61%), augite (25%), olivine (5%), chlorite (5% ), magnetite (3%), pyrite (1%). Zoned plagioclase is in advanced stage of sericitization; augite is lavender-brown and probably titaniferous; chlorite is an olive highly birefringent variety probably derived from alteration of olivine; pyrite is in places intergrown with magnetite. Grain size: 1 mm. Fabre: hypidio· morphic granular. Rock : olivine gabbro. El Paso Natural Gas #1 West core Bureau of Economic Geology Texas Mortgage and Loan Labradorite? (64%), augite 05%), chlorite (15%), magnetite or ilmenite (6%), apatite (tr). Plagioclase is in advanced stage of sericitization; augite is in large skeletal crystals; magnetite or ilmenite contains hematite lamellae; two types of chlorite are present: (a) low-birefringent green chlorite and (b) a moderately birefringent olive-brown chlorite. Grain size: 1 to 5 mm. Fabric: hypidiomorphic granular. Rock: leuco·gabbro. Lion #1 Bridwell cuttings 8750-8810 Bureau of Economic Geology (1) Fragments of calcareous arkose, limestone, and dolomite that show no evidence of meta· morphism. (2) One fragment of dolomite that shows evidence of weak metamorphism-rhombic fabric is blurred and rhombs lose their integrity; the rock is spotted with tiny points of brown iron oxide. Lion #1 Bridwell cuttings 8810-8950 Bureau of Economic Geology Similar to other slides in this interval-rock is an altered leuco-gabbro. Lion # 1 Bridwell cuttings 8815-8948 Bureau of Economic Geology Oligoclase (69%), chlorite 00%), augite (8%), am phi bole? (7%), magnetite or ilmenite (3%), calcite (3%1, sphene (tr), apatite (tr). Zoned plagioclase is partly sericitizcd, partly chloritized, and, locally, altered to fine-grained masses of epidote-sodic character results from saussuritization of a more calcic variety; chlorite occurs in granular masses from alteration of plagioclase and in sheaf-like masses from alteration of a ferromagnesian mineral-probably augite; amphibole? is colorless and probably secondary. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular-metasomatic. Rock: altered (saussuritized) leuco-gabbro. Lion # 1 Bridwell core 8951-53 Humble Oil & Rig. Co. Labradorite (68%) , augite (8%), ch lo rite (8%), olivine (6%), serpentine (5%), magnetite or ilmenite (4%J, biotite (1%), apatite (tr). Zoned plagioclase is partly sericitized; olivine occurs in large masses and is mostly altered to serpentine and an olive-colored chlorite; augite and olivine accommodate themselves to plagioclase; a green low-birefringent chlorite and an olive-colored moderately birefringent chlorite are commonly mixed; probably the latter is an intermediate stage between a primary ferromagnesian mineral and the common chlorites; biotite is a pale orange-brown variety that fringes augite and the opaque mineral; the opaque min­ eral, magnetite or ilmenite, occurs locally in skeletal or dendritic masses; apatite is in long needles. Grain size: 1 to 5 mm. Fabric: hypidiomorphic granular. Rock: lcuco-olivine gabbro. Phillips #l·A Stevens cuttings 8190-8200 Bureau of Economic Geology Groundmass (83%), chlorite (8%), altered feldspar (3%), leucoxene (3%), sericite (2%), Cryptocrystalline to microcystalline quartz-alkali feldspar groundmass is locally microspher­ulitic and locally shows coarsenings; quartz, microperthite, and plagioclase? form phenocrysts. Grain size: groundmass mostly less than 0.01 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyoliie porphyry. Bureau of Economic Geology, The Unfrersity of Texas Shell # 1 '.\ichols cuttings 9030-40 Shell Oil Co. Groundma..s (6-1;<-l. plagioclase (15'.C l. sericite •.10'< l. chlorite 15'.Cl. calcite (5';<) . leu. coxene (l ~), zircon (tr). apatite (tr). ~licrocrystalline microgranular groundma..'5 of quartz and fddsp3r is p3rtJy sericitized: plagiorlase pht""nocrysts are in adva1wed stage of sericitiz.a· tion. Grain s.ize: gr0tmdmas.s 0.0:::! mm: phenocrysts up to 0.5 mm. Fabric: porphyritic. Rock: altered rh>·olite p<>rphr.L BRE\\STER COl"'.\TY Dodson (Hinton l ::1 Texas cuttings 4513-23 : 4-00'.!-07: Stanolind Oil & Gas Co. _\merican Syndicate 46'.!l·'.!4: 46-13·50~ 4658.71 ..\ series of slides containmg small fragment;. \lic.rodine microperthite and albite (97%\, an.alcitt> 13~). amphibole lt; 1. rnagnetit.t" 1.)r ilmenite (tr I. apalite 1tr). cakitt" ctr). Feld:o.par is. partly kaolinized: microrline mirroperthite i~ the dominant feld5'par in mo5't ~Iides : apatite ocC'u~ in needle~. Grain :_;.ize: 0.5 mm. Fabric: hypidit,morphic granular. Rock: analcitc micro­s1·enite. [Tenia,,·->-not basement.] Dod;on (Hinton l ::1 Texas cuttings 6100·6-IOO Bureau of Economic Ge-ology :\merlcan Syndic..lt~· ..\lbite and alkali feld;par 184'.C l. quartz \IO;<-l. albli amphibole--arfwd><>nite"? (3'C\. cal· cite (~<(-) . aegirine q :C \. zirCt'n \tr 1• fluorite 1. trl. magnetite or ilmenite 1 tr). Feld:'p3r is probably mostly alkali feldspar. zoned. and psrth-kaolinized: quartz is inter>titial to feldspar laths: laths of feldspar C<>mmt>nl,· show a parallel structure. Grain size: feldspar laths O.'.! x 1 mm maximum. Fabric: ranges from hypidiomorphic granular to trachytoid. Rock : aegin"n~ ar.frendsonite? micrtlgranitt?. [Tertiary?-not L~ement.] BRISCOE COl''.'>TY Amerada #I Hamilton cutting.~ 8775 Bureau of Economic Geology Oligoclase-andesine (4~'.C l. lwrnblende (35''(·l. quarlz (!'.!<(-\. bit>tite (IO'.C l. chlorite 0 %), ~phene •.tr'· apatite 1tr1. Plag:ioclax> i:o. almost C'l'ntpletely :::.eridtized: blne·grt"'t'n hornblende i:; partly ahert"d h' chlorite: biotite pleochwi::.m i~ pale brown to brown: in some fragments feld::.par i:' cru~hed and ~ranulated and hflrnblendf' i::' fr~1yed. Grain s.ize: O.~ to ~ mm. Fabric: hypidiomorphic granular with cataclastic element:>-. Rock: quen amphibole l'fcurs as small prism:_;. in ma~.,;:,es of chlorite and is probably seconda.,·. Grain size: 0.5 to '.! mm. Fabric: hypidiomorphic granular. Rock: /euco·gabbro. Hunt #1 Ritchie cuttings 7510·90 Humble Oil & Rig. Co. ..\ndesine·labradorite 161 'Cl . chlorite I l~;<-l. alkali feld;par t 8;<-l. augite 18'.C \, magnetite or ilmenite 14'.C '. amphibole 13;<-l. iddingsite? (3'.C l, biotite (1r;. l. apatite (trl . Pla~iodase is zoned and partIv ;ericitized: locally it has alkali feld;par rims: colorles..< seconda"· ~mphibole occur:; in ~heave:o. or felt~ of small prism.s: biotite, red-bro'fliD. rims maµ-netite or ilmenite; idding-s ite"? apparently indicates the former p~nre of olhine: apatite i:o. in nee-dle5 or prisms. Grain size: 1 to 3 mm. Fauric : hypidiomorphic granular. Rock: altered lt>uco-gabbro. Basement Rocks, Texas-New Mexico Hunt #1 Ritchie cuttings 7590-7635 Bureau of Economic Geology (l) Talc (78%), dolomite (20%), serpentine (2%) . Patches of dolomite orcur in a mass of talc with minor serpentine. Grain size: dolomite 0.1 mm. Fabric: metasomatic. Rock: dolomite-talc rock. (2) Quartz, calcite, feldspar, bleached biotite, chlorite, magnetite or ilmenite, leucoxene, apa­tite, zircon, amphibole?. Percentages of minerals vary widely in different fragments. Chlorite occurs in tufts: amphibole? occurs as tiny needles sprouting through the rock and orcurring within calrite, quartz, and feldspar. Grain size: 0.05 to 0.2 mm. Fabric: relict elastic-incipient crystalloblastic. Rock: meta-arkose. Hunt #1 Ritchie cuttings 7590-7610 Humble Oil & Rfg. Co. Dolomite, talr, rhert, diopside?. Slide shows fragments of dolomite, dolomite partly altered to talc, and dolomite almost completely altered to talc. Locally dolomite shows (l) linear zones that have more or less optical rontinuity and may be solution paths, (2) large equi-extinguish­ing masses with inclm,ions of more or less rhombic dolomite without optical continuity, (3) recrystallization into elongate grains. Sporadic 1trains of diopside? occur within the dolomite. One fragment of recrystallized chert is present. Grain size: less than 0.1 mm. Fabric: crystallo­blastic (granoblastic to sieve). Rock: metadolomite. Hunt #1 Ritchie cuttings 7610-50 Humble Oil & Rfg. Co. (l) Dolomite 000%), diopside (tr), pyrite (tr). Grain size : 0.02 to 0.1 mm. Fabric: grano­blastic. Rock: metadolomite. (2) Groundmass, quartz, carbonate, pyroxene?. Brown groundmass partly replaced by carbonate is composed in part of small high relief laths; quartz grains appear to be phenocrysts; pyroxene? also appears to be a phenocryst (only one tiny fragment of this rock in the slide). Grain size: groundmass less than 0.02 mm ; phenocrysts up to 0.2 mm. Fabric: porphyritic?. Rock: volcanic rock. (3) Quartz, feldspar, carbonate, magnetite-ilmenite, red iron oxide. Quartz occurs as large round pebbles in a finer matrix of quartz and feldspar stained with iron oxide. In some fragments there are masses of recrystallized spherulitic chert and masses of carbonate. Grain size: matrix less than 0.1 mm; pebbles up to 2 mm. Fabric: essentially elastic-matrix does not appear to have recrystallized; some signs of weak metamorphism. Rock: feldspathic metasandstone. Hunt #1 Ritchie cuttings 7650-7710 Humble Oil & Rig. Co. Groundmass (70%), quartz and feldspar (15%), carbonate (5%), magnetite or ilmenite, leucoxene, and red iron oxide (7%), amphibole? (3o/o), bleached biotite (tr), tourmaline (tr), apatite (tr), zircon (tr) . Fine brown groundmass is composed mostly of fine quartz and clay minerals with some incipient sericite; angular fragments of quartz, plagioclase, and microcline occur throu1thout the groundmass; patches of carbonate are common; amphibole? occurs as bundles of fine needles or hairs of crystalloblastic habit in quartz, carbonate, and feldspar. Mineral percentages vary considerably in different fragments. Grain size: groundmass less than 0.02 mm : fragments! quartz-feldspar ranges up to 0.2 mm. Fabric: elastic with incipient crystallo­blastic elements (sieve). Rock: fragments range from argillite to weakly metamorphosed siltstone and sandstone. Hunt #1 Ritchie cuttings 7710-20 Humble Oil & Rig. Co. (l) Dolomite, diopside, tremolite, serpentine. Diopside, tremolite, and relict dolomite occur in a mosaic and all are partly replaced by serpentine. Grain size: 0.2 to l mm. Fabric: rrystallo­blastic ( 1tranoblastic to sieve). Rock : diopside-tremolite-dolomite horn/els. (2) Relict altered dolomite grains that locally contain concentrations of tremolite occur in a mass of serpentine. Serpentine shows outline of old dolomite mosaic. Hunt #1 Ritchie cuttings 7720-40 Humble Oil & Rfg. Co. Same as 7650-7710 but with more sericite in groundmass and no amphibole? needles. Hunt #1 Ritchie cuttings 7635· 7730 Bureau of Economic Geology Similar to 7590-7635 (2) . Bureau of Economic Geology, The University of Texas Hunt #1 Ritchie cuttings 7730-45 Bureau of Economic Geology Dolomite, serpentine, tremolite. Older stained and discolored dolomite occurs with clear re­crystallized carbonate; fibrous tremolite has formed at the expense of dolomite in some frag· ments. In one fragment tiny fibrous amphibole? occurs in serpentine; granular patches of a semi-opaque mineral form small rings filled with serpentine and give the slide a pocked appear· ance. Grain size: 0.05 to 0.2 mm. Fabric: crystalloblastic (sievcl-metasomutic. l\ork: serJJC1'tin· i:ed tremolite-dolomite horn/els. Hunt #1 Ritchie cuttings 7740-60 Humble Oil & Rig. Co. Same as 7650-7710. Hunt #1 Ritchie cuttings 7760-70 Humble Oil & Rig. Co. Dolomite, serpentine, talc, tremolite. Dolomite is locally sheared, recrystallized, and altered to tremolite, talc, and serpentine; some late carbonate in veinlets cuts metamorphic minerals. Grain size: 0.05 to 0.2 mm. Fabric: crystalloblastic (sieve)-metasomatic. Rock: serpentinized talc· dolomite rock. Hunt # l Ritchie cuttin gs 7770-90 Humble Oil & Rig. Co. Same as 7650-7710. Hunt #1 Ritchie cuttings 7790-7890 Bureau of Economic Geology Plagioclase (57%), hornblende (25%), alkali feldspar (5%), magnetite or ilmenite (5%), chlorite (4%) , biotite (3%), apatite 0 % ), epidote (tr), sphene (tr), augite (tr). Zoned pla~ioclase is in an advanced stage of sericitization ; yellow-green hornblende occurs as large grains and as felts of small prisms; some colorless am phi bole surrounds pyroxene relicts; partly kaolinized alkali feldspar occurs as anhedral grains and locally rims plagioclase; biotite is red· brown; apatite occurs partly in long needles and partly in more or less equant grains; a few grains of epidote are present as result of alteration of feldspar. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock : altered diorite. Hunt #1 Ritchie cuttings 7790-7890 Humble Oil & Rig. Co. Sodic plagioclase (60%), augite (15%), magnetite or ilmenite (8%), chlorite (7%), amphibole (7%), biotite (3%), sphene (tr), apatite (tr). Plagioclase is almost completely sericitized, but there is some unsericitizcd sodic. pla~ioclase-albite'?; sodic nature of the plagioclase is probably due to alteration of a more calcic variety; augite is partly altered to amphibole, with pale green· brow n to ~rcen pleochroism, which in turn is partly altered to biotite; chlorite is in granular mass.rs : ma~netite or ilmenite i~ loca lly frin~ed with red-brown biotite. Grain size: up to 1 cm. Fabric: hypidiomorphic granular. Ho<'k: altered lcuco-albitc' gabbro. Hunt #2 Ritchie cuttings 7130-7560 Bureau of Economic Geology Plagioclase (62%), biotite 00% ), augite (8%), chlorite (7%), magnetite or ilmenite (5%) , olivine (5%) , amphibole (2%), apatite 0 %), calcite (tr), unidentified secondary mineral (tr). Plagioclase is variably sericitized in different fragments and shows zonation from labradorite cores to sodic rims; red-brown biotite occurs as a fringe around opaque mineral and as large plates; augite is locally a very deep brown color; greenish-yellow chlorite replaces feldspar; olivine is locally partly altered to an olive-brown highly birefringent mineral which in turn is altered in part to a pale brown highly birefringent mineral; amphibole is secondary; apatite is in larµ:e grains. A moderately bire£ringent mineral with moderate relief and positive optic sign occurring within altered plagiorlase was not identified. Grain size : 1 to 3 mm. Fabric: hy pidio· morphic granular. Rock: leuco-olirine gabbro. Hunt #2 Ritchie cuttings 7590-7710 Bureau of Economic Geology Dolomite 186~(;), serpentine 00% ), anhydrite (3'/o), talc (J % ) . Dolomite is variably altered to serpentine in different fraJ!ml'nts; large porphyroblastic anhydrite crystals are present in one fragment: !orally dolomit<' i> altt•rrd to talc. Grain size: 0.02 to 0.1 mm; anhydrite up to 4 mm. Fabric: metasomatic. l\ock: scrpentinized dolomite. (Photomicrograph, Pl. IX, C.) Hunt #2 Ritchie cuttin gs 7630-40 Humble Oil & Rig. Co. Andesine-lahradorite (66%), chlorite (8%), hiotite (7%), augite (5%), olivine (5%), magne­tite or ilmenite (4%), apatite (2%) , amphibole (2%), carbonate 0 %), pyrite (tr), epidote Basement Rocks, Texas-New Mexico (tr). Partly sericitized zoned plagioclase ranges from albite through oligoclase, most is albite; there are three varieties of chlorite--green low-birefringent, olive low-birefringent, and olive moderate· to high-birefringent-these seem to be stages in the alteration of the ferromagnesian minerals; very red-brown biotite in part envelops opaque mineral and in part is fringed with amphibole; apatite is in very large grains. Grain size: 1 to 3 mm. Fabric : hypidiomorphic granu· lar. Rock: leuco-olivine gabbro. Hunt #2 Ritchie cuttings 7640-60 Humble Oil & Rig. Co. Groundmass (90%), magnetite or ilmenite, quartz, and feldspar (10%). Brown fine-grained groundmass is an aggregate of quartz, feldspar, and clay minerals showing variable degree of reconstitution to sericite-sericite occurs in tiny fibers locally showing parallelism; magnetite or ilmenite is in fine scattered grains; quartz and feldspar occur in angular sand and silt-sized grains. Grain size: groundmass less than 0.02 mm; fragmental quartz and feldspar up to 0.1 mm. Fabric: elastic with incipient crystalloblastic elements. Rock: ranges from argillite to meta· argillite. Hunt #2 Ritchie cuttings 7660·7700 Humble Oil & Rig. Co. Dolomite, diopside, tremolite, serpentine. Minerals are present to varying degree in different fra~ments; tremolite ocn1rs as corroded grains in an almost completely serpentinized fragment; diopside is locally altered to serpentine. Grain size: dolomite about 0.1 mm; diopside up to 1 mm. Fabric: crystalloblastic (sieve). Rock: serpentinized diopside-dolomite horn/els. Hunt #2 Ritchie cuttings 7700-20 Humble Oil & Rfg. Co. Groundmass (82%), alkali feldspar (10%), clinozoisite (3%), magnetite or ilmenite, leucoxene, red iron oxide (3%1, calcite (!%), biotite (!%),apatite (tr). Microcrystalline to cryptocrys· talline groundmass is composed mostly of microgranular alkali feldspar and tiny prisms of clinozoisite; phenocrysts are partly kaolinized alkali feldspar and partly oxidized biotite. Grain size: groundmass cryptocrystalline to 0.05 mm; phenocrysts 0.5 to 1 mm. Fabric: porphyritic. Rock: altered trachyte porphyry. Hunt #2 Ritchie cuttings 7710-20 Bureau of Economic Geology Groundmass (86%), alkali feldspar (8%), magnetite or ilmenite (3o/o), leucoxene (!%),quartz 0%), calcite 0 % ), scheelite? (tr), red iron oxide (tr), apatite (tr). Brown-stained ground· mas.' consists of a cryptocrystalline mass, probably alkali feldspar (on basis of relief), containing needles and short prisms of am phi bole (10%) and clusters and masses of fine epidote grains that outline flowage structure (4o/o); alkali feldspar phenocrysts are in an advanced stage of kaolinization; quartz and calcite occur in a cross-cutting vein Jet; magnetite or ilmenite occurs in tiny scattered grains; red iron oxide forms a pervading stain; scheelite? replaces plagioclase adjoining quartz-calcite veinlet and also occurs in the veinlet. Grain size: groundmass mostly cryptor.rystalline; amphibole needles up to 0.2 mm long; epidote grains mostly less than 0.05 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: altered trachyte porphyry. Hunt #2 Ritchie cuttings 7720-50 Humble Oil & Rfg. Co. Groundmass (75%), alkali feldspar and albite 05% ), magnetite or ilmenite, lcucoxene, red iron oxide (5%), clinozoisite and epidote ( 4%), quartz (1 % ) , axinite? (tr), apatite (tr). Cryptocrystalline to mirroszranular groundmass shows relict vitroclastic structure with the out­ line of the devitrified shards plainly visible; red iron oxide stain, grains of magnetite or ilmenite, and blobs of leucoxene occur throu~hout; albite and alkali feldspar occur as partly seriritized "phenocrysts"; clinozoisite (3%) and epidote 0 %) occur as masses and tiny prisms throughout groundmass; the rock is cut by a quartz-calcite vein let. Presence of clinozoisite and axinite? may indicate beginnings of metamorphism. Grain size: groundmass up to 0.02 mm; feldspar up to 1 mm. Fabric: relict vitroclastic. Rock: trachyte? tuff. Hunt #2 Ritchie cuttings 7720-60 Bureau of Economic Geology Groundmass (84%), microperthite (12%) , magnetite or ilmenite (2%), leucoxene (1%) , red iron oxide (1% ) , quartz (tr), calcite (tr). Most of the groundmass is obscured by red iron oxide, kaolinite, or masses of fine sericite, locally it is cryptocrystalline, in some fragments it appears to be vitroclastic, in some fragments it appears to be microlitic; microperthite occurs as partly kaolinized phenocrysts; one small quartz phenocryst is present. Grain size: groundmass rrypto­crystalline to 0.02 mm: phenocrysts up to 1 mm. Fabric: porphyritic?, vitroclastic?. Rock: apparently fragments of both tuff and flow rocks are present. Burt>au of Economic Geology, The University of Texas Hunl #10 Ritchie <'Uttings i920-8165 Humble Oil & Rfg. Co. \ll Labradorite \i5'7cl, augite \8<;(-l, olh·ine (5%), magnetite or ilmenite (4o/o), talc (4%), rhlorite \:!'."( I, biotite \)C( 1, pyrite \ 1 '7c l, apatite llr l. Pla~iodase is zoned and partly $eri~·itized. outer zone~ may be and~ine or more sodir varieties.; talr '? or possibly mica? O<'<'UI> in fibrou> masses with rhlorite and appea1> lo ~ partly altered to rhlorite: red­ brown biotite. Joral1y ahert"d. to a green variety. occurs around magnetite or ilrnrnite. Grain sizt-: 0.5 to ~ mm. Fabric: hypidiomorphir i:ranular. Rock: /euco-olii-ine gabbro. \~.l Plagioda>e \i5'/c l. biotite \BC(\. magnetite or ilmenite \i'lo). augite (4o/o), rhlorite ( ~c( 1, epidote \:!'."(-\. Zoned and partly ,oeriritized plagioda..rrete grains and strings of grains and is easil~· ronfu,oed with augite. Grain sire: 0.2 to 0.5 mm. Fabric: subophitic. Rock: leuco-albite diabase. ~lidslates #I Hickok & Reynolds cuttings 8120-50 Shell Oil Co. Labradorite? \i in brecciated fragments: alkali feldspar is probably microperthite; fine granular epidote replaces large volumes of lhe rock; calcite occurs in brecciated fragments ; biotite is green-brown and partly altered to chlorite: hornblende pleochroi;;m is yellow-green lo green; red iron oxide and magnetite occur in ,·einle~ in the shattered fragments .. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphir granular with local cataclasis. Rock: granodiorite. Dunnigan ;::: I Ellis cuttings 5320-65 Stanolind Oil & Gas Co. Groundmass •90'."(-l, alkali feldspar? 15%), quartz (5'7c ·1, magnetite or ilmenite (Ir), red iron oxide \trl. Groundma..;;;; is stained and altered (kaolinized\ and in one fragment shows a micrographic fabric: >econdary quartz fills ,·esicles: sericitired feldspar phenocrysts are so altered a> to be unidentifiable. Grain size: groundma..;;;; 0.02 to 0.05 mm; phenorrysts up 10 l mm. Fabric: porphyritic. Rock: rh)·o/ite porphyr)·. Shamrock ;=I Thompson cuttings 3110-15 Bureau of Economic Geology :\ndesine-labradorite \58% l, augite (25'7c), magnetite or ilmenite (IO'}'o), chlorile (5o/o), alkali feld;;par 1~r;.1, biotite (trl, hornblende (tr.l, sphene (trl, apatite (trl. Partly sericit­ized pla!!iocla.-e >how> an indistinct zoning: augite is obscured by unidentified alteration prod· urts: oJi,·e-drab rhlorite is finely fibrous: alkali feldspar is interstitial to plagioclase and localJ~· s.ho"·s H'ry fine micrographic structure v.·hich indicates the presence of minor quartz; biotite is red-brown: hornblende is green-brown and mantles augite; apatite is in myriad needles. Grain >ize : 0.5 to 2 mm. Fabric: hrpidiomorphic granular. Roel<: gabbro. Basement Rocks, Texas-New Mexico Shamrock #1 Thompson cuttings 3130-35 Bureau of Economic Geology Labradorite (56o/o), augite, hornblende, biotite, chlorite, and mica '! (35%), magnetite or ilmenite (6o/o), alkali feldspar and quartz (2o/o), unidentified needles (1% ). Twinned plagioclase laths show a more or less radial pattern; corroded augite relicts are mostly altered to a green-brown finely fibrous mineral with moderate birefringence that may be a mica; green-brown hornblende is associated with the altered pyroxene; interstitial alkali feldspar and quartz are late-crystallizing minor constituents. Grain size: 0.2 to l mm. Fabric : subophitic. Rock: altered diabase. Shamrock #1 Thompson cuttings 3156-59 Bureau of Economic Geology Albite and alkali feldspar (87o/o), quartz (8o/o), magnetite or ilmenite (3o/o), hornblende (2% ), bleached biotite (tr), calcite (tr) , sphene (tr), apatite (tr), zircon (tr). Plagioclase is partly sericitized and partly kaolinized; alkali feldspar is kaolinized; hornblende is green-brown. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock : albite grarwdiorite. Stanolind # 1 Griffin core 5602-04 Stanolind Oil & Gas Co. Groundmass (85%) , quartz (8%) , microperthite (7%) , calcite (tr), sphene ( trl, leucoxene (tr). Microcrystalline groundmass shows flowage, micrographic, and microspherulitic structures; microperthite and quartz occur as phenocrysts, and the quartz, in particular, is rounded and embayed. Grain size : groundmass less than 0.02 mm ; phenocrysts up to 3 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Stanolind # l Griffin core 5613-14 Humble Oil & Rfg. Co. Groundmass (85o/o) , microperthite (9o/o), quartz (6o/o), red iron oxide (tr) , magnetite or ilmenite (tr), sphene (tr}, leucoxene (tr) , zircon (tr) . Stained groundmass shows pronounced flowage and contains (1) large and small spherulites and spherulitic areas, (2) cryptocrystalline areas, (3) lenses and layers of coarser quartz-alkali feldspar, (4) micrographic areas ; quartz and microperthite occur as phenocrysts with the quartz in general rounded and embayed. Grain size: ground mass cryptocrystalline to 0.1 mm ; phenocrysts 0.4 to 1 mm; spherulites up to 2 mm in diameter. Fabric : porphyritic. Rock: rhyolite porphyry. Stanolind #1 Griffin core 5613-14 Stanolind Oil & Gas Co. Groundmass (85%), microperthite (10% ), quartz (5% ). Similar to 5602-04. Microcrystalline groundmass shows flowage, microspherulitic, and micrographic structures; feldspar phenocrysts are subhedral, quartz phenocrysts are round and embayed. Grain size: groundmass less than 0.02 mm; phenocrysts up to 2 mm. Fabric : porphyritic. Rock : rhyolite porphyry. Stanolind # l Griffin core 5646-4 7 Humble Oil & Rfg. Co. Groundmass (84% ) , quartz (9o/o), microperthite (6%), leucoxene (lo/o), sphene (tr), mag­netite or ilmenite (tr), apatite (tr). Cryptocrystalline to microcrystalline groundmass is locally micrographic and microspherulitic and contains lenses of coarser quartz and alkali feldspar; flowage is prominent and the whole is stained with ocherous iron oxide ; quartz occurs as round and embayed phenocrysts; microperthite phenocrysts show incipient kaolinization; sphene is mostly altered to leucoxene. Grain size : groundmass cryptocrystalline to 0.1 mm; phenocrysts 0.3 to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Stanolind # 1 Griffin cuttings 5646-48 Bureau of Economic Geology Groundmass (85%), quartz 00%), microperthite (5% ), magnetite or ilmenite (tr), red iron oxide (tr} . Brown partly kaolinized groundmass is cryptocrystalline to microcrystalline and, locally, microspherulitic; flowage is well developed. Quartz is in em bayed and corroded phenocrysts; microperthite phenocrysts are kaolinized. Grain size: groundmass less than 0.02 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Tipton & Waggoner #1 McConnell cuttings 3060 Bureau of Economic Geology Labradorite (66o/o), augite (10%), amphibole 00%), magnetite or ilmenite (8o/o), alkali feldspar (5%), quartz Oo/o), biotite (tr), apatite (tr). Plagioclase is variably sericitized in different fragments and shows a vague zoning; augite is a brown-tinted variety partly altered to secondary green amphibole; there are two kinds of am phi bole in the slide-a secondary green amphibole is predominant but a green-brown primary amphibole is also present ; alkali feldspar micrographically intergrown with quartz occurs between plagioclase grains; biotite, yellow-brown to brown pleochroism, is associated with the green amphibole. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: altered leuco-microgabbro. Bureau of Economic Geology, The University of Texas Tipton & Waggoner # 1 McConnell cuttings 3068 Bureau of Economic Geology Similar to 3060. CASTRO COUNTY Anderson-Prichard # 1 Fowler-McDaniel cuttings 9650 Humble Oil & Rfg. Co. Plagioclase (61%), chlorite (10%), olivine (8%), augite (5%), iddingsite (5%), biotite (4%) , amphibole (3%), magnetite or ilmenite (3%), talc? 0%), apatite (tr). Zoned and partly sericitized plagioclase has calcic cores Oabradorite) and sodic rims (albite) ; chlorite is composed of two varieties-a green low-birefringent chlorite (2%) in granular masses and an olive low-birefringent chlorite (8%) in fibro-lamellar masses locally showing a mesh structure after olivine; red-brown biotite fringes opaque minerals and olivine ; talc? or mica is a fibrous mineral apparently representing an intermediate alteration stage between primary ferromag­nesian mineral and chlorite; three kinds of amphibole are present-pleochroism pale green to green (probably a secondary variety), a variety with brown to green pleochroism, and a red­brown variety associated with red-brown biotite. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular. Rock : leuco-olivine gabbro. Sun # 1 Haberer cuttings 8890-8900 Shell Oil Co. Labradorite (66%), magnetite or ilmenite (15%) , chlorite (10%), epidote (8%), apatite (1 % ) , biotite (tr). Partly kaolinized plagioclase is in twinned laths; original ferromagnesian minerals are altered to chlorite and epidote ; biotite is almost completely altered to chlorite. Grain size: 0.2 to 0.5 mm with some plagioclase laths up to 2 mm long. Fabric: subophitic. Rock: altered diabase. Sun #1 Haberer cuttings 8890-8900 Shell Oil Co. Plagioclase (60%), chlorite. (39%), ]eucoxene 0%), magnetite or ilmenite (tr). Rock is thoroughly altered but plagioclase is preserved as twinned laths. Grain size: 0.2 mm. Fabric: probably ophitic or subophitic. Rock: altered diabase?. Sun # 1 Haberer cuttings 8900-10 Shell Oil Co. Labradorite (75%), albite (14%), chlorite (5%), augite (2%), calcite (2%), biotite (1%), apatite (1%l. Partly sericitized calcic plagioclase (!abradorite) is mantled with more sodic p]agioclase (albite) which shows more advanced kaolinization; biotite is a reddish-brown variety partly altered to chlorite. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: leuco-microgabbro. Sun #1 Haberer core 9124-25 Bureau of Economic Geology Groundmass (91%), alkali feldspar and plagioclase (4%), leucoxene (3%), red iron oxide (2%), zircon (tr), calcite (tr\, rock fragments (tr), sphene (tr) . Groundmass is composed of irresolvable greenish material showing flowage structure and containing elongate lenses and blebs of colorless cryptocrystalline material with low index of refraction. Grain size: cryptocrystalline to 0.01 mm. Fabric: microgranular. Rock: altered volcanic rock (probably extrusive). Sun # 1 Herring cuttings 9730·90 Bureau of Economic Geology (I) Fragments of rrinoidal limestone. (2) Fragment of altered gab bro. (3) Quartz, feldspar, fine sericite·chlorite-quartz.fe!dspar mass, magnetic or ilmenite, leucox­ene, carbonate, rock fragment. Quartz occurs mostly in angular fragments but some are sub-round; feldspar is present as altered fragments; magnetite or ilmenite is finely dis­seminated ; fine sericite-chlorite-quartz-feldspar mass shows a rude foliation in one frag­ment; rock fragments are micrographic rhyolite; mineral percentages vary widely in different fragments. Grain size : fine sericite-chlorite-quartz-feldspar mass is less than 0.02 mm; quartz and feldspar fragments 0.2 to 0.4 mm. Fabric: elastic with incipient crystalloblastic elements in some fragments. Rock: mostly arkose and sandstone showing vague indications of metamorphism with one fragment of clay· slate. Basement Rocks, Texas-New Mexico Sun # 1 Herring cuttings 9790.10065 Bureau of Economic Geology Labradorite (48%), augite (33%), magnetite or ilmenit~ (8% ), chlorite (4%), biotite (2%), amphibole (2%), serpentine (2%), leucoxene (1 % ) , rutile (tr), pyrite (tr), apatite (tr). Zoned plagioclase shows advanced sericitization and, locally, indications of development of more sodic phases, probably through alteration; augite includes irregular or skeletal masses of magnetite or ilmenite; am phi bole is a secondary green variety; biotite is red-brown and fringes grains of the magnetite or ilmenite. Grain size : 2 to 4 mm. Fabric: hypidiomorphic granular. Rock: gabbro. Sun #1 Herring cuttings 10065-10135 Bureau of Economic Geology (1) Al bite and alkali feldspar (84%), quartz (5%), muscovite (5%), carbonate (3%), magnetite or ilmenite (3%), epidote (tr), biotite (tr), amphibole '! (tr). Feldspar is mostly al bite; quartz is in angular grains ; poikiloblastic muscovite is locally porphyro­blastic; biotite is green; fibrous radiating mineral is probably amphibole; mineral per· centages are extremely variable in different fragments. Fabric: granoblastic-poikiloblastic. Rock: horn/els. (Photomicrograph, Pl. IX, D.) (2) Quartz, feldspar, quartz-feldspar matrix, magnetite or ilmenite. Angular quartz and feld· spar grains are set in a very fine quartz-feldspar matrix-individual grains are resolved only with difficulty; magnetite or ilmenite is finely disseminated. Grain size: quartz and feldspar fragments 0.05 to 0.2 mm; matrix less than 0.02 mm-no evidence of metamorphism. Fabric : elastic. Rock : siltstone. Sun #1 Herring core 10114-28 Bureau of Economic Geology Antigorite and chlorite? (72%), calcite (25%), magnetite or ilmenite (3%). Pale _brownish fibrolamellar antigorite is inseparably mixed with a fibrous low·birefringent mineral, probably a chlorite, with a higher index of refraction than antigorite. Calcite is in veinlets and masses of grains. Magnetite or ilmenite is dendritic. Grain size: serpentine less than 0.02 mm; calcite 0.2 mm. Fabric: librolamellar. Rock: serpentinite. Sun # 1 Herring core 10128-33 Bureau of Economic Geology Thoroughly altered aphanitic green rock. Semi.opaque brown material (48%), green mica (15%) , nontronite (8%), talc (10% ), calcite (7%), epidote (5%), chlorite (5%) , pyrite (2%). Rock has a parallel banded structure marked by veinlets of calcite. The semi-opaque material is probably a clay derived from alteration of one of the other secondary minerals -apparently epidote. The talc and the green mica appear to be the same mineral and the color varies from predominantly pinkish or colorless to pale greenish to, locally, grass-green. It occurs in "pockets" with or without chlorite. The nontronite, with a distinct "crepe paper" appearance is intimately associated with calcite in veinlets. Grain size: 0.05 to 0.2 mm. Fabric: metasomatic-banded. Rock: altered ferromagnesian-rich rock. Sun # 1 Herring cuttings 10130-40 Stanolind Oil & Gas Co. Original nature of rock obscured by a pale yellow-green fibrous mineral with moderate bire· fringence which is apparently derived from alteration of feldspar. Grain size: 0.01 to 0.02 mm. Sun # 1 Herring cuttings 10135-10500 Bureau of Economic Geology (l) Plagioclase (50%), olivine 05%), chlorite (10%), augite 00%), magnetite or ilmenite (6%), talc (5%), biotite (2%), carbonate (1%), amphibole 0 % ), apatite (tr). Plagio­clase is sericitized; biotite is red-brown; amphibole is a secondary green variety; carbonate is the product of alteration of original ferromagnesian constituents. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: olivine gabbro. (2) Plagioclase (53%), amphibole (30%), biotite (8%), magnetite or ilmenite (7%), chlorite (2% ), apatite (tr). Plagioclase is in an advanced stage of sericitization; green-brown am­phibole occurs as discrete grains and as fibrous masses; biotite is red-brown. Grain size: 0.2 mm. Fabric: subophitic. Rock: diabase. Sun #1 Herring core 10447-59 Honolulu Oil Corp. Labradorite (75%), augite (10%), magnetite or ilmenite (5%), olivine (4%), chlorite (~%), biotite (2%), leucoxene (l% ) , apatite (tr), sphene (tr). Zoned plagioclase shows incipient alteration to sericite; two chlorites are present, an olive-drab variety from alteration of, and replacing, olivine and a green variety fringing pyroxene and olivine; intensely red-brown biotite fringes magnetite; sphene is partly altered to leucoxene. Grain size: 1 to 3 mm. Fabric : hypidio­morphic granular. Rock: leuco-olivine gabbro. Bureau of Economic Geology, The University of Texas Sun # 1 Herring core 10449-54 Bureau of Economic Geology Andesine-labradorite (61%), chlorite 00%) , aui:ite (8%), magnetite or ilmenite (8%), olivine (8'/o), biotite (4%l. apatite 0%), epidote (tr), pyrite or pyrrhotite (tr). Plagioclase shows a vague zoning and is partly sericitized; augite is pale pink-brown; olivine is partly altered to a hip:hly birefrini:ent green mineral (a chlorite?) and partly altered to a moderately birefringent olive-drab chlorite ; biotite pleochroism is pale to intense red-brown and locally it grows around the opaque mineral; chlorite may be divided into ( l) high-birefringent green chlorite ( 10%), (2) moderate-birefringent olive-drab chlorile (2%), and (3) low-birefringent green chlorite (tr); a substantial sericite content is included in the plagioclase total. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: olivine gabbro. Sun #1 Herring cuttings 10460-10500 Stanolind Oil & Gas Co. Labradorite (53%), augite (40%), magnetite or ilmenite (4%), biotite (2%), chlorite 0%), apatite (1r) . Plap:ioclase is partly altered to epidote and sericite; augite is brownish and ap­parently titaniferous: biotite pleochroism is pale brown lo dark red-brown: a pa lite occurs in grains and needles. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: gabbro. Sun #1 Herring cuttings 10500 Shell Oil Co. Labradorite? (69%), olivine 05%), alkali feldspar (5%), augite (3%), chlorite (3%), magne­tite or ilmenite (3%), biotile 0%), apatite 0 %) . Plagioclase is zoned and partly ahered to sericile, the outer zones of the zoned plagioclase grains are sodic and probably approach albite in composition; alkali feldspar occurs as small grains between plagioclase subhedrons and as rims on plagioclase grains; alkali feldspar is a dirty brown color as a result of incipient kaolini· ation; olivine is partly altered to a moderately birefringent rhlorile ; biotite is an intensely red­brown variety and rims the opaque mineral ; some apatite occurs in needles. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: leuco-olivine gabbro. CHILDRESS COUNTY Stanolind # l Owens core 7380-84 Bureau of Economic Geology Groundmass (86%), plagioclase (8%), magnetite or ilmenite (3%), red iron oxide 0%), chlorite 0 % ) , calcite 0 % ) , apatite (tr). Groundmass ranges from microspherulitic to micro· granular ; the spherulitic areas are mostly alkali feldspar and the microgranular material is mostly feldspar with subordinate quartz, magnetite, and red iron oxide; plagioclase phenocrysts are almost completely sericitized; magnetite or ilmenite is partly ahered to red iron oxide; cal· cite occurs in grains and \'einlets. Grain size: groundmass, spherulites up to 0.5 mm, microgranu­ lar material up to 0.1 mm; phenocrysts up to 3 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Stanolind # 1 Owens cuttings 7380-84 Shell Oil Co. Groundmass (87%), feldspar (5%), quartz (5%), sericile (3%) , leucoxene (tr), brown iron oxide (tr). Micrographic groundmass is altered and stained; groundmass contains quartz, and some quartz is present as larger grains or groups of grains in a mosaic (probably secondary); phenocrysts are altered so as to obscure their mineralogy. Grain size; groundmass 0.05 to 0.1 mm; phenocrysts up to 0.5 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Stanolind #1 Owens cuttings 7381-84 Stanolind Oil & Gas Co. Groundmass (89%) , albite? (5%), quartz (5%), chlorite 0%), leucoxene (tr), red iron oxide (tr), magnetite or ilmenite (tr), calcite (tr). Groundmass is obscured by heavy iron oxide stain; pla1doclase phenocrysts are in advanced stage of sericitization; probably most of the larger quartz grains and mosaic are secondary. Grain size: groundmass less than 0.05 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. CLAY COUNTY Bridwell #26 Edrington cuttings 3273 Humble Oil & Rfg. Co. Quartz and plairioclase (82%), biotite (8%), ser1c1te (7%), epidote (3%), leucoxene (tr), calcite (Ir), sphene (tr), apatite (tr), zircon (tr). Quartz and partly sericitized plagioclase occur in angular fragments; biolite is present as tiny plates as a result of reconstitution of ori~inal argillaceous intergranular material; sericile occurs as fine oriented fibers; epidote occurs as fine scattered grains and some large masses. Grain size: 0.02 to 0.2 mm. Fabric: relict clastic­incipient crystalloblastic. Rock: phyllitic metagraywacke. Basement Rocks, Texas-New Mexico Bridwell #l·A Edrington core 02183-84~ Stanolind Oil & Gas Co. Albite (38%), quartz (30%), alkali feldspar-mostly microcline microperthite (20%), sericite (6%), biotite (2%), epidote (2%), chlorite (2%) , magnetite or ilmenite (tr), zircon (tr) , apatite (tr), rhyolite? fragment (tr) . Plagioclase occurs in finely twinned grains; epidote is in tiny clusters of grains and in larger grains throughout the slide; sericite occurs from alteration of feldspar (within feldspar grains) and as intergranular fibers; zircon is rounded; biotite pleochroism is pale brown to deep green. Grain size : 0.2 to 0.5 mm. Fabric: relict clastic­incipient crystalloblastic (a low-grade quartzofeldspathic rock with the micaceous-argillaceous constituents completely reconstituted but no recrystallization of quartz and feldspar). Rock: meta· arkose. Goldsmith #l Republic Nat. Gas cuttings 2560-65 Bureau of Economic Geology Oligoclase ( 50%), microcline microperthite ( 35%), quartz 00%), biotite ( 3%), calcite O % ) , chlorite (1%), apatite (tr), zircon (tr). Plagioclase is partly sericitized; biotite is a deeply colored green-brown variety. Grain size: l to 2 mm. Fabric: hypidiomorphic granular. Rock : granodiorite. Perkins # l Stine core 2935 Humble Oil & Rfg. Co. Albite-oligoclase and microcline (60%), quartz (25% ), chlorite (5%), sericite (4%), epidote (3%), chert (2%), leucoxene 0%), apatite (tr), zircon (tr) . Plagioclase is partly sericitized ; microcline is locally microperthitic; quartz occurs as individual grains and as fragments of mosaic; epidote occurs in stnngs and lines of tiny grains that outline quartz and feldspar grain boundaries. Quartz and feldspar grains occur in a fine intergranular mass of quartz-feldspar­epidote·chlorite-sericite. Grain size: quartz and feldspar grains O.l to l mm ; reconstituted intergranular material 0.02 mm and less. Fdbric: relict elastic-incipient crystalloblastic. Rock: metagraywacke. Texas #41 Byers cuttings 4250-55 Bureau of Economic Geology l\Hcrocline microperthite, quartz, chlorite, magnetite. Slide is composed mostly of individual mineral fragments; only two small rock fragments are present. Grain size: 0.5 to 2 mm. Fabric : ? Rock: granite. COCHRAN COUNTY Cosden # l Barker cuttings 8600-10 Bureau of Economic Geology Plagioclase (72%) , magnetite or ilmenite 00%), chlorite 00% ), augite ( 4%), apatite (2%), quartz (2% ), rutile? (tr). Plagioclase occurs in non-oriented sericitized laths, index of refrac· tion suggests it is albite but the habit indicates a more calcic variety-the high soda content may be a result of the alteration ; chlorite occurs in very large masses and includes small grains of rutile?; quartz appears to be secondary; augite occurs as relicts surrounded by chlorite. Grain size: plagioclase laths a\'erage 0.5 mm and reach a maximum of 2 mm long. Fabric : hypidio· morphic granular. Rock: altered leuco-albite gabbro. Humble #I lllasten cuttings 10775-88 Shell Oil Co. Quartz and alkali feldspar 000% ), leucoxene (tr), pyrite (tr), calcite (tr). Microcrystalline quartz-feldspar rock showing Aowage structure. Grain size: less than 0.02 mm. Fabric: micro· granular. Rock : rhyolite. Humble # l lllasten cuttings 10775-88 Shell Oil Co. Groundmass (70%), microperthite (25% ), pyrite (4%), calcite 0%). Cryptocrystalline ground­mass shows well-de\'eloped Aowage structure; phenocrysts are microperthite. Grain size: groundmass less than 0.01 mm; phenocrysts up to l mm. Fabric: porphyritic. Rock: rhyolite porphyry. Humble #I Masten core 10788 Bureau of Economic Geology Groundmass (93%), microperthite (4% ), calcite (3%), albite (tr), sericite (tr), quartz (tr), leucoxene (tr), pyrite (tr), apatite (tr) . Groundmass is microcrystalline to cryptocrystalline and shows well-developed flowage structure; microperthite, albite, and quartz occur as pheno­ crysts; calcite occurs in masses and veinlets parallel and normal to Aowage structure. Grain size: groundmass less than 0.02 mm; phenocrysts up to 3 mm. Fabric: porphyritic. Rock : rhyolite porphyry. (Photomicrograph, Pl. VIII, D. ) Bureau of Economic Geology, The University of Texas Humble #1 Westheimer core 739,3.94 Bureau of Economic Geology Groundmass (85%), microperthite (7%), calcite (3%), quartz (2%), anhydrite 0%), albite 0 % ) , magnetite or ilmenite 0 % ) , leucoxene (tr), sericite (tr), hematite (tr), zircon (tr). Microcrystalline to cryptocrystalline groundmnss shows flowage structure; microperthite and albite occur as phenocrysts; anhydrite occurs with quartz and calcite in a vug. Grain size: groundmass mostly less than 0.02 mm; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Humble #1 Westheimer core 7393-94 Honolulu Oil Corp. Groundmass (74%), microperthite 00%), quartz 00%), calcite (2%), anhydrite (2%), sericite 0 % ), magnetite or ilmenite 0%), leucoxene (tr), sphene (tr), zircon (tr). Ground· mass is microcrystalline to cryptocrystalline with flowage structure well developed in cryptocrys· talline areas; there is considerable secondary? quartz in the coarser areas; phenocrysts are mostly microperthite; some are anti·perthitic; one phenocryst has an albite core and a micro· perthite border; calcite and anhydrite occur in local coarse areas that originally may have been vesicles or pumicy openings; most of the quartz is in veinlets and local coarsenings. Grain size : groundmass cryptocrystalline to 0.2 mm; phenocrysts up to 2 mm. Fabric : porphyritic. Rock : rhyolite porphyry. Shell # l Pittman cuttings 11490-94 Bureau of Economic Geology Plagioclase (55%), quartz (25%), microperthite 05%), leucoxene (4%), chlorite 0%), apatite (tr). Plagioclase is almost completely sericitized; microperthite is partly kaolinized; quartz occurs as masses of small grains between feldspar subhedrons. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: altered J?ranodiorite. Stanolind # 5 Edwards cuttings 12620-30 Shell Oil Co. Quartz and feldspar (100%), chlorite (tr), leucoxene (tr) . Microcrystalline to cryptocrystalline mass shows very pronounced flow structure; some coarser quartz is in augen and elongate lenses up to 0.2 mm long. Grain size: mostly less than 0.01 mm. Fabric: microgranular?; large part of rock is cryptocrystalline. Rock : rhyolite?. Stanolind # 5 Edwards cuttings 12620-30 Shell Oil Co. Microperthite (95%), senc1te (3%), calcite 0%), leucoxene (1%) . Only one very small fragment. Grain size: l mm. Fabric: ? . Rock: probably a rhyolite phenocryst. Stanolind # l Reed core 12678 Bureau of Economic Geology Groundmass (85%), quartz (8%), microperthite (3%), oligoclase (3%), leucoxene 0%), chlorite (tr), zircon (tr). Quartz-aikali feldspar groundmass is in part microspherulitic and in part micrographic with, locally, a vague flowage structure; quartz is in embayed pheno­ crysts; plagioclase and microperthite occur as phenocrysts. Grain size: groundmass cryptocrystal­line to 0.05 mm but mostly less than 0.01 mm; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Stanolind # 1 Slaughter cuttings 10770 Shell Oil Co. Groundmass (89%), microperthite (5%), leucoxene ( 4%) , quartz (2%), magnetite or ilmenite (tr), sericite (tr) . Groundmass is micrographic quartz.feldspar intergrowth; microperthite occurs as phenocrysts; quartz (2%) occurs also as local coarsenings. Grain size: groundmass less than 0.05 mm; phenocrysts up to 0.3 mm. Fabric: porphyritic. Rock : rhyolite. Stanolind # 1 Slaughter cuttings 10820-39 Shell Oil Co. Groundmass (83%), albite 05%), sericite 0 %) , chlorite 0%), magnetite or ilmenite (tr), 1"t1coxcne (tr I, zircon (tr), apatite (tr) . Micrographic groundmass contains twinned partly kaolinized al bite phcnocrysts veined with sericite. Grain size: groundmass less than 0.01 mm; phenocrysts up to l mm. Fabric: porphyritic. Rock: rhyolite porphyry. Stanolind #I Slaughter core 10839-40 Stanolind Oil & Gas Co. Groundma"" (82%), albite 00%), biotite (1%), chlorite (3%) , qua1tz (2%), magnetite or ilmenite (2'/'o I, lcucoxcne (tr), epidote (tr), zircon (tr), rutile (tr), apatite (tr), red iron oxide (tr). ~licrocrystalline to cryptocrystalline groundmass shows micrographic structure and is pervaded with rc in shea,-e,_ plates, and fibrous masses throughout the slide­ lorally it shows paralld orientation: garnet relic-ts occur in a mass of chlorite and sericite indicatinJ! a n•tro;.!rf•s..;;h·e mt"tamorphism: biotite pleochroism is pale brown to very dark brown. Grain !'izr: O.::! to I mm with ~arnets up to~ mm. Fabric: crystalloblastir--catarlastic (apparently a rrystallobl.:i:"-tic mt'dium-~racfe ~arnetift'rous schis.t has suffered rataclastic retrograde meta­ morphism l . Ro.-k: biotitc·gamet schist. (Photomicrograph, PI. V, A.) )[c[lreath & Suggett #I Whaley cuttings 2312--+0 Bureau of Economic Geology Quartz, feldspar, chlorite, biotite, magnetite or ilmenite, garnet. Mass of quartz and feldspar contains rhlorite anJ biotite showin~ a rude orientation: f!arnet remnants orcur in masses of dark chlorite. Grain site: o.~ to 0.5 mm. Fabric: CrYstalloblastic. Rock: biotite·garnet-chlorite ~~ . )!cElreath & Suggett #I Whaley core 2330 Bureau of Economic Geology Albite-oligoclase and quartz (66'C ), biotite (15%), chlorite (10%), garnet (8%), magnetite or ilmenite (I fc 1. apatite (tr). Lorally crushed plaf!iorlase-quartz mosaic is composed predomi­nantly of plagioclas.e; rhlorite occurs in oriented plates, shea\"es, and fibrous masses as.sociated Basement Rocks, Texas-New Mexico with relict garnets; biotite, yellow-brown to brown pleochroism, occurs in oriented plates partly altered to chlorite; relict garnet porphyroblasls O<'Ctir in nests of chlorile-biolitc-sericite and are more or less concentrated in layers, Grain size: 0.2 to 1 mm. Fabric: !'rystallobln>lit·-1·a1udu>tic (retrograde metamorphism, garnet->chlorile). Rock: garnet-biotite-oligoclasc schist. Muenster #1 Josten cuttings 3160 Bureau of Economic Geology Quartz (40%), oligoclase (35%), hornblende (12%), biotite (6%), alkali feldspar (5%), calcite (1o/o), apatite 0 '7c), magnetite or ilmenite (tr), Quartz-oligorlase mo>aic contains oriented prisms .of partly altered blue-green hornblende and oriented plates of brown biotite. Slide is composed of very small fragments and the mineral percentages abo\'e are only an estimate. Grain size: 0.1 to 0.2 mm. Fabric: crystallohlastic. Rork: homblencle-oligoclase schist. Muenster #1 Josten cuttings 3165 Bureau of Economic Geology Quartz and albite-oligoclase (80%), biotite 02%), chlorile (5%), magnetite or ilmenite (1% ) , calcite 0 % ), apatite 0%). Plap;ioclase and quartz occur in a mosaic with grains showing dimensional orientation; J!reen-brown biotite partly altered to chlorite occurs in oriented plates. Grain size: 0.1to0.2 mm. Fabric: lcpidoblastic. Rock: biotite schist. Muenster #1 Josten cuttings 3578-3652 Bureau of Economic Geology Quartz and oligoclase (71%), hornblende 05%), biotite (7%), calr.ite (4%l, magnetite or ilmenite O o/o), red iron oxide 0%), apatite 0 %), pyrite (tr), sphene (tr). The quartz-olip;o­clase mosaic is romrosed rredominantly of quartz; oriented hornblende prisms are faded and in rart alternd to calcite; biotite forms oriented green-brown plates. Grain size: 0.1 to 0.2 mm. Fabric : crystalloblastic, Rock : homblende-oligoclase schist. Phillips #1-CT Atcheson core 2263-71? Bureau of Economic Geology Rock is a conglomerate. Rock and mineral fragments occur in a brown microcrystalline to cryptocrystalline matrix that is probably a micaceous, chloritic paste. (1) Rock fragment-I cm long; essentially a monomineralic fragment composed of non-oriented colorless amphibole grains up to I mm and veined with chlorite. (2) Rock fragment-8 mm lonp;; amphibole (87%), chlorite 00%), muscovite (3% ); rock has pronounced schistosity. (3) Slate or phyllite frag­ments. (4) Fragments of quartz mosaic. (5) Grains of magnetite or ilmenite, sphene, and calcite. (6) Some completely sericitized fragments, The conglomerate is composed mostly of fragments of basic rocks-for the most part amrhibole-rich rocks, schistose or massive, and amphibole fragments of varied grain size and in various stages of alteration to chlorite and, to a lesser extent, zoisite. Fragments are set in a brownish-green matrix which also contains quartz pebbles. Note lack of feldspar. Rock: weakly metamorphosed pebble conglomerate-a conglomeratic metagray­wacke. (Photomicrograph, PL V, D.) Phillips #3 Atcheson cuttings 2165-70 Bureau of Economic Geology Quartz (77%), biotite (8o/o), epidote (8o/o), muscovite (5%), hornblende 0%), chlorite (1%) , magnetite or ilmenite (tr) . Quartz mosaic locally shows granulation and dimensional orientation; muscovite, biotite (red-brown), epidote, chlorite, and hornblende occur together in patches; muscovite tends to be porphyroblastic; biotite is locally bleached and sheared; hornblende rleochroism is yellow-green to green. Mineral rercentages vary in dilTerent fragments. Grain size: 0.1to0.2 mm. Fabric: crystalloblastic (granoblastic to sieve). Rock: metaquartzite. Phillips #3 Dangle cuttings 2517-19 Bureau of Economic Geology Microcline microperthite (69%), oligoclase (15%), quartz OOo/o), biotite (2%), calcite (2%) , chlorite 0 % ), magnetite or ilmenite (1 % ) , apatite (tr), zircon (tr) , Microcline shows very fine perthitic de\"elopment; quartz locally is myrmekitically to micrographically intergrown with feldspar; hiotite pleochroism is yellow-brown to almost black ; chlorite occurs with calcite as a result of alteration of a ferroma~nesian mineral. Grain size: 0.5 to 2 mm. Fabric: xenomorphic g:ranular-poikil itic. Rock: granite. Phillips #3 Dangle core 2517-19 Humble Oil & Rig. Co. 111icrocline microperthite (64o/o), albite 05%), calcite (8%), quartz (7% ), biotite <.3%), magnetite or ilmenite (2%), chlorite 0%), apatite (tr), zircon (tr). Microcline shows ex­tremely fine perthitic development and occurs in large grains with al bite; ferromagnesian minerals and their alteration products plus the accessory minerals occur in "nests" between large feldspars; calrite occurs in part in veinlets and in part was derived from alteration of feldspar and hornblende?; biotite pleochroism is yellow-brown to very dark brown. Grain size: 1 to 6 mllL Fabric: xenomorphic granular. Rock: granite. Bureau of Economic Geology, The Unfrersity of Texas Phillips #I Fielder cuttings 2935-40 Bureau of Economic Geology ~licrodine microperthite (76':(-l, quartz (15%), albite (57ol, rhlorite (3'/'o), biotite 0%), calcite (tr\. leurnxene (tr\. magnetite or ilmenite (tr), apatite (tr\, zircon (tr). l\lirrorline mkroperthite is in large poikilitir grains: red-brown biotite is considerably altered; fine granular rhlorite orcnn; in veinlets with calcite and locally replaces feldspar. Grain size: 0.5 to 3 mm. Fabric: xenomorphir granular-poikilitir. Rofk: granite. Phillips #I Reitar core 3216 Bureau of Economic Geology ~lierorline mirroperthite (7~'7r \. quartz l15% \. oligorlase (5".'c l. r hlorite \3'7c \. myrmekite t~'lc 1. ralrite ( l<"'r). magnetite or ilmenite 09ci ), biotite (l':r). zircon (tr\. leuroxene t td. sphene (tr\. apatite 1tr\. Quartz. plagiorlase. and myrmekite for the most part occur as small grains along the borders of large mierorline grains; biotite pleorhroism is pale red-brown to almost black and. with the other heavy minerals.. it occurs in nests between large microcline grains. Grain size: average 5 mm. Fabric: h~·pidiomorphir granular. Rork: granite. Phillips #2-A Reitar core 3253-56 Bureau of Economic Geology )lirrorline microperthite (65% l, oligorlase (10<;(-), quartz (10%), calcite (5%), m)Tmekite l3'l-l. rhlorite (2<(-l, biotite (4'/cl, magnetite or ilmenite !1%), zircon (tr), sphene (tr). ~lineral relation> same a> 3216. Rork: granite. COTTLE COUNTY Anderson-Prichard #I Lynch cuttings 5810-~0 Shell Oil Co. _.\]bite ( 7~'/c l , quartz ( 15':(-\. biotite \5% \. rhlorite (5'/c \, magnetite or ilmenite (3<;1-), apa­tite ltr• . zircon \tr\. Plagioda:;e is zoned and shows advanced sericitization; original ferromal;­nesian minerals have altered to chlorite and leucoxene. Grain size: 0.5 mm. Fabric: hypidiomor· phic granular. Rock: leuco-albite-quart: microdiorite. Anden;on-Prichard # 1 Lynch cuttings 5820·30 Bureau of Economic Geology Albite-oligoclase (67% \. quartz (15% \. microcline (8'/c l. rhlorite (6<;(-l, biotite (2%), magnetite or ilmenite (~'l'cl. apatite \tr ·•. red iron oxide (trl. Plagioclase is zoned and ,·ar­iably seriritized; rhlorite is in large laths probably as a result of alteration of biotite; biotite plt"oehroi:_;.m is ye How-brown to dark brown-it is partly altered to chlorite : microcline fiHs in around plagiocla::oe and quartz grain~. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic gran· ular. Rock: microgranodiorite. General Crude =13-1 Swenson core 5410-23 Bureau of Economic Geology .-\lbite-oligoclase \51':(-l. rhlorite \ 10'/o), alkali feldspar (8<:'c l, ouartz (7% \, epidote (5%), cakite 15rt-•. biotite t 5~1. amphibt.)le ~.ic;c. I, ilmenite 1~'ic 1. lt~ucoxene ~Ired ferwmagnesian minerals fn_)m which it was derin·d as. an alteration product: epidote occur.--as grains and mas.._~s as a res.uh of alteration of plagioclase and ferromagnesian minerals: green-brown hiolite occur:::;. in aggregate::. of tiny m1n-orit"nted platt"s. with chlorite and t"pidote and. is appart"ntly a secondary mint"ral : pale green amphibole relicts. are engulfed by chlorite and calcite: bleached biotite i> partly altered to rhlorite. Grain size: 0.5 to 4 mm. Fabric: hypidiomorphic granular-mt:>ta~omatic. Rock: altered quart; diorite. General Crud(" =13-1 ~wt·n~lll core 5719-~:l Bureau of Economic Geology Feldspar and ~(·ricite ( 79:C 1. biotite 1W'C1. quartz (7% \. magnetite or ilmenite and leucoxene t 5~c l. rhakt>Jonic ~ilit·a (1':c1. apatite l tr\. Fdd:"-par i~ mo5-tly a partly s:eriritized plagioda.se: ~ricite t)ft'Ur~ a~ ~1 n~:-ccurs as small pbh":0-witlwut preft"rrt"d orientation: quartz i~ in angular to subangular scat­tered grain:-": ch~1kedonic silica showing spherulitic extinction fill:; cross-cutting n•ffilets. In ~ummary. rock con:"-ists of clear angular quartz grains and "nests." of biotite in a mass of seri­citizeJ feld:'par grain:0 and intergranular sericite. Grain size: 0.01 to 0.1 mm. Fabric: relict elastic -t-ry~tallohla:>-tit·. Hock: \t•ry low ;r~!de .~aicilt' ph~·l/ite. Basement Rocks, Texas-New Mexico General Crude #33-1 Swenson core 5449-52 Bureau of Economic Geology Microcline and albite (53%), quartz (38%), biotite (7%), magnetite or ilmenite (2%), chlorite (tr), apatite (tr), zircon (tr). Quartz, Rlbite, and microcline form a mosaic in which there are large porphyroblasts of albite and, to a lesser extent, microcline partly replaced by al bite; biotite showing pale to very dark brown pleochroism occurs in scattered plates; magnetite or ilmenite occurs in grains partly altered to red iron oxide; apatite is in clusters of prisms and grains. Some feldspathic material was probably introduced. Grain size: mosaic 0.1 to O.~ mm; porphyroblasts l to 2 mm. Fabric: porphyroblastic. Rock: metarkosite. Humble #1-J Matador core 8082.87 Humble Oil & Refg. Co. Plagioclase (51%), quartz (20%), sericite (20%), microcline (5%), chlorite (3%), calcite (1%), pyrite (tr), leucoxene (tr), rock fragments (tr), apatite (tr), zircon (tr). Plagioclase is almost completely sericitized and grains merge with intergranular sericite so that their boundaries are difficult to distinguish; quartz is in scattered angular grains; sericite, locally showinl( rude parallel orientation, occurs as masses of fibers and originated through recon· constitution of intergranular argillaceous material (some fine quartz and feldspar are included in this figure) ; microcline shows incipient kaolinization; calcite occurs with quartz in veinlets; rock fragments are granitic. Grain size: quartz and feldspar fragments 0.1 to 0.5 mm; inter· granular material mostly less than 0.02. Fabric: relict elastic-incipient crystalloblastic. Rock : sericite phyllite. Jones & Stasney #I Wiley cuttings 4850-55 Bureau of Economic Geology Albite (50%), sericite and very fine granular quartz and feldspar (32%), quartz (12%), rock fragments (3%), chlorite (2%), magnetite or ilmenite, leucoxene, and red iron oxide (1%), apatite (tr), zircon (tr). Quartz and albite occur as angular grains in a mat of finely fibrous sericite and as a fine granular mass of quartz and feldspar; sericite is also present as a rrsult of plagioclase alteration; chlorite occurs with sericite as fine plates and fibers and in one frag­ment occurs abundantly as oriented fibers so that the rock might be called phyllite; rock fragments are mostly chert and quartzite. Grain size: 0.2 to 0.4 mm. Fabric: relict clastic­incipient crystalloblastic. Quartz and feldspar have not recrystallized but intergranular material has been reconstituted to sericite and chlorite. Rock: meta-arkose. Jones & Stasney #I Wiley cuttings 4900·05 Bureau of Economic Geology Similar to 4850-55 hut with calcite locally replacing part of the rock and an increase in the opaque iron and/or titanium minerals. Jones & Stasney #I Wiley cuttings 4950-55 Bureau of Economic Geology Albite·oligoclase (73% l, microcline microperthite ( 10%), quartz (10%), chlorite (3% i, sericite and fine granular quartz and feldspar (3%), epidote (I%), magnetite or ilmenite (tr), muscovite (tr), apatite (tr), zircon (tr). Albite-oligoclase occurs as angular !(rains and locally is in an advanced stage of sericitization; quartz and microcline microperthite occur as angular grains; chlorite occurs with sericite as intergranular plates and fibers; muscovite is evidently a second cycle mineral and not, like sericite and chlorite, the result of reconstitu· tion. Grain size: 0.2 to 0.4 mm. Fabric: relict elastic-incipient crystalloblastic. Rock: meta­ arkose. Jones & Stasney # l Wiley cuttings 5000-05 Bureau of Economic Geology (l) Fragments similar to 4850-55 and 4950-55; one fragment shows pronounced dimensional orientation of grains. (2) A few fragments of altered igneous rock: plagioclase (59%), chlorite (20%), epidote (20%), magnetite or ilmenite (1%) , calcite (tr). Plagioclase occurs as non-oriented laths and is partly altered to sericite and epidote-on basis of index of refraction it is albite, probably as a result of alteration (calcic plagioclase-+albite and epidote); epidote occurs in magses and clusters of small grains, mostly from alteration of plagioclase. Grain size: plagioclase laths 0.2 to 0.5 mm long. Fabric: relict subophitic. Rock: altered diabase. Jones & Stasney # 1 Wiley cuttings 5050·55 Bureau of Economic Geology Contains fragments similar to 4850-55 and 4950-55. (Photomicrograph, Pl. V, C.) Jones & Stasney # l Wiley cuttings 5100·05 Bureau of Economic Geologv Albite-oligoclase and microcline microperthite (83%), quartz (10%), hematite?, red iron oxide, and leucoxene (3%), sericite (3%), chlorite (1 %>, rock fragments (tr), epidote (tr), apatite Bureau of Economic Geology, The University of Texas (tr), zircon (tr). Partly sericitized plagiocla«• and rartly kaolinized microcline microperthite occur with quartz as angular grains separated by fibers of sericite and chlorite and by rims of hematite? and grains of leucoxene. Rock fragments are apparently rhyolite? and chert. Grain size: 0.1 to 0.5 mm. Fabric: relict elastic-incipient crystalloblastic. Rock: mcta·arkose. Jones & Stasney # 1 Wiley cuttings 5150·55 Bureau of Economic Geology Contains fragments similar to types previously described. Jones & Stasney #1 Wiley cuttings 5200·05 Bureau of Economic Geology Contains fragments similar to those previously described, but a number of fragments show a higher hematite? content. Jones & Stasney #1 Wiley cuttings 5245 Bureau of Economic Geology Contains fragments similar to types previously described, but locally opaque iron and/ or tita­ nium minerals (magnetite or ilmenite, hematite?, leucoxene, and pyrite) are so concentrated as to comprise as much as 20 percent of the rock fragment. Merry Bros. & Perini # 1 Pursell cuttings 4659.4740 Humble Oil & Rfg. Co. (1) Alkali feldspar and albite (62'/o) , quartz (20%), biotite (8%), sericite (5%), epidote (2%), chlorite (2%), calcite (1% ), ilmenite (tr), leucoxene (tr), zircon (tr), apatite (tr). Albite is partly sericitized; quartz and feldspar are in angular fragments which commonly show dimensional orientation; grcf':n-brown biotite occurs as reconstituted inter­ granular material, locally poorly developed plates have formed; sericite occurs as a result of alteration of feldspar and as reronstituted inlergranular fibers. Grain size: 0.1 to 0.2 mm. Fabric : relict elastic-incipient crystalloblastic (quartz and feldspar are not recrys­tallized). Rock: meta-arkose. (2) Feldspar (65%), quartz (25%l, sericite and muscovite (6%), chlorite (4%), calcite (tr), epidote (trl, apatite (tr). Feldspar is mostly albite, locally porphyroblastic; sericite is in part from alteration of plagioclase and in part from reconstitution; there are some muscovite flakes. Grain size: 0.05 to 0.2 mm. Fabric: granoblastic. Rock: metarkosite. (This rork seems to be a more highly metamorphosed equivalent of (1) above in which quartz and feldspar have recrystallized.) Ramsey #1 Lynch cuttings 5670-80 Bureau of Economic Geology Oligoclase (52%), quartz (10%), biotite (10'/o) , chlorite (10%), am phi bole (8%), augite (4%), alkali feldspar (3%), epidote (2%), magnetite or ilmenite 0%), calcite (tr), two unidentifi<"d accessory minerals (tr). Zoned plagioclase is partly sericitized and locally con­tains inclusions of fine needles of an unidentified mineral ; biotite pleochroism is yellow or red-brown to almost opaque; amphibole, pale hrown to deep green-brown plcochroism, occurs in skeletal grains and is partly altered to chlorite; epidote is the result of alteration of amphi· bole. Grain size: 0.2 to 0.5 mm. Fabric: hypidiomorphic granular. Rock: microgranodiorite. Seaboard and Shamrock #1 Tapper core 6655 Humble Oil & Rfg. Co. Albite and microcline microperthite (68%), quartz (20%), sericite (8%), chlorite (2%), chert (l'j'c), cpidote (l% ) , rhyolite (tr), carbonate (trl, sphene (tr), ilmenite (tr), leucoxene (tr), red iron oxide (tr), apatite (tr), zircon (tr). Feldspar and quartz occur as angular grains ns.r.;ociat<'d wi th dH~r1 and rhyolite rock fragments; grains are .separated by ~wricite, chlorite, and rpidote. Grain size: 0.1 to 0.5 mm. Fabric: relict elastic-incipient crystalloblastic (quartz and feldspar are not recrystallized ). Rock: meta-arkose. CRANE COUNTY Atlantic #2-A lini\'ersity core 11642-45 Bureau of Economic Geology Hornblende (35<;t ), oligoclase-ande>inc (22%), alkali feldspar 05%), biotite (12%), quartz (lO'Jol, myrmt•kite (2% ), sphene (2%), pyrite 0%), leucoxene (1%), calcite (tr), apatite ( tr). zircon ( tr I. Jl ornblend e, pleochroi~m yellow-green to green, commonly occurs in large poikilitic grains, and commonly it includes small highly birefringent needles of an unidentified mineral; biotite pleochroism is beige to rich brown; alkali feldspar is probably orthoclase; sphenc is partly altered to leuroxene. Grain size: 1 to 2 mm. Fabric: hypidiomorphic granular­poorly de\'elopcd. Rock: mela-granodiorite. Basement Rocks, Texas-New Mexico Lomand #3 Tubbs cuttings 7120-60 Stanolind Oil & Gas Co. Microcline (60%), quartz (25%), albite 05%), muscovite (tr), calcite (tr), rutile (tr), zircon (tr). Slide is composed of only small fragments. Grain size: 2 to 3 mm. Fabric: ? Rock: granite. CROCKETT COUNTY Amerada #l·D Shannon cuttings 7825-30 Shell Oil Co Albite (40%), microcline (29%), quartz (20%), muscovite and sericite (8%), biotite (2%), magnetite or ilmenite ( l % ) , apatite (tr), leucoxene (tr). Plagioclase is partly sericitized; biotite is a green·brown variety. Grain size: 0.2 mm. Fabric: xenomorphic granular. Rock: albite microgranodiorite. Amerada #l·D Shannon cuttings 7830-35 Shell Oil Co. Microcline (37%), albite (32%), quartz (25%), muscovite and senc1te (2%), magnetite or ilmenite (2%), biotite (1% ) , chlorite (1% ) , leucoxene (tr), calcite (tr), pyrite (tr), zircon (tr). Plagioclase is partly sericitized. Grain size: 0.2 to 0.5 mm. Fabric : xenomorphic granular. Rock: microgranite. Amerada #1-D Shannon cuttings 7834-35 Bureau of Economic Geology Microcline (36%), oligoclase (27%), quartz (25%), biotite (7%), chlorite (3%), magnetite or ilmenite (2%), muscovite (tr), pyrite (tr), apatite (tr) , calcite (tr). Plagioclase is partly sericitized and kaolinized; biotite is very deeply colored; chlorite, bright green, was derived from alteration of biotite; muscovite apparently replaces plagioclase. Grain size: 0.5 mm. Fabric: hypidiomorphic granular-poorly developed. Rock: microgranite. CROSBY COUNTY Deep Rock # 2 Morgan Jones cuttings 8760-70 Bureau of Economic Geology Fragments of quartz and microperthite; original rock probably granitic. Humble #I Irvin core 9947 Bureau of Economic Geology Andesine-labradorite (60%), hornblende (28%), chlorite (4%), ilmenite (3%), alkali feld­spar (3%), sphene 0 %), epidote (1%), apatite (tr). Twinned plugioclnse subhedrons show a vague zoning with sericitized cores; oriented prisms of hornblende, yellow-green to grass­green pleochroism, are partly altered to chlorite; alkali feldspar is in a sponge containing plagioclase inclusions; sphene occurs in large grains and as rims on ilmenite; the whole rock is fractured and brecciated. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular­gneissic. Rock: diorite gneiss. (Photomicrograph, Pl. VI, B.) (Note: Hand specimen of the core shows a granitic stringer I inch thick cutting the diorite at a high angle.) Humble #I Montgomery cuttings 9700-07 Bureau of Economic Geology Albite-oligoclase (34% ), quartz (25%), microcline (20%), calcite OS%), chlorite (4%), muscovite (2%J, apatite (tr). Plagioclasc is in an advanced stage of sericitization; quartz is fractured, crushed, brecciated; calcite fills fractures and cements brcccin; muscovite is warped and bent. Grain size: less than 0.05 mm (crushed) to 2 mm. Fabric: cataclastic. Hock: cata­clastically altered granodiorite. (Note: Polished core section indicates the granodiorite occurs as pebbles in a conglomerate.) Humble #1 Montgomery core 9704-07 Humble Oil & Hfg. Co. Polished core section shows the rock to be a pebble conglomerate. Albite-oligoclase (57%), quartz (20%), calcite (7o/o), sericite (6%), rock fragments (5% ), ilmenite (4%), leucoxene (1% ), zircon (tr), amphibole (tr), apatite (tr). Fractured plagioclase grains and subordinate alkali feldspar are cut by a mesh of sericite veinlets; quartz occurs in patches of sutured mosaic and sporadic individual grains; calcite vein lets fill fractures and locally appear to be sheared; rock fragments are mostly volcanic rocks; ilmenite is rimmed with leucoxene. Grain size: I to 3 mm. Fabric: clastic-cataclastic. Rock: metaconglomerate. Bureau of Economic Geology, The University of Texas Humble # l Montgomery cuttings Shell Oil Co. Albite (66%), quartz (20o/o), sericite (8o/o), biotite (4o/o), calcite (1%), leucoxene (lo/o), zircon (tr}, apatite (tr) . Plagioclase is partly sericitized; biotite is bleached. Grain size: 0.5 to 2 mm. Fabric : hypidiomorphic granular. Rock: leuco·quartz diorite. Ohio #1 Morgan Jones cuttings 863!>-36 Shell Oil Co. Albite (85o/o), quartz 05%), magnetite or ilmenite (tr), apatite (tr). Grain size and fabric similar to 8635-36 (next below). Rock: leuco-albite·quartz diorite?. Ohio # 1 Morgan Jones cuttings 8635-36 Shell Oil Co. Microperthite (84o/o), quartz (15%), amphibole 0%). Small fragment; plagioclase in micro· perthite occurs as irregular veins and irregular twinned masses; amphibole is deeply colored and only one grain is present. Grain size: 2 lo 4 mm. Fabric: hypidiomorphic granular. Rock: granite. Ohio # 1 Morgan Jones cuttings 8636-38 Shell Oil Co. Microperthite (88% ), quartz (lOo/o), altered biotite (2%). Grain size: 2 to 5 mm. Fabric: hypidiomorphic granular. Rock: granite. Summation of the three poor thin sections of cuttings from the Ohio # 1 Morgan Jones 8635·36 and 8636-38 indicates a rock that is probably a granite or granodiorite. CULBERSON COUNTY Magnolia #1-A Cowden core 8760-70 Bureau of Economic Geology Oligoclase (73%), microcline microperthite (15%), biotite (5%), calcite (4%), chlorite (3o/o), apatite (tr), sericite (tr), quartz (tr), zircon (tr). Plagioclase is zoned and probably ranges from al bite to andesine; there are a few flakes of sericite from alteration of feldspar but for the most part the feldspar is kaolinized; calcite is associated with and replaces biotite and chlorite and fills interstices between feldspar subhedrons; biotite is a pale brown to reddish­brown variety that is partly altered to chlorite: quartz occurs with calcite as a secondary open­s pace filling; apatite occurs in long needles and in more or less equant grains. Grain size: 2 mm: Fabric: hypidiomorphic granular. Rock: syenodiorite. [This rock occurs as a tabular intrusive body within the Pennsylvanian section-not basement.] DALLAM COUNTY Pure #1 Sneed Heirs cuttings 6775-79 Bureau of Economic Geology Microcline microperthite (63%) , quartz (20%), plagioclase 05%), biotite (2o/o), magnetite or ilmenite (tr), leucoxene (tr), sphene (tr), pyrite (tr), apatite (tr). Plagioclase is partly sericitized; quartz is fractured and fractures are filled with pyrite; sphene is partly altered to leucoxene; biotite is red-brown. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Texas #1 Capitol Freehold Land Trust core 6168-69 Stanolind Oil & Gas Co. Groundmass (76%), albite (15%), quartz (7% ), magnetite (2o/o), calcite (tr), chlorite (tr}, fluorite (tr), zircon (tr), apatite (tr) . Microcrystalline micrographic groundmass is stained red and shot through with vein lets containing quartz, calcite, and fluorite; albite phenocrysts are partly altered to seririte; some of the quartz is in veinlets and some occurs as round phenocrysts. Grain size: groundmass up lo 0.05 mm; phenocrysts up to 3 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Texas #1 Capitol Freehold Land Trust cuttings 6169 Bureau of Economic Geology Groundmass (77o/o), plagioclase (15o/o), magnetite or ilmenite (5o/o), quartz (2%), chlorite (1% ) . Groundmass shows advanced kaolinization; it is mostly micrographic with equi·extin· guishing patches 0.2 mm in diameter; plagioclase phenocrysts show advanced sericitization; maf?:netite or ilmenite ocrurs in large masses and in vugs with secondary quartz and apatite. Grain size : f!J"Oundmass 0.1 to 0.2 mm; phenocrysts 1 to 3 mm. Fabric: porphyritic. Rock : rhyolite porphyry. Basement Rocks, Texas-New Mexico DEAF SMITH COUNTY Humble #1 Hyslop cuttings 7750-7800 Humble Oil & Rfg. Co. Labradorite (54% ), augite (30% ). magnetite or ilmenite 00% ), chlorite (4% ), calcite (2% ), apatite (tr) . Plagioclase is in zoned laths in a more or less triangular pattern; augite is locally in continuous large grains enclosing plagiorlase laths and locally in small grains between plagio· clase laths; chlorite occurs in masses and veinlets and has a rather high birefringence; apatite is in needles. Grain size: plagioclase 0.2 to 0.4 mm ; continuous augite grains up to 2 mm. Fabric: ranges from ophitic to subophitic. Rock: diabase. Humble #1 Hyslop core 7803-05 Humble Oil & Rfg. Co. Groundmass (87%), al bite (5%), mirroperthite (4%), quartz (3%), leucoxene 0 % ) , red iron oxide (tr), ilm•nite (tr), ralrite (tr), apatite (tr), zircon (tr). Groundmass is a line granular mass of quartz and alkali feldspar showing flowage structure around phenocrysts ; al bite is in twinned and rorroded phenocrysts; microperthite with poor perthitic development occurs as phenocrysts ; quartz is present as corroded and em bayed phenocrysts. Grain size: groundmass 0.01 to 0.02 mm; phenorrysts up to 3 mm. Fabric: porphyritic. Rock : rhyolite porphyry. DENTON COUNTY Hunt #1 Martin core Stanolind Oil & Gas Co. Andesine (69%), augite 00%), hornblende 00%), chlorite (4%), magnetite or ilmenite (2%), epidote (3% ), biotite (2%) , pyrite (tr), apatite (tr) . Plagioclase subhedrons show a rude orientation; augite is partly altered; hornblende pleochroism is pale brown to brown-i;reen; epidote occurs in grains, groups of grains, and veinlets; magnetite or ilmenite accommodates itself to the other grains in the slide and is evidently late-magmatic; pyrite is in a veinlet. Grain size: 0.3 to 0.5 mm. Fabric: hypidiomorphic granular. Rock: microdiorite. Jenkins, Kelsey, Jones & Eubanks #1 Waide core 1882 Bureau of Economic Geology Oligoclase and quartz (74% ), biotite (12% ), epidote (8% ), garnet ( 4% ), chlorite 0 % ) , calcite (1 % ) , sphene (tr) , apatite (tr) , zircon (Ir) . Vaguely zoned plagioclase and quartz form an aggregate in which plagiorlase is predominant; euhedral and subhedral garnet crystals occur in nests of brown biotite and epidote-the biotite-epidote-garnet nests are more or less concen· trated in layers. Grain size: 0.5 to l mm. Fabric: crystalloblastic-gneissic Rock: garnet-epidote­biotite-quartz-oligoclase gneiss. Texas #1 Yeatts core 2013 Bureau of Economic Geology Andesine (60% ), amphibole (35% ), magnetite or ilmenite (4% ), biotite (1% ), apatite (tr). Andesine forms a granular aggregate of twinned and untwinned grains; two varieties of am­phibole are present: (1) green-brown hornblende in more or less oriented prisms and (2) color­less amphibole-cummingtonite? thinly rimmed with blue-green hornblende; the green-brown hornblende is more or less concentrated in layers; biotite commonly fringes the opaque mineral. Grain size: 0.2 mm. Fabric: crystalloblastic-gneissic. Rock: hornblende-andesine gneiss or amphibolite. DICKENS COUNTY Humble #3 Matador core 7735-37 Humble Oil & Rfg. Co. Andesine (43% ), biotite (20%), hornblende (15%), quartz 02%), alkali feldspar (8% ), pyrite and magnetite or ilmenite (2%), apatite (trl, epidote (tr). Zoned plagioclase locally has seriritized cores; biotite is reddish brown; hornblende pleochroism is yellow-green to green; epidote is deri\"ed from alteration of plagioclase ; alkali feldspar is apparently microperthite. Grain size: 0.2 to 2 mm. Fabric: hypidiomorphic granular. Rock: quartz diorite. Humble #3 Matador core 7735.37 Humble Oil & Rfg. Co. !'lficrographically intergrown quartz and microcline microperthite (99%), magnetite or ilmenite 0 % ), chlorite (trl, sphene (tr), musco\"ite (tr), calcite (tr), bleached biotite (tr), zircon (tr), pyrite (tr). Grain size : intergrowth 0.2 mm ; equi-extinguishing patches up to 5 mm. Fabric: micrographic. Rock: micrographic granite. Bureau of Economic Geology, The University of Texas Humble #2-F Matador core 7535-42 Humble Oil & Rig. Co. Rock is transversed by a distinct plane separating coarser and finer types. 0) Plagioclase (54%), quartz (20%), microperthite (10%), sericite 00%), calcite (3%), red iron oxide (2%), leucoxene 0%), magnetite or ilmenite (tr), chlorite (tr), sphene (tr), epidote (tr), apatite (tr), zircon (tr). Rock is composed of angular grains of partly sericitized plagioclase, quartz, and microperthite in a mass of intergranular fibers of sericite; red iron oxide outlines grain boundaries; quartz-calcite veinlets are parallel to the bedding plane. Grain size: 0.01 to 0.1 mm. Fabric: clastic-crystalloblastic; no recrystallization of quartz and feldspar. Rock: meta-arkose. (2) Similar to (1) above but with more sericite and epidote. Grain size: mostly less than O.OZ mm. Fabric: clastic-crystalloblastic. Rock: meta-argillite. Humble #2-F Matador core 7 535-42 Humble Oil &Rig. Co. Same as 7535-42. Humble #1-G Matador cuttings 8230-40 Shell Oil Co. Microperthite (69%), quartz (30%), altered ferromagnesian mineral 0%), biotite (tr), chlorite (tr), apatite (tr). Microperthite is partly kaolinized; one grain shows micrographio intergrowth with quartz; quartz is severely strained; biotite is partly altered to chlorite; ferro­magnesian mineral is so altered as to be nearly opaque. Grain size: 0.1 to 2 mm. Fabric: ranges from hypidiomorphic granular to micrographic. Rock : granite. Humble #1-G Matador core 8246-48 Humble Oil & Rig. Co. Quartz (52%), microcline microperthite (25%), oligoclase 08%), biotite (4%), chlorite 0%), ilmenite (tr), leucoxene (tr), fluorite (tr), sphene (tr), zircon (tr), apatite (tr). Quartz is in large grains showing strain and "welded" structure and in micrographic intergrowth with potassium feldspar; green-brown biotite is in thin deeply colored plates. Grain size: 2 to 6 mm. Fabric: ranges from hypidiomorphic granular to micrographic. Rock: quartzose granite. Humble #1-G Matador core 8246-48 Humble Oil & Rig. Co. Quartz (50%), microcline microperthite (37%), albite (7%), biotite (4%), leucoxene 0%), sphene 0 %), ilmenite (tr), chlorite (tr), apatite (tr). Quartz occurs as large grains and in a micrographic intergrowth with microcline microperthite; albite shows incipient alteration to sericite: biotite is in thin green-brown plate. Grain size: l to 6 mm. Fabric: ranges from hypidio­ morphic granular to micrographic. Rock: quartzose granite. Livermore #I Bird rorc 8390 Humble Oil & Rig. Co. Quartz (52% ), oligoclase 05%), microperthite 05%), biotite (6%), calcite (6%), chlorite (3%), magnetite or ilmenite (3%), hornblende (tr), apatite (tr), sphene (tr), zircon (tr). Quartz is in part in large grains that show strain and "welded" structure; zoned plagioclase is variably sericitized; chlorite and calcite are apparently derived from alteration o[ hornblende; biotite is reddish brown; sphene is in spongy networks. Grain size: 2 to 8 mm. Fabric: hypidio­morphic granular. Rock: quartzose granite. Magnolia #1 Wiley cuttings 7700-16 Bureau of Economic Geology Oligocla>e-andesine? (50%), biotite (20%), quartz (20%), magnetite or ilmenite and red-brown iron oxide (7%), calcite (3%), epidote (tr), apatite (tr), chlorite (tr), pyrite (tr), leucoxene (tr). Slide is composed of only a few small grains. Red-brown iron oxide stains and obscures the slide; pla~ioclase shows incipient alteration to sericite; biotite is a deeply colored green-brown variety and is !orally in a parallel orientation; grains are fractured and biotite is locally reduced to masses that are not easily resolved. Grain size: 0.05 to 0.2 mm. Fabric: cataclastic. Rock: bioti te phyllite. !l!a~nolia #1 Wiley cuttings 7710-16 Bureau of Economic Geology Quartz (75%), chlorite 06%), sericite (5%), magnetite or ilmenite (4%). Only one small fraµ;mrnt present. Quartz is fractured ancl crushed throughout; r.hlorite and sericite is intergran­ ular. Grain size: 0.2 mm. Fabric: cataclastic. Rock: cataclastically altered metaquartzite. !llagnolia #1 Wiley cuttings 7710-16 Humble Oil & Rig. Co. Oligoclase (58%), biotite (20%), quartz (15%), chlorite (4%), alkali feldspar (3%), epidote (tr), magnetite or ilmenite (tr), apatite (tr), zircon (tr) . Plagioclase shows incipient sericitiza­tion and is anti-perthitic; green-brown biotite occurs in masses of small non-oriented plates; Basement Rocks, Texas-New Mexico most of alkali feldspar is present in anti-perthite; chlorite is derived from alteration of biotite ; epidote is secondary in veinlets. Grain size: 0.1 mm. Fabric: granoblastic (recrystallized quartz and feldspar; reconstituted argillaceous fraction). Rock: metarkosite. Magnolia #1 Wiley cuttings 7716 Shell Oil Co. Albite (71%), quartz (15%), sericite (5%), chlorite (5%), epidote (3%), leucoxene 0%), magnetite or ilmenite (tr), apatite (tr), calcite (tr). Albite is in twinned and untwinned grains, partly sericitized, and may include some alkali feldspar; sericite is unequally distributed in the slide and there are barren areas, areas with sporadic flakes, and masses of sericite showing flowage structure; epidote is also unequally distributed through the slide. Grain size: 0.02 to 0.10 mm. Fabric: crystalloblastic (granoblastic to sieve). Rock: phyllitic metarkosite. National Petroleum Association #1 Blackwell cuttings 8370-80 Shell Oil Co. Microperthite (79%), plagioclase (10%), quartz (10%), sericite (lo/a), chlorite (tr), altered ferromagnesian mineral (tr). Plagioclase is zoned and rimmed with alkali feldspar; it probably ranges from oligoclase to albite ; feldspar is partly sericitized. Grain size: 2.0 mm. Fabric: hypidiomorphic granular. Rock: granite. National Petroleum Association # 1 Blackwell cuttings 8370-80 Shell Oil Co. Quartz (40%), microperthite (39%), albite (15%), pyroxene? (4%), hiotite (2%), magnetite or ilmenite (tr), calcite (tr), chlorite (tr). Small fragment is a quartz-rich chip in which high quartz content is not critical in light of preceding slide; biotite, brown to very dark brown pleochroism, is rimmed with magnetite or ilmenite. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. National Petroleum Association #1 Blackwell cuttings 8380-84 Shell Oil Co. Albite (66%), microcline (25%), quartz (5%), biotite (2%), sericite (2%), leucoxene (tr), chlorite (tr), red and yellow iron oxide (tr). Some microcline remnants are completely sur­rounded by albite; biotite, pale to dark brown pleochroism, is partly altered to chlorite. Grain size: l to 3 mm. Fabric: hypidiomorphic granular. Rock : a/bite syenodiorite. Union of Cal. #1 Elliot cuttings 8110-17 Shell Oil Co. Albite (85%), quartz (11%), biotite (2%), chlorite (1%), magnetite or ilmenite 0%), apatite (tr). Biotite is partly bleached. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular­poorly developed. Rock: leuco-albite-quartz diorite. Woodward #1 Williamson cuttings 7780-87 Shell Oil Co. Alkali feldspar (50%), albite (25%), quartz (20%), sericite (3%), leuce>xene 0%), chlorite ( 1 % ) , magnetite or ilmenite (tr), zircon (tr). Feldspar is largely kaolinized; quartz occurs in angular grains; there is Jess intergranular material than in the preceding slide. Grain size: l mm. Fabric: elastic. Rock: meta-arkose. Woodward #1 Williamson cuttings 7780-87 Shell Oil Co. Quartz (35%), alhite (35%), fine intergranular quartz-feldspar-sericite mass contammg epi­dote grains 05%), microcline (8%), epidote (4%), sericite (3%). Larger quartz and feldspar grains are surrounded by a fine mass of quartz-feldspar-sericite-epidote; sericite also occurs as scattered flakes; epidotc abo occurs as small grains around the periphery of the larger quartz and feldspar grains. Grain size: about l mm (fine quartz-feldspar·sericite mass is less than 0.02 mm) . Fabric: elastic, but shows elements of reconstitution under low-grade metamorphism. Rock: meta-arkose. DONLEY COUNTY Doswell #1 Mcllfurty cuttings 537075 Humble Oil & Rfg. Co. Groundmass (58%), albite (27%), microperthite (8%), magnetite or ilmenite (3%), chlorite (2%), calcite (1'7(;), red iron oxide (1% ) apatite (tr), zircon (tr). Brown microspherulitic groundmass is apparently composed mostly of alkali feldspar and shows pronounced flowage structure; locally it is microgranular; corroded and ~mbayed phenocrysts are al bite, partly sericitized, and microperthite; calcite and chlorite replace an original ferromagnesian phcn­ocryst; magnetite or ilmenite occurs as scattered grains and in veinlets mostly oxidized to Bureau of Economic Geology, The University of Texas red iron oxide. Grain size: groundmass microspherulites are mostly 0.01 to 0.02 mm but sporadic spherulites reach l mm in diameter; phenocrysts are up to 3 mm. Fabric: porphyritic. Rock: trachyte porphyry. Honolulu #1 Ozier core Honolulu Oil Corp. l\licroperthite (55%), quartz \20'/c), albite (20%), chlorite (2%), biotite 0%), sphene 0%), leucoxene (l% ) , fluorite (tr), apatite (tr), magnetite or ilmenite (tr), calcite (tr) . Plagio· clase shows incipient alteration to sericite; biotite, pale to drab-brown pleochroism, is partly altered to chlorite. Grain size: 0.5 mm to l cm, average l to 3 mm. Fabric: hypidiomorphic granular. Rock: granite. Honolulu # l Ozier core 5890-93 Stanolind Oil & Gas Co. Oligoclase (60%), microperthite (20%), quartz (15%), chlorite (3%), biotite 0%), magnetite or ilmenite (1 % ) , leucoxene (tr), fluorite (tr), zircon (trI, calcite (tr). Plagioclase is partly sericitized; biotite, a deep brown variety, is partly altered to chlorite. Grain size: l to 3 mm. Fabric: hypidiomorphic granular. Rock: gran odiorite. Humble #1 Roach core 5265 Humble Oil & Rig. Co. Microperthite (43%), albite (30%), quartz 05%), chlorite (5%), sericite (3%), calcite (2%), magnetite or ilmenite (2% .l, leucoxene (tr), red iron oxide (tr), apatite (tr), zircon (tr). Partly kaolinized microperthite is locally micrographicully intergrown with quartz; albite occurs in large subhedrons and smaller laths; quartz is in angular grains; sericite occurs from alteration of plagioclase and as intergranular fil>ers; calcite is in patches and veinlets; mag· netite or ilmenite is in scattered grains; apatite forms small needles and prisms. Grain size: 0~2 to 0.5 mm with plagioclase subhedrons up to 2 mm. Fal>ric: hypidiomorphic granular. Rock: microgran.ite porphyry. Hunt #5 Ritchie cuttings 6465-85 Bureau of Economic Geology Groundmass (70%l, albite (25%), biotite (2'/o), leucoxene (2%), magnetite or ilmenite (1%), calcite (tr). l\licrographic quartz-alkali feldspar groundmass is obscured by red iron oxide; albite phenocrysts are altered and stained; biotite is green-brown; calcite replaces part of the groundmass. Grain size: phenocrysts 0.2 to 2 mm; groundmass 0.01 to 0.05 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Hunt #5 Ritchie cunings 6485-95 Bureau of Economic Geology Unmetamorphosed sandy dolomite. Hunt #5 Ritchie cullings 6495-6505 Bureau of Economic Geology Groundmass (60%), feldspar (35%\, red iron oxide (3%\, l>iotite (2%), magnetite or ilmen· ite (tr), apatite (tr). l\licrographic? quartz-alkali feldspar groundmass is heavily stained with red iron oxide and flecked with leucoxene; feldspar and biotite phenocrysts are stained and altered. Grain size: groundmass 0.01 to 0.05 mm ; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Placid #1 Kelly cuttings 6960-7050 Humble Oil & Rfg. Co . ..\nde>ine-lahradorite q2% l. aui:ite (25% l, iddingsite? (8%), chlorite (8%), magnetite or ilm~nite \5% I. olivine \3% I. alkali feldspar (2'fr), sericite \2% l, hornblende (2% I. biotite (2'/o), calcite 0%), apatite (trl . Zoned plagioclase shows incipient alteration to sericite; rare oJi,·inr rt>lict:o: are prr~t~nt hut most of it has con,·erted to iddingsite'!: biotite is red-brown; lwrnhl<"nde. st"condary, !"hows pale p:r("r-11 to green pleochroism, alkali feldspar is interstitial to pl erit:ite locally occur::. as small masses : calcite replaces ferromagnesian minerals ane ( 76~c \, quartz 02% \, magnetite or ilmenite and leucoxene !.'i%1. amphiboJe·' 15% 1, epidote (!'fr\, calcite (!o/o), chlorite (tr), sphene (tr), apatite (tr I. Partly kaolin;zec] fragments of alkali feldspar, subordinate plagioclase, and quartz c·on~littllt' the ma~~ of rock; amphibole '! needles and prisms are unequally distributed­in ~ome fragnwnb they compose UJ' to 15o/o of the rock and in others they are absent; Basement Rocks, Texas-New Mexico stratification is expressed in grain size and mineral percenta~es; rock is very similar to rocks in the Ritchie wells in Briscoe County. Grain size: variable 0.02 to 0.1 mm. Fabric: relict elastic-incipient crystalloblastic (sie\'e). Rock: metasiltstohe or horn/els. Shamrock #1 Adair cuttings 53-15-50 Bureau of Economic Geology Slide shows mostly mineral grains with only a few rock fragments. Al bite (45% l, quartz (30%) , alkali feldspar (25%l, magnetite or ilmenite (tr), apatite (trl, calcite (tr), zircon (tr). Quartz shows intense strain; alkali feldspar is kaolinized. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular with local cataclastic elements (crushing and brecciation). Rock: cataclastically altered albite granodiorite. Shamrock # 1 Adair core 5349-52; 5352-58 Stanolind Oil & Gas Co. Quartz (40% l, al bite (36%), alkali feldspar (20%), calcite (3%), magnetite or ilmenite (l'f.:1, red iron oxide (tr), muscovite (tr), apatite (tr). Strained quartz shows lamellae ex· tending to edge of grains; alkali feldspar is partly kaolinized and replaced by calcite; calcite also occurs in veinlets; parts of rock seem to have been brecciated. Grain size: 1 to 3 mm. Fabric: hypidiomorphic granular. Rock : albite granodiorite. Shamrock # 1 Adair core Honolulu Oil Corp. Alhite (40%), alkali feldspar (20'/'c l, quartz (15% l, sericite (10%), chlorite (8'/c), epidote (4%), magnetite or ilmenite (2%), leucoxene 0 %), zircon (tr). Plngioclase is partly seri· citized; epidote occurs with chlorite in scattered grains and in veinlets and as very fine grains in plagioclase as a result of alteration; some chlorite is apparently also derived from alteration of biotite. Grain size: 1 to 5 mm. Fabric: hypidiomorphic granular. Rock : a/bite granodiorite. Stanolind # 1 Broome core 6748-53 Bureau of Economic Geology Oligoclase (66%), quartz (15%1, chlorite (10%1, epidote (5%), alkali feldspar (3%) , apatite (I%), magnetite or ilmenite (tr), sphene (tr), zircon (tr). Plagioclase is anti-perthite -!'ome ~rains contain small rhombic or rectangular bodies, irregu1ar masses, and veinlets of alkali feldspar, some plagioclase is in advanced stage of sericitization; biotite, green-brown to \'Cry dark brown pleochroism, is partly altered to chlorite. Grain size: 0.5 to 4 mm. Fabric: hypidiomorphic granular. Rock: quartz diorite. Stanolind # 1 Broome core 6748-53 Stanolind Oil & Gas Co. Oligoclase (58%), quartz (35%), chlorite (4%), epidote (2%), magnetite or ilmenite (1%), red iron oxide (trl, pyrite (tr), zircon (tr), apatite (tr), biotite (tr). Plagioclase is partly sericitized (perhaps 7% of rock is sericite); biotite occurs in remnants and most of it is altered to chlorite; epidote commonly occurs in grains within the mass of chlorite; pyrite is in veinlets. Grain size: average 2 to 3 mm, maximum 1 cm. Fabric : xenomorphic granular. Rock: leuco-quartz diorite. Stanolind # 1 Broome core 6751-53 Stanolind Oil & Gas Co. Oligoclase (50'7c), alkali feldspar (26%), quartz (15% ), biotite (7%), chlorite (2%), apatite (tr), epidote (tr), pyrite (tr), leucoxene (tr), myrmekite (tr), magnetite or ilmenite (tr), calcite (tr). Biotite, pleochroism pale brown to deep red-brown, is partly altered to chlorite. Grain size: 2 to 5 mm. Fabric: hypidiomorphic granular. Rock : granodiorite. Stanolind # 1 Lewis cuttings 4085-86 Bureau of Economic Geology Oligoclase (57%), quartz (20%), alkali feldspar 05%), biotite (4%), chlorite (2%), leu­coxene 0%), magnetite or ilmenite 0%), apatite (tr), myrmekite (tr). Plagioclase shows a variable degree of alteration to sericite; biotite, pale brown to dark reddish-brown pleochroism, is partly altered to chlorite. Grain size: 0.2 to 2 mm. Fabric : hypidiomorphic granular. Rock: granodiorite. Stanolind # l Lewis core utopn Stanolind Oil & Gas Co. Alkali feldspar (40%), oligoclase (34%), quartz (20%), biotite (5o/o), chlorite 0%) , calcite (tr), zircon (tr), pyrite (tr) . Biotite pleochroism is pale brown to deep red-brown; zircon forms halos in biotite. Grain size: l mm. Fabric : xenomorphic granular. Rock: granite. Bureau of Economic Geology, The University of Texas Stanolind # 1 Lewis core "middle" Stanolind Oil & Gas Co. Alkali feldspar (42%), oligoclase (35%), quartz (15%), biotite (6%), magnetite or ilmen· ite (2%), pyrite (tr), apatite (tr), zircon (tr). Feldspar is partly kaolinized; biotite, pale red­brown to very dark brown pleochroism, shows incipient alteration to chlorite. Grain size: 1 mm. Fabric: xenomorphic granular. Rock: granite. Stanolind # 1 Lewis core "bottom" Stanolind Oil & Gas Co. Oligoclase ( 48%), quartz (25%), hornblende (20%), pyroxene? ( 4%), magnetite or ilmenite (2%), calcite (1%), apatite (tr) . Hornblende, yellow-brown to deep yellow-green pleochroism, occurs as poikilitic equant grains; pyroxene? is partly altered to calcite. Grain size: 0.2 to 0.5 mm. Fabric: xenomorphic granular. Rock : quartz diorite. Welch #1 Lazy RG Ranch cuttings 5160-7020 Humble Oil & Rfg Co. Albite (48%), microperthite (30%), quartz (13%), biotite (2%), chlorite (2%), magnetite or ilmenite 0%), hornblende 0%), sphene (1%), epidote 0%), calcite 0%), apatite (tr). Al bite is partly sericitized; some microperthite occurs as large host grains containing inclusions of the other rock-making minerals including microperthite; biotite pleochroism is yellow-brown to very dark brown; epidote and chlorite occur together from alteration of ferromagnesian minerals; hornblende pleochroism is yellow-green to green. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular-poikilitic. Rock: albite granodiorite. ECTOR COUNTY Phillips #15 Embar cuttings 8503 Bureau of Economic Geology Quartz (46%), microcline (30%), biotite (10%), calcite (10%), albite (3%), pyrite 0%), apatite (tr). Microcline is in small grains in a mosaic with quartz and in larger poikilitic grains; partly oxidized dark brown biotite is in oriented laths; calcite occurs in layers parallel to the foliation. Grain size: 0.1 to 0.2 mm. Fabric: crystalloblastic. Rock: biotite-calcite­ microcline schist. Phillips #23 Embar cuttings 8550-55 Bureau of Economic Geology Quartz (71%), microcline (15%), albite-oligoclase (5%), phlogopite or biotite (4%), calcite (4%), magnetite or ilmenite 0%), apatite (tr). Quartz shows parallel lines of inclusions and Boehm lamellae; mica pleochroi;m is colorless to very pale brown and it may be phlogopite or biotite; plagioclase is sericitized ; mica locally shows a parallel orientation. Grain size: 0.2 to 0.5 mm. Fabric: granoblastic. Rock: metaquartzite. Phillips # 1-J TXL cuttings 11170-80 Shell Oil Co. Oligoclase (50%), quartz (30%), hornblende 03%), chlorite (5%), biotite (2%), calcite (tr!, ma11netite or ilmenite (tr), apatite (tr). Plagioclase is partly sericitized; hornblende plco..Juoirn1 is deep yellow-green to deep green; biotite, green-brown, is partly altered to chlorite. Grain size : 0.2 to 0.5 mm. Fabric: xenomorphic granular. Rock: leuco-quartz micro­ diorite. Phillips #1-J TXL cuttings 11190-11215 Shell Oil Co. ..\]bite (SO';;.), quartz (35%), biotite 05%), microcline (tr), apatite (tr), zircon (tr). Plagiocla~c i:0; partly sericitized ; biotite is an intensely colored green-brown variety. Grain size: 0.2 lo 1 mm. Fabric: hypidiomorphic granular. Rock: leuco-albite-quartz microdiorite. Phillips #l·J TXL cuttings 11210-15 Bureau of Economic Geology Oligocla>c (5i%) , quartz (15%). biotite (10%) , hornblende (10%), microcline (3%), magne­tite or ilmenite (2% ), chlorite (2%), apatite (1'%). calcite (tr), epidote (tr). Plagioclase is partly srricitized: microclint> occur:-; in part as inclusion~ in plagiocla~e; biotite, green-brown, is partly altered to chlorite; hornblende pleochroism is yellow-green to blue-green. Grain ~izP: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: quartz microdioritc. Texas #6 Cowden cuttings 8560-70 Bureau of Economic Geology ;\licrocline 130-100%) , quartz (0-35%) , plagioclase (0-5%), biotite (0·5%), pyrite (0-5%), musco,·ite W--1%) , magnetite or ilmenite (0-2%), apatite 0%), zircon (tr). Percentages of minerals vary widely in different fragments; biotite, pale brown to brown pleochroism, is Basement Rocks, Texas-New Mexico locally in parallel orientation; plagioclase occurs in only one fragment and is sericitized. Grain size: 0.05 to 0.5 mm. Fabric : crystalloblastic. Rock : granite gneiss or arkose gneiss. EL PASO COUNTY Jones # 1 Sorely cuttings 2213-20 Bureau of Economy Geology Oligoclase-andesine (39%), hornblende (30%), quartz 02%), biotite (8%), chlorite? (6%), magnetite or ilmenite (3%), alkali feldspar (2%), apatite (tr). Zoned plagioclase is partly sericitized; green-brown hornblende is partly altered to blue-green hornblende ; biotite pleo­chroism is pale to dark brown with a reddish tinge; greenish-brown moderately birefringent mineral in plagioclase cleavages and replacing plagioclase and hornblende may be a chlorite; alkali feldspar occurs as spongy masses and in veinlets between plagioclase grains. Grain size: 0.5 to 2 mm. Fabric : hypidiomorphic granular. Rock: quartz diorite. FISHER COUNTY General Crude #1 Aiken cuttings 7505 Bureau of Economic Geology Microcline microperthite (78%), albite 00%) , quartz 00%), magnetite or ilmenite (2%), calcite (tr), sphene (tr), chlorite (tr), leucoxene (tr), apatite (tr). Plagioclase is partly altered to sericite; calcite occurs in veinlets. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular. Rock: granite. General Crude # 12 Flanagan cuttings 6743-46 Bureau of Economic Geology Quartz (61%), albite and microcline (20%), biotite (10%), epidote (7%), muscovite (2%), sphene (tr), chlorite (tr), apatite (tr). Quartz occurs in a mosaic with feldspar; albite is partly sericitized and may include some oligoclase; mica shows a parallel orientation; slide consists of very small fragments, and mineral percentages are a very rude estimate. Grain size: 0.1 to 0.2 mm. Fabric : crystalloblastic. Rock: epidote-biotite schist. (Photomicrograph Pl. VII, C.) Humble #1 Crowley core 7085 Humble Oil & Rfg. Co. Quartz (52%), microcline (20%), oligoclase-andesine (12%), biotite (10%), epidote (4%), muscovite (2%), sphene (tr), apatite (tr). Quartz and feldspar form a mosaic ; biotite is in non-oriented plates (apparently, in light of succeeeding slide, this section is cut nearly parallel to the foliation); epidote is in scattered grains. Grain size : 0.1 to 0.5 mm. Fabric: crystallo­blastic. Rock: biotite schist. Humble #1 Crowley core 7085 Humble Oil & Rfg. Co. Quartz (42%), oligoclase (25%), microcline 05%), biotite 02%), epidote (3%) , muscovite (2%), calcite (l% ) , magnetite (tr), chlorite (tr), apatite (tr). Oriented plates of green-brown hiotite are dispersed in a quartz-feldspar mosaic. Grain size: 0.2 to 0.5 mm. Fabric: lepidoblastic. Rock : biotite schist. Lion # 1 Huddleston cuttings 5920-40 Bureau of Economic Geology Quartz (60%), microcline (39%), calcite 0%), chlorite (tr), magnetite or ilmenite (tr), sericite (tr), fluorite (tr), leucoxene (tr), red iron oxide (tr), zircon (tr) . Microcline, including minor sodic plagioclase, occurs in a mosaic with quartz. Mineral percentages are highly variable in different fragments ; quartz and feldspar are unequally distributed in rude layers in some fragments. Grain size: 0.05 to 0.2 mm. Fabric : granoblastic-gneissic. Rock : arkose gneiss. Skelly and Lion # 1 Lanning cuttings 6140-60 Bureau of Economic Geology Alkali feldspar (72%), quartz (20%) , sericite-muscovite (2%), pyrite (2%), biotite 0%), leucoxene (l% ) , calcite (l% ) , chlorite (l% ) , apatite (tr). Alkali feldspar forms a mosaic with quartz, most probably it is microcline (vague quadrille twinning) ; quartz is more or less con· centrated in layers; sericite-muscovite is in parallel oriented fibers and plates; pale green-brown biotite flakes show parallel orientation. Grain size: 0.02 to 0.1 mm. Fabric: granoblastic-gneissic. Rock : arkose gneiss. Bureau of Economic Geology, The University of Texas Texas #I Stephens cuttings 6151 Bureau of Economic Geology Quartz (54'."'cl . oligoclase (25':(-l, microcline (12%), biotite (7%), muscovite (1%), magnetite or ilmenite \!% l, dolomite (tr\, apatite (tr), zircon (tr). Plagioclase is partly sericitized; biotite. golden brown to almost black plf'Ochroism, occurs in oriented plates. Grain size: 0.1 to 0.5 mm. Fabric g:ranoblastir-g:ne-is.si<'. Rock: arlwse gnt•i_..s. Texas # 1-C Stephens cuttings 6280-90 Shell Oil Co. Microcline and albite (50'7cl , quartz (38o/o), chlorite (8%), epidote (4%), apatite (tr), calcite (tr), sphene (trl, zircon (tr). lllicrocline, albite, and quartz occur in a mosaic; tbe feldspar shows incipient kaolinization; chlorite occurs in rudely oriented fibers; epidote is in tiny grains between feldspar and quartz grains. Grain size: 0.1 to 0.2 mm. Fabric: granoblastic-gneissic. Rock : arkose gneiss. FLOYD COUNTY Chiles #I Strickler cuttings 7730-35 Shell Oil Co. Olig:oclase (95%), senc1te (-lo/o), calcite (!%), amphibole (tr), biotite (tr), epidote (tr), rutile (tr). Plagioclase is partly sericitized ; am phi bole pleochroism is pale tan to deep green. Grain size: I mm. Fabric: hypidiomorphic granular. Rock: leuco-microdiorite. Chiles #I Strickler cuttings i755-57 Shell Oil Co. Oligoclase-andesine (95%) , sericite (3% ), biotite (!'Jc) , chlorite (I%l, amphibole (tr), leucoxene (tr). apatite (tr). Biotite, pale to dark brown pleochroism, is partly altered to chlorite; am phi bole plf'OChroism is pale to deep green. Grain size: I mm. Fabric: hypidiomorphic granular. Rock: leuco-microdiorite. Houston #I Lackey cuttings 10390-95 Bureau of Economic Geology Labradorite (59<;'c), augite \25':(-), chlorite? 00%), biotite (3'7c), chlorite 0%), magnetite or ilmenite \!%),apatite (1%l. Plagioclase is zoned and partly sericitized; chlorite? is an oli,.e-green mineral with moderate birefringence that seems to have originated through alteration of oli,·ine: biotite is red-brown; apatite is in needles. Grain size: 1 to 3 mm. Fabric: hypidio­morphic granular. Rock: gabbro. Lh·ermore #I Krause core 7834-35 Honolulu Oil Corp. Oligoclase (64%). quartz (IO<;'cl, chlorite (8%l. calcite (8% ), biotite (5%), sericite (3%), alkali feldspar (2';(-), pyrite (tr), apatite (tr), sphene (tr), magnetite or ilmenite (tr), leucox­ene (tr I, zircon \tr l. Plagioclase occurs in large twinned subhedrons; chlorite is derived from alteration of biotite but also occur.s in granular masses associated with calcite; calcite occurs in veinlets and with chlorite and seric-ite in a network of fine veinlets; biotite pleochroism is pale brown to dark green-brown: zircon forms weak halos in biotite: alkali feldspar is interstitial to plagioclase subhedrons. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: altered leuco-quart; diorite. Lh·ermore # 1 Krause core 7835-36 Honolulu Oil Corp. (I) Similar to 7834-35. (!?\ ~lirroC'rystalline to rryptorrystaHine matrix rontaining carbonate grains, angular frag;ments of quartz and plagioclase, bent and altered biotite plates, zircon, and chlorite. This is probably a crnshed phase of (I l : it is composed mostly of \'ery fine crushed quartz and feldspar. The calcite is largely secondary and fills cracks and replaces matrix. Livermore # 1 Krause core 7836-38 Honolulu Oil Corp. Oli~oclase I 63C:C l . calcite IIO'lc I, chlorite (IO'Jc l. quartz (8lastic. Rock: chlorite schist. Bureau of Economic Geology, The Universitl of Texas Honolulu #2 Whittaker core 5296-97 Bureau of Economic Geology Quartz (50%), microcline and albite (42%), chlorite (4%), muscovite (3%), calcite 0%), magnetite or ilmenite (tr), fluorite (trl, apatite (tr). Quartz forms an unevenly sized gran­oblastic mosaic with feldspar; albite is partly sericitized and locally contains masses of small mica plates and chlorite; muscovite is in local concentrations where it shows a more or less parallel orientation. Rock shows folded layers of different grain size and mineral composition. Grain size: 0.02 to 0.5 mm. Fabric: crystalloblastic-gneissic. Rock: arkose gneiss. Honolulu #3 Whittaker core 5656-59 Bureau of Economic Geology Microcline (45%), quartz (40%), chlorite (4%), calcite (4%), sericite-muscovite (3%), apatite (2%), sphene and leucoxene (1%), red iron oxide 0%), magnetite or ilmenite (tr), zircon (tr). Microcline is fractured and fractures are healed with alkali feldspar; quartz occurs unequally distributed in patches of grains; chlorite includes some bleached biotite; calcite occurs in patches within feldspar grains and with chlorite in veinlets; sericite-muscovite occurs in masses and in veinlets, commonly it is associated with chlorite. Locally there is a rude foliation. In this rock fractured large microcline grains and patches of quartz were invaded by a finer microcline-chlorite-mica-calcite assemblage. Grain size: broken microcline and coarse quartz 0.5 to 2 mm; fine microcline-chlorite-mica-calcite assemblage 0.2 mm. Fabric: ranges from xenomorphic granular to gneissic ( protoclastic?). Rock: granite gneiss. Honolulu #4 Whittaker core 5840 Bureau of Economic Geology Quartz (73%), microcline and albite (20%), chlorite (3%), magnetite or ilmenite (2%), biotite 0%), calcite 0%), muscovite (tr), sphene (tr), apatite (tr). Quartz forms a well­sized granoblastic mosaic with feldspar; microcline is predominant over al bite; chlorite is after biotite and is also in veinlets with calcite; green-brown biotite and chlorite are both in skeletal poikilitic grains showing rude orientation. Grain size : 0.1 to 0.2 mm. Fabric: granoblastic. Rock: metaquartzite. Hunt #1 McElmurray cuttings 5443-5561 Humble Oil & Rig. Co. Quartz (54%), microcline (35%), plagioclase (8%), muscovite (3%), calcite (tr), magnetite or ilmenite (tr), apatite (tr). Plagioclase is completely sericitized; quartz and feldspar form a mosaic showing a crude dimensional orientation. Grain size: mostly 0.1 to 0.2 mm. Fabric: granoblastic (recrystallized quartz and feldspar). Rock: metarkosite. Seaboa~d # 1 Earwood cuttings 6886-90 Bureau of Economic Geology Microcline and albite (50%), quartz (41%), biotite, chlorite, and hornblende (5%), magne· tile or ilmenite (3%), calcite 0 %), apatite (tr). Feldspar is predominantly microcline; quartz. showing dimensional orientation in some fragments, forms a granoblastic aggregate with feldspar; green-brown biotite is partly altered to chlorite; green hornblende is partly altered to rhlorite and shows parallel orientation in some fragments. Mineral percenta~es vary widely in different fragments. Grain size : 0.1to0.5 mm. Fabric : granoblastic (foliation very poorly developed). Rock : metarkosite or arkose gneiss. Seaboard #l·A Hanks cuttings 6100-10 Bureau of Economic Geology Microcline microperthite (65%), oligoclase (20%), biotite (8%), quartz (7%), apatite (tr), pyrite (tr), zircon (tr). l\licrocline microperthite is locally poikilitic and includes quartz and oriented biotite plates; red-brown biotitc shows sub-parallel orientation; apatite is in rods that cross grain boundaries. Grain size: 0.5 mm. Fabric: xenomorphic granular-poikilitic. Rock: microgranite. (Contains one fragment similar to #1-A Hanks 6140). Seaboard # 1-A Hanks cuttings 6140 Bureau of Economic Geology Microcline microperthite (60%), quartz (20%), oligoclase (20%), apatite (tr). Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic to xenomorphic granular. Rock: granite. (Also contains fragments similar to #1-A Hanks 6100-6l10.) Seaboard # l Jordan cuttings 6430-34 Bureau of Economic Geology Alk~li feldspar (45%), plagioclase (25%), biotite (15%), calcite 02%), magnettte or il­!"cmte (~%), py~oxene _(tr), apatite_ <.tr). Alkali feldspar shows a vague quadrille twinning; rntermed1ate plagooclase 1s rn part senc1ttzed; pyroxene 1s completely altered to calcite. Rock is similar to 6415 but contains more alkali feldspnr, shows sericitization of plagioclase, and con­tains no unaltered pyroxene. Grain size : 0.5 to l mm. Fabric : xenomorphic granular-gneissic. Rock: syenite gneiss. Basement Rocks, Texas-New Mexico Seaboard #I Jordan cuttings 6445 Bureau of Economic CPology Andesine (35%), alkali feldspar (24%), augite (20%), biotite 05%), magnetiie or ilmenite (3%), calcite (3%), apatite (tr). Plagiodase shows a vague zoning; alkali feld"par has a vague and blurred extinction that suggests quadrille twinning; augite has a faint greenish pleochroism; biotite, golden-brown to very dark brown pleochroism, shows a rude orientation in some fragments; calcite occurs in veinlets and masses. Mineral percentages vary considerably in different fragments. Grain size: 0.5 to I mm. Fabric: gneissic. Rock: biotite·augite·andesine gneiss or syenodiorite gneiss. OLDHAM COUNTY Alhough #I Matador cuttings 4850-4915 Bureau of Economic Geology Microcline microperthite (78%), quartz (20%), chlorite 0%), leucoxene 0%), bleached biotite (tr), magnetite or ilmenite (tr). Microcline microperthite, in advanced stage of kaoliniza­tion, is micrographically intergrown with quartz-intergrowth is mostly irregular hut locally cuneiform; fine acicular inclusions in the quartz and feldspar were not identified. Grain size: 1 to 2 mm; intergrowth 0.1 to 0.2 mm. Fabric : micrographic. Rock: micrographic granite. Albough #1 Matador cuttings 4850-4910 Humble Oil & Rfg. Co. Microperthite (78%), quartz (20%), magnetite or ilmenite 0%), chlorite 0%l, bleached biotite (tr), leucoxene (tr), biotite (tr), amphibole (tr), fluorite (tr), apatite (tr). Microperthite is in part intergrown with quartz in an irregular to cuneiform pattern; biotite is red-brown ; amphibole is yellow-green. Grain size: 1 to 2 mm; intergrowth 0.01 to 0.5 mm. Fabric : micro­graphic. Rock: micrographic granite. Albough #2 Matador cuttings 4090-4115 Humble Oil & Rfg. Co. Microcline microperthite (76%), quartz (20%), magnetite or ilmenite (2%), chlorite 0%), calcite 0 % ) , leucoxene (tr), sphene (tr) , fluorite (tr) , zircon (tr). Microcline microperthite is in part intergrown with quartz in an irregular to cuneiform pattern; calcite occurs as veinlets and scattered grains. Grain size: 0.5 to 2 mm; intergrowth 0.02 to 0.2 mm. Fabric: micrographic. Rock : micrographic granite. Humble #1-E Matador core 6280 Stanolind Oil & Gas Co. Microperthite (67%), quartz (20%), albite 00%), magnetite (2%), chlorite 0%), red iron oxide (tr), epidote (tr) . Epidote is in veinlets and radiating structures. Grain size : 3 to 6 mm. Fabric: hypidiomorphic granular. Rock: granite. Livermore # 1 Moser cuttings 6880 Humble Oil & Rfg. Co. Groundmass (86%), albite (7%), chlorite (4%), ilmenite 0%), leucoxene 0%), red iron oxide 0 % ) , epidote (tr), sphene (tr), apatite (tr) . Microgranular quartz-alkali feldspar ground­mass is locally cryptocrystalline and locally contains pods of interlocking feldspar laths; albite phenocrysts are locally fractured; leucoxene occurs as scattered masses and surrounds ilmenite; epidote occurs as veinlets and replacement bodies. Grain size: groundmass 0.01 to 0.1 mm; phenocrysts 0.5 to I mm. Fabric: porphyritic. Rock: rhyolite porphyry. Livermore # 1 Moser cuttings 6880-84 Humble Oil & Rfg. Co. Groundma« sociated with red-brown biotite; fibrous green amphibole is a secondary alteration mineral derived from augite; chlorite is (l) a common green variety and (2) an olive-colored variety from alteration of olivine; zeolite---thompsonite and an unidentified feldspathoid fill a vug and constitute an amygdule. Grain size: l to 6 mm. Fabric: hypidiomorphic granular. Rock : olivine gabbro. Gulf #1-A Keliehor core 9627-28 Bureau of Economic Geology Augite (42%), labradorite (32%), olivine (7%), magnetite or ilmenite (5%), iddingsite? (5%), serpentine (3%), am phi bole (3%), chlorite (2%), red-brown biotite (1% ), apatite (tr). Plagioclase shows vague zonation; augite is tinted brown ; olivine is altered to serpen­tine and iddingsite?; secondary amphibole is pale green; red-brown biotite occurs in asso­ ciation with magnetite or ilmenite grains; pale green-brown hiotite occurs in sheaves. Grain size: 1 mm to l cm. Fabric: hypidiomorphic granular. Rock: olivine gabbro. (Photomicro­graph, Pl. IX, B.) Stanolind #1 Jarrell core 8160-61 lf2 Stanolind Oil & Gas Co. Groundmass (82%), alkali feldspar (12%), albite (3%), chlorite (3%), magnetite or ilmenite (tr) , epidote (tr), sericite (tr) , leucoxene (tr) , zircon (tr) , red iron oxide (tr). Microcrystalline to cryptocrystalline quartz-feldspar groundmass shows flowage structure and locally is micro­graphic; cryptocrystalline areas surround phenocrysts; albite and alkali feldspar phenocrysts occur as rounded and corroded subhedrons. Grain size: groundmass is microcrystalline to crypto­crystalline; phenocrysts up to 3 mm. Fabric: porphyritic. Rock : rhyolite porphyry. Bureau of Economic Geology, The University of Texas Stanolind #1 Jarrell cuttings 8161 Y2 Bureau of Economic Geology Groundmass (88%), plagioclase 00%), chlorite 0%), leucoxene (1% ) , apatite (tr). Brown microcrystalline to cryptocrystalline groundmass is micrographic to microspherulitic and shows pronounced flowage structure; plagioclase phenocrysts are completely sericitized; chlorite is derived from biotite. Grain size: groundmass in large part cryptocrystnlline, equi-extinguishing micrographic areas reach 0.1 mm; phenocrysts 0.2 to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Sunray # 1 Kimbrough cuttings 8870-80 Shell Oil Co. Grains of altered feldspar, quartz, chloritized biotite, apatite, leucoxene, and magnetite or ilmenite. Grain size: 0.1 to 1 mm. Fabric: sub-micrographic. Rock: micrographic granite. U. S. Smelting & Refining #1-A Osborner cuttings 9700-10 Bureau of Economic Geology Groundmass (79%), plagioclase (20%), leuxocene 0%), apatite (tr), zircon (tr). Brown quartz-alkali feldspar groundmass is in part micrographic and in part microspherulitic; plagio­clase occurs as partly sericitized phenocrysts and probably some alkali feldspar is included in this total; slide contains one large fragment of rhyolite porphyry and a number of fragments of arkose containing round rhyolite sand grains. Grain size: groundmass, micrographic patches up to 0.2 mm; microspherulites up to 0.02 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. PECOS COUNTY Anderson-Prichard #2 Boren core 4858 Humble Oil & Rig. Co. Albite-oligoclase (40%), microcline (36%), quartz (10%), biotite (10%), chlorite (4%), sphene (tr), apatite (tr), zircon (tr) _ Plagioclase is partly seriticized and locally myrmekitic: rock shows two distinct layers separated by grain size and mineral content-one layer, 1 to 2 mm grain size, is rich in quartz-microcline ; the second layer, 0.5 mm grain size, is rich in plagioclase and biotite; biotite is reddish brown and occurs as oriented plates concentrated in the finer-grained layer. Grain size: 0.5 to 2 mm. Fabric : crystalloblastic-gneissic. Rock: granodiorite gneiss. Anderson-Prichard #2 Masterson core 4570 Bureau of Economic Geology Andesine-labradorite (50%), amphibole (25%), magnetite or ilmenite 00%), brownish chlorite? (7%), green chlorite (5% ), apatite (2%), calcite 0%), sphalerite (tr). Plagioclase is transected by a network of veinlets of brownish mineral that is probably chlorite; amphibole, pale green to pale brown pleochroism, occurs in bent, frayed prisms and is partly altered to chlorite; apatite is in unusually large grains; the resinous opaque mineral is probably sphalerite (blackjack). Grain size: 2 to 10 mm. Fabric : hypidiomorphic granular. Rock: altered diorite. Anderson-Prichard #I-A Masterson cuttings 4560-64 Bureau of Economic Geology Quartz (40%), microcline (29%), hematite (20%), sericitized plagioclase (5%), finely crushed material (3%), biotite relicts (2%), chlorite 0%), calcite (tr), apatite (tr), zircon (tr), pyrite (tr), magnetite or ilmenite (tr). Quartz occurs in large fractured grains; hematite is in 1arw· masst>B-; sericite almost completely replaces plagioclase..Mineral percentages ahove are not too sif!nificant; some fragments are microcline-quartz-sericite-plagioclase rock, others show the sam<' rock fra£'turrJ and altered and containing introduced hematite. Grain size: 0.5 to 2 mm. Fabric: xcnomorphic granular. Rock: altered granite. Childress Royalty # 1 Masterson cuttings 4600-05 Bureau of Economic Geology Hornblende (45%), labradorite (40%), magnetite or ilmenite (6%), chlorite (3%), pyrite (2%), biotite (2%), red and yellow iron oxide 0%), calcite 0%), leucoxene (tr), apatite (tr I. Plaµiocla~e is variably altered to f'.er icite or st'ricite and very fine granular epidote or clina ozoi:-;ite: it indudes fine nt'rdles of an unidentified mineral; hornhlende, yelJow-grecn to green pleochroism, locally is so altered as to be nearly opaque; commonly it shows grain centers altered to a fibrous brownish hii::hly birefringent mineral and fringes of unaltered green or blue-~reen hornblende; commonly the hornblende includes fine grains of magnetite or il­ menite: magnetite or ilmenite is intergrown with pyrite; biotite: pleochroism is pa1e brown to dark red-brown; chlorite is in veinlets. Grain size: I to 3 mm. Fabric: hypidiomorphic granular. Rork: altered hornblende gabbro. Basement Rocks, Texas-New Mexico Gulf #1 Garvin cuttings 4525-30 Bureau of Economic Geology Oligoclase ( 47%), quartz ( 40%), microcline (10%), chlorite (1% ) , calcite (1% ) , leucoxene 0%), ilmenite (tr), zircon (tr). Plagioclase is partly sericitized; chlorite is in granular masses; ilmenite is partly altered to leucoxene. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granodiorite. Gulf #1 Millar core Stanolind Oil & Gas Co. Microcline microperthite (73%), quartz (20%), oligoclase (5%), biotite (1%), chlorite 0%), zircon (tr), magnetite or ilmenite (tr), sericite (tr). Quartz is in a mosaic with feldspar; biotite in part poikilitic, shows pale brown to red-brown pleochroism. Grain size: 0.1 to 1 mm. Fabric: xenomorphic granular. Rock: microgranite. Gulf #1 Millar core 4536-38 Bureau of Economic Geology 0) Microcline microperthite (67%), quartz (25% ), albite (5%), biotite 0%), chlorite 0%), magnetite or ilmenite 0 % ) , apatite (tr), myrmekite (tr), leucoxene (tr), pyrite (tr). Red-brown biotite is partly altered to chlorite. Grain size: 0.2 to 0.5 mm. Fabric: xeno· morphic granular. Rock: microgranite. (2) Microcline microperthite (58%), quartz (35%), plagioclase (5%), leucoxene 0 %) , magnetite or ilmenite 0 % ) , zircon (tr), biotite (tr). Quartz is strained and locally granulated; plagioclase shows incipient alteration to sericite. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular-cataclastic. Rock : cataclastically altered granite. Gulf #2 Millar cuttings 4489-93 Bureau of Economic Geology There appear to be several rock types represented in the fragments in this slide: (1) quartz. alkali feldspar mosaic ranging from 100% quartz to 100% feldspar, (2) quartz grains in a mass of chlorite, (3) quartz grains cemented with pyrite, (4) quartz grains in a microcrystalline matrix. Number 1 is probably representative of basement and is a microgranite (grain size: 0.2 to 0.4 mm; fabric : xenomorphic granular). Number 4 consists of angular quartz, apatite, magnetite or ilmenite, biotite relicts, zircon, red iron oxide, and a microcrystalline quartz·chlorite~ sericite-chalcedony matrix. In one fragment the biotite plates are oriented. Grain size: 0.5 mm. Fabric: clastic-cataclastic? Rock: metasandstone?. Gulf # 3 Millar cuttings 4396-4404 Bureau of Economic Geology Alkali feldspar and alhite (55%), quartz (25%), chalcedonic silica (20%), calcite (tr), apa· tite (tr). Albite is partly sericitized, alkali feldspar is partly kaolinized; the chalcedonic silica is in small grains and masses of grains; quartz is embayed by chalcedonic material and locally shows a sutured fabric. Grain size: 1 to 2 mm ( chalcedonic material less than 0.02 mm). Fabric: hypidiomorphic granular-metasomatic. Rock: altered granite or granodiorite. <:111£ # 1 O'Sullivan cuttings 4630-35 Bureau of Economic Geology Labradorite (72%), hypersthene 00%), quartz (5%), biotite (4%) , magnetite or ilmenite (4%), hornblende (4%), pyrite 0%>. apatite (tr), chlorite (tr), sphene (tr), epidote (tr), zircon (tr) . Plagioclase is very calcic, near bytownite, with maximum extinction angle about 40 degrees; it commonly shows two directions of twinning and is variably sericitized; in hornblende-rich fragments it is almost completely sericitized; quartz may be largely second­ary; hornblende is largely restricted to one fragment and shows yellow-green to green pleo­chroism; biotite is very deep red-brown; chlorite is derived from alteration of hornblende. Grain size: 0.2 to 0.5 mm. Fabric: hypidiomorphic granular. Rock: leuco-microgabbro. Humble #1 B. F. Smith cuttings 5167-68 Bureau of Economic Geology Plagioclase (60%), hornblende (35%), magnetite or ilmenite (5%), sericite (included in feldspar), zoisite and epidote (included in feldspar), apatite (tr) . Plagioclase, probably originally andesine or labradorite, is completely altered to fine masses of sericite and epidote; hornblende, yellow-green to green pleochroism, shows blue-green borders. Grain size : 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: altered diorite or microdiorite. Humble #1-L University core 5630-40 Bureau of Economic Geology Oligoclase (56%), microcline (20% ), biotite (6%), chlorite (5%), quartz (5%), magnetite or ilmenite (4%), sphene (2%), apatite (1%), calcite (1 % ), myrmekite (tr), amphibole (tr), pyrite (tr), zircon (tr). Plagioclase shows incipient sericitization; three types of biotite are present-pleochroism pale brown to dark brown, pleochroism pale brown to reddish brown, Bureau of Economic Geology, The University of Texas and pleochroism yellow-Lrown to dark green; chlorite is an alteration product of biotite and amphibole; amphibole relicts are surrounded hy chlorite and calcite; apatite and sphene occur in strings and roughly defined layers parallel to the foliation. Grain size: 0.5 mm. Fabric: crystalloblastic-gneissic. Rock: quartz-biotite-microcline-oligoclasc gneiss or granodi­ orite gneiss. HumLle #1 Wilson core 5235 Bureau of Economic Geology Quartz (55%), microcline (30%), albite 00%), muscovite (2%), magnetite or ilmenite (2%), leucoxene (I%), apatite (tr), chlorite (tr). l\Hcrocline shows only minor perthitic development; plagioclase shows incipient alteration to sericite; some magnetite grains are rimmed with red iron oxide. Grain size: very irregular with large microclinc grains and elongated lenses of smaller quartz-microcline-albite grains, range 0.05 to 2 mm. Fabric: gneissic-poorly developed layering of coarser and finer material. Rock: granite gneiss. Los Nietos # 1-B University cuttings 5550-60 Bureau of Economic Geology l\!icrocline (67%), quartz (20',1o), oligoclase (7%), biotite (4%), chlorite 0%), pyrite(!%), sphene (tr), zircon (tr), calcite (tr), apatite (tr), magnetite or ilmenite (trl. lliotite is dark red-brown. Grain size: 0.5 to 1 mm. Fabric: xenomorphic granular-poikilitic. Rock: micro­ granite. Los Nietos # 1-B University cuttings 5615-20 Bureau of Economic Geology Same as 5550-60. McCandless #1-10 Atlantic cuttings Shell Oil Co. l\!icrocline (53%), quartz (30%), biotite 00%), albite (5%), sericite (2%), apatite (tr), mag­netite or ilmenite (tr), red iron oxide (tr). Quartz occurs in large strained grains; albite is partly sericitized; biotite pleochroism is pale red-brown to very dark brown. Grain size : 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: microgranite. McCandless #1 Turney cuttings 4980-86 Bureau of Economic Geology l\licrocline (61%), quartz (25%), plagioclase 00%), magnetite or ilmenite (3%), red iron oxide 0%). l\licrocline is in part in lenses or incipient augen and in part crushed and drawn out between recrystallized quartz masses; quartz is recrystallized and concentrated in lentic­ular masses of interlocking mosaic; plagioc1ase is in fragments in advanced stages of sericiti­zatlon but showing less cataclastic alteration than microcline; red iron oxide stain pervades crushed material. Grain size: feldspar and quartz: 0.2 to 1 mm; crush material less than 0.1 mm. FaLric : cataclastic: Rock : cataclastically altered granite partly converted to granite gneiss. McCandless #1 University core 5513 Bureau of Economic Geology Microcline (35<;~), albite (30%), quartz (20%), biotite (4%), epidote (4%), calcite (2%), magnetite or ilmenite (2',)"o), myrmekite (2%), muscovite (1%) , apatite (tr), zircon (tr). l'lagioclase is partly sericitized ; biotite pleochroism is pale yellow-Lrown to very dark Lrown; calcite is in ,-einlets and scattered grains and partly replaces pla~ioclase and primary ferro­magnesian mineral; magnetite or ilmenite is locally altered to red iron oxide; cpidote occurs in small masses of grains and '"rosettes" of radiating needles; it is commonly associated with calcite. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: microgranite. l\lagnolia # 3 Fromme core 4667-82 Bureau of Economic Geology Kaolinized feldspar (55%), red iron oxide (25%), fine crush zone 00%), magnetite or il­m~nite (-!<;(-) , Liotite relicts (3%), quartz (3%), apatite (tr). Plagioclase and alkali feldspar arc in advanced stages of kaolinization; red iron oxide replaces the ferromagnesian minerals. Grain size: 0.5 to 2 mm. Fabric: relict hypidiomorphic granular. Rock : altered syenite. lllagnolia # 3 F rommc core 4682-97 Bureau of Economic Geology Thoroughly altered rock. Chlorite (-10%), semi-opaque brownish-white clay mineral (30%), fcld>par \l·Vi'o ), magnetite or ilmenite (5%), quartz (5%), rnlcite (5%), apatite 0%), unaltf'n•d hiotite (tr). Chlorite occurs in large granular masses; F-Cmi·opaque brownish-white clay replaces a larl!e body of the rock, including biotite and feldspar. Grain size: 0.5 mm. Fabric: relict hypidiomorphic granular. Rock: thoroughly altered igneous rock (note prismatic apatite), probably a chloritized and kaolinized microsyenite. Basement Rocks, Texas-New Mexico Magnolia #2-96 Powell-State cuttings 4695-4700 Bureau of Economic Geology Two small fragments composed of large grains of strained quartz. Pan American #1-4 MacDer cuttings 5290-95 Bureau of Economic Geology Microcline and oligoclase-andes.ine (72%), quartz (25%), red iron oxide (2%), chlorite (l'fo), sericite (tr) , zircon (tr). Feldspar is mostly microcline, locally microperthitic, and occurs as broken augen and in crushed areas with quartz; locally twinned plagioclase grains show bending; quartz occurs in some coarser dimensionally oriented masses (recrystallized) and in fine crush areas with feldspar; red iron oxide occurs as veinlets. Grain size: from crushed ma­terial less than 0.02 mm to augen up to 0.5 mm. Fabric: cataclastic. Rock: granite gneiss. Phillips # 1 Pascoe cuttings 4630-40 Bureau of Economic Geology Andesine (45%), hornblende (2<>%), calcite (15%), biotite (IO'fo), chlorite (5%), magnetite or ilmenite (5'fo), pyroxene? (tr), pyrite (tr), apatite (tr). Plagioclase is partly sericitized; hornblende pleochroism is yellow-brown to very dark green-brown-it is an intensely colored variety that is locally oxidized and semi-opaque; biotite pleochroism is reddish brown to very dark brown; calcite and blue-green chlorite locally invade and replace masses of the rock. Grain size: 0.5 mm. Fabric: hypidiomorphic granular. Rock: microdiorite. Phillips #1-A Puckett "B" cuttings 16510-25 Bureau of Economic Geology Microcline microperthite (65%), quartz (20%), calcite (10%), biotite (3%), pyrite (l'fo), sericite 0 % ) , apatite (tr), zircon (tr). Microcline microperthite and quartz occur as crushed and broken grains; secondary calcite has invaded the crushed areas; biotite is altered; sericite occurs in vein lets; mineral percentages are not significant as slide is composed largely of mineral fragments rather than rock fragments. Grain size: 1 to 3 mm. Fabric: xenomorphic granular--<:ataclastic. Rock: cataclastically altered granite. Phillips # 1-C Puckett core 14923 Bureau of Economic Geology Microcline microperthite (58%), quartz 08%), oligoclase (15%) , chlorite (5%), biotite (3%), magnetite or ilmenite (l'fo), hornblende (tr), muscovite (tr), calcite (tr), sphene (tr), pyrite (tr), apatite (tr), zircon (tr). Plagioclase is locally myrmekitically intergrown with quartz and occurs as large individuals and as small grains in a matrix between large micro­clines; biotite is dark red-brown; chlorite is brown and nearly opaque and results from alter­ation of ferromagnesian mineral; hornblende was observed completely enveloped by biotite in one grain; pyrite occurs along grain boundaries; calcite is in a veinlet. Grain size: large quartz and feldspars 2 to 6 mm; smaller feldspar 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: granite. Phillips #1-D Puckett core 14309-20 Bureau of Economic Geology Microcline microperthite (64%), quartz (20%), oligoclase 00%), chlorite (3%), biotite (2%), magn.,tite or ilmcnitr ll%l, hornblende (tr), calcite (trl, >phcne (tr), apatite (tr), zircon (tr). Oligodase shows incipient sericitization, locally is myrmekitically intergrown with quartz and occurs mostly as smaller grains between large microclines; red-brown biotite is partly altered to chlorite; dark green hornblende relicts occur in chlorite masses; some zircons are large. Grain size: plagioclasc 0.5 to 1 mm; microcline and quartz 2 to 8 mm. Fabric: hypidiomorphic granular. Rock: granite. Shell (Humphreys) #1 University cuttings 5200-04 Bureau of Economic Geology Very small fragments of microcline, albite, quartz, pyrite, red iron oxide, green hornblende, and biotite. Grain size: 0.1 to 0.2 mm. Rock: microgranite. Standard of Texas #1-3 !l!acDer cuttings 5350-60 Bureau of Economic Geology Microcline microperthite (50%), albite (25%), quartz (20%), magnetite or ilmenite (2%), sericite (2%), chlorite 0%), zircon (tr). Perthitic structure is very fine and obscures microcline twinning; al bite is finely twinned; quartz occurs as smeared and strained lenses in some frag­ ments, is crushed (with feldspar) in some fragments, is rutilated in some fragments, and is in clear unstrained grains in other fragments; sericite is in scattered flakes. Grain size: 0.5 to 1 mm with finer crush zones. Fabric: xenomorphic granular-cataclastic. Rock: microgranite. Bureau of Economic Geology, The University of Texas Standard of Texas #1·4 l\lacDer cuttings 5280-5313 Bureau of Economic Geology ;\lirrorline (47'/c), olip:oclase (30%), quartz ( 10% ), biotite (5"7r), hornblende (3%), magnetite or ilmenite (2%), muscovite and sericite (1%), chlorite (1%), calcite (1%), pyrite (tr), apatite (tr), maroon iron oxide (tr), sphene (tr), zircon (tr). Microrline is ]orally microper­thitic, plagioclase is partly sericitized; biotite pleochroism is pale to deep brown; hornblende pleochroism is colorless to pale green; chlorite is from alteration of biotite: muscovite and sericite occur as scattered plates and flakes and are derived from alteration of plagioclase. Grain size: 0.5 to l mm. Fabric: xenomorphic granular. Rock: microgranite. Note: Two fragments in this slide consist of microcrystalline quartz-alkali feldspar-sericite groundmass containing seri· citized oligoclase phenocrysts and showing flow structure, apatite, biotite relicts, quartz pheno­crysts. Plagioclase phenocrysts harn sericitized rims; groundmass less than 0.05 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Stanolind # 1 Conry-Davis cuttings 5745 Bureau of Economic Geology Andesine (61%), quartz 05%), hornblende (15%), biotite (3%), ilmenite (3%), chlorite 12%). epidote 0%), sphene (tr), apatite (tr) . Hornblende pleochroism is yellow to deep green; biotite is a brown variety : sphene rims ilmenite. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock : quartz diorite. Stanolind #l·A Hinyard core 6473-73Y,, Stanolind Oil & Gas Co. Quartz (67%), albite (30%), muscovite and sericite (3%), apatite (tr), leucoxene (tr), calcite (tr), epidote (tr), zircon (tr), sphene (tr), magnetite or ilmenite (tr), chlorite (tr). Plap:ioclase is partly sericitized and partly replaced by calcite; quartz occurs in a mosaic with plagioclase. Grain size: 0.5 mm. Fabric : granoblastic (quartz and feldspar recrystallized). Rork: metarkosite. Stanolind #l·A Hinyard core 6487 -89 Stanolind Oil & Gas Co. Quartz ( 50% l. oligoclase ( 40%), biotite (7%), sericite (3%), sphenc (tr). zircon ( trl . apatite (tr), chlorite (tr). Biotite pleochroism is pale brown to deep red-brown. Grain size: 0.5 to 1 mm. Fabric: granoblastic (quartz and feldspar recrystallized). Rock: metarkosite. Stanolind # l·A Hinyard core 6721-24 Stanolind Oil & Gas Co. (1) Oligoclase (59<;Cl , hornblende (25%), biotite (15%l, calcite 0%l, pyrite (tr), magne· tite or ilmenite (tr), sphene (tr), apatite (tr), zircon (tr), sericite (tr). Plagioclase is partly altered to sericite; hornblende, yellow-green to green pleochroism, is in rudely oriented grains; biotite, pale brown to dark brown pleochroism, commonly shows a rude orientation. although some grains have grown normal to the foliation; zircon forms halos in biotite. Grain size: 0.5 mm. Fabric: crystalloblastic. Rock: biotite amphibolite. (2) Quartz (69%), oligoclase (25%), biotite (2%l, sericite (2%), hornblende 0%), calcite 0%) . Quartz is in a mosaic with feldspar: plagioclase is partly seriritized and occurs in twinned and untwinned grains; biotite pleochroism is pale brown to dark brown; hornblende pleochroism is yellow-green to green. Grain size: 2 mm. Fabric: granoblastic. Rock: felds­pathic metaquart:ite. Superior # 1 Cordova Union cuttings 4860-65 Bureau of Economic Geology Feldspar (75%), quartz (20%), calcite (3%), muscovite 0%), pyrite 0%), amphibole (tr), epidote (tr), apatite (tr). Feldspar is probably mostly albite; it is altered to sericite and partly replaced by calcite; quartz is in massive strained grains ; amphibole occurs as sheaves of needles in vicinity of a veinlet: epidote prisms are associated with the amphibo]e; muscovite i~ in scattered plates. Grain size: 0.5 to 2 mm. Fabric : hypidiomorphic granular. Rock: altered albite granodiorite. Union of California #1-C Heiner cuttings 6145-50 Bureau of Economic Geology (1) 60% of total fra~ments in slide: andesine? (54%), hornblende (25%l, biotite (10%). maimetite or ilmenite (7%), apatite (3%), pyrite (I'7c). Hornblende is blue-green; biotite is red-brown. Grain size: 0.5 mm. Fabric : hypidiomorphic granular. Rock : microdiorite. (2) 40% of total fragments in slide: microcline (37~~ ), quartz (30%), albite-oligoclase (25%), m)Tmekite 18'/t), zircon (tr). Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Union of California #1-C Heiner cuttings 6155-60 Bureau of Economic Geology (I) 90-::C of total fragments in slide: andesine and labradorite (55%), hornblende (20%), magnetite or ilmenite (7%), biotite (6%), augite (8%), calcite (4%), apatite (tr) . Basement Rocks, Texas-New Mexico Andesine occurs in fragments that do not contain augite, labradorite is in augite-bearing fragments; hornblende is in yellow-green masses surrounding augite and apparently derived from augite and in blue-green prisms in fragments without augite; augite is not present in all fragments; calcite is in veinlets. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granu· Jar. Rock: fragments of microdiorite and microgabbro about equal in number. (2) 10% of slide: microcline, quartz, oligoclase, myrmekite, and magnetite or ilmenite. Grain size : 0.5 to l mm. Fabric : ? Rock: microgranite. POTTER COUNTY Amarillo Oil & Gas #3 Masterson core 2750 Bureau of Economic Geology Groundmass (78%), albite 00%), chlorite (7%), quartz-chlorite-epidote veinlets (3%) , magne· tite or ilmenite (2%), zircon (tr) . Groundmass is a cryptocrystalline to microgranular aggregate of alkali feldspar stained with iron oxide; local coarsenings as parallel oriented lenses and layers show that quartz, chlorite, and epidote are minor constituent' of th e ~roundmnss; crypto· crystalline areas show relict crystallitic structures; al bite forms twinned phenocrysts; chlorite is after biotite; quartz-chlorite·epidote veinlets cut the rock and one occupies a microfault that shows 2 mm of apparent displacement. Grain size: groundmass cryptocrystalline to 0.02 mm; coarsenings up to 0.1 mm; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: trachyte porphyry. Amarillo Oil & Gas #3 Masterson core 2765 Bureau of Economic Geology Plagioclase (66%), nontronite? (20%), magnetite or ilmenite (7%) , alkali feldspar (3o/o), quartz (2o/o), hornblende (1%), augite 0%), pyrite (tr), chlorite (tr), rutile (tr), apatite (tr). Radial laths of plagioclase are andesine or labradorite; augite and green-brown hornblende occur as corroded relict pyrogenic minerals in a mass of brown moderately birefringent mineral that appears to be nontronite; quartz and alkali feldspar are in micrographic inter growths between plagioclase laths; tiny grains in nontronite mass are probably rutile; apatite occurs in long needles. Grain size: 0.2 to 1 mm. Fabric: subophitic. Rock: altered leuco-diabase. Amarillo Oil & Gas #5 Masterson cuttings 2200 Bureau of Economic Geology Microcline microperthite ( 42%), albite-oligoclase ( 40%), quartz 05%), calcite (2%), biotite (l% ) , pyrite (tr), sphene (tr), leucoxene (tr), apatite (tr). Plagioclase shows sericitized cores mantled with clear albite; quartz occurs in fractured and broken grains between feldspar grains; calcite fills fractures; biotite is a green-brown variety that is partly bleached. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular. Rock: granite. Canadian River #4-B Masters<>n core 1943-52 Stanolind Oil & Gas Co. Groundmass (90o/o), albite (8%), magnetite or ilmenite 0%), calcite 0%), apatite (tr), zircon (tr), rutile? (tr l. Groundmass can be divided into a low-index cryptocrystalline area (60%) and a coarser area of quartz and feldspar (0.05 to 0.1 mm) that may be a cavity filling (30%). Plagioclase phenocrysts are partly sericitized; calcite occurs in the coarser part of the i:roundmass; rutile? occurs as hair-like inclusions in quartz in the coarser part of the groundmass. Grain size: !(roundmass cryptocrystalline to 0.1 mm ; phenocrysts 0.5 to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Canadian River #4-B Masterson core 1952 Humble Oil & Rfg. Co. Groundmass (89%), albite (8o/o), magnetite or ilmenite (2o/o), red iron oxide 0%), calcite (tr), leucoxene (tr), apatite (tr). Cryptocrystalline to microcrystalline quartz-alkali feldspar groundmass shows what appears to be a relict sub-parallel shard or vitroclastic structure; al bite forms phenocrysts whose long dimension is parallel to the groundmass structure. Grain size: groundmass cryptocrystalline to 0.1 mm; phenocrysts up to 2 mm. Fabric: relict vitroclastic? Rock : rhyolite tufj. Colorado Interstate #25-A Bivins cuttings 2460-67 Bureau of Economic Geology Groundmass, feldspar, calcite, chalcedony, quartz. Heavily stained quartz-alkali feldspar ground­mass is mostly micrographic, locally microspherulitic; leucoxene forms tiny plates throughout the groundmass; deeply kaolinized phenocrysts of plagioclase and alkali feldspar are present in some fragments; calcite as masses and vein lets replaces parts of the groundmass; chalcedony lines a cavity in one fragment; quartz grains fill a cavity in one fragment; a biaxial positive moderately birefringent mineral in a cavity in one fragment was not identified; mineral per­centages are highly variable in different fragments. Grain size: groundmass less than 0.01 to 0.05 mm ; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Bureau of Economic Geology, The University of Texas Colorado Interstate cuttings 2546-52 Bureau of Economic Geology #25-A Bivins Plagioclase-labradorite?, olivine, augite, hypersthene, magnetite or ilmenite, iddingsite. Zoned plagioclase is variably sericitized in different fragments; augite shows a very fine diallage structure; mineral percentages and degree of alteration vary widely in different fragments. Grain size: 1 to 2 mm. Fabric: ranges from hypidiomorphic granular to ophitic. Rock: leuco­olivine gabbro. Colorado Interstate cuttings 2610-60 Bureau of Economic Geology #25-A Bivins Labradorite (80%), augite (15%), olivine (3%), iddingsite (2%) , magnetite or ilmenite (tr), serpentine"! (tr), biotite (tr), apatite (tr). Zoned plagioclase is variably sericitized; augite occurs as large individuals including plap;ioclase laths and as smaller grains inter­stitial to plagioclase laths; locally it shows a very fine diallage structure and locally it is partly altered to a brown moderately to highly birefringent mineral that may be hornblende ; idding­site and serpentine? are the result of alteration of olivine; red-brown biotite rims magnetite­ilmcnite: mineral percentages vary in different fragments. Grain size: 0.5 to 1 mm. Fabric: ranges from hypidiomorphic granular to ophitic. Rock: leuco-olivine diabase. Colorado Interstate cuttings 2767-76 Bureau of Economic Geology #25-A Bivins (1) Plagioclase-labradorite? (63%), augite (20%), magnetite or ilmenite (10%), chlorite? (5%). hornblende 0%), quartz (1%), apatite (trl. Zoned plagioclase occurs as tiny laths in a more or less triangular or radial pattern; magnetite or ilmenite and augite occur interstitial to plagioclase laths; augite commonly shows an altered core of brownish chlorite?; chlorite? is a brownish fibrous mineral with moderate birefringence that ap­parently replaces pla!!ioclase and augite; hornblende is a brown variety. Grain size: 0.1 to 0.2 mm. Fabric: subophitic. Rock: diabase. (2) Fragments of almost opaque rock composed mostly of fine granular magnetite-ilmenite, with subordinate plag:ioclase microlites, and brownish chlorite. Grain size: less than 0.02 mm. Fabric: microgranu1ar. Rock : iron ore. Colorado Interstate cuttings 2875-83 Bureau of Economic Geology #25-A Bivins (1) Lahradorite (54%) , augite (35%l, magnetite or ilmenite (10%), chlorite? (1%), apatite (tr) . Plagioclase shows incipient alteration to sericite ; augite is lavender-brown; magnetite or ilmenite occurs in ~cattered grains; ch1orite? is a dirty brown variety. Grain size: 0.1 to 0.5 mm. Fabric: subophitic. Rock: diabase. (2) Ground mass (93%), augite (5%), plagioclase (2%). Groundmass is mostly fine granular magnetite or ilmenite with subordinate feldspar microlites; augite and plagioclase occur as scattered or sporadic phenocr~·sts. Grain size: groundmass less than 0.02 mm; pheno­crysts up to 0.2 mm. Fabric: microgranular. Rock: iron ore. Colorado Interstate cuttings 2956-62 Bureau of Economic Geology #25-A Bivins Labradorite (63%) , magnetite or ilmenite (20%), augite (10%), olivine (4%), chlorite? (3%), apatite (tr l . Plagioclase is zoned: magnetite or ilmenite grains occur between plagioclase laths; au{!ite is lavender-brown and shows a fine dial1age structure; olivine? is heavily stained and partly altered to chlorite?: chlorite? is a green-brown fibrous moderately birefringent mineral that has developed at the expense of the ferromagnesian minerals-it occurs in the parallel or diallage structure in the pyroxene and within the olivine; apatite is in long needles. Grain size: 0.5 to l mm. Fabric: subophitic. Rock : olivine diabase. Colorado Interstate cuttings 3002-10 Bureau of Economic Geology #25-A Bivins Groundmass (95% ), serpentine and nontronite? "phenocrysts" (5% ). Groundmass is an ag· gregate of serpentine, magnetite or ilmenite, plagioclase laths, epidote?, and nontronite?; original phenocrysts of augite and plagioclase are altered to serpentine and nontronite. Grain size: groundmass less than 0.02 mm : phenocrysts 0.2 to 0.5 mm. Fabric : metasomatic-relict subophitic '?. Rock : altered basalt porphyry. Hasement Rocks, Texas-New Mexico Colorado Interstate cuttings 3010-30 Bureau of E<'onomic Geology #25-A Bivins Groundmass (95%J, feldspar (5%), apatite (tr). J\licrographic quartz-alkali foldspnr·mag­netite or ilmenite-leucoxene groundmass is heavily stained with red iron oxide aud the feld­spar is partly kaolinized; feldspar phenocrysts are heavily kaolinized. Grain size: groundmass 0.01 to 0.05 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Colorado Interstate cuttings 2419-2~ Bureau of Economic Geology #41-B Masterson Microcline microperthite (66%), quartz (18%), albite-oligocluse 05%), magnetite or ii· m<"nite and leucoxene 0%), sphene (tr), apatite (trl, zircon (trl. Grain size: 0.5 to 1 mm. Fabric: xenomorphic granular. Rock: microgrcmite. Emerald #I Masterson rnttings 2065:!: Bureau of Economic Geology Groundmass (80%), alkali feldspar and al bite ( 18%), magnetite or ilmenite (1 % ) , biotite (1%) . Microgranular to microspherulitic quartz-alkali feldspar groundmass shows a peculiar "ropy" flowage structure and in some fragments deformed shard" characteril"tic of welded tuffs; grains of alkali feldspar and albite appear as "phenocrysts." Grain size: groundmass 0.01 to 0.02 mm: "'phenocrysts" up to I mm. Fabric: relict vitroclastic-welded ., . Rock: rhyolite tuff (possibly a welded tuff) . Emerald #I Masterson cuttings 2125-30 Bureau of Economic Geology Very small fragments appear to be rhyolite porphyry; slide very poor. Greatn Amarillo Oil #I Masterson cuttings 27Ti Bureau of Economic Geology (I l l1meous rock fragments, mostly sheared quartz, quartz, and microclinr. (2 I Volcanic rock fragments, mostly with a groundma>s composed of alkali feldspar, ch lo rite, leucoxene, and minor quartz and containing $rriticized alhite phenocrysts in some frag­ ments. Grain size: groundmass less than 0.02 mm; phenocrysts up to 0.2 mm. Fabric: microgranular-relirt vitroclastic. Rock: fragments of trachyte tu ff and trachyte porphyry. Prairie #I Bivins core 2585-95 Bureau of Economic Geology Lahradorite (58%), augite (30%), olivine (7%), alkali feldspar (2%), magnetite or ilmenite (2%), iddingsite (I%), biotite (tr), chlorite (tr), apatite (tr), zeolite? (tr). There are two varieties of pyroxene present: (1) large optically continuous irregular individuals of augite, pinkish brown in color with weak pleochroism and showing a faint diallage structure; (2) large optically continuous individuals, more marked shagreen surface than the first variety, mark­edly pleochroic in the pinkish-brown to green hypersthene formula but with positive optic sign; indistinctly zoned plagioclase occurs in laths and tablets, commonly within augite grains; olivine is locally partly altered to iddingsite; alkali feldspar occurs between plagio­clase subhedrons; zeolite? forms a narrow veinlet. Grain size: 0.5 to 2 mm. Fabric: ophitic. Rock: olivine diabase. (Photomicrographs, PL X, A and B). Ranch Creek # 1 Masterson cuttings 2480 Bureau of Economic Geology Groundmass, feldspar grains, chlorite, iron oxide, leucoxene, apatite. Red-stainrd and kaolinized potassium feldspar and albite groundmass is crytorrystalline to microgranular; kaolinized feldspar grains occur as phenocrysts. Slide is very poor and fragments are few. Grain size: groundmass cryptocrystalline to 0.1 mm; phenocrysts up to 0.5 mm. Fabric: in some fragments relict vitroclastic, in some fragments porphyritic. Rock: trachyte tuff and trach.yte porphyry frag­ ments. Sinclair-Prairie #I Bush cuttings 5300-5700 Stanolind Oil & Gas Co. Groundmass (58%). albite (15%), microperthite 05%). quartz (6%), calcite (4%l, sericite 0%), magnetite or ilmenite 0%), leucoxene (trl, chlorite (trl, apatite (trl, zircon (tr). Groundmass can be divided into a red-stained micrographic area (407': ) au in clusters of grains; plagioclase shows incipient alteration to sericite; biotite, pale to very dark brown pleochroism, is partly altered to chlorite; calcite partly replaces plagioclase; red iron oxide occurs in vein lets and along grain boundaries; sericite-muscovite occurs in a vein let and within plagioclase grains (sericite only) as a result of alteration. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular-poorly developed; larger grains are surrounded by clusters of small grains. Rock: granite. RANDALL COUNTY Placid #I Greeley cuttings 8240-44 Bureau of Economic Geology Groundmass (72%), quartz (10%), albite (8%), microperthite (8%), chlorite (2%), leucoxene (tr), apatite (tr). Brown micrographic groundmass contains local microspherulitic patches; quartz occurs as corroded, embayed phenocrysts; microperthite and plagioclase, probably al bite, are phenocrysts; chlorite occurs as a replacement of biotite. Grain size: groundmass 0.05 to 0.1 mm; phenocrysts up to 0.2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. ROBERTS COUNTY Phillips #I Jenkie cuttings 11737 Humble Oil & Rig. Co. Albite·oligoclase (52%), quartz (22% ), microperthite (20%), chlorite (3%), biotite (2%), leucoxene (1%), magnetite or ilmenite (tr), apatite (tr), zircon (tr). Plagioclase is in an advanced stage of sericitization; red-brown biotite is partly altered to chlorite; leucoxene is locally in diamond-shaped grains after sphene. Grain size: I to 3 mm. Fabric: hypidiomorphic granular. Rock: granodiorite. RUNNELS COUNTY Superior #I McDowell cuttings 6283 Bureau of Economic Geology Alhitc 186%), biotite (7%), quartz (4%), amphibole 0%), muscovite 0%), leucoxene (!%), picdmontite? (tr), apatite (tr), zircon (tr), pyrite (trl, ilmenite (tr), fluorite (tr), alkali feldspar (tr), rutile? (tr). Plagioclase occurs in a mosaic of twinned grains with a trace of alkali feldspar present between plagioclase grains and as anti-perthite; biotite, colorless to red­hrown pleochroism, occurs in short equant plates, commonly showing tiny pleochroic halos; bundles of elongate colorless prisms occur in a crystalloblastic habit in one fragment and appear to be amphihole-anthophyllite?; one grain of a mineral pleochroic in pink occurs in this same fragment and may he piedmontite. Grain size: 0.2 to 0.5 mm average, 2 mm maximum. Fabric: predominantly hypidiomorphic granular bnt with elements of a sieve fabric evident in one fragment. Rork: a/bite horn/els?. SAN SABA COUNTY Cayce #I Moore cuttings 1656·69 Bureau of Economic Geology Microcline (46o/r). albite-oligoclase (30%), quartz (22%), hiotite (2%), chlorite (tr), sericite· muscovite (tr), sphene (tr), apatite (tr), zircon (tr). Plagioclase shows incipient sericitization; sericite·musrovite ocrurs in irregular plates and fihers: hiotite pleochroism is yelJow.brown to almost black. Grain size: 0.5 to 2 mm. Fabric: xenomorphic granular-poikilitic. Rock: granite. SCHLEICHER COUNTY Atlantic # 1 Roberts core 7751 Bureau of Economic Geology Mirrocline microperthite (72%), quartz 08%), oligoclase (5%), calcite (3%), chlorite (]%), magnetite or ilmenite and rell iron oxide 0%), leucoxene (tr), zircon (tr) . Quartz is ]orally micrographically interizrown with microrline mirroperthite and commonly quartz grains are connected hy Yeinletg along feldspar boundaries; chlorite is after hiotite; red iron oxide occurs in \'t>inlrts. Grain size: 0.5 to 4 mm. Fahric: xenomorphic granular--mirrog:raphir. Rork: f!rnn;t,.. B11rra11 of Economic Geology, The University of Texas Humble #I Spenrer •·ore 6909-10 Bureau of Economic Geology lllicrocline microperthite (i0%), quartz (20%), albite (8%), calcite (2%), muscovite (tr), bleached biotite? (tr), pyrite (tr). zir<"on (tr) . .!llicrocline mirroperthite shows incipient kaolini­zation and abundant carlsbad twins; quartz is locally micrographically intergrown with feldspar and in some areas penetrates and ~mbays feldspar in almost skeletal !?rains; plagioclase is murky with incipient sericite but locally has clear rims; bleached biotite? is assorinted with masses of calcite. Grain size: 2 mm to 1 cm. Fabric: xenomorphic j?ranular. Rock: granite. Humble #1 Stanford rore 9020 Bureau of Economic Geology ~licrocline microperthite (64%), quartz (18% ), albite (15%), chlorite (2%), leucoxene 0%). bleached biotite (tr), zirron (trl . .!llirrorline microperthite grains locally show micrographic borders: quartz orrurs as individual grains. and in micrographic intergrowths ; zircon is in clusters of small grains. Grain size: fine granular quartz-feldspar vein let 0.05 to 0.1 mm; rock 2 to 5 mm. Fabric: xenomorphic granular-mirrographir. Rock : granite. Phillips #1 Callan <'uttings 6058-65 Bureau of Economic Geology Chlorite (38%), sericite (25%), labradorite-bytownite (15%), magnetite or ilmenite and pyrite (10%) , olivine (5%), augite (5<;'r), serpentine (2%). Olivine relicts occur wthin masses of ~reenish-brown rhlorite that shows a characteris.tic mesh structure; green chlorite is present in minor amounts: plag1oc1ase is in part completely sericitized and in part shows sericitized cores with lented fibers is probably talr but possibly is sericite or brucite; red iron oxide occurs in spots from alteration of pyrite. Grain size: dolomite mosaic 0.02 to 0.05 mm: vein dolomite is up to 0.1 mm. Fahri•·: ratarlastic-rrystalloblastir. Rork : metadolomite. (Photo­micrograph, Pl. \'II. D.) :'tanolind (Superior and T ntex ., #I Jordan cuttings 8920-21 Bureau of Economic Geology Oli!"oclase (61%l. quartz (20'/(l, mirrocline 05%l, biotite (2%), epidote (2%), chlorite ftr\, mal!netitr or ilmenite (trl, sphene (tr), apatite (tr), zircon (tr). Plagioclase shows incipient alteration to sericite; hiotite, pale yellow to \'ery dark brown pleochroism, is partly altered to chloritr: rpidote is in mas.re in needles and stubby prism>. Grain size : 0.02 to 0.1 mm with sporadic feldspar up to 2 mm. Fabric: xenomorphic granular. Rock: porphyritic al bite sycnodiorite. Smith #2 Farren cuttings 3056 Stanolind Oil & Gas Co. Microperthite (93% ), quartz (6%), aegirine-augite (l')'o), apatite (tr). Grain size: up to 3 mm. Fabric: hypidiomorphic granular. Rock: granite. Basement Rocks, Texas-New Mexico Smith #2 Farren cuttings 3065-70 Stanolind Oil & Gas Co. Rock is breccia composed of fragments of quartz, microperthite, magnetite or ilmenite, biotite, zircon, and a fine intergranular matrix. Grain size : 0.5 mm. Fabric: cataclastic. Rock: brecciated granite. Smith #2 Farren cuttings 3125 Stanolind Oil & Gas Co. Microperthite (65%), quartz (30%), hornblende (3%), chlorite (1%), magnetite or ilmenite 0%), apatite (tr). Hornblende pleochroism is pale green-brown to deep green. Grain size: 1 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Smith #2 Farren cuttings 3125 Stanolind Oil & Gas Co. OJ Microperthite (62%), quartz (30%), chlorite (5% ), magnetite or ilmenite (2%), biotite 0%), leucoxene (tr), sphene (tr). Rock shows incipient brecciation. Grain size: 1to2 mm. Fabric: hypidiomorphic granular. Rock: granite. (2) Microperthite (66%), quartz (25%), amphibole (7%), magnetite or ilmenite 0%), apatite (1 % ) . Amphibole pleochroism is a brown to deep brown-green; apatite is abundant in needles. Grain size : 0.2 to 0.3 mm. Fabric : xenomorphic granular. Rock : microgranite. Smith #2 Farren cuttings 3877-83 Stanolind Oil & Gas Co. Fragment is apparently from a miarolitic cavity and consists of carbonate, radiating and con­voluted masses of chlorite, biotite, and green pyroxene (probably aegirine-augite). Smith #2 Farren cuttings 4286 Stanolind Oil & Gas Co. Microperthite (55%), quartz (30%), albite (10%), amphibole (5%), biotite (1%), zircon (tr). Amphibole pleochroism is brown to deep brown with a border of riebeckite (blue). Grain size: 0.2 mm. Fabric: xenomorphic granular. Rock: microgranite. Thomas & McFarland #1 Kachelhoffer cuttings 2280 Bureau of Economic Geology Microcline microperthite, plagioclase, anhydrite, quartz, magnetite or ilmenite, red iron oxide. Microcline microperthite and sodic plagioclase occur as fractured grains veined with red iron oxide and anhydrite; masses of anhydrite are also present in the rock. Grain size: 0.5 to 4 mm. Fabric : ? . Rock : granite. Thomas & McFarland #1 Kachelhoffer cuttings 2325 Bureau of Economic Geology Microcline microperthite, albite·oligoclase, quartz, hornblende, biotite, magnetite or ilmenite, apatite, zircon. Plagioclase shows indistinct zoning; hornblende is green-brown; biotite is brown. Slide is composed mostly of individual mineral fragments. Grain size: 0.2 to 2 mm. Fabric: ?. Rock: granite. WICHITA COUNTY Continental and Magnolia #1 Beach cuttings 3180 Bureau of Economic Geology Magnetite or ilmenite, quartz, sericite, epidote, calcite, sphene. Slide is poor and composed of very fine cuttings; sericite fibers appear to be oriented; magnetite or ilmenite content is very high. Grain size: less than 0.1 mm. Fabric: crystalloblastic?. Rock: epidote-sericite phyllite?. Frabar-Hodges # 1 George core 3318 Humble Oil & Rig. Co. Chlorite and epidote (83%), calcite (8%), mica? (4%), quartz (3%), pyrite and magnetite (2%), relict plagioclase? (tr). Tiny eidote and chlorite plates form the mass of the rock; epidote also occurs in large grains sporadically throughout the rock and in quartz-calcite vein­lets; quartz in veinlets includes myriad tiny chlorite plates; the strongly birefringent platy mineral is probably a mica; pyrite and magnetite occurs as scattered grains and cubes; one grain apparently of relict twinned plagioclase almost entirely engulfed by chlorite and epidote was observed. Grain size: mostly less than 0.02 mm; mica? plates up to 0.2 mm long. Fabric: metasomatic. Rock : chlorite-epidote rock. Gulf #1 Miller core 4231-35 Humble Oil & Rig Co. Microcline microperthite and albite-oligoclase (68%), quartz 05%), sericite (6%), rock frag­ments (5%), biotite (3%), calcite (2%), chlorite 0%), sphene (tr), epidote (tr), sphene and Bureau of Economic Geofogy, The University of Texas ilmenite (tr), apatite (tr), zircon (tr). Microcline microperthite and variably sericitized albite­oligoclase occur as angular grains; quartz is in angular grains; sericite occurs as intergranular fibers as a result of reconstitution and from alteration of plagioclase; green biotite occurs as small plates and fibers concentrated in nests with chlorite; rock fragments are chert, microgranite, and quartz mosaic ; sphene-ilmenite occur together as single grains. Grain size: up to 0.5 mm. Fabric : relict elastic-incipient crystalloblastic (no recrystallization of quartz and feldspar but growth of biotite and sericite attests to reconstitution). Rock: meta-arkose. Texas #44 Skinner core 2860 Bureau of Economic Geology Albite and microcline (62%), quartz 05%), rock fragments 00%), quartz-feldspar-chlorite­sericite matrix (8%), chlorite (3%), calcite 0%), pyrite 0%), muscovite (tr), apatite (tr), zircon (tr). Feldspar and quartz occur in angular fractured and strained grains; microcline is microperthitic in some grains; rock fragments are chert, volcanic rocks, metasedimentary rocks, and granitic rocks; matrix may be a crush matrix or partly reconstituted intergranular sedimen­tary material ; chlorite is in masses and veinlets; pyrite occurs with quartz and chlorite in veinlets and is partly altered to red iron oxide. Grain size: less than 0.02 to 0.5 mm. Fabric: cataclastic. Rock: meta-arkose or metagraywacke. Texas #44 Skinner core 2860-65 Humble Oil & Rfg. Co. Microcline microperthite and albite-oligoclase (73%), quartz 05%), rock fragments ( 4%), chlorite (3%), epidote (3%), magnetite or ilmenite 0%), sericite 0 %) , leucoxene (tr), calcite (tr), apatite (tr), zircon (tr). Feldspar occurs in both angular and round grains ; quartz is in angular grains and in veinlets; epidote occurs with quartz in veinlets and as scattered grains in the rock: rock fragments are granitic and volcanic; chlorite forms intergranular plates and masses with sericite fibers. Grain size: up to 0.5 mm with sporadic rock and feldspar fragments up to 2 mm. Fabric: relict elastic-incipient crystalloblastic (no recrystallization of quartz and feldspar) . Rock: meta-arkose. WILBARGER COUNTY Barkley-Meadows #14-A Stephens core 3007 Humble Oil & Rfg. Co. Labradorite (63%), augite (25%), biotite (4%), magnetite or ilmenite (4%), hornblende (3% ), hypersthene 0 % ), iddingsite? (tr), pyrite (tr), apatite (tr) . Plagioclase is zoned; augite shows a parallel schiller structure and locally is fringed with hornblende showing pale brown to green pleochroism; biotite is red-brown and fringes the opaque mineral; hypersthene is partly altered to iddingsite?. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: leuco-microgabbro. Gulf #6-E Blackman core 3515-33 Bureau of Economic Geology Quartz (51%), albite (25% ), microcline 05%), biotite (7%), muscovite (1%), sericite 0%), apatite (tr) , zircon (tr). Quartz and feldspar occur in a mosaic, plagioclase is partly sericitized; muscovite and biotite occur as equant non-oriented plates; biotite pleochroism is pale brown to almost black. Grain size: 0.2 to 0.4 mm. Fabric: granoblastic (thoroughly recrystallized rock). Rock : metarkosite. (Photomicrograph, Pl. V, B.) Humble #6 Stevens cuttings 2810-15 Bureau of Economic Geology Andesine (60%), hornblende (40%), magnetite or ilmenite (tr), apatite (tr). Plagioclase sub­hedrons are partly sericitized ; green hornblende occurs in frayed and felty masses ; locally it shows a brown fringe or interior patch. Grain size: up to 2 mm. Fabric: hypidiomorphic granular. Rock: diorite. Texas #1 Main core 4725 Humble Oil & Rfg. Co. Microcline (56%), quartz (25%), albite (8%), biotite (8%), muscovite (2%), magnetite or ilmenite and leucoxene 0 % ), apatite (tr), zircon (tr) . Microcline and quartz form a mosaic showing a rude dimensional orientation; plagioclase is more or less concentrated in a layer that is also richer in muscovite ; biotite and muscovite are in oriented plates. Grain size: 0.2 to 0.5 mm. Fabric : lepidoblastic. Rock : biotite-quartz-microcline schist. WILLIAMSON COUNTY Shell and Sinclair #1 Purcell core 9474-79 Bureau of Economic Geology Microcline ( 40%), albite-oligoclase (35%), quartz 08%), biotite ( 4%), magnetite or ilmenite (2%), muscovite 0 % ), chlorite (tr), sphene (tr), calcite (tr), apatite (tr), zircon (tr). Basement Rocks, Texas-New Mexico Plagioclase is partly altered to sericite, locally it is replaced by muscovite; quartz is commonly welded and undulose; mahogany brown biotite, partly altered to chlorite, is locally crinkled and bent. The rock is cut by a number of fractures, one of which shows a 3·mm apparent displacement. There is an over-all dimensional orientation of grains. Grain size: 0.5 to 5 mm. Fabric: hypidio­morphic granular-gneissic. Rock: granite gnei.ss. WINKLER COUNTY Gulf #46-E Keystone cuttings 9995-10000 Bureau of Economic Geology Microcline microperthite (54%), quartz (20%), albite-oligoclase 05%), chlorite (4%), hornblende (2%), calcite (2%), biotite (2%), pyrite 0%), zircon (tr), apatite (tr). Quartz is strained, granulated, and streaked out in incipient gneissic development; hornblende and biotite occur as faded relicts mostly altered to chlorite and calcite. Grain size: 0.05 mm (granulated quartz) to 5 mm (microcline). Fabric: hypidiomorphic granular--cataclastic. Rock: granite partly converted to granite gneiss. Gulf #46.E Keystone cuttings 10000·05 Bureau of Economic Geology Microcline microperthite and oligoclase (67%), quartz (25%), biotite (8%), apatite (tr), zircon (tr). Feldspar is mostly microcline microperthite, plagioclase is sericitized; biotite, very dark green-brown variety, is so oxidized as to be nearly opaque. Grain size: 0.5 to 2 mm. Fabric: xenomorphic granular. Rock: granite. Gulf #50-E Keystone cuttings 10080-90 Bureau of Economic Geology Microcline microperthite (37%), oligoclase (35%), quartz (15%), chlorite (7%), biotite (5%), calcite 0%), apatite (tr), sphene (tr), hornblende (tr), zircon (tr). Plagioclase is partly sericitized; biotite pleochroism is yellow-brown to very dark brown; chlorite is derived from alteration of hornblende. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Gulf #62-E Keystone cuttings 9650·60 Bureau of Economic Geology Microcline microperthite (53%), albite-oligoclase (25%), quartz 05%), chlorite (3%), biotite (3%), hornblende 0%), apatite (tr). Plagioclase shows incipient sericitization; bier lite, pale brown to brown pleochroism, is partly altered to a greenish-brown moderate to high­birefringent chlorite; blue-green hornblende shows alteration to the same type of chlorite. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Gulf #69-E Keystone cuttings 9865·71 Bureau of Economic Geology Microcline and microperthite and albite-oligoclase (77%), quartz (20%), biotite (2%), sericite-muscovite 0%), calcite (tr). Quartz is granulated; slide shows only a very few frag­ments. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite or grano• diorite. Gulf #70-E Keystone cuttings 9744-48 Bureau of Economic Geology Microcline (56%), quartz (20%), albite-oligoclase 05%), biotite (3%), chlorite (2%), hornblende 0%), sphene 0%), sericite-muscovite 0%), calcite 0%), zircon (tr), ilmenite (tr), leucoxene (tr), apatite (tr). Microcline is locally microperthitic; quartz shows incipient granulation; hornblende, green to dark green pleochroism, is partly altered to calrite and chlorite; biotite pleochroism is golden brown to very dark brown; ilmenite is enveloped by leucoxene. Grain size: 0.2 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Gulf #73-E Keystone cuttings 9830-40 Bureau of Economic Geology Oligoclase (58%), microcline microperthite 05%), quartz 00%), biotite (10%), horn­blende (4%), epidote 0%), chlorite (1%), sphene 0%), calcite (tr), leucoxene (tr), pyrite (tr), apatite (tr), zircon (tr) . Biotite pleochroism is yellow-brown to brown; horn­blende pleochroism is yellow-green to green. Grain size: 0.5 to 2 mm. Fabric: hypidiomor­phic granular. Rock: granodiorite. Gulf #75-E Keystone cuttings 9800-10 Bureau of Economic Geology Microcline, albite, quartz, sericite, biotite, chlorite, red iron oxide, apatite. Slide is composed mostly of mineral fragments; microcline is locally microperthitic; plagioclase is largely seri­citized; biotite is_ bleac~e~; serici~e occurs in masses from alteration of plagioclase. Grain size: 0.5 to 2 mm. Fabnc: hyp1d1omorph1c granular. Rock: granite. Bureau of Economic Geology, The University of Texas Phillips #2 Bashara core 9915 Stanolind Oil & Gas Co. Oligoclase-andesine (68%), quartz 05%), biotite 00%) microperthite (5%), chlorite 0%), leucoxene 0%l, apatite (trl, zircon (trl, pyrite (tr). calcite (tr). Plagioclase is partly sericitized; red-brown biotite occurs in oriented plates and is partly altered to chlorite; quartz is markedly strained; calcite is in veinlets. Grain size: 0.5 to 1 mm. Fabric: gneissic (biotite is oriented but there is also a tendency for orientation of long axes of quartz and feldspar grains: no separation of dark minerals, quartz, and feldspar has taken place; some evidence of relict hypidiomorphic fabric is present). Rock: granodiorite gneiss. Phillips #4 Walton cuttings 9705-10 Bureau of Economic Geology lllirroperthite (79%1, albite·oligoclase 05%1, quartz (5%), apatite (1%) , muscovite (tr). Plagioclase is locally seriritized: slide is composed mostly of mineral fragments (largely feldspar) and estimated mode is not necessarily representative. Grain size: 1 to 4 mm. Fabric :?. Rock: probably a granite. Phillips #5 Walton cuttings 9685-90 Bureau of Economic Geology Quartz (48%1. albite (25%1, biotite 05%). amphibole (7%), chlorite (2%), magnetite or ilmenite (l'lc), calcite 0 % ), pyrite (1% ) , apatite (tr). Biotite, red-brown, occurs in oriented plates; hornblende, partly altered to chlorite, is in oriented prisms: calcite occurs as veinlets. Grain size : 0.5 mm. Fabric: crystalloblastic. Rock: amphibole-biotite-albite-quartz schist. Richardson & Bass #10-E Walton core 9857 Stanolind Oil & Gas Co. Oligoclase·andesine (40% ), microperthite (27%), quartz (20%), biotite 00%), sericite (2%), calcite 0%1, pyrite (tr), apatite (trl, zircon (tr) . Biotite pleochroism is pale brown to red-brown: calcite is in veinlets. Grain size: 1 to 2 mm. Fabric: xenomorphic granular. Rock: granodiorite. Richardson & Bass #10-E Walton core 9858 Stanolind Oil & Gas Co. Oligoclase (74%1, quartz 05%), biotite (8%), musco>ite 0%), microcline 0%) pyrite 0%1, leucoxene (tr), fluorite (tr), z.ircon (tr), apatite (tr), calcite (tr) . Quartz is severely strained, locally crushed; biotite, pale to dark red-brown p)eochroism, is in non-oriented laths and shows zircon halos ; plagioclase is partly sericitized; microcline remnant is completely en­veloped by plagioclase. Grain size: 5 mm. Fabric : hypidiomorphic granular. Rock: cataclastically altered quart: diorite. Sinclair #6-A Walton cuttings 9955-65 Bureau of Economic Geology Albite-oligoclase (62%), quartz (20%), microcline mirroperthite 05%), sericite-muscovite (2%), calcite (!%),zircon (tr), sphene (trl, magnetite or ilmenite (tr). Plagioclase shows incipient alteration to sericite; microcline is only locally microperthitic: quarlz shows strain and incipient granulation: sericite-muscovite occurs as small fibers in plagioclase and small flakes scattered in the rock. Grain size: 0.5 to 1 mm. Fabric : xenomorphic granular. Rock: microgranodiorite. YOAKUM COUNTY Continental # l Rodgers core 13015 Bureau of Economic Geology Groundmass (82%1. oligoclase? and alkali feldspar 05'7c), leucoxene (3%l. zircon (tr), apatite (tr), rock fra~ment·chert (tr), sphene (tr), pyrite (tr), calcite (tr). Groundmass is a microcrystalline mass of feldspar, sericite, and chlorite; chlorite also occurs as larger ma..es: sericite also occurs as an interlacing network of fibers ; plagioclase and alkali feld­spar occur as scattered grains; rock shows a parallel layering expressed in distribution of ser­irite and size of feldspar grains: there is a faint indication of vitroclastic structure. Grain size: groundma;;s less than 0.05 mm: feldspar fra!'ments up to 1 mm. Fabric: pyroclastic. Rock: trachrandesite cuff. (Photomicrograph, Pl. \1II, C.) Continental # 1 Rodgers core 13015 Bureau of Economic GeologJ Groundmass (91%), albite and alkali feldspar (3o/o l, calcite (trl, leucoxene (tr), apatite (tr). zircon (tr' . Groundmass is microcrystalline and composed of quartz-feldspar-chlorite­sericite; partly resorbed phenocryst consists of an al bite core and an alkali feldspar rim. Grain size : groundmass les.s than 0.02 mm; phenocrysl up lo 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry (possibly a fragment in the luff). Basement Rocks, Texas-New Mexico Continental # 1 Rodgers core 13016 Bureau of Economic Geology Groundmass (90%), microperthite (8%), plagioclase (2%), chlorite (tr), leucoxcne (tr), apatite (tr), zircon (tr). Microcrystalline groundmass is composed of quartz, feldspar, and sericite fibers and stained with iron oxide; angular fragments of microperthite and plagioclase are irregularly distributed in the groundmass. Grain size : ground mass less than 0.05 mm; feldspar grains up to 1 mm. Fabric : pyroclastic. Rock: rhyolite wff. Fikes & Murchison #17-C Elliott cuttings 11200-05 Bureau of Economic Geology Microperthite (82%), quartz (18%), leucoxene (tr), sericite (tr), zircon (tr). Microper· thite, possibly including some plagioclase, occurs in an irregular to cuneiform intcrgrowth with quartz. Grain size: 0.5 to 2 mm. Fabric: micrographic. Rock : micrographic granite. PART 2-SOUTHEAST NEW MEXICO CHAVES COUNTY Amerada #1-RA State cuttings 11580-90 Shell Oil Co. Groundmass (95%), tourmaline (5%). Microcrystalline quartz-alkali feldspar groundmass contains radial aggregates of gray to colorless tourmaline. Grain size: groundmass less than 0.02 mm; tourmaline aggregates up to 0.2 mm. Fabric: microgranular. Rock: tourmalinized rhyolite. Amerada #1-RA State cuttingo 11594 Bureau of Economic Geology Groundmnss (94%), albite (3%), alkali feldspar 0 %l, leucoxene 0 %l, calcite 0 %) , quartz (tr) . Groundmass is composed of quartz and alkali feldspar and shows very fine flow structure; al bite and alkali feldspar also occur as phenocrysts; quartz occurs mostly as a part of the groundmass but is also present in sporadic larger grains. Grain size: groundmass 0.01 to 0.02 mm ; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Amerada #1-RA State cuttings 11620-21 Shell Oil Co. Slide contains two grains: one is similar to 11580-90 but without the tourmaline; the other is a breccia or conglomerate of angular rhyolite fragments 0.5 to 3 mm in diameter. Fragments for the most part have a microcrystalline groundmass but (a) one contains tourmaline, (b) one is micrographic, (c) one has small sporadic albite phenocrysts, and (d) one shows small scat· tered grains of opaque mineral that is probably ilmenite. Amerada #1-RA State cuttings 11621 Bureau of Economic Geology Microcrystalline to cryptocrystalline quartz-feldspar mass contains sporadic sericite fibers and thin quartz veinlets. Rock: rhyolite. Barnsdall #1-A State core 12034-40 Bureau of Economic Geology Labradorite (54%), semi-opaque mineral-partly devitrified glass? (30%), chlorite (10%), magnetite or ilmenite (5%), unidentified altered remnants (1%), carbonate (tr), biotite, (tr), pyrite (tr). Plagioclase is present as microlites; semi-opaque gray-brown mineral proves to be anisotropic on very thin edge under high illumination and may be a partly devitrified glass; chlorite? is an olive-green secondary mineral with moderate to high birefringence that shows a concentric banded structure; it commonly surrounds colorless, moderate to highly birefringent, one-cleavage, uniaxial-negative fragments that were not identified. Grain size: 0.1to0.2 mm. Fabric: hyalopilitic?. Rock: altered basalt. Barnsdall # l·A State cuttings Shell Oil Co. Labradorite (78%), chlorite (6%), biotite? (6%), augite (5%), magnetite or ilmenite (4%), amphibole 0 % ) . Biotite?, pale green to pale brown pleochroism, is a highly birefringent micaceous mineral partly altered to an olive-drab chlorite; augite is brown and probably tita· niferous; amphibole occurs in tiny needles. Grain size: 0.1 to 0.2 mm. Fabric: subophitic. Rock: diabase. Black # 1 Shildneck cuttings 6850-60 Shell Oil Co. Microcline (58%), albite 05%), quartz (15%), chlorite 00%), leucoxene (2%), biotite (tr), zircon (tr). Biotite is almost completely altered to chlorite. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock: grcmite. Black #1 Shildneck cuttings 6860-70 Shell Oil Co. Quartz (35%), al bite (30%), microcline (22%), amphibole 00%), chlorite (3%), apatite (tr). Am phi bole is deeply colored with brown to green pleochroism but this deep color may be due in part to excess thickness of poor slide. Grain size: 0.5 mrn. Fabric: hypidiomorphic ~ranu}ar. Rock: granodiorite. Black # 1 Shildneek cuttings 6860-70 Shell Oil Co. Labradorit•· t65%), chlorite (20%), augite (8%), magnetite or ilmenite (5%), epidote (1%), lcucoX<·ne 0%J. Plagioclase is in non-oriented laths; augite is violet-brown in color and Basement Rocks, Texas-New Mexico probably titaniferous; epidote occurs in small masses in plagioclase and chlorite; chlorite is derived from alteration of original ferromagnesian minerals. G1ain size: 0.5 mm. Fabric: sub­ophitic. Rock: altered leuco.diabase. Continental #I Lankford cuttings 8040-50 Shell Oil Co. Quartz (74%), alkali feldspar 00%), muscovite and sericite (5%), pyrite (5%), biotite (4%), magnetite or ilmenite (2%), rutile (tr), apatite (tr), zircon (tr) . Some large muscovite plates show bending, rucking; biotite, an olive· brown variety, is smeared out into streaks; feldspar is altered. Grain size: 0.1 to 0.2 mm. Fabric: ranges from granoblastic to lepidoblastic. Rock : metaquartzite (grading into a mica schist or phyllite). Continental #I Lankford cuttings 8075-95 Stanolind Oil & Gas Co. Quartz (82%), feldspar (8%), biotite (6%), pyrite (3%), muscovite 0%), apatite (tr), rutile (tr), zircon (tr). Quartz occurs as round grains in a mosaic; feldspar, microcline and albite, occurs as large poikilitic grains and as smaller grains in the mosaic; one porphyrqblast of muscovite is present; biotite, pale brown to very red-brown pleochroism, occurs in short equant plates. Grain size: 0.1 to 0.2 mm. Fabric: granoblastic. Rock: metaquartzite. Continental # l Lankford cuttings 8090-95 Shell Oil Co. Quartz (84%), alkali feldspar (10%), biotite (3%), pyrite (2%) , muscovite 0%), calcite (tr), apatite (tr). Quartz is in an even-sized mosaic ; biotite is in short stubby prisms. Grain size: 0.2 to 0.4 mm. Fabric: granoblastic. Rock: metaquartzite. Dekalb # l Lewis core 5635-38 Bureau of Economic Geology Plagioclase (45%l, microcline microperthite (30%), quartz 05%) , hornblende (3%), chlorite (3%), epidote (2%), magnetite or ilmenite 0%), sphene (1%), leucoxene (tr), calcite (tr), apatite (tr), zircon (tr) . Zoned plagioclase shows cores altered to epidote and sericite and, locally, mantles of alkali feldspar-composition probably ranges from andesine through albite: quartz is interstitial to plagioclase subhedrons and locally is partly granulated; hornblende pleochroism is pale green to green; chlorite occurs with calcite and is derived from alteration of biotite and hornblende; sphene is in individual crystals and also mantles magnetite or ilmenite; locally it is partly altered to ]eucoxene. Grain size : feldspar 2 to 6 mm ; quartz 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granodiorite. Dekalb and l\lagnolia #I White core 7458-63 Bureau of Economic Geology Microcline microperthite (39%), quartz (20%), al bite (15%), line granular quartz-alkali feld­spar (15%), rhlorite (8% ), leucoxene (2%l, pyrite 0%), apatite (tr), zircon (tr). Micro­cline microperthite occurs as large individuals which locally are fractured; al bite is in ad­vanced stage of sericitization; a line granular mass of quartz and alkali feldspar containing chlorite and cut by pyrite veinlets occurs between larger quartz and feldspar grains in the area of fracturing and hrecciation; some of the alkali feldspar shows rhombic cross sections char­ acteristic of adularia; chlorite occurs in masses containing spots of pyrite and 1cucoxenc; lcucuxene occurs with chlorite and also as a replacement of sphene in diamond·shapcd crys­tals. Grain size: 2 to 6 mm; line granular quartz and feldspar 0.1 to 0.2 mm. Fabric: hypi­diomorphic granular. Rock: graf!ite. Franklin, Aston & Fair #1 Orchard Park cuttings 5350-60 Shell Oil Co. Fragments of quartz, sericitized alhite, and microcline, with a trace of chlorite. Grain size: 0.5 to l mm. Franklin, Aston & Fair #1 Orchard Park cuttings 5360·70 Shell Oil Co. Fragments of sericitized mierocline microperthite and quartz u p to 2 mm. Franklin, Aston & Fair #1 Orchard Park cuttings 5400­10 Shell Oil Co. Plagioclase (67%), quartz (30%), biotite (3%), sericite (included in feldspar), zircon (tr). Plagioclase has a completely sericitized center and an unaltered albite rim; biotite pleo­chroism is pale red-brown to very dark red-brown. Grain size: l to 2 mm. Fabric: hypidiomor· phic granular. Rock: granodiorite. Bureau of Economic Geology, The University of Texas Franklin, Aston & Fair # l Orchard Park cuttings 5440-50 Shell Oil Co. Microcline microperthite (50%), quartz (50%), chlorite (tr), biotite (tr), apatite (tr). Biotite is altered to chlorite. Franklin, Aston & Fair #1 Orchard Park cuttings 5480-90 Shell Oil Co. One fragment only. Red-brown biotite, sericitized albite, microcline, magnetite or ilmenite, apatite. Franklin, Aston & Fair #1 Orchard Park cuttings 5500-10 Shell Oil Coe Albite (75%), biotite (l5o/o), quartz (lOo/ol, pyrite (tr), zircon (tr), apatite (tr). Albite is partly sericitized; biotite is red-brown variety; pyrite occurs in biotite cleavages; apatite and zircon occur mostly in biotite with zircon making intense halos. Grain size: 2 mm. Fabric: hypidiomorphic granular. Rock: granodiorite. Franklin, Aston & Fair # l Orchard Park cuttings 5650-60 Shell Oil Co. Same as 5500-10 but with about 4% microcline and a trace of fl uorite. Franklin, Aston & Fair # l Orchard Park cuttings 5800-10 Shell Oil Co. Microcline microperthite (42%1, albite (40o/ol, quartz (l5o/o), biotite (3%), muscovite (tr). Al bite is partly sericitized. Grain size: 2 to 4 mm. Fabric: hypidiomorphic granular. Rock: granodiorite. Franklin, Aston & Fair # l Orchard Park core 5814-27 Bureau of Economic Geology Microcline microperthite (51%), quartz (25%), oligoclase (17%), chlorite (5o/o), leucoxene (l% ) , fluorite (l% ) , zircon (tr\, apatite (tr) , sphene (tr\. Plagioclase is partly sericitized; leucoxene and sphene occur within masses of chlorite derived from alteration of biotite; fluorite is in a veinlet. Grain size: 2 to 6 mm. Fabric: hypidiomorphic granular. Rock: granite. Gulf #1 Jennings core 8300 Bureau of Economic Geology Microperthite (35%l. oligoclase (33% 1, quartz (20o/o), chlorite (4o/o), calcite (3%) , biotite (2o/ol, leucoxene (2%1, sphene (1%), apatite (tr), zircon (tr), fluorite (tr) . Plagioclase is partly sericitized and microperthite shows incipient kaolinization ; ca1cite replaces plagioclase; chlorite is derived from alteration of green-brown biotite and contains small scattered grains of leucoxene: >phene is altered in part to lcucoxene. Grain size: 0.5 to 3 mm. Fabric: hypidio­morphic granular. Rock : granite. Gulf #1 Jennings core 8319 Humble Oil & Rfg. Co. ~licroperthite (34%) , al bite (25% l, quartz (25%), chlorite (8% ), fluorite (3%), sericite (3% I. leucoxene ( l '.C). calcite \l%l, apatite (tr), zircon (tr) . Chlorite is after biotite; Jeu roxene occurs as 5rnal1 grains in the chlorite; calcite replaces p]agioclase; fluorite occurs in lar~e mas...-e-s: sericite is derh·ed from alteration of plagiocJase. Grain size: 1 to 6 mm. Fabric: hypitliomorphic granular. Rock: granite. Gulf #I Jennings core 8319 Bureau of Economic Geology \lirroperthite t :>~<;{ l. albite (30%1, quartz (30o/o), chlorite (5%), calcite (2%) , leucoxene (Io/cl, pyrite (tr l. apatite (tr), zircon (tr). Plagioclase is in large twinned grains; chlorite apparently was derin·d from alteralion of biotite; leuroxene occurs in masses and in small ~rains included in chlorite: calcite in part replaces feldspar; apatite and zircon show zonal ~rowth. Grain size: I mm to I cm. Fabric: hypidiomorphic granular. Rock: granite. Gulf #1 State-Cha,es U core 3100± Bureau of Economic Geology Groundma's (89<'(). calcite (7ed of calcite-chlorite-zoisite-feldspar and a semi-opaque mineral that is greenish in reAecteJ lif!ht: the f!roundmass shows a stratification or flowage structure; microcline and pla~ioclase occur as round grains ; euhedral rhombic or prismatic zoisite crysta1s are dispersed through the slide. Grain size: groundmass less than 0.05 mm; feldspar and zoisite 0.1 to 0.2 mm. Fabric: pyroclastic!, porphyritic?. Rock: altered volcanic rook, tufl or flow. Basement Rocks, Texas-New Mexico Honolulu #I Hinkle-Federal core 7310-15 Bureau of Economic Geology Microcline microperthite (42%), albite (25%), quartz (25%), biotite (3%), chlorite (2%), magnetite and ilmenite (2%), epidote 0%), sphene (tr), apatite (tr), calcite (tr). Plagioclase is sericitized; quartz shows strain and granulation with development of some fine cruslwd areas: biotite occurs in partly bleached plates, pale brown to brown pleochroism, and is largely altered to chlorite; rhlorite occurs in veinlets with epidote and as a result of alteration of biotite; magnetite and ilmenite are unevenly distributed in layers; ilmenite is surrounded by leucoxene or sphene. Grain size: irregular, 0.1 mm to 1 cm. Fabric: hypidiomorphic granular with cataclastic elements. Rock: granite. Honolulu #1 Hinkle-Federal core 7310-15 (cut parallel to core) Humble Oil & Rig. Co. Albite (40%), quartz (30%), biotite (15%), epidote (7%), magnetite or ilmenite (4%), chlorite (3%), apatite (1 %) , sphene (tr). Plagioclase is zoned and shows incipient sericitiza­ tion; epidote occurs in scattered grains, large masses, and tiny mosses in plagioclase; biotite, pale to olive-drab pleochroism, is partly altered to chlorite; sphene occurs ns very thin rims on opaque mineral. Grain size: wide range from 0.1 to 2 mm; average 0.2 to 0.5 mm. Fabric: hypidiomorphic granular. Rock : albite-quartz microdiorite. Honolulu #1 Hinkle-Federal core 7310-15 (cut normal to core) Humble Oil & Rig. Co. Same as above. Honolulu #1 Levick-State cuttings 7210-15 Bureau of Economic Geology Albite (35%), microcline microperthite (30%), chlorite (12%), quartz (10%), epidote (8%), leucoxene (2%), calcite (2%l, magnetite or ilmenite 0%), apatite (tr), sphene (tr). Plagio­clase is altered to various combinations of sericite, epidote, and chlorite and probably originally was more calcic than albite; chlorite occurs as a result of alteration of plagioclase and in masses with epidote ; quartz is sutured and partly granulated. Grain size: 0.5 to 2 mm. Fabric: hypidio­morphic granular-cataclastic. Rock: cataclastically altered al bite granodiorite. Honolulu #1 McConkey Estate cuttings 5970-80 Shell Oil Co. Groundmass (87%), albite (8%) , leucoxene (2%), chlorite (1%), sphene 0%), calcite (1%), apatite (tr). Albite ocrurs in the quartz-alkali feldspar groundmass and as phenocrysts partly altered to sericite: apatite occurs in clusters of grains; sphene is partly altered to leucoxene. Grain size : groundmass less than 0.05 mm; phenocrysts up to 1 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Honolulu #1 McConkey Estate cuttings 6340-50 Shell Oil Co. Alkali feldspar and albite·oligoclase (68%), quartz (20%), calcite (8%), leucoxene (2%), sericite and muscovite (2%), chlorite (tr). Quartz is strained; calcite is in veinlets and partly replaces feldspar. Grain size: 0.2 to 1 mm. Fabric: hypidiomorphic granular. Rock: micro­granodiorite. Honolulu #1 McConkey Estate cuttings 6350-60 Shell Oil Co. Alkali feldspar (83%), quartz 00%), chlorite (4%), sericite (3%), calcite (tr), zircon (tr). Feldspar is partly sericitized. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock : granite. Honolulu #1 McConkey Estate core 6364·71 Bureau of Economic Geology Groundmass (79%), quartz 00%), al bite ( 5%), microperthite ( 5%), chlorite 0 % ) , magne­tite or ilmenite (tr), leucoxene (tr), pyrite (tr), sericite·muscovite (tr), apatite (tr), zircon (tr) . Groundmass is a fine granular mosaic of quartz and alkali feldspar; albite, microperthite, and quartz orcur as phenocrysts, locally poikilitic, containing inclusions of quartz and alkali feldspar and locally porphyroblastic (albite only); sericite and chlorite occur as fibers and plates in the p:roundmass. Grain size: groundmass 0.02 to 0.1 mm; phenocry•ts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Humble #1 Gorman-Federal core 5848-49 Bureau of Economic Geology ?1ficro.rline microperthite (58%), qu~rtz (20%), al bite 05%), chlorite (3%), magnetite or ilmenite (2%), leucoxene 0%), calcite 0%), pyrite (tr), biotite (tr), sphene (tr), scapolite? (tr), zircon (tr), apatite (tr) . Quartz fills interstices between feldspar grains and shows a rude Bureau of Economic Geology, The University of Texas dimensional orientation; al bite is partly sericitized; chlorite is the result of alteration of biotite and a1so occurs in masses around opaque miners]; sphene occurs in spongy masses of small subhedral grains around opaque mineral and is locally associated with scapolite?. Grain size: 0.5 to 2 mm. Fabric: xenomorphic granular. Rock: granite. Humble #1-N State core 3476 Stanolind Oil & Gas Co. Labradorite? (59%), sericite 05%), calcite 05%), augite (4%), magnetite or ilmenite (2%), chlorite (2%), glass (2%), pyrite 0 % ), apatite (tr). Most of plagioclase is in laths but one large phenocryst is present; sericite replaces a large part of the rock; calcite fills amygdules and re­places part of the mass of the rock; apatite occurs as needles. There is a local quartz-biotite­sericite-calcite-altered feldspar-chlorite area in the slide that is probably part of a vein or an area of deuteric alteration; it is separated from the diabase proper by calcite. Grain size: laths of feldspar 0.1 by 0.5 mm; phenocryst 1 cm. Fabric: subophitic. Rock: diabase [possibly Tertiary]. Humble #1-N State core 3500-03 Stanolind Oil & Gas Co. Labradorite (73%), augite (8%), olivine (7%), magnetite or ilmenite (5%), chlorite (3%), biotite 0%), pyrite 0%), alkali feldspar 0%), sericite 0%), apatite (tr), analcite (tr). Plagioclase is zoned; augite is brownish violet and probably titaniferous; olivine is partly altered to a peculiar olive chlorite? that is highly birefringent; intensely red-brown biotite borders magnetite or ilmenite. Grain size: 0.5 to 1 mm. Fabric: subophitic. Rock: analcitic olivine diabase [possibly Tertiary]. Humble #1-N State core 3804-09 Bureau of Economic Geology Labradorite (63%), sericite 00%,) calcite (8%), chlorite (5%), quartz (4%), augite (3%), alkali feldspar (2%), leucoxene (2%), biotite 0 % ) , magnetite or ilmenite 0 % ) , pyrite ( 1% ) , zeolite (tr). Plagioclase laths are partly sericitized; sericite in part replaces plagioclase and in part occurs in masses with calcite and chlorite surrounding augite remnants; biotite is in small flakes around magnetite or ilmenite; calcite fills amygdules; amygdules constitute about 10% of the slide and locally show a quartz center or contain pyrite; alkali feldspar occurs in inter· stices of plagioclase subhedrons. Grain size: plagioclase laths 0.1 by 0.5 mm; amygdules up to I cm. Fabric: subophitic. Rock: altered diabase [possibly Tertiary]. Humble #I-N State core 3835 Stanolind Oil & Gas Co. Quartz (84%), calcite (7%), albite (4%), pyrite (3%), sericite 0%), chlorite 0%), leucoxene (tr), rutile (tr), zircon (tr) . Quartz occurs in a well-sized equi·granular mosaic; calcite is dispersed through the slide. Grain size : 0.1 to 0.2 mm. Fabric: granoblastic. Rock: metaquart:ite. Humble #1-N State core 3936 Stanolind Oil & Gas Co. 0) 60% of slide: labradorite (52%), chlorite (30%), calcite (25%), magnetite or ilmenite (3% ), pyrite (tr). Grain size: 0.1 to 0.4 mm. Fabric: subophitic. Rock: altered diabase. (2) 40% of slide: albite (45% ), quartz (30%), calcite 00%), biotite 00%), chlorite (5%). Grain size: 0.1 to 0.2 mm. Fabric: xenomorphic granular. Rock: microgranodiorite [possibly Tertiary]. Humble #1-N State core 3939 Stanolind Oil & Gas Co. l..abradorite? (70%), senc1te 05% ), amygdules (8%), augite (5%l, magnetite or ilmenite (2%), chlorite (tr). Sericite with minor associated chlorite replaces large volumes of the rock; the rock is characterized by round amygdules composed for the most part of calcite but locally of pyrite and quartz; sericite commonly forms a rim around the outside of the amygdule. Grain size: feldspar laths 0.1 by 0.5 mm; amygdules up to 2 mm. Fabric: subophitic. Rock: diabase [possibly Tertiary]. Humble #l·U State cuttings 7847-51 Bureau of Economic Geology Microperthite (56%), albite (20%), quartz (20%), chlorite (4%), apatite (tr), zircon (tr). Plagioclase is in an advanced stage of sericitization; microperthite is partly kaolinized; chlorite is derived from alteration of biotite. Grain size: 0.2 to 2 mm. Fabric: xenomorphic granular. Rock: granite. Basement Rocks, Texas-New Mexico Humble #1-Y State cuttings 7425-30 Bureau of Economic Geology Microperthite (40%), oligoclase (38%), quartz (10%), biotite (6%), hornblende (3%), chlorite (1%), ilmenite 0%), sphene 0%), epidote (tr), apatite (tr). Plagioclase is partly sericitized; biotite pleochroism is pale brown to deep rich brown; hornblende pleochroism is yellow-green to green. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock : granite. Magnolia #1 Black Hills Unit cuttings 5930-40 Shell Oil Co. Groundmass (98%), quartz (1%), pyrite 0 %), biotite (tr), apatite (tr), tourmaline (tr). Cryptocrystalline mass contains angular fragments of quartz (less than 0.05 mm) and biotite flakes; it is composed of sericite, chlorite, quartz, and feldspar where constituents are coarse enough to be recognized. Grain size: groundmass mostly cryptocrystalline; angular quartz frag­ments up to 0.05 mm. Fabric: ? . Rock : argillite? Magnolia #1-B O'Brien core 7665-66 Bureau of Economic Geology Albite-oligoclase and microcline microperthite (74%), quartz 00%), chlorite 00%), ilmenite (3%), calcite (3%), leucoxene (tr), sphene (tr), epidote (tr), apatite (tr), zircon (tr). Feld­spar is mostly a partly sericitized albite-oligoclase which is commonly rimmed with microperthite; quartz is interstitial to feldspar and coarsely micrographic; chlorite, strongly pleochroic in yellow­brown to deep green, occurs in veinlets, masses, and plates; locally it contains biotite and horn­blende relicts; caldte occurs with chlorite as result of alteration of ferromagnesian minerals; ilmenite i:rains include masses of small sphene crystals; epidote occurs with chlorite in the cores of some plagioclase grains; apatite is in part in long needles; zircon is in part in skeletal and needle-like crystals with serrate edges. Grain size: 0.2 to 2 mm. Fabric: hypidiomorphic granular. Rock: granodiorite. Magnolia #1 Shaw-Federal core 12070 Bureau of Economic Geology Groundma•s (64%), microcline microperthite 05%), andesine 00%), augite (4%), chlorite (3%), magnetite or ilmenite (2%), calcite (2%), amphibole (tr), apatite (tr), zircon (tr). Groundmass is a micrographic intergrowth of quartz and alkali feldspar; microcline microper· thite occurs a• phenocrysts; phenocrysts of zoned plagioclase show sericitized cores-mostly it is andesine ; augite phenocrysts are fractured ; chlorite is in fine fibrous masses; magnetite or ilmenite occurs as larger scattered i>;rains and as finely disseminated grains in the groundmass; calcite is derived from alteration of ferromagnesian minerals; commonly it forms a core mass fringed with green am phi bole. Grain size: groundmass 0.1 to 0.2 mm; phenocrysts 0.5 to 3 mm. Fabric: porphyritic. Rock: augite-andesine-microgranite porphyry. Magnolia #1-Z State cuttings 8728 Bureau of Economic Geology Albite-oligoclase (45%), microperthite (30%), quartz (20%), chlorite (4%), carbonate 0 % ), leucoxene (tr), sphene (tr), apatite (tr), zircon (tr). Plagioclase is partly sericitized; chlorite is after biotite; there are only a few mineral grains in each fragment in this slide and the per­centages are not very significant. Grain size: 0.5 to 4 mm. Fabric: hypidiomorphic granular. Rock : granodiorite. Magnolia # l Turney-Federal core 5321-24 Bureau of Economic Geology Albite (70%), quartz (15%) , alkali feldspar (8%), chlorite (3%), leucoxene (2% ), epidote (1 % ) , sphene 0 % ), apatite (tr). Plagioclase is partly sericitized; quartz appears to be late and fills voids between feldspar grains; a brown kaolinized alkali feldspar rims these voids and separates quartz from plagioclase; chlorite forms from alteration of biotite and is rudely oriented. Grain size: 0.5 mm. Fabric: hypidiomorphic granular-gneissic. Rock: al bite granodioritc gneiss. Magnolia #1 Turney-Federal core 5321-24 Bureau of Economic Geology Oligoclase-andesine (42%), fine granular epidote-chlorite-magnetite material (25%), chlorite (8%), alkali feldspar (8%), epidote (7%), magnetite or ilmenite (5%), amphibole (5%), apatite (tr), pyrite (tr), sphene (tr). Partly kaolinized plagioclase is concentrated in layers; fine-grained epidote-chlorite-magnetite material occurs in layers, elongate lenses, and between plagioclase grains; some. altered amphibole grains are included in this material; faded green hornblende occurs as onented grains; alkali feldspar is almost completely kaolinized. Grain size: 0.2 to 0.5 mm. Fabric: crystalloblastic-<:ataclastic-gneissic. Rock: evidote-chloritc­oligoclase gneiss. Bureau of Economic Geology, The University of Texas Olson #I Noble Trust core 7630-60 Humble Oil & Rfg. Co. Sericite (80'fo), quartz 05%), magnetite or ilmenite (3'fo), epidote (2'fo), leucoxene (tr), rutile (tr), calcite (tr), chlorite (tr). Sericite is in fine oriented fibers which in some layers are so closely packed as to appear a solid mass of sericite; quartz occurs in small angular to subangular grains with long axes aligned; quartz is irregularly distributed through the layers. Thin distinct layers (1 to 2 mm thick) are visible when the thin section is viewed mega>copically. These layers under the microscope are expressed by variations in grain size and miea content. Grain size: mostly less than 0.02 mm. Fabric: lepidoblastic. Rock: sericite phyllite. Olson # l Noble Trust core 7630-60 Humble Oil & Rfg. Co. Same as preceding slide but with a large feldspar pebble, more quartz and less well-defined layers. Olson #I Noble Trust core 8030 Ilureau of Economic Geology Sericite (82%), quartz (8% ), feldspar (5'fo), magnetite or ilmenite and leucoxene (4'fo), cakite (I';CJ, epidote (tr I, apatite (tr), zircon (trJ. Sericite occurs as oriented fibers which, locally, are so densely packed as to resemble a solid mass; quartz and feldspar occur as individual grains within the mass of sericite; quartz is mostly in angular grains and shows an orientation of long axes; the opaque minerals are in a large number of very small scattered grains. A mega­scopic examination of the thin section shows a very fine layering and cross-bedding with the layers on the order of several millimeters. Grain size: mostly less than 0.02 mm. Fabric: lepido· blas:ic. Rock: sericite phyllite. Olson #I Noble Trust core 8030'! Ilureau of Economic Geology Sericite (64%), plagioclase and quartz (30%), chlorite (3'fo), epidote (2'fo), magnetite or ilmenite (I%), rock fragment (tr) . Sericite in a network of anastomosing fibers flows around quartz and plagioclase grains; plagioclase occurs as augen and in smaller grains. Grain size: quartz and feldspar 0.1 to 1 mm. Fabric : lepidoblastic. Rock: sericite phyllite. Richfield # 1 Comanche Unit core 6128 Bureau of Economic Geology Groundmass (78%), quartz (7%), microperthite (5'fo), albite (5'fo), chlorite (3%,J, leucoxene (2'fo), apatite (tr), zircon (tr), calcite (tr) . The alkali feldspar-quartz-sericite groundmass shows Ao wage structure; quartz grains and fragments of sutured mosaic occur as pherlocrysts with plagioclase and microperthite ; leucoxene and chlorite result from biotite alteration. Grain size: groundmass 0.01 to 0.02 mm; phenocrysts up to 2 mm. Fabric : porphyritic. Rock : rhyolite porphyry. Richfield # l l\lullis core 12143-53 Stanolind Oil & Gas Co. Albite (56'fo), alkali feldspar (IO'fo), biotite (10%), chlorite (lO'fo), quartz (10'fo), leucox­ene (2%), magnetite or ilmenite (l'fo), calcite (l'fo), pyrite (tr), sericite (included in albite estimate), apatite (tr), zircon (tr), radioactive mineral (tr). Zoned plagioclase is partly re· placed by sericite and calcite; biotite, brown to dark brown pleochroism, is partly altered to chlorite; radioactive mineral is opaque---nearly l mm in diameter-with a large halo. Grain size: 2 to 5 mm. Fabric: hypidiomorphic granular. Rock: albite granodiorite. Richfield #1 l\lullis core 12143-53 Bureau of Economic Geology Oligoclase (3-1%), quartz (20'fo) , microcline microperthite (15'fo), biotite 05'loJ, chlorite (10%), amphibole (3'fo), leucoxene (2%), magnetite or ilmenite (l'fo), apatite (tr), zircon (tr), pyrite (tr), sphene (tr), calcite (tr). Plagioclase is partly sericitized; biotite, pale to dark brown pleochroism, is partly altered to chlorite; pale green to almost colorless amphibole relicts are altered to calcite and chlorite ; calcite also replaces plagioclase and occurs in vein­Iets. Grain size: l to 3 mm. Fabric : hypidiomorphic granular. Rock : granodiorite. Richfield #1-A Trigg cuttings 9980-90 Shell Oil Co. Al bite (70%), chlorite (20%), sericite (7%), magnetite or ilmenite (I%), sphene (I'lo), calcite 0%), apatite (tr), zircon (trJ. Albite is partly sericitized; chlerite is derived from biotite. Grain size: l mm. Fabric : hypidiomorphic granular. Rock: altered albite diorite. Richfield #1-A Trigg cuttings 9980-90 Shell Oil Co. Plagioclase (5i'fo), microperthite (30%), chlorite (7'fo), biotite (5'fo), quartz (!% ),zircon (tr), apatite (tr). Plagioclase is partly sericitized; biotite is almost completely altered to chlorite. Grain size: l to 3 mm. Fabric: hypidiomorphic granular. Rock: syenodiorite. Basement Rocks, Texas-New Mexico Richfield #1-A Trigg cuttings 9980-93 Stanolind Oil & Gas Co. Plagioclase and microperthite (91%), quartz (5%), biotite (2%), hornblende 0%), chlorite (l%, ) calcite (tr), sericite (tr), apatite (tr). Plagioclase is zoned with a core of andesine and an al bite rim (estimated from relief); microperthite is partly kaolinized; hornblende pleo· chroism is brown to deep green; biotite pleochroism is brown to dark brown. Grain size: l to 2 mm average, with grains up to 4 mm. Fabric: hypidiomorphic granular. Rock: gran· odiorile. Richfield #1·3 White core 9046-47 Bureau of Economic Geology Oligoclase? (62%), quartz 05%), chlorite (15%), calcite (3%), pyrite (3%), leucoxene (2%), apatite (tr). Plagioclase is sericitized; calcite and pyrite occur in veinlets and in scattered grains; chloritization is intense near veinlets. Grain size: 0.1 to 0.2 mm. Fabric: meta· somatic. Rock: chloritized quartz microdiorite. Sanders # l Sanders cuttings 494-0-50 Shell Oil Co. Scattered fragments show leucoxene, chlorite, quartz, and alkali feldspar. Grain size: about 0.2 mm. Sanders # l Sanders cuttings 524-0-50 Shell Oil Co. Microcrystalline to cryptocrystalline mass contains oriented senc1te flakes and is cut by thin quartz veinlets. It is partly replaced by masses of calcite. One angular quartz fragment 0.5 mm long is present. Fabric: cataclastic. Rock: mylonite? or a cataclastically altered rhyolite? Sanders # l Sanders cuttings 5260-70 Shell Oil Co. Microcrystalline to cryptocrystalline mass contains streaks and veinlets of sericite showing rudely parallel orientation. Embayed and broken quartz fragments up to l mm long make up about 10% of the slide and are probably phenocrysts. Calcite replaces part of the groundmass. Rock: cataclastically altered rhyolite. Sanders # l Sanders cuttings 5270-80 Shell Oil Co. Essentially same as 5260-70 but with altered feldspar phenocrysts (1%), and a trace of chlorite and leucoxene. Sanders # l Sanders cuttings 5280-90 Shell Oil Co. Essentially same as 5270-80 hut with (a) prismatic masses of leucoxene parallel to the general foliation; (b) some coarser masses of intergrown quartz and alkali feldspar; and (c) a trace of rutile and zircon. Sanders #1 Sanders summary Rock is a cataclastically altered rhyolite porphyry with a foliation developed through cata· clasis. Sun # l Pinion core 1850 Bureau of Economic Geology Al bite (51%) , epidote (20%), groundmass 05%), chlorite 00%), quartz (2%), calcite (l% ) , biotile (l% ) . Al bite occurs as twinned subhedral phenocrysts, partly altered to epidote, and as microlites in the groundmass (original feldspar probably was more calcic); epidote occurs throughout the slide replacing feldspar and filling cavities; groundmass is composed of albite microlites, closely paoked spheruliies, and fine granular epidote; chlorite occurs with epidote as cavity fillings and shows remarkable interference colors; quartz is secondary and occurs with coarse epidote; green biotite fills cavities and is secondary. Grain size: from tiny microlites up to 2-mm phenocrysls; spherulites 0.1 to 0.2 mm. Fabric: microspherulitic-por· phyritic. Rock: epidotized chloritized albite andesite porphyry. Union and Dekalb # l State cuttings 7575 Bureau of Economic Geology Albite-oligoclase <34$"o), quartz (20%), hornblende (20%), microcline (15%), biotite 00%), magnetite or ilmenite 0 %), epidote (tr), sphene (tr), calcite (tr), apatite (tr), zircon (tr). Plagioclase is partly sericitized; hornblende shows yellow-green to green pleochroism and locally is poikilitic; bio_tite is green-brown. Grain size: 0.5 to 2 mm. Fabric : xenomorphic granular. Rock : granvdionte. Bureau of Economic Geology, The University of Texas Union and Dekalb #I Stale cuttings 7575·80 Bureau of Economic Geology Microdine (66%), plagioclase 05% l, quartz 05%), hornblende 0%), biotite 0%), calcite (I'7c), magnetite or ilmenite 0 % l, chlorite (tr), leucoxene (tr), apatite (Ir), zircon (tr). lllicrocline is partly kaolinized; plagioclase is sericitized; biotite is ~reen·brown; hornblende shows yellow·green to green pleochroism; calrite replaces feldspar locally. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Union and Dekalb # 1 State cuttings 7575-80 Bureau of Economic Geology lllicrocline (-19%) , oligoclase·andesine (20%) , quartz 05%), hornblende 02%), biotite (2%), magnetite or ilmenite 0%). epidote 0%l, chlorite (trl, leucoxene (tr), sphene (tr), apatite (tr). Microcline is sporadically microperthitic; zoned plagioclase grains in the oligoclase­ andesine range commonly show sericitiU'd cores; locally grains in advanced stage of sericitization appear to be more sodic; hornblende pleochroism is yellow-green to green. Grain size: 0.5 to 2 mm. Fabric: xenomorphic granular. Rock: granite. Union and Dekalb # 1 State cuttin;;s 7580 Bureau of Economic Geology lllirrocline microperthite and olil!oclase (75%), quartz 00%), hornblende (8%), biotite (4%), ilmenite (2%), calcite 0%), leucoxene (tr), sphene (tr), epidote (tr). Feldspar is predominantly microcline microperthite; plagioclase is partly sericitized; hornblende pleo­ chroism is yellow.green to green; biotite is {!Teen-brown; ilmenite is partly altered to leucoxene. Grain size : 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. DEBACA COUNTY Cities Production #1 Hobson cuttings 6720-30 Bureau of Economic Geology lllicrocline microperthite (52%), oligoclase (25%), quartz (15%), biotite (3%), magnetite or ilmenite (2%), muscovite and sericite 0%), chlorite 0%), calcite 0%), zircon (tr), apatite (tr) . Plagioclase is partly sericitized; green-brown biotite shows rude orientation; calcite is from alteration of original ferromagnesian mineral. Grain size: rock contains fragments showing a grain size of 0.2 to 0.5 mm and fragments with a grain size of 0.5 to 3 mm (mostly just quartz and feldspar without accessory minerals). Fabric: xenomorphic granular. Rock: microgranite and granite. Pure #1 Fee-Federal core 6467 Bureau of Economic Geology Microcline (36%), oligoclase (35%), quartz (20%), muscovite (4%), biotite (3%), ilmenite (1%), chlorite 0%), sphene (tr), fluorite (tr), leucoxene (tr), apatite (tr). Microcline is locally microperthitic; plagioclase is in part sericitized; quartz is in large strained masses; biotite, pale brown to brown pleochroism, is in part altered to chlorite; leucoxene occurs within chlorite masses; sphene locally surrounds the opaque mineral. Grain size: 2 to 5 mm. Fabric: hypidiomorphic granular. Rock : granite. South Basin Oil Company #1 Good core 4774-79 Bureau of Economic Geology Quartz (66%), hornblende 00%), biotite (9%), oligoclase (8%), epidote (5%), magnetite or ilmenite (2%). Quartz occurs in a mosaic and shows dimensional orientation; blue-green hornblende occurs as porphyroblasts with a planar orientation; biotite is in oriented plates (green·brown); oligoclase is partly sericitized and is unequally distributed in lenses and layers; epidote occurs as indi"idual grains and strings of grains. Grain size: mosaic 0.05 to 0.1 mm; porphyroblasts up to 1 mm. Fabric: porphyroblastic. Rock : epidote·biotite-hornblende schist. Woolworth & Hawkins #1 Myrick cuttings 6080-90 Bureau of Economic Geology Albite·oligoclase (75%), hornblende (12%), magnetite or ilmenite (4'}'c,), quartz (3%), alkali feldspar (3%), biotite (2%), apatite (1%), pyrite (tr), epidote (tr). Zoned plagioclase is \'ariably sericitized in different fragments and locally is rimmed with alkali feldspar; hornblende is considerably altered; biotite is a green-brown variety partly altered to chlorite. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock : altered leuco·diorite? Woolworth & Hawkins #1 .M}Tick cuttings 6080-90 Bureau of Economic Geology Plagioclase, hornblende, pyroxene, quartz, biotite, chlorite, magnetite or ilmenite, apatite. Plagio­clase is sericitized; hornblende and plagioclase are altered. Only a few fragments in the slide. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: altered quartz diorite. Basement Rocks, Texas-New Mexico Woolworth & Hawkins #1 Myrick cuttings 6168-74 Bureau of Economic Geology Oligoclase (61%), hornblende (15%), quartz 00%), pyroxene (5%), alkali feldspar (4%), magnetite or ilmenite (3%), biotite (2%), pyrite (tr), apatite (tr). Plagioclase is sericitized; hornblende, yellow-green to green pleochroism, occurs in prismatic grains and felty masses and locally is altered to a fine granular brownish mass of unidentified mineral which is rimmed with secondary green amphibole; larger hornblende prisms locally show a brown core; pyroxene is in faded partly oxidized grains; partly kaolinized alkali feldspar is commonly micrographic with quartz; biotite pleochroism is pale reddish brown to dark green; apatite is in l'art in needles. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: quartz diorite. Woolworth & Hawkins #1 Myrick cuttings 6173-74 Bureau of Economic Geology Sericitized plagioclase, altered hornblende, pyroxene, quartz, biotite, chlorite, magnetite or ilmenite, apatite. Mineral percentages are not significant. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: altered quortz diorite. EDDY COUNTY Continental #1 Thurman-Federal cuttings 10760-65 Bureau of Economic Geology Andesine to albite (57%), chlorite (20%), magnetite or ilmenite (10%), calcite (8%), leucox· ene (3%), apatite (2%), pyrite (tr). Anorthite content of the plagioclae rims and as smaller subhedrons--cores are commonly altered to sericite or sericite and epidote: biotite showing yellow-brown to very deep brown pleochroism is partly altered to chlorite; hornblende is mostly altered to chlorite. Estimation of the relative amounts of pla1?ioclase and alkali feldspar is rnlnerable. Grain size: I to 4 mm, nriable. Fabric: hypidio· morphic granular. Rock: granite. Southern Union and Magnolia #1 Elliott core 9886-8i~f Bureau of Economic Geology 1\!icrocl ine (40'7c). albite (35%), quartz (20%). chlorite 0%1, bleached biotite 0%1 , musco· vite (tr\, epidote (tr\, magnetite or ilmenite (trl, sphene (tr), calcite (trl, zircon (tr). Albite shows indpient !"eriritization and in part is in ,·ery Ion~ narrow tablets. Grain size: 0.2 to l mm average with sporadic plagioclase grains up to 4 mm. Fabric: hypidiomorphic granular. Rock: microgranite. Amerada # 5 Corri~an cuttings 7803 Bureau of Economic Geology Oli,orlase 160%) _ microrline microperthite (20':{-). quartz 00%), biotite (5%), hornblende (2% I. ilmenite (~'l'c\. sphene elf to the quartz and plagiorlase grains and commonly shows feathery projections into quartz and plagioclase !'rain boundaries; biotite pleochroism is pale oli,·e-brown to ,-ery dark hrown: hornblende pleorhroi~m is yellow-~reen to ~ret>n: zircon forms halos in biotite: ilmenite is rommonly em·eloped by leucoxene. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular with microrline showin~ a poikilitic tendency. Rock: granodion"te. Amerada #6 Corrigan cuttings 7687 Bttreau of Economic Geology ~1icrocline microperthite (66%), oliirnclase (20%), quartz 00%), biotite (2~), magnetite or ilmenite (1 '7o), calcite (l'lc), apatite (tr). Plagioclase is partly sericitized: quartz is in part myrmekitically inter~rown with plagioclase: biotite occurs as bleached and altered rem· nants: calcite replace:; plagioclase: mirroc1ine shows the same "late cf!·stallizin~ features11 as in #5 Corri~an, accommodates it5elf to pla~ioclase and quartz grains, and commonly orrurs as a "spongy"' host. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular. Rock: granite. Amerada #7 Corrigan cuttings 763~ Bureau of Economic Geology Mirrocline (.j()'(-). oligoclase (38%), quartz (10% ), biotite (8% ), leucoxene (2%), ilmenite 0%l, calcite (!<;1:), apatite (tr). :\[icrocline is only locally perthitic and occurs in irregular grains which accommodate themsehes to quartz and plagioclase grain boundaries; plagioclase sho,,·s a ,·arird dep:ree of alteration to seririte; biotitc pleochroism is pale brown to ,·ery dark hrown: locall,-quartz is myrmekitically interl!rown with plagioclase. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Amerada #II Corrigan cuttings 7 410 Bureau of Economic Geology ~licrocline 153%1, oligoclase (25%), quartz (15%), biotite (5%1, leucoxene 0%), epidote (I<;(), apatite (tr I, calcite (tr). :\licrocline is locally microperthitic and occurs in large grains. Basement Rocks, Texas-New Mexico and as irregular projections into quartz and plagioclase grain boundaries; plagioclase is partly altered to sericite or scricite and epidote; quartz locally is myrmekitic with pla~iocla'c; biotite pleochroism is pale brown to very dark brown. Grain size: 0.5 to 2 mm. Fabric : hypidiomorphic granular. Rock: granite. Amerada #4 Hare cuttings 7928 Bureau of Economic Geology Oligoclase (58%), quanz 05-%), microcline 00%), biotite (8% ), hornblende (5% l, leu· coxene (2%), calcite 0%), apatite 0%), magnetite or ilmenite (tr), zircon (tr). Plagio· clase occurs as small clear twinned subhedrons and as large irregular twinned grains, porphy­roblastic in habit, zoned, and including myriad small grains of the other mineral constituents of the rock; microcline occurs as grains and as irregular feathery bodies along quartz and plagioclase grain boundaries; amount of quartz varies widely in different fragments; hornblende, yellow-green to deep green pleochroism, occurs in poikilitic grains; biotite pleochroism is pale to deep brown. One fragment only shows feldspar, quartz, biotite, and apatite in a matrix of what appears to be chalcedonic silica and a fibrous pale brownish mineral that is probably sericite; apparently this is an altered phase (silicification-sericitization l of the normal igneous rock described above. From meager amount of material available for study it appears that the "phenocrysts" are actually porphyroblasts that have grown perhaps under the influence of late solutions. Rock appears to be meta-igneous to some degree. Grain size: 0.2 to 0.4 mm with large porphyroblastic feldspars up to 3 mm. Fabric: in most fragments xenomorphic to hypidio­morphic granular, in some fragments gneissic. Rock: microgranodiorite porphyry. Amerada #5 Hare cuttings 7844 Bureau of Economic Geology Oligoclase (52%), microcline (30%), quartz 02%), biotite (4%), magnetite or ilmenite (I%), leucoxene 0 % ) , apatite (tr). Plagioclase is partly sericitized; microcline, locally microperthitic, corrodes and embays quartz and plagioclase; quartz is locally myrmekitic. Grain size: 0.5 to 1 mm. Fabric : hypidiomorphic granular. Rock: microgranodiorite. Amerada # 7 Phillips cuttings 10211 Bureau of Economic Geology Microperthite (40%), oligoclase (35%), quartz (20%), leucoxene (2%), biotite (1 %) , pyrite 0%), sericite 0 %) , apatite (tr), zircon (trl. The potassium feldspar is very finely perthitic; sericite occurs as srattcred flakes or fihers. Grain size: 0.1 to 1 mm. Fabric: poikilitic, round discrete quartz grains occur in a sponge of microperthite; plagioclase retains its integrity of grain. Rock : microgranite. Amerada #7 Phillips cuttings 10214 Bureau of Economic Geology Microcline (56%), quartz (30%), albite 00%), ma~netite or ilmenite (3%), biotite 0%), leucoxene (tr), sphene (tr). Grain size: 0.05 to 0.2 mm. Fabric : xenomorphic granular­poikilitic. Rock: microgranite. Amerada #3-A Phillips cuttings 11006 Bureau of Economic Geology Al bite (51 % ) , microcline (30%) , quartz 05%), biotite (2%), chlorite 0 % ) , magnetite or ilmenite 0 % ), leucoxcne (tr), sphene (tr), apatite (tr), zircon (tr). Plai;ioclase shows incipient alteration to sericite; ratio of plagioclase to microcline varies widely in different fragments. Grain size: 0.2 to 0.5 mm with sporadic grains up to 2 mm. Fabric: hypidiomorphic granular-poikilitic. Rock : al bite microgranodiorite. Amerada #1 State BTA core 11716 Ilureau of Economic Geology Groundmass (98%), quartz (2%), sericite (tr), calcite (tr), pyrite (tr) , zircon (tr). Ground· mass is micrographic with equi-extinguishing patches less than 0.1 mm in diameter; quartz an~ult of solutions) ; musco\'ite orcurs in scattered small plates: pyrite is ronC'entrated along grain boundaries and may be laq!ely se<>ondary: maµnetite or ilmenite is in fine scattered grains. Grain size: 0.1 to 0.3 mm. Fabric: xenomorphic granular. Rock: microgranite. Amerada #10 Wood cuttings 7661 Bureau of Economic Geology Same as #9 Wood but slide contains only 10% plagioclase and markedly elongated zircons. Basement Rocks, Texas-New Mexico Cities Service #3-S State cuttings 8030-34 Bureau of Economic Geology Oligoclase (57%), microcline microperthite (25%), quartz (12%), biotite (4%), magnetite or ilmenite (!%), leucoxene (1%), epidote (tr), apatite (tr). Plagioclase is partly "''ricititcd: quartz contains fine needles of rutile? ; biotite is green-brown; leucoxene envdops ilmenite. Grain size: 0.3 to 1 mm. Fabric: hypidiomorphic granular-poorly developed. Rock: microgran­ odiorite. Continental # 1 Burger B-28 core 9373 Bureau of Economic Geology Feldspar (39%), biotite (20%), calcite (15%), chlorite (10%), sericite (10%), quartz (3%), leucoxene (2%), ilmenite (1%), epidote (tr), apatite (tr). Feldspar is almost completely sericitized and makes up the mass of the rock; green-brown biotitc occurs in tiny flakes; quartz is in small "eyes"; ilmenite is partly altered to leucoxene. This rock consists of a mass of secondary minerals, chlorite-calcite-biotite-epidote-sericite, that completely obscure the original fabric. Parallel, anastomosing, calcite veinlets and concentrations of biotite-chlorite in layers and lenses give the rock a banded or layered character. Grain size: 0.1 to 0.2 mm. Fabric: metasomatic. Rock: probably an altered microdiorite. Continental #1 Burger B-28 core 9376 Bureau of Economic Geology Pla~ioclase (76%), chlorite (15%) , pyrite, magnetite or ilmenite, and leucoxene (5%), calr.ite (3%) , biotite (lo/o ) , apatite (tr). Original nature of plagioclase is obscured by sericitization, only the shape of grains and twinning remain ; chlorite occurs in non-oriented fibers and plates; opaque minerals occur in veinlets and in scattered grains. Grain size: 0.1 to 2 mm. Fabric: relict hypidiomorphic granular. Rock : altered microdiorite. Continental #I Burger B-28 core 9379 Bureau of Economic Geology Microcline microperthite (68%), quartz (20%), albite (10%), biotite (1%), calcite (1%), magnetite or ilmenite (tr), chlorite (tr), zircon (tr), sphene (tr), apatite (tr) . Plagioclase has a sericitized core and a clear border; biotite pleochroism is pale green to deep brown; chlorite is a brown variety. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Hand specimen of core shows that this granite occurs in a narrow stringer intruding altered microdiorite. Continental #1-E Lockhart A-27 core 7791 Bureau of Economic Geology Al bite (52%), quartz (30%), microcline (15%), biotite (2%), pyrite and magnetite or ilmenite (1 % ) , leucoxene (tr), zircon (tr), apatite (tr), muscovite (tr). Green-brown biotite occurs in small scattered plates and is commonly surrounded by leucoxene. Grain size: 0.5 to I mm. Fabric: hypidiomorphic granular. Rock: a/bite microgranodiorite. (Photomicrograph, Pl. IV, C.) Continental #6-E Lockhart B-11 cuttings 8058-65 Bureau of Economic Geology Microcline microperthite (49%), oligoclase (35%), quartz (10%), sericite (3%), biotite (2%), calcite (1%), pyrite (tr), zircon (tr), leucoxene (tr), magnetite or ilmenite (tr). Plagioclase shows incipient alteration to sericite; sericite and muscovite occur as (a) fine fibers as a result of plagioclase alteration and (b) as flakes and masses distributed through the rock; biotite is a red-hrown variety. Grain size: 0.5 to I mm. Fabric: hypidiomorphic granular-poikilitic. Rock: microgranite. Continental #4 Lockhart B-12 cuttings 8202 Bureau of Economic Geology Microcline (63%), quartz (20%), a!bite-oligoclase (15%), calcite (2%), magnetite or il­menite (tr), muscovite (tr), leucoxene (tr). Calcite 1s ma cross-cutting veinlet. Grain size: 0.5 to 2 mm. Fabric: xenomorphic granular. Rock: granite. Continental #4 Lockhart B-12 cuttings 8202 Bureau of Economic Geology Andesinc-lahradorite (56%), hornblende (30%), biotite (5%), epidote (3%), chlorite (2%), calcite (2%) , magnetite or ilmenite (2%), sphene (tr), apatite (tr). Hornblende showing yellow-green to dark j!reen pleochroism is locally poikilitic; biotite is green-hrown; calcite occurs in n veinlet. Grain size: 0.5 to 1 mm with sporadic grains up to 2 mm. Fabric: xenomorphic granular. Rock : microdiorite. Bureau of Economic Geology, The University of Texas Continental #1-A Lockhart B-13 cuttings 7514-39 Bureau of Economic Geology :\licrodine microperthite ('ii%), olig:oclase (20o/o), ma1metitc or ilmenite (4o/o), chlorite (4~o I, calcite \I~ I, quartz (trJ, apatite (tr). Brownish chlorite fills breccia zones; calcite in pJrt replaces pla;;.iodase; quartz on·urs as veinkts in brecda zo1u.·s. Grain size: 0.5 to 2 mm. Fabric xenomorphic granular with local brecciation. Rock: s1..-nite. Continental #J-..\ Lockhart B-13 cuttings 7585-90 Bureau of Economic Geology Microcline microperthite (6~%), oligoclase (20%), augite (!Oo/o), magnetite or ilmenite \7<:"<), red iron oxide (I% I, pyrite (tr), sphene (tr), zircon (tr)_ Green pyroxene shows schiller structure. Grain size: 0.5 to 2 mm. Fabric: xenomorphic granular. Rock: syenite. Continental #I Warren A-29 core 9361-91 Bureau of Economic Geology Oligoclase-andesine (3-t7o), microcline microperthite (25%), quartz (15%), amphibole 00%), biotite \7<;< 1, magnetite or ilmenite l3':c), epidote (3'7o ), pyroxene remnants~ (2%), calcite 0 7o I, myrmekite \tr J, zircon (trJ, sphene (tr). Plagioclase is indistinctly zoned; rarely quaru anu alkali leld; par sho"· micrographic inter growth; amphibole is composed about equally of hornblende with yellow-green to blue-green pleochroism and a pale, almost colorless, amphibole apparently deri,·ed from alteration of the blue-green hornblende; 5% of the hiotite occurs as red­ bruwn plates around c.p:.ique mirwral. the rt"mainclase schi.st. (2, 65'() '.\licrocline (59% l, quartz (20%), albite-oligocla..«! ( 15% l, ilmenite, leucoxene and red iron oxide \3<;(-I, chlorite (2<;(;), sericite ll%l, apatite (tr), zircon (tr) . Grain size: Yariable. "°me fragments 0.1to0.2 mm; others 0.5 to 2 mm. Fabric: xenomorphic granular. Reick ; microgranite. Basement Rocks, Texas-New Mexico Gulf #5-A Carson core 7881 Bureau of Economic Geology Microcline microperthite (59%), oligoclase-andesine 05%), quartz (0%), biotite (8%), horn­blende (5%), magnetite or ilmenite (2%), sphene (2%l, chlorite (1%), cpidote (tr), apatite (tr), pyrite (tr), zircon (tr). Plagioclase shows a vague zoning; hornulendc is yellow-i:reen; biotite is green-brown and partly altered to chlorite; zircon forms halos in biotite. Grain size: 2 to 6 mm. Fauric: hypidiomorphic granular. Rock: granite. Gulf #4-A Cole-State cuttings 7649 Bureau of Economic Geology Microcline (55%), albite (20%), quartz (20%), Ieucoxene (3%l, magnetite or ilmenite (1%), sericite-muscovite (1%), epidote (tr), calcite (tr), apatite (tr). Plagioclase occurs as grains in a mosaic with quartz and microcline and as larger poikilitic grains (percentages of plagioclase and mir.rocline vary considerably in different frag:mcnts in this slide): leucoxene is in Sl'atterrrl grains and veinlets; grains of maj!;netite or ilmenite are commonly surrounded by epidote. Grain size: 0.1 to 0.2 with sporadic pln~iodase porphyroblasts up to 1 mm. Fabric: xenomorphic granular-poikilitic. Rock: microgranite. Gulf #5-F Graham-State core 9820 Bureau of Economic Geology Albite (40%), microcl ine (30%), quartz (20%), muscovite (4%l, hiotile (2%), calcite (2% l. leucoxene (1%), pyrite (!%),red iron oxide (tr), zircon (tr). Plagioclase is partly sericitized and for the most part occurs as discrete grains in a mosaic, although there are sporadic large poikiloblastic al bite j!;rains as well; biotite is ~reen-brown ; ilmenite is partly altered to leucoxene ; pyrite occurs in grains and along grain boundaries. Grain size: 0.2 to 0.5 mm. Fabric: hypidio­morphic granular. Rock: al bite micrograrwdiorite. Gulf #7 King core 8051-60 Bureau of Economic Geology S-.irite and chlorite (37%), microcline (30%). nuartz 00%), hemntite (10%), biolite (5%), yellow isotropic cavity filling ( 4% ), ilmenite (2%), Ieucoxene (2%), apatite (tr), zircon (tr). Sericite and chlorite occur as a fine fibrous intergranular mass probably in larj!;e part derived from alteration of plagioclase, chlorite is subordinate to sericite; microcline is locally microper­ thitic and, in part, occurs as corroded grains within the sericite-chlorite mass; quartz occurs in fractured grains; biotite is frayed, faded, bent, wavy, and locally so stained with hematite as to be opaque; cavities are filled with an unidentified isotropic yellow mineral. Grain size: broken quartz and feldspar grains 0.5 to 2 mm; intergranular mass, less than 0.05 mm. Fabric: cata­clastic. Rock: brecciated granite. Gulf #7 King core 8060 Bureau of Economic Geology Microcline (58%), oligoclase (20%), quartz (15%), biotite (3%), magnetite or ilmenite 0%), epidote 0 %) , sericite 0%), tourmaline 0 %) , zircon (tr), leucoxene (tr), apatite (tr). Plagioclase is partly sericitized; biotite shows yellow-brown to brown pleochroism; epidote is commonly associated with magnetite or ilmenite. Grain size: 0.2 to 0.5 mm. Fabric: hypidio­morphic granular-poorly developed. Rock: microgranite. Gulf #7 King core 8063 Humble Oil & Rig. Co. Microcline (56%), quartz (20%), albite (15%), senc1te (4% ), biotite (3o/o), magnetite or ilmenite (2%l, apatite (tr). Some of the microcline is microperthitic but this is only locally developed; plagioclase is partly sericitized; $Cattered flakes of biotite, pale green-brown lo almost black pleochroism, show very dark halos. Grain size: 0.5 mm. Fabric: xenomorphic granular. Rock : microgranite. Gulf #2 Stichter core 7980 Bureau of Economic Geology Microcline (60%), olip;oclase (15% ), quartz 02%), biotite (10%) , leucoxene (2%) . calcite (1% ), magnetite or ilmenite (tr), apatite (tr), zircon (tr), chlorite (tr), hornblende (tr). Microcline is locally microperthitic; plagioclase is partly sericitized; biotite shows yellow-green to dark ~reen pleochroism; zircons form halos in biotite. Grain size: 0.5 mm. Fabric : xenomorphic granular. Rock: microgranite. Humble #1 Keinath-Federal cuttings 9951-54 Shell Oil Co. Microcline microperthite (57%), quartz (30%), biotite (6%), albite (5%), magnetite or ilmenite (2%), hornblende (tr), rutile (tr), sphene (trl, zircon (tr), apatite (tr). Biotite pleochroism is green-brown to almost black. Grain size: 0.2 to 0.5 mm. Fabric: xenomorphic granular. Rock: microgranite. Bureau of Economic Geology, The University of Texas Humble #1 Keinath-Federal core 9951-54 Bureau of Economic Geology Oligoclase (41%), microcline (30%), quartz (20%), biotite (5%), hornblende (2%), ilmenite (1% ) , sphene ( 1 % ) , apatite (tr), zircon (tr). Plagioclase occurs in large grains and locally includes microcline; biotite is green-brown; hornblende pleochroism is yellow-green to green; sphene in part surrounds ilmenite. Grain size: 0.2 to 2 mm. Fabric: hypidiomorphic granular. Rock: granodiorite. Humble #10 Greenwood cuttings 7710 Bureau of Economic Geology Microcline (56%), quartz (25%), albite (15%) , muscovite and sericite (2%), magnetite or ilmenite (2%), zircon (tr) , red iron oxide (tr), apatite (tr). Plagioclase shows incipient altera· tion to sericite; zircon crystals are markedly elongate. Grain size: 0.1 to 1 mm. Fabric: xeno­morphic granular. Rock: microgranite. Humble #11 Greenwood cuttings 7495-7500 Bureau of Economic Geology Microcline and oligoclase (71 %) , quartz (25%), magnetite or ilmenite (2%), biotite 0%), muscovite and sericite (1%), calcite (tr), zircon (tr). The feldspar is probably more than half microcline but the two feldspars are difficult to estimate quantitatively; biotite is green-brown. Grain size: 0.1 to 0.2 mm. Fabric: xenomorphic granular (microcline is late and accommodates itself to the plagioclase and quartz). Rock: microgranite. Humble #3-V State cuttings 7665-70 Bureau of Economic Geology Microcline microperthite (75%), oligoclase (10%), quartz (10%), hornblende (2%), biotite (2%), ilmenite (1%), chlorite (tr), leucoxene (tr), apatite (tr), zircon (tr). Plagioclase is locally myrmekitic; quartz contains inclusions of fine needles; hornblende is deeply colored­pleochroism green-brown to dark green-brown; biotite, pale to dark brown pleochroism, is partly oxidized and partly altered to chlorite; ilmenite is enveloped by leucoxene. Grain size: 0.5 to 2 mm. Fabric : hypidiomorphic granular. Rock: granite. Humble #5-V State cuttings 8395-99 Bureau of Economic Geology Microcline microperthite (75%), quartz OS.%), oligoclase (10%), biotite (tr), leuroxene (tr), red iron oxide (tr), pyrite (tr), apatite (tr). Plagioclase shows incipient sericite development; biotite is a semi-opaque partly oxidized relict. Grain size: 1 to 3 mm. Fabric: hypidiomorphic granular-poorly developed. Rock: granite. Humble #6-V State core 7705 Bureau of Economic Geology Microcline (39%), oligoclase (30%), quartz (20%), biotite (8%), leucoxene (2%), calcite (1%), magnetite or ilmenite (tr), red iron oxide (tr), apatite (tr), zircon (tr). Plagioclase is partly sericitized; biotite shows an almost colorless to reddish-brown pleochroism. Grain size: 0.5 to 3 mm. Fabric: hypidiomorphic granular. Rock: granite. Humble #8-V State cuttings 7560-65 Bureau of Economic Geology Microcline, quartz, oligoclase-andesine, biotite, chlorite, magnetite or ilmenite, leucoxene, calcite, apatite. Slide is composed mostly of mineral fragments and no valid mode estimate can be made; plagioclase is partly sericitized; biotite pleochroism is green-brown to almost black; calcite is in a cross-cutting veinlet. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite or granodiorite. Humble #9-V State cuttings 8235-40 Bureau of Economic Geology Microcline (56%), albite (20%), quartz (20%), biotite (2%), chlorite 0%), ilmenite (1%), lcucoxene (trl, apatite (tr). Plagioclase shows incipient sericite development; biotite pleo­chroism is pale brown to very dark brown; ilmenite is enveloped in leucoxene. Grain size: 0.5 to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Magnolia # 17 Carson core 8155± Bureau of Economic Geology Albite (45%), microcline microperthite (34%), quartz (10%), biotite (5%), calcite (4%), magnetite or ilmenite (1% ) , leucoxene (1% ) , zircon (tr), apatite (tr), Plagioclase is partly alL1·r<'lu>ti<": hiotite is green-brown. Grain size: 0.2 to 0.5 mm. Fabric : xenomorphic granular-poikilitir. Rock : micro­granodiorite. Skelly # 1 Stichter cuttings 8052-53 Bureau of Economic Geology Oligoclase (48%), microcline (30%), quartz (15%), biotite (3%), sericite-muscovite (1 % ) , magnetite or ilmenite (1%), calcite (1%), leucoxene (lo/o), zircon (tr) . Biotitc pleoc·hroism is yellow·brown to dark brown ; sericite·muscovite occurs as fibers within p]agioclase grains (alteration) and as scattered plates. Grain size: 0.2 to 1 mm. Fabric: hypidiomorphic granular. Rock : microgranodiorite. Stanolind #1 W. H. Jones cuttings 10570-80 Shell Oil Co. lllicrocline microperthite (55%), al bite (30%), quartz (15%), calcite (tr), biotite (tr), ma~netite or ilmenite (tr). ~licrocline is to a large extent replaced by albite to form a twinned perthitic intergrowth; biotite is a deep green variety; calcite is in a veinlet. Grain size: 0.5 to 1 mm. Fabric: hypidiomorphic granular. Rock : microgranite. Stanolind # 11-X State core 8150 Bureau of Economic Geology Microcline. microperthite (57% ), quartz (25%), al bite (10%), biotite (4%), leucoxene (2%), chlorite (1o/o), magnetite or ilmenite (1 % ) , apatite (tr), zircon (tr). In part of the slide quartz is in a microg:raphir. intergrowth with rnicrodine rnicroperthite; biotite, pale to dark brown pleochroism, is partly altered to chlorite; zoned zircons form halos in biotite. Grain size: 0.5 to 5 mm. Fabric: hypidiomorphic granular-micrographic. Rock: granite. Texas #1 Blinebry cuttings 7510-15 Bureau of Economic Geology J\licrocline microperthite (46%). oligoclase (35%), quartz 05%), biotite (2%), magnetite or ilmenite (2~~), apatite (tr), zircon (tr). Plagioclase is partly sericitized; biotite is green-brown. Grain size: 0.2 to 1 mm. Fabric: hypidiomorphic granular. Rock: microgranite. Texas #4 Blinebry cuttings 8370-75 Bureau of Economic Geology lllicrocline mi r roperthite (47%). andesine (30'/o), biotite 00%), quartz (5%). hornblende (3'/o). ma~netite or ilmenite (2%), calcite (2%), red iron oxide (1 o/o), zircon (tr) . Plagio­rlasc is indistinrtly zoned; hornblende pleochroi'm is yellow-green to green; biotite is red­hrown: minerals are unequally distributed in different fragments-some are mostly plagio­ clase and ~orne are mostly mirrocline rnicroperthite; some fragments are brecciated; one fra~ment of au~ite-apatite i' in the slide and indicates presence of more basic rocks in the well. Grain size: 0.S to 2 mm. Fabric: hypidiomorphic granular. Rock: granite. Basement Rocks, Texas-New Mexico Texas #2 Lockhart cuttings 7590-95 Ilureau of Economic Geology Microcline microperthite (63%), oligoclase (15%), quartz (15%), biotite (4%), ma,:netite or ilmenite (3%), zircon (tr), apatite (tr). Biotite pleochroism is pale brown to brown. Grain size: 0.2 to 2 mm. Fabric: xenomorphic granular-poikilitic. Rock: granite. LINCOLN COUNTY Standard of Texas #1 Heard-Federal cuttings 7800·70 Bureau of Economic Geology Labradorite (51%), olivine (20%), augite (10%), chlorite (10%), magnetite or ilmenite (5%), biotite (3%), pyrite or pyrrhotite 0%), epidote (tr). Plagioclase is partly sericitized ; olivine is heavily veined with a mesh of opaque veinlets and is partly altered to an olive-colored chlorite; there are two kinds of chlorite, an olive-colored highly birefringent variety principally derived from alteration of olivine and a common green low-birefringent type; augite is apparently late and accommodates itself to olivine and labradorite, occupying inter-feldspar areas and wrapping around olivine; locally it is almost vermicular; augite also includes closely spaced fine linear opaque inclusions that give the mineral a "diallage" look; biotite, a very red variety, occurs around the opaque minerals; epidote is in small scattered grains in plagioclase as a result of alteration of the plagioclase. Grain size: 1 to 3 mm. Fabric: hypidiomorphic granular. Rock: olivine gabbro. Standard of Texas #1 Heard-Federal core 8050 Bureau of Economic Geology Labradorite (54%), olivine 08%), olivine alteration product (12%), augite (5%), chlorite (5%), magnetite or ilmenite (4%), biotite (2%). Plagioclase is partly sericitized and shows vague zoning; olivine is meshed with veins of opaque mineral and is partly altered to a highly birefringent pale olive-brown to olive-green mineral-chlorite?; augite rims olivine and accom­modates itself to plagioclase laths; red-brown biotite fringes opaque mineral. Grain size: 2 to 8 mm. Fabric: hypidiomorphic granular. Rock: olivine gabbro. Stanolind #1 Picacho Unit cuttings 2685-90 Shell Oil Co. Quartz (67%), microcline microperthite (30%), al bite (2%), calcite (1%), magnetite or ilmenite (tr), rutile (tr). Some of the microcline microperthite grains appear to be round. Grain size: 0.1to0.2 mm. Fabric: granoblastic. Rock: metarkosite. Stanolind #1 Picacho Unit cuttings 2685-90 Shell Oil Co. Albite (59%), quartz (30%), alkali feldspar-microperthite? (10%), magnetite or ilmenite (1%), leucoxene (tr), calcite (tr), rutile (tr), zircon (tr). Grain size: 0.1 to 0.2 mm. Fabric: granoblastic. Rock: metarkosite. Stanolind #1 Picacho Unit cuttings 2740-45 Shell Oil Co. Quartz (58%), microcline microperthite (35%), calcite (4%), magnetite or ilmenite (2%), leucoxene (1% ) . Microcline is only sparsely perthitic and shows incipient kaolinization; some round feldspar grains show overgrowths. Grain size : 0.2 mm. Fabric: granoblastic. Rock: metarkosite. OTERO COUNTY Flynn et al. # 1 Donohue cuttings 1685-88 Stanolind Oil & Gas Co. Alkali feldspar and zeolite (86%), analcite (6%), aegirine (5%), magnetite or ilmenite (2%), clinozoisite (1% ) , eudialite? (tr), pyrite (tr). Fibrous zeolite is replacing feldspar. Grain size : 0.1 to 0.5 mm. Fabric: microlitic. Rock: analcite-aegirine microsyenite [probably Tertiary]. Hunt & Turner #1 McMillan core 2175 Bureau of Economic Geology Groundmass (80%), albite and alkali feldspar (8%), quartz (5%), magnetite or ilmenite (3%), calcite (3%), red iron oxide 0%), sphene (tr). Micrographic quartz-alkali feldspar ground­mass shows poorly developed flowage structure; feldspar phenocrysts are almost completely kaolinized but al bite can be distinguished by chessboard twins; quartz occurs as embayed and corroded phcnocrysts and is locally intergrown with feldspar phenocrysts. Grain size: groundmass less than 0.05 mm; phenocrysts up to 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Bureau of Economic Geology, The University of Texas Hunt & Turner #1 Mcl\lillan core 2175 Bureau of Economic Geology Alkali feldspar (62%), quartz (20%), calcite (7o/o), magnetite or ilmenite (6o/o), yellow iron oxide? (3o/o I, chlorite (1% ), leucoxene (1 o/o), zircon (tr), apatite (tr). Alkali feldspar is partly kaolinized and is micrographically intergrown with quartz-the intergrowth is mostly irregular but locally is cuneiform; quartz is in part in angular irregular-edged grains and in part in a fine micrographic intergrowth; magnetite or ilmenite is partly altered to red iron oxide; calcite is in cross-cutting Yeinlets ; a canary-yeHow mineral forms a stain in vicinity of calcite veinlets and may be an iron oxide. Grain size: 0.5 to 1 mm. Fabric: micrographic. Rock: micrographic microgranite. Standard of Texas #1 Scarp Unit cuttings 2592-97 Bureau of Economic Geology Labradorite (-10% l, hornblende (15%), sericite (15% l, augite (10%), magnetite (5%), epidote (5%1. biotite (-l% ), chlorite (4o/ol, quartz (2%), apatite (tr) . Plagioclase is in advanced stage of sericitization and in some places has gone over completely to masses of sericite; augite is a titaniferous violet-brown variety; hornblende, almost colorless to green pleochroism, occurs in large frayed grains (primary) and in mas.. und zoisitt•; cl1lorilP ('ornmon ly t•11clo~t·s ma~Sl'S of amphil>ole which on·1irs as (al flukes and prisms surrounded by chlorite and (b) continous grains, colorless to pale green, partly altered to chlorite and f>pidott>. Crain ~ize: l to 2 mm. Fahric: hypidiomorpltic granular. Rock: gab bro. QUAY COU"lTY Stanolind #I Fuller ('()f'f> 6740-47 Bureau of Economic Geology Groundmass (74C:fc.). seridte·muscovite {15o/o), magnetite or ilmef"!ilr., leuroxcne and quartz (6%1. red iron oxide (5%). Groundmass is composed mostly of alkali feldspar showing very fine radial or spherulitic extinction phenomena with minor sericite and scattered magnetite or ilmen ite grains: masses of sericite J!rading into poikiloOlastic muscovite orcur throughout the slide and locally have a shape suggesting they are after former feldspar grains; angular quartz !(rains are scattered through the rock. Grain size: 0.01 to 0.1 mm. Fabric: relict pyro­clastic-incipient crystallohlastic (poikilohlastic). RoC"k: rh.yolite metatu/J. ROOSEVELT COUNTY ,\u oc-nirs as ~ l) yellow·~reen chlorite in masses resuhin,:?: from alteration of bio· tite an_\! hornblen_de and, m part, replaci_ng plagioclase 01%) and (2) green chlorite from ulteratton 0£ h10tJf f> f.J'Jc): hornhJenh matrix (21 % ) , sericite (10%), quartz (5%), calcite (4%), lcu<'oxene (4%1, pyrite (1%), apatite (tr), zircon (tr). Plagioclase is varial>ly serir.itized ; fine crush matrix is composed of sericite, quartz, and feldspar; sericite occurs as fibers in matrix and in fibrous masses as a feldspar alteration product; quartz occurs as patrhes of strained and sutured grains and secondarily in small patches and along grain boundaries; ralcite occurs in crushed zones and in part replaces feldspar; leucoxene is in grains and vein lets: pyrite is I orally associated with leueoxene. Grain size: mineral fragments 0.2 to 2 mm: crnsh matrix less than 0.05 mm out without a rlear separation. Fabric: ca ta· ('lastit'. Rock: brerdated leuco-albite syenodiorite. Magnolia #1 A. K. Smith cuttings 10000·16 Shell Oil Co Ground mass (92%), microperthite (8%). Mirrocrystalline to cryptocrystalline quartz·alkali feJd. spar groundmass shows well-developed Ho wage structure; stringers of coarser quartz grains and finely dispersed opaque mineral are present; microperthite is present as a single phenocryst. Grain size: groundmass microcrysta!line to eryptocrystalline; phenocryst 1 mm. Fabric: porphy­ritic. Rock: rhyolite porphyry. !\lagnolia #I A. K. Smith cutlings 10000·16 Shell Oil Co. Groundmass (65% ), microperthite (35%), Ieucoxene (tr), pyrite (tr). Micrographic to micro­spherulitic to cryptocrystalline groundmass shows well·developed flowage structure, particularly around phenocrysts. Two large phenorrysts are apparently alkali feldspar in advanced stage of perthitization and are largely composed of patches of twinned all>ite. Grain size: groundmass, mostly less than 0.02 mm; phenoerysts 2 mm. Fabric: porphyritic. Rock: rhyolite porphyry. Mid·Continent #I Strickland core 7508-13 Bureau of Economic Geology Sericite (78%), chlorite 00%), magnetite or ilmenite (7%), calcite (5%). Rock is thor­oughly stained and allered; sericite has replaced most of the original feldspar but outlines of laths up to 0.5 mm can be seen; calcite or:cnrs as vein lets. Grain size: (original igneous rock) 0.1 to 0.5 mm. Fabric: relict sul>ophitic?, microlitic?. Rock: altered andesite? or diabase? Mid·Continent #I Strickland core 7513 Bureau of Economic Geology Plagioclase (53%), actinolite (~5%), mal(netite or ilmenite (6c/r, ), chlorite (3%), calcite (3% ), quartz (tr), rutile? (tr), red iron oxide (tr). Plagioclase microlites and laths are al­ most completely sericitized but index of refraction indicates an intermediate to calcic type; green fibrous actinolite replaces the original ferromag:nesian mineral-probably pyroxene­and occurs between plagioclase laths; magnetite or ilmenite is in scattered grains; calcite occurs in Yeinlets; <]uartz is secondary. Grain size: 0.1 to 0.2 mm. Fabric: relict subophitic. Rock: altered diabase. :11iric: ophitic. Rock : allered albite diabase. Shell #I Harwood cuttings 7935-55 Bureau of Economic Geology Alkali feldspar (7.3%), quartz (25%), chlorite (!%), leucoxene 0%), magnetite or ilmenite (tr\, red iron oxide (tr), amphibole (tr). Alkali feldspar and quartz occur as a fine granular mass 1dth ahout 1% of th~ alkali feldspar present '" phenocryst>. Grain >ize: 0.01 to 0.{12 mm. Fabric: microgranular. Rock: rhyo/ite. Shell #I Hanrnod ntttings 7950-55 Shell Oil Co Ro<"k is a tlne mas:-: of quartz and alkali feldspar with shreds of rhlorite and biotite and flerks of le11coxf'n1>. Grain size: le-~=--than 0.02 mm. Fahric: microgranular. Rock: rhyolite. ' Rasemnit Rocks, r..xas-Nt'w MPxico Shell #I Saunders cuttings 8640-79 Burcnu of Et·nnomir Geology Alkali feld,par (72%>. quartz (25%), chlorite (!'fr) , leucoxenc (I ')'ol, n•d iron oxide ( l~rl , zircon (trl. magrwtitr or ilmt·nite (tr}. Alkali feldspar and quartz o(·cur as u hnc µ:ranular mass, <·ontaininµ: 011f" alkali feldspar phenocryst; mag1wtitc or ilmt'nite is almost. complt•tt>ly converted to red iron oxide which forms a pervading stain. Grain size: 0.01to0.02 mm. Fabric: microgranular. Rock : rhyulite. Shell #I Saunders core 8650 Bureau of Economic Geoloµy Groundmass (81 % ) , microperthite (8'/r), olil!oeiase (3% ), rhlorite (3%), hydromuscovite" (2%), fluorite (!'lo ), magnetite or ilmenite (1 '/o), red iron oxide (1'1o), zircon (tr), leuroxene (tr), apatite (tr), sphene (tr). Microcrystalline groundmass is <·om1u1st~ cleava~es thnl may lw n11 ·1mphibole or a mica ; ~phene envelops ilmenite. Grain size: 0.5 to 2 mm. Fabric: ~nris.'t bridge. Dobrin (1952. p. 109) summarizes data on susceptihilit\· mcasurrmr11ts given by Heiland (19401 and Birch (1942) , and the range of measured susceptibility ml· ues of various rock types is more in agree­ment with that obtained in thi:; study, al· though correspondence of maximum and minimum rnlue; ;, poor. Dobrin (1952, p. 109) also present,; calculated suscepti· bilities based on Slichter's method ( 1929) and Stearn's figures 11929) which are much higher than the observed values on the Frost bridge. Nettleton (1940) and Dobrin ( 1952), following Slichter (1929) conclude that the calculated values are more reliable than measurements made in too-strong fields. In the writer's opinion there is possi­bility of error in the use of the magne· tite content of an average of a particular rock type (that is, "the average granite") as a basis for calculation of magnetic sus· ceptibility. Stearn (1929, pp. 330-331) does not indicate how he constructed the tables of values of magnetite-ilmenite con­tent of various rock types which have been used in calculation of magnetic sus· ceptibility, but in the experience of pe­trographers concerned with modal analy· sis of rocks, there can be, and commonly is, a wide range in the amount of resolv· able magnetite-ilmenite in a particular type of rock. It is reasonable to assume a similar variation in cryptocrystalline mag· netite-ilmenite. The average value may not be significant for purposes of calculation. Nettleton and Elkins (1944) calculated magnetite content and probable intensity of magnetization from published analyses of igneous rocks for which density and data to compute normative magnetite were available. These 1,450 normative magne­tite and density values were analyzed sta· tistically according to the C.I.P.W. and Iddings' classifications of igneous rocks (Nettleton and Elkins, 1944, pp. 68-71), and the median magnetite value for each class or division of igneous rocks in these two classifications was used to calculate intensity of magnetization. The major weakness in these calculations, as realized by Nettleton and Elkins and pointed out by Duane Reno in the discussion follow· ing the paper, is the use of normative values. Normative and modal magnetite may differ considerably. This study by Nettleton and Elkins is a worthy contri· bution because it presents for use approxi­mate magnetic data for a large number of rocks before a large body of experi­mental data on directly measured values is available. Basement Rocks, Texas-New Mexico Considerable difference in magnetite­ilmenite content may exist between simi­lar rock types from separated localities and between samples of similar rock from one body. In this research it was found that two samples of diorite separated by a 3-foot interval in one core yielded mag· netic susceptibility values of 74 and 150; granite samples from within a 7-foot in­terval ranged from 950 to 1,200; two dia­base samples from the same well, sep· arated by an interval of 300 feet, showed values of 340 and 3,900; two samples of gabbro from 200 feet apart in the same well ranged from 1,200 to 1,900. Of course, such variations are not always present, and some determinations on samples from the same core are in close agreement. Large masses of igneous rock are con· sidered to be in general more homoge· neous than masses of metamorphic or sedimentary rocks, and considering the tremendous volumes of material, the ho· mogeneity is truly remarkable; in detail the percentages of magnetite-ilmenite may show considerable variation from one sample to another. Chayes (1950) dis­cusses the question of homogeneity of igneous masses and presents data from micrometric analyses. In the writer's opin­ion, reservations must be made in calcu­lations when using figures purporting to be the "magnetite-ilmenite content of the average granite" or the "average magne­tite-ilmenite content of gabbros." Geo­logically and mathematically these figures may be extremely vulnerable. It was noted that the values derived from measurements on the Frost magnetic susceptibility bridge are in general lower than calculated values derived from Stearn's tables on magnetite-ilmenite con· tent of rocks, notwithstanding that the Frost bridge operates at a field strength only about 40 percent greater than that of the earth. Possibly Stearn's figures are in general too high or possibly ilmenite makes up a higher proportion of the total magnetite-ilmenite content than Stearn allowed. In any case, the source of these geologic data should be reviewed. Conclusions.-It appears that a rock sample in a random well core, or in fact any single random sample, will not give satisfactory information on magnetic sus· ceptibility of the rock mass penetrated. This is not surprising because most scien­tists have learned the danger in relying on a single sample. The theory of sampling has long been applied to geologic prob­lems, and the value of systematic sampling has been many times demonstrated. The mining geologist in investigation of the content of a particular mineral in a rock mass must depend on large numbers of systematic samples. Magnetic susceptibil­ity is related for the most part to the mag­netite-ilmenite content of a rock. A single core sample will not give reliable infor­mation on the magnetite-ilmenite content of a rock mass any more than a single assay will give reliable information on, for example, the copper content of an ore body. The magnetite-ilmenite content of a particular granite is probably less varied than the copper content of an ore body, but the analogy is valid. The over·all magnetic susceptibility of a particular rock mass can be determined only by thorough and systematic sampling much as was done by Steenland and Wool­lard (1952, pp. 1075-1093) in their in­vestigation of the Cortlandt Complex. Pre· cambrian rock masses in subsurface can­not be so investigated. In areas that show little variation in rock type as determined from relatively closely spaced wells, such as in the Brunson area of Lea County, Ne"' Mexico, there is a significant varia­tion in magnetic sus<;eptibility. In the Brunson area 18 susceptibility determina­tions on granite and granodiorite that show little variation in general mineral composition in thin section, ranged from 25 to 1,300 for granite and 40 to 4,800 for granodiorite. It is concluded that magnetic suscepti­bility as measured in this study will not be of direct value to exploration geo­physicists concerned with evaluating the magnetic effects of the basement. These determinations are of value only in that Bureau of Economic Geology, The University of Texas they add to the store of knowledge on magnetic susceptibility of rocks. Kalash· nikov and Kapitsa ( 1952) state that me­chanical stresses applied to rocks cause a decrease in susceptibility and conclude that magnetic susceptibility as measured in the laboratory is not the same as it is in situ because of changes in the mechani­cal conditions of the surrounding medium. This is more reason for caution in at­tempting to assign susceptibility values for classes of rocks on the basis of labora­tory research and calculations. Mooney and Bleifuss ( 1953) measured susceptibility of a number of rock types on the outcrop and give a mean suscepti­bility and a coefficient of variation for various rock types. They state that the mean susceptibility of a rock formation will determine the general level of mag­netic intensity and the variability will de· termine the relief of the magnetic con· tours over a particular rock type. It is their opinion that, notwithstanding the overlap of individual susceptibility values of different rock types, the mean suscepti­bility of the rock types studied differs significantly. On the basis of values given in Table 1 of this paper, a mean sus· ceptibility for rhyolite porphyry (range 18 to 72) is significant but a mean sus­ceptibility for granite (range 21 to 1,400) is of dubious significance because of the variation. Published data show that the mean susceptibility of rhyolite in west Texas cannot be applied to rhyolite from other areas. Evidently each rock unit is an individual problem in sampling and measurement, and values obtained in an area where susceptibility of a basement unit can be determined through adequate sampling and measurement cannot be car­ried over to a similar rock unit that is concealed. It has been assumed in magnetic pros­pecting that sedimentary rocks have little or no effect on the magnetometer and that the anomalies are due entirely to trends within the basement or to intru­sive igneous rocks. While there are excep· tions to this assumption, it is in general valid. The problem of magnetic anomalies and their interpretation has been ap­proached from two directions: ( 1) by making assumptions based on magnetic susceptibilities of rock types obtained by calculations and measurements and (2) by field work with the magnetometer. It is concluded herein that sampling control adequate for determination of the mag­netic susceptibility of masses of concealed basement rocks does not exist and prob­ably will not be practical. Basic research with the magnetometer in the field will probably produce data of more value in the study of magnetic anomalies. BIBLIOGRAPHY Brncu, FRANCIS (editor) (1942) Handbook of physical constants: Geol. Soc. Amer., Special Paper 36, 325 pp. CHAYES, F. (1950) Composition of the granites of Westerly and Bradford, Rhode Island: Amer. Jour. Sci., vol. 248, pp. 378-407. CoLLINCWOOD, D. M. (1930) Magnetic suscepti· bility and magnetite content of sands and shales: Bull. Amer. Assoc. Petr. Geol., vol. 14, pp. 1187-1190. DonRIN, M. B. (1952) Introduction to geophysi· cal prospecting, McGraw-Hill Book Company, New York, 435 pp. FLAWN, P. T. (1953) Magnetic susceptibility measurements in west Texas and southeast New Mexico: Proc. Geophys. Soc. Tulsa, vol. 1, pp. 55-58. GRENET, G. (1930) Sur Jes proprietes magnetiques des roches: Annal es de Physique, Serie 10, vol. 13, pp. 263-348. HAALCK, IIANS (1942) Der Gesteinsmagnetismus, Becker und Erler, Leipzig, 90 pp. HAWES, JULIAN (1952) A magnetic study of the Spavinaw granite area, Oklahoma: Geophyhics, vol. 17, pp. 27-45. HEILAND, C. A. (1940) Geophysical exploration, Prentice-Hall, Inc., New York, 1013 pp. KALASHNIKOV, A. G., and KAPITSA, S. P. (1952) Magnitnaya vosprimchivost' ~ornykh porod pri­ uprugikh napryazheniyakh [Magnetic susrepti· bility of rocks under elastic stresses] : Akad. Nauk SSSR Doklady, tom 86, pp. 521-523. KATO, Y os10 ( 1941) Investigation of the mag· netic properties of the rocks constituting the earth's crust (2d report), On the susceptibility of the rock (Part I): Sci. Repts. of the Tohuko Imperial University, Sendai, Japan. First Se­ries, Vol. XXIX, pp. 602--628. MoONEY, II. W. (1952) Magnetic susceptibility measurements in Minnesota, Part I, Technique of measurement: Geophysics, vol. 17, pp. 531­ 543. ----and Bu1russ, RODNEY (1953) Mag­ netic susceptibility measurements in Minnesota, Part II, Analysis of field results: Geophysics, vol. 18, pp. 383-393. Basement Rocks, Texas-New Mexico NAGATA, TAKES! (1940) Some physical properties of lavas of volcanoes Asama and Mihara. II Magnetic susceptibility: Bull. Earthquake Re· search Institute, Tokyo Imperial University, Vol. XVII, pp. 102-135. NETTLETON, L. L. (1940) Geophysical prospect· ing for oil, McGraw-Hill Book Company, New York, 444 pp. ----and EI KINS, T. A. (1944) Associa· tion of magnetic and density contrasts with ig­neous rock classifications: Geophysics, vol. 9, pp. 60-68. SLICHTER, L. B. (1929) Certain aspects of mag­netic surveying, in Geophysical prospecting, 1929 : Trans. Amer. Inst. Min. Met. Eng., vol. 81, pp. 238-260. STEARN, N. H. (1929) A background for the ap­plication of geomagnetics to exploration, in Geophysical prospecting, 1929: Trans. Amer. Inst. Min. Met. Eng., vol. 81, pp. 315-344. STEENLAND, N. C., and WooLLARD, G. P. (1952) Gravity and magnetic investigation of the struc· lure of the Cortlandt Complex, New York: Bull. Geo I. Soc. Amer., vol. 63, pp. 107 5-1104. WERNER, STURE (1945) Determinations of mag· netic susceptibility of ores and rocks from Swedish iron ore deposits: Sver. Geo!. U ders. Arsbok 39, 79 pp. ~ TADLE 1. Measured magnetic susceptibility values.1 All x IO·• cgs units/unit volume. RllYOLITE AND QUARTZ SYENITE QUARTZ DIORITE GRANITE' GRANODIOHITE AND SYENITE AND DIORITE BASALT AND GADDRO DIABASE -­-­··--· --·-­··--­(1) 21 (28) 40 (42) 37 (47) 51 (55) 220' (63) 80 (2) 25 (29) 49 (43) 58 (48) 74 (56) 1200' (64) llO (3) 27 (30) 69 (44) 1300 (49) llO (4) 29 (31) 88 (45) 4600 (50) 130 (57) 1600 (65) 340 (58) 1900 (66) 1700 (5) 31 (32) 730' (51) 150 (6) 32 (33) 750' SYF.NOGABBRO (52) 1400 (59) 2200 (67) 3300 (60) 2300 (68) 3900 (7) 32 (34) 790 (46) 170 (53) 1700' (8) 33' (35) 960 (54) 2200 (9) 34 (36) 1300 (61) 2400' (62) 6900' (10) 37 (37) 1300' (11) 43 (38) 3500' (12) 67 (39) 4000 (13) 180 (40) 4800 (14) 180 (15) 250' GRANODIORITE (16) 250' G:\EISS RllYOLJTE POHPHYHY (69) 18 (70) 20 (71) 26 (72) 33 (73) 40 (74) 51 (75) 53' (76) 60 (77) 66 (78) 67 (79) 69 (80) 72 RHYOLITE TUFF (81) 44 MISCELLANEOUS ROCKS (82) andesite porphyry .. (83) andesite porphyry ... (84) andesite luff .. (85) latite tuff . (86) serpentinite (87) sericite phyllite (88) sericite phyllitc (89) sericite phyllite ( 90) metaquartzite (91) metarkosite (92) amphibolite (93) amphibolite (94) biotite-microcline· plagioclase gneiss . (95) quartz-hornblende­plagioclase gneiss . 68 4500 9100 51 3200 44 53 57 67 72 3200 13000' 6500 52 g>.., " "'::: c '-. ~ c ;:, c ;i;:;· I;") "' c c ~ ...., ;:,-. "' (17) 610 (41) 9000 (18) 670 (19) 750 (20) 870 (21) 950 (96) epidote-hornblende­biotite schist 430 ~ ;:; · "'..... ~· (22) 1200 (23) 1300 ~ (24) 1400 ~ GRANITE GNEISS (25) 30 ~ (26) 43 MYLONITIZED GRANITE (27) 290 1 Parenlhe1ized number preceding 1utceplibility value refen to well name io Table 2. 2 Rock family name8 include fine-grained e'tuivalenh 1uch as microgranite and micrograoile porphyry, microgabbro, etc. • Sample i1 cutting•; all otben are core 1ample1. t>EPTll (in feet) AND l': A. Biotite schist (Jamison No. l Webb, Taylor County, Texas, 6081 feet) 199 B. Same as (A) but with crossed nicols ... 199 C. Biotite schist (General Crude No. 12 Flanagan, Fisher County, Texas, 6743 to 6746 feet, cuttings) ........... ...... . . 155 D. Metadolomite (Humble No. l Nachlinger, Scurry County, Texas, 8266 feet) 196 B, biotite; Q, quartz; A, apatite; MG, magnetite; P, plagioclase; GP, gypsum cement (plaster-of-paris); D, sheared dolomite; T, talc Bureau of Eco110111ic Geology, The University of Texas PLATE VIII Photomirrographs of rocks from the Panhandle volcanic terrane All x45 APPENDIX II, PAGE A. Rhyolite porphyry (Superior No. 5.J-9 Gray Ranch, Oldham County, Texas, 7181 to 7187 feet) 184 B. l\licrospherulitic quartz-alkali feldspar groundmass in rhyolite porphyry (Standard of Texas No. I-A Palm, Armstrong County, Texas, 6UO to 6141 feet\............................................................ 125 C. Trachyandesite tuff (Continental No. 1 Rodgers, Yoakum County, Texas, 13,015 feet) . 204 D. Flowage structure in rhyolite porphyry (Humble No. 1 Masten, Cochran County, Texas, IO,i88 feet) 137 Q, quartz; P, plagioclase A B c A B D Basement Rocks, Texas-New Mexico PLATE IX Photomicrographs of rocks from the Swisher :rahbroic terrnnr All x45 APl'f:NOIX II, PA~I: A. Olivine diabase (Standard of Texas No. l Johnson, Swisher County, Texas, 9193 to 9200 feet) 198 B. Olivine gabbro (Gull No. 1-A Keliehor, Parmer County, Texas, 9627 to 9628 feet) 185 C. Serpentinized dolomite (Hunt No. 2 Ritchie, Briscoe County, Texas, 7590 to 7710 feet) 1 . 130 D. Hornfels (Sun No. l Herring, Castro County, Texas, 10,065 to 10,135 feet, cuttings) 135 0, olivine; A, augite, P, plagioclase; I, iddingsite: MG, magnetite; S, serpentine; D, dolomite; F, al bite and alkali feldspar; Q, quartz ; MS, muscovite 1Thi1 well bottomt!d in ,·olranic rocks cmd thf'refore i!I considered "·ith thr. Panhandle \'Olcanic terrane (Table l); bowe,·er, lhf' 19C(118Dce of gabbro aad contact metamorphosf'd sf'dimentary rocks f'ncountered hi11:her in the buc-mf'nl sf'clion is typical of tht' Swither pbbroic ternne proper. Bureau of Economic Geology, The University of Texas PLATE X Photomicrographs ot rocks from the Wichita igneous prodnre All x45 APPENDIX II, PAGE A. Oli\'ine diabase (Prairie No. l Bivins, Potter County, Texas, 2585 to 2595 feet)............. 193 B. Same as (A) but with crossed nicols 193 C. Granite (Kerr-lllcGee No. 3 Berneta, Hartley County, Texas, 5878 to 5881 feet) 166 D. Cuneiform quartz·potassium feldspar intergrowth in micrographic granite (Superio•· No. 4 Matador, Oldham County, Texas, 7074 feet) 185 0 , olivine; A, augite ; P, plagioclase : Mp, microperthite; Q, quartz A B c Index Abilene Minimum : 63, 67 Adair No. 1, Shamrock: 153 age determinations: 8, 9, 26, 27, 31, 38, 48, 50 age of-Panhandle volcanic terrane: 43 Red River mobile belt: 38 Swisher gabbroic terrane : 46 Texas era ton: 31 Van Horn mobile belt : 34 Wichita igneous province : 50 Agee No. 1, Nu-Enamel: 178 Aiken No. l, General Crude: 155 A. K. Smith No. 1, Magnolia : 230 Albough No. 1 Matador: 183 No. 2 Matador: lll.3 Alexander No. 1, Lubbock Machine & Supply : 51 Altman No. 1, Honolulu : 159 Amanda No. 1, Gulf: 29, 220 Amarillo Mountains: 9, 21, 37, 47, 48, 50, 51 Amarillo Oil & Gas No. 3 Masterson: 191 No. 5 Masterson: 191 Amarillo uplift : 38, 39, 43, 51, 52, 53, 55, 57, 67 Amerada No. 1 Birney: 37, 181 No. 5 Corrigan: 216 No. 6 Corrigan: 216 No. 7 Corrigan : 216 No. 11 Corrigan: 216 No. 1Hamilton:128 No. 4 Hare: 217 No. 5 Hare : 217 No. 1 Hughes : 165 No. 1-D Jones: 158 No. 2-A Jones: 158 No. 1 Kurfees: 162 No. 7 Phillips: 217 No. 3-A Phillips: 217 No. l·D Shannon: 147 No. 1 State BTA: 42, 217, 218 No. 1-RA State: 206 No. 5-F State·Graham: 218 No. 1 Stribling: 174 No. 1 Walden : 218 No. 3 Walden : 218 No. 4 Walden: 218 No. 5 Wood : 218 No. 6 Wood : 218 No. 9Wood:218 No. 10 Wood: 218 American Land No. 1 Roseborough: 43, 169 Anadarko basin: 55 Andrews County: 7, 30 petrographic reports on wells in: 119-123 Anderson-Prichard No. 2 Boren : 186 No. 1Fowler-McDaniel : 134 No. 1 Gettys: 45, 172 No. 1 Lynch: 144 No. l ·A Masterson: 186 No. 2 Masterson: 186 No. 1 Rich: 199 Appalachian µ:eosyncline: 59 Appalachian-Ouachita trend : 70 Arbuckle Mountains: 23, 46, 47, 50, 51, 53 uplift: 52 Archer County: 36 petrographic reports on wells in: 123 Argo-Herring No. 5, Shell: 223 No. 9, Shell : 223 No. 4-A, Shell: 223 No. 6·A, Shell: 223 No. 10-A, Shell: 224 Armstrong County: 41, 42, 44, 45, 49 petrographic reports on wells in : 124-126 Atcheson No. 1-CT, Phillips: 143 No. 3, Phillips: 143 Atlantic Refining Company No. 1 Roberts : 31, 195 No. 1-10 McCandless: 188 No. 2-A University: 146 Atlas Life No. 1, J. L. Hamon (Coroco): 199 Austral No. 1 Saddler: 229 Bailey County: 23, 41, 45, 46, 56 petrographic reports on wells in: 126-128 Bailey No. 3, Holt: 49, 50, 160 Baker No. 5-B, Skelly: 226 Balcones fault zone: 7, 25, 32, 58, 62 Bankline No. l·A Elliott: 174 Baringer Hill pegmatite : 26 uraninite from: 27 Barker No. 1, Cosden: 42, 44, 137 Barkley-Meadows No. 14-A Stephens: 48, 202 Barnsdall No. l·A State: 206 No. 2 Davenport: 139 Bartley No. 1-A, Phillips: 177 Barton No. 1, Sinclair: 28, 225 basement, definition of: 11 surface: 53-57 Bashara No. 2, Phillips: 204 basin-and-range structure: 25 Bateman No. 4, Humble: 171 Baugh No. 1, Honolulu: 197 Beach No. 1, Continental and Magnolia: 201 Beavers No. 1, Bock-Anderson: 159 Beck No. 1, Stanolind: 166 Bell-Federal No. 1, Signal: 231 Bend arch: 63 axis: 31, 52 Berneta No. 2, Kerr-McGee: 166 No. 3, Kerr-McGee: 166 Berry No. 1, Continental: 48, 141 Best No. 1, Tidewater: 232 No. 1 Tindall : 200 B. F. Smith No. l, Humble: 187 Big Four Ranch No. l, Seaboard: 172 Big Bend National Park: 59 Big Branch gneiss: 27 Big Chief No. 1 Deloache: 167 Bird No. 1, Livermore: 150 No. 1, Stanolind : 159 Birney No. 1, Amerada : 37, 181 Bivins No. 25-A, Colorado Interstate : 42, 45, 48, 191, 192, 193 No. 1-GG, Phillips: 166 No. 1, Prairie: 193 Black No. 1 Shildneck : 206 Black Hills Unit No. 1, Magnolia: 29, 211 Blackman No. 6-E, Gulf: 202 Blackwell No. 1, National Petroleum Association: 151 Blinebry No. 1, Texas: 226 No. 4, Texas: 226 Bliss sandstone: 34 Bureau of Economic Geology, The University of Texas Bock-Anderson No. 1 Beavers: 159 Bolt No. 1, Humble: 30, 171 Boone No. 1, Tidewater: 232 Boquillas area : 60 Canyon: 59 Boren No. 2, Anderson-Prichard: 186 bouguer anomaly maps: 63 Bowers No. 1, Sidwell: 161 Bravo dome: 48, 49, 53 Brandenburger No. 1, Cochran & Steward: 177 Brewster County: 61, 66 petrographic reports on wells in: 128 Bridwell No. l, Lion: 45, 127 No. 1-A Edrington : 137 No. 26 Edrington: 136 No. 1 Houghton: 165 No. 2-A Houghton: 165 Briscoe County: 9, 23, 42, 43, 44, 45 petrographic reports on wells in: 128-132 Broome No. l, Stanolind: 153 Brown Geophysical Company : 63 Brown No. I, Magnolia: 229, 230 No l, Superior of Cal.: 141 Brunson area: IS, 28, 29 Brunson No. 4, Sinclair: 225 No. 5, Sinclair: 225 Bryan No. 1, LT.LO. : 197 Bullington No. 1, Phillips : 123 Burger B-28 No. I, Continental: 219 Burnett No. I, Ohio : 172 Burro Hills Unit No. I, Magnolia: 29 Bush No. I, Sinclair-Prairie: 193, 194 No. I, Standard of Texas: 194 Byers No. 41, Texas: 137 Byrd No. I, Humble and Stanolind: 162 Caldwell County: 58 California No. I Theison : 169 Callan No. I, Phillips: 196 Cambro-Ordovician rocks: 54 Cameron No. 1, Superior: 139 Campbell No. I, Humble: 42, 44, 167, 168 Canadian River No. 4-B Masterson: 191 Capitol Freehold Land Trust No. 1, Texas: 148 Carrizo Mountain group: 60 Carson County: 41, 48, 49, S7 petrop:raphic reports on wells in : 132-134 Carson No. 5-A, Gulf : 221 No. 17, Map:nolia : 222 Carter No. I, Shell: 224 Castro County : 9, 41, 43, 45 Cayce No. I Moore: 19S Central Basin Platform: 7, IS, 28, 32, 34, 53, SS, S6, 63, 66 Central Stable Region: 22 Charles No. I Sztykgold: 37, 51, 178, 179 Chase, Gerald: 47 Chaves County: 9, 28, 29, 41, 42, 55 petrographic reports on wells in: 206-215 Chesher No. I, Shell: 224 Childress County: 25, 49 Childress Royalty No. I Masterson: 186 Chiles No. I Strickler: IS6 Cities Production No. I Hob•on: 214 Cities Service No. 3-S State : 219 No. 1 Whittemore: 132 Clarke No. I, Shell : 159 Clay County: 36 petrographic reports on wells in: 136-137 Oemcnts No. I, Honolulu and Sinclair: 162 Clemmie No. I, Phillips : IS7 Coahuila, Mexico : 59 Cobb, H. S.: 233 Cobb No. I, Texas: 179 Cobb-Federal No. I, Richardson & Bass: 53, 216 Cochran County: 41, 42, 44, 46, S6 petrographic reports on wells in: 137-B9 Cochran & Steward No. 1Brandenburger:177 Coke County: S3, S4 petrographic reports on wells in: 139 Coleman County: 29, 39, 41 petrographic reports on wells in: 139-141 Cole-State No. 4-A, Gulf: 221 Collin County: 67 Collingsworth County: 25, 49, Sl, 57 petrographic reports on wells in: 141 Collins No. 1, Shell and Texas: 122 Colorado Interstate No. 25-A Bivins: 42, 4S, 48, 191, 192, 193 No. 41-B Masterson: 193 Comanche County: 37 petrographic reports on wells in: 141 Comanche Unit No. 1, Richfield: 212 Concho arch: 12, 31, S3 County: 41 platform: 12, 31, S3 Conrin,o; No. I, Prrsidio: 194 Conry-Davis No. 1, Stanolind: 190 contact metamorphic rocks, Swisher gabbroic ter­rane: 45-46 continental joints: 70, 71 Continental No. I Berry: 48, 141 No. l Burger B-28: 219 No. l Lankford: 29, 207 No. I-A Lockhart B-13: 220 No. 1-E Lockhart A-27: 219 No. 4 Lockhart B-12: 219 No. 6-E Lockhart B-ll: 219 No. I McCutcheon : 170 No. I Martin: 171 No. I Rodgers: 204, 205 No. I Thurman-Federal: 21S No. I Warren A-29: 28, 220 No. 2 Warren B-29: 220 No. I Whaley: 141 Continental and Magnolia No. I Beach: 201 continents, orip;in of: 69 Cooke County: 8, 36, 48 petrographic reports on wells in: 141-144 Corbin No. 1, Stanolind: 125, 126 Cordilleran trend : 70 Cordova Union No. I, Superior: 190 Corrigan No. 5, Amerada: 216 No. 6, Amerada: 216 No. 7, Amerada: 216 No. II, Amerada: 216 Cosden No. I Barker: 42, 44, 137 Cottle County : 8, 37, 40, S3 petrographic reports on wells in: 144-146 Coursey No. I, Kadane & Sons: 142 Cowden No. I-A, Magnolia: 148 No. 6, Texas : 29, IS4 Cox No. I, Shell: 121 Crane County, petrographic reports on wells in: 146-147 craton, definition of: 21 Texas: 7, 9, 12, 22, 23, 25-32, 40, 50, 51, 56, 66, 68, 69, 70 cratonic margin: 7, 8, 10, 25, 32, 34, 63, 66, 67, 68 Criner Hills: 38 uplift: 52 Crockett County : 63 petrographic reports on wells in: 147 Crosby County : 8, 30, 36, 37 petrographic reports on wells in: 147-148 Crosbyton dome: 66 Crowley No. 1, Humble: 155 Culberson County: 8, 33, 34, 55, 63, 66 petrographic reports on wells in: 148 Curry County : 41 Dallam County : 41, 49 petrographic reports on wells in : 148 Dangle No. 3, Phillips : 143 Daniel No. 1, Standard of Texas: 157 Davenport No. 2, Barnsdall : 139 No. 7, Lesh-McCall: 178 Davis No. 1-A, Union and Cities Service: 159 Deaf Smith County: 41, 42, 67 petrographic reports on wells in: 149 Debaca County: 29, 55 petrographic reports on wells in: 214-215 Deep Rock No. 2 Morgan Jones : 147 definitions-basement : 11 craton: 21 fabric : 115 Fisher metasedimentary terrane : 39 mobile belt: 21 Panhandle volcanic terrane: 41 Red River mobile belt: 36 rock nomenclature: 116-118 Swisher gabbroic terrane : 43 terrane: 21 texture: 115 Van Horn mobile belt : 32 Wichita igneous province : 46 Dekalb No. 1 Lewis : 207 Dekalb and Magnolia No. I White: 207 Dela ware basin: 32, 53, 54, 63, 66 Deloache No. 1, Big Chief: 167 Denton County: 8, 36, 38 petrographic reports on wells in: 149 Diablo Platform: 32, 63 Dickens County : 8, 23, 39, 40, 54, 66 petrographic reports on wells in : 149-151 Dodson (Hinton) No. 1 Texas American Syndi­cate : 128 Donald No. 1, Gulf Prod. : 142 Donley County: 9, 25, 41, 43, 45, 49, 57 petrographic reports on wells in : 151-154 Donohue No. I, Flynn et al. : 227 Doswell No. McMurty : 151 Dunnigan No. 1 Ellis : 132 Earwood No. 1, Seaboard : 182 Eastern Platform: 54, 63 Ector County : 7, 28, 29, 30 petrographic reports on wells in : 154-155 Eddy County: 29, 53, 55 petrographic reports on wells in: 215-216 Edrington No. 1-A, Bridwell: 137 No. 26, Bridwell : 136 Edwards No. 5, Stanolind : 138 Electra uplift: 8, 36, 39, 55 Ellenburger group: 54 Elliott No. 1-A, Banklinc: 174 No. 1, Southern Union and Magnolia: 216 No. 1, Union of Cal.: 151 Ellis No. 1, Dunnigan : 132 El Paso County, petrographic reports on wells in: 155 El Paso Natural Gas No. 1 West Texas Mortgage and Loan: 45, 126, 127 Elwood No. 2, Sun : 177, 178 Elwood Estate No. I, Honolulu and Signal: 167 Embar area : 29, 30 field: 15, 28 Embar No. 15, Phillips : 29, 154 No. 23, Phillips : 29, 154 Emerald No. l Masterson : 193 Espy No. I, Welch : 34, 70, 195 fabric, defined: ll5 terms, definition of : ll5-ll6 Farren No. 2, Smith: 49, 50, 62, 200, 201 Farris No. I, Humble: 175 faults in basement rocks: 56-57 Fee No. l·G, Humble : 159 Fee-Federal No. I, Pure: 214 Fette No. 31, Hollandsworth : 36, 142 Fielder No. I, Phillips: 144 Fields No. 1, Phillips : 178 Fisher County: 8, 23, 39, 63, 67 petro:>;raphic reports on wells in : 155-156 Fisher metasedimentary. terrane: 7, 8, 23, 39-41, 63 gravity anomalies: 66, 67 Fisher No. 2, Stanolind : 163, 164 Flanagan No. 12, General Crude: 155 Floyd County : 8, 9, 23, 36, 37, 43, 55, 56 petrographic reports on wells in : 156-157 Flynn et al. No. 1 Donohue: 227 Foard County : 8, 36, 37 petrographic reports on wells in: 157-158 Fort Stockton high : 7, 15, 28, 29, 30, 34, 55, 63, 66 Fort Worth basin: 63 Fowler-McDaniel No. I, Anderson-Prichard: 134 Frabar-Hodges No. I George: 201 Franklin, Aston & Fair No. l Orchard Park: 207 208 ' Franklin Mountains: ll, 23, 34, 43, 53, 68 Fromme No. 3, Magnolia: 188 Frost, C. H.: 2.13 Frost Airborne Survoys, Inc.: 233 Frost Geophysical Corporation: 233 Frost magnetic susceptibility bridge : 233, 234 Fuller No. 1, Stanolind: 41, 229 Gaines County : 55 petrographic report on wells in : 158 Gallagher & Lawson No. I Terry: 37, 141 Garland-Anthony No. I Hammons: 38, 185 Garvin No. I, Gulf: 187 Garza County: 66 petrographic reports on well s in : 158-159 General Crude No. I Aiken: 155 No. 12 Flanagan: 155 No. 82-1 Jones : 170 No. IP. Jones : 170 No. 13-1 Swenson: 144 No. 33-1 Swenson: 145 No. 43-1 Swenson : 181 George No. 1, Frabar-Hodges: 201 No. 1, Schenk et al.: WO Gettys No. I, Anderson-Prichard: 45, 172 Gill No. 1, Killam (Chandler): 139, 140 Gillespie County, petrographic reports on wells in: 159 Glasscock County, petrographic reports on wells in: 159 Glenn No. 1, Shell : 225 No. 3, Shell: 225 Bureau of Eco/lomic Geology, The University of Texas glossary, prlroi:rnphic terms: JO, 15 Goldsmith No. I Hcpublic Nut. Gu• : 1:17 Goldstt•in, Aui:ust, Jr.: HI Goldston No. I-A Lumuirth·Stutc: 229 Good No. I South Basin: 29, 214 Gormun-Fedt·ral No. I, llumlilt•: 209 Grady Best No. I, Tiilewatl'r: 2:>2 Gruham-Stute No. 5-F, Gulf: 221 gra vity anomnlle~­Fb:.lwr mrtasedimcntary trrran e: 66 Panha ndle \'oll'nnic tt'rrnnt' : 67 Red River mouile bclt: 66 Swisher i:abhroie lt'rrant.': 67 Texas rraton: 63 Vnn Horn mobile belt: 66 Wichita i~rwous prndnfr: 67 ~ravity data: 63 · 67 Gray County: ·II, .J.2 , ·18, W. 50, 62 petrographic reports on wrlls in: 159-·162 Gray Randi No. 5.J.-9. Suprrior: 184 Greater Amarillo Oil No. I ~lastcrsou: 19.~ Grerlt•y No. l, Placid: 19'1 Green No. I, Stanolind: 183, 184 Grrenwood No. JO, llumblt• : 222 No. Il, Humble : 222 Grrer County: 67 Griffin No. I, Stanolind: 1:>3 Grisham-Hunter No. I, Sinclair-Prairie: 122 Gulf No. I Amanda: 29, 220 No. 6-E Rlackman: 202 No. 5-A Carson: 221 No. 4-A Cole-State: 221 No. 1 Garvin: 187 No. 5-F Graham-State: 221 No. I Jennin~s: 208 No. I-A Krliehor: 185 No. 46-E Keystone: 203 No. 50-E Keystone : 203 No. 62-E Keystone : 203 No. 70-E Keystone: 203 No. 73-E Keystone: 203 No. 75-E Keystone: 203 No. I Kilµon•: 180 No. 7 Kini:: 221 No. I Millar: 187 No. 2 ~lillar: 187 No. 3 Millar: 187 No. I ~lillrr: 201 No. I O'Sullivan : 187 No. State-Chaves U: 208 No. 2 Stirht.r: 221 No. 1-R Swenson: J.~8 No. 9-E University "Z": 119 Gulf Oil Corporation. thin srrtions from: 119 Gulf Prod. No. I Donald : J.12 Haberer No. I, Sun: 4S, 13.J. Hale County: 9, 23, 37, .J.I, 43, 55, 56 pt'lroµraphic reports on wells in: 162-165 Hall County: 25, 49 petrograrhi<' report~ on \\·rlls in: 165 llalsrll No. I, Honolulu: 172, In Hamilton, W. B.: 50 Hamilton No. I, Amerada: 128 Hammons No. I, Garland-Anthony: .18, 185 H•mon (Coroco) No. I Atlas Life: 199 Hanks No. 1-A, Sea hoard: 182 Hare No. 4, Amerada: 217 No. 5, Amerada: 217 Hartley County : 41, .J.9, 67 petrographic reports on wells in: 165-167 Harwood No. I, Shell : 230 llnssie Hunt Trust No. I Helms: 42, 45., 124, 125 No. I J. L. Cattle Co.: 124 Hayden No. I, Thousand Islands : 159 l laymond houldl'r heJ: 59 Ilazt'I snndstont': :H llrnnl-Ft·.dl'rnl No. I. Stnndard of 'foxns: ·18, 227 hrdn-ocru ton: 2~. 70 llt'µi No. I, Stanolind: IM llcim•r No. l·C, Union of Cnlifornin: 19 lleithold No. I, Skelly: 161 l!t·lms No. I, Hassie Hunt Trust : 42, 45, 124, 125 No. I, Sun and Ohio: 197 lkmphill County: 49 Herring No. I, Sun: 45, 134, 135, 136 Hirkok & Reynolds No. I, Midstates: 132 No. I Rawling: 139 Ilinkle-Fedt'ral No. I, Honolulu: 209 Hinton No. I. Humphreys: 178 No. ~' Pt>n rose: :?2:~ llinynrd Cattle Company No. I, Stanolind: 29, 190 Hobgood No. I, Humule: 42, 44, 168, 169 llohs;on No.], Citit'S Production : 214 !lo..kley County: ·I I, ·l2, 44, 46 petroµ:ruphir n•ports on wells in: 167-169 Hollandsworth No. :n Ft•ttt" .36, l.J.2 Holt No. 3 Rail<'y: 49, 50, 160 Honolulu No. I Altman: 159 No. I Ilnugh : 197 No. I Halsell : 172, 17.1 No. I Hinkle-Federal : 209 No. I King: 167 No. I l.t•virk-State: 209 No. I-A Lockett : 167 No. I Mcl~onkey Estate: 42, 209 No. I Nasworthy: 199 No. I Ozier: 152 No. I Pool: 197 No. 1 Rhoades: 175 No. 2 Whittaker: 181, 182 No.:> Whittaker: 182 No. 4 Whittaker: 182 llonoluln and Signal No. I Elwood Estate: 167 Honolulu and Sinclair No. I (:Jements: 162 llonoluln nncl Sunray No. I Moore: 167 Honolulu Oil Corporation, thin sections from: 119 !lopping No. I, Stanolincl: 174 lloughton No. I, Bridwell : 165 No. 2-A, Rridwell: 165 Houston No. I Lat·key: 156 Howard County: 6.~ Howard No. I, Superior: 184 Huddleston No. I, Lion: 155 Hudspeth Connty : 8, .~3. 43, 55, 66, 68 petrographic reports on wells in : 169 lluero Mountain>: 23, 34, 53 Hu!(hes No. I, Amerada: 165 Humble No. 4 Bateman: 171 No. I B. F. Smith: 187 No. 1 Belt: 30, 171 No. I Carnpliell: 42, 44, 167, 168 No. I Crowley: 155 No. I Farris: 175 No. 1-G Fee: 159 No. I Gorman-Federal : 209 No. IO Greenwood: 222 No. II Greenwood: 222 No. I Hougood : 42, 44, 168, 169 No. 1 Hyslop: 42, 149 No. I Irvin: 37, 147 Basement Rocks, Texas-New Mexico No. l Jackson: 173 No. l Keinath-Federal: 221, 222 No. 3 Lineberry: 120 No. 1 Masten: 137 No. 1-D Matador: 37, 181 No. 1-E Matador: 49, 50, 183 No. 1-G Matador: 150 No. 1-H Matador: 181 No. 1-J Matador: 145 No. 2-F Matador: 150 No. 2-H Matador: 181 No. 3 Matador: 149 No. 1 Montgomery: 37, 147, 148 No. 1 Morris: 197 No. 1 Nachlinger: 39, 40, 196 No. 1 Nanny: 198 No. 9 Odam: 139 No. 10 Odam: 139 No. l Pearson: 29, 215 No. l Pinson: 120 No. 1 Pratt: 177 No. 1-B Reynolds Cattle Company: 3'4 No. 1 Roach: 152 No. 1Ross: 172 No. 1 Scarborough: 120 No. 2 Shelton : 165, 166 No. 1 Smith: 187 No. 1Spencer : 196 No. 14 Spires : 170 No. 1 Stanford: 196 No. 1-N State: 210 No. 1-U State: 210 No. 3-V State : 222 No. 5-V State: 222 No. 6-V State: 222 No. 8-V State: 222 No. 9-V State: 222 No. 1-Y State : 211 No. 6 Stevens: 202 No. 1-L University : 187 No. 1 Weaver : 165 No. 1 Westheimer: 138 No. 1 Wilson : 60, 188 Humble and Stanolind No. l Byrd: 162 Humble Oil & Refining Company, thin sections from: 119 Humphreys No. 1Hinton: 178 Hunt No. 1McElmurray : 182 No. 1Martin : 149 No. 1 Presidio Trust : 34, 70, 194 No. 1 Ritchie: 45, 128, 129, BO No. 2 Ritchie: 42, 45, 130, 131 No. 4 Ritchie: 125 No. 5 Ritchie: 45, 152 No. 10 Ritchie: 132 Hunt Trust No. 1 Helms: 42, 45, 124, 125 No. 1 J. L. Cattle Co.: 124 Hunt & Turner No. 1 l\frMillan: 43, 227, 228 Hunter & Hunter No. 1 Steele : 39, 170 Hutchinson, R. 111.: 27 Hutchinson County: 49 Hyslop No. l, Humble: 42, 149 igneous activity, Tertiary: 62 igneous rock nomenclature: 16, 17 igneous rocks of Ouachita foldbelt : 60 Iron Mountain : 26. 27 Irvin No. 1, Humble: 37, 147 LT.LO. No. 1 Bryan : 197 Jackson No. l, Humble: 173 No. 1, Seaboard: 173 Jaffe, H. W.: 27, 31 Jamison No. 1Wcl>b:1911, 199 J arrell No. 1, Stanolind : 185, 186 JefT Davis County: 66 petro~raphic reports on wells in: 170 Jenkie No. 1, Phillips: 195 Jenkins No. l, Texas: 158 Jen kins, Kelsey, Jones & Eubanks No. 1 Waide: 149 Jennings No. 1, Gull: 208 J. L. Cattle Co. No. 1, Hassie Hunt Trust: 124 J. L. Hamon (Coroco) No. 1 Atlas Life: 199 Johnson No. 1, Magnolia: 176 No. 1-E, Smith : 162 No. 1, Standard o( Texas : 198 No. 3, Texas: 158 Jones County: 8 petrographic reports on wells in: 170 Jones No. 1-D, Amerada: 158 No. 2-A, Amerada: 158 No. 2, Deep Rock: 147 No. l, General Crude: 170 No. 82-1, General Crude: 170 No. 1, Ohio: 148 No. l,SanJuan: 173 No. 1 Sorely: 155 No. l, Stanolind: 226 Jones & Stasney No. l Wiley: 1'45, 146 Jordan No. l, Seaboard: 182, 183 No. 1, Stanolind (Superior and Intex): 196 Josten No. 1, Muenster : 143 KachelhofTer No. 1, Thomas & McFarland: 201 Kadane &Sons No. l Coursey: 142 Kary No. l, Phillips : 176 Kathryn No. l, Phillips: 197 Keahey No. 2, Phillips: 160 Keinath-Federal No. 1, Humble: 221, 222 Keliehor No. 1-A, Gull: 185 No. 1, Standard of Texas: 163 Kelly No. 1, Placid: 45, 152 Kent County: 8, 23, 39 petrographic reports on wells in: 170 Kerr-McGee No. 2 Berneta: 166 No. 3 Bern eta: 166 No. 1 Shelton: 166 Keystone area: 29, 30 field: 15, 28 Keystone No. 46-E, Gull: 203 No. 50-E, Gull: 203 No. 62-E, Gull: 203 No. 70-E, Gull : 203 No. n-E, Gulf: 203 No. 75-E, Gulf: 203 Kilgore No. 1, Gull: ISO Killam (Chandler) No. I Gill: 139, 140 Kimhle County: 29, 30 petrographic reports on wells in : 171 Kimbrough No. I, Sunray: 45, 186 Kinney County: 60, 61 King County, petrographic reports on well s in: 171-172 King No. 7, Gull: 221 No. l, Honolulu: 176 King, Philip B.: 14 Kiowa County : 67 Knox County: 66 petrographic report on wells in: 172 Kober, Leopold: 20 Krause No. l , Livermore: 156, 157 Kurfees No. 1, Amerada: 162. Bureau of Economic Geology, The University of Texas Lackey No. 1, Houston: 156 Lamb County: 9, 23, 41, 43, 45, 56 petrographic reporls on wells in : 172-174 Lam birth-Slate No. 1-A, Goldston: 229 Lampasas County: 28 pelro~raphic reports on wells in: 174 Langlie No. 1, Olson and Atlantic: 223 Lankford No. 1, Continental: 29, 207 No. l, Pure: 166 No. 1, Skelly and Lion: 155 Lann in:; No. 1, Skelley and Lion: 155 Lanoria quartzite: 34 Latham No. 4, l\fognolia: 160 Lazy HG llanch No. I, Wcleh: 154 Lea County: 9, 28, 29, 34, 41, 42 petrographic reports on wells in: 216-227 Lemons No. I, Texas: 179 Lesh-McCall No. 7 Davenport: 178 Levick-State No. I, Honolulu : 209 Lewis No. I, Dekalb: 207 No. 1, Stanolind : 153, 154 Libb Wallis No. I, Phillips: 198 Lincoln County : 29, 48, 55 petrographic reports on wells in: 227 Lineberry No. 3, Humble: 120 Lion No. I Bridwell: 45, 127 No. I Huddleston: 155 Livermore No. I Bird: 150 No. I Krause : 1S6, 157 No. 1 Moser: 183 Livingston No. 4, Shell: 224 Llanoria: 58, 59, 61 Llano uplift: 7, 9, 12, 23, 25, 26-27, 28, 29, 30, 31, 32, 38, 40, so, 53, 63, 68, 70 Lorketl No. 1-A, Honolulu : 167 Lockhart No. 2, Texas: 227 A-27 No. 1-E, Continental: 219 B-11 No. 6-E, Continental: 219 R-12 No. 4, Continental: 219 B-13 No. 1-A, Continental: 220 Loffiand No. 3 Tubbs: 147 Lonsdale, John T.: 14 Los Nictos No. 1-ll University: Hl8 Loving County: 63, 66 Luhbock County: .37, 41 pclro;:raphic reports on wr lls in: 174-176 Lubbock Machine & Supply No. I Alexander: 51 Lu<'as No. I, Southern Union: 2.31 Lulin~ area: 60 field: !:II Lulinl(·l\1cxia-Talco fault system: 58 Lynch No. I, Andcrson-Pri<"hard: 144 No. I, llamsey: 146 Lynn County : 41 pelro:o;raphie reports on wells in: 176-177 MarDcr No. 1-3. Standard of Texas: 189 No. 1-·1, Pan-American : 189 No. 1··1, Standard of Texas: 190 magnetic sus('rptihility: 10, 15 mcnsurt:'ments: 23:i-2 ~2 Jisc11ssio11 of roek values: 235-2.38 mras11rc Petroleum Company: 70 Puckett "B" No. 1-A, Phillips: 189 No. 1-C, Phillips: 31, 189 No. ).]),Phillips, 189 Pump Stntion Hills: 23, M, 4.3, 5.3, 68 Pun-t•ll No. I, Slwll nnd Siudair: .~O. 202 Pursell No. I, Merry Bros. & Perini: 146 Pure No. I Fee-Federal: 21'1 No. 1 Lankford: 166 No. 1 Sneed Heirs: 148 Quay County: 41 petrol(raphic reports on wells in: 229 Quintana No. 1-A Moore: 60 Ramsey No. I Lynch: 146 Ranch Creek No. 1 Masterson: 193 Randall County: 41 petrographic reports on wells in: 195 Rawling No. l, Hickock & Reynolds : 139 Red I\fountain i:neiss : 27 Red River mobile belt: 7, 8, 9, 22, 2.3, 24, 25, 30, 31, 36-39, 43, 48, 50, 51, 52, 57, 68, 69-70 p;ravity anomalies of: 66 Red River uplift: 8, 23, 25, 36, 38, 39, 52, 53, 55 Reed No. I, Stanolind: 138 Reitar No. I, Phillips: 144 No. 2-A, Phillips: 144 Republic Nat. Gas No. l, Goldsmith: 137 Reynolds Cattle Company No. 1-B, Humble : 34 Rhoades No. I, Honolulu : 175 Rich No. l, Anderson-Prichard: 199 Richardson No. 1 Schwartz: 199 Richardson & Bass No. I Cobb-Federal: 53, 216 No. 10-E Walton: 204 Richfield No. l Comanche Unit : 212 No. 1-A Trigg: 212, 213 No. 1-3 White: 213 Riley formation : 54 Ritchie No. I, Hunt: 45, 128, 129, 130 No. 2, Hunt: 42, 45, 130, 131 No. 4, Hunt: 125 No. 5, Hunt : 45, 152 No. 10, Hunt : 132 Roach No. l, Humble: 152 Roberts County: 49 petrographic reports on wells in: 195 Roberts No. ), Atlantic: 31, 195 rock nomenclature, definitions: 116-118 rock types ol- Fisher metasedimentary terrane : 39 Panhandle volcanic terrane: 41 Red River mobile belt : 36 Swisher terrane: 44 Wichita igneous province: 49 Rodgers No. I, Continental: 204, 205 Rollins, .T. C.: 233 Roosevelt County: 7, 9, 30, 41, 43, 55. 56, 66 petrographic reports on wells in: 229-2:l2 Roseborough No. I, Ameriran Land: 4.3, 169 Rosiwal modal analysis: 119 Ross No. I, Humble: 172 No. 2-A, Ohio: 172 Roth, Robert: 12 Roxunn No. I Mathews : 157, 158 Runnrls County: ;~o. 39, 40, 41 petro~raphic reports on wells in: 195 Saddler No. 1, Austral : 229 San Andres Mountains: 23 Sanders No. 1, Sanders: 213 San Juan No. l Jones : 173 San Marcos arch: 31 San Saba County, petrographic reports on wells in: 195 Saunders No. I, Shell: 231 Scarborough No. I, Humble: 120 No. 1-E, Shell: 121 Scarp Unit No. 1, Standard of Texas: 228, 229 Schenck et al. No. 1 George: 200 Schleicher County: 31, S4 petrographic reports on wells in: 195-196 Schwartz No. l, Richardson: 199 Scurry County: 8, 39 petrographic reports on wells in: 196-197 Seaboard No. 1 Big Four Ranch: 172 No. 1 Earwood: 182 No. 1-A Hanks: 182 No. l Jackson: 173 No. 1 Jordan: 182, 183 Seaboard and Shamrock No. 1 Tapper: 146 No. 1 University "C": 169 Shackelford County: 37 petrographic reports on wells in: 197 Shamrock No. 1 Adair: lS.3 No. 1 McCracken: 161 No. 1Taylor : 161 No. 1 Thompson: 132, 133 Shannon No. 1-D, Amerada: 147 Shaw-Federal No. 1, Magnolia : 28, 211 Shell No. 4·A Argo-Herring: 22'3 No. S Argo-Herring : 223 No. 6-A Aq:o-Herring: 223 No. 9 Argo-Herring: 223 No. 10-A Argo-Herring: 224 No. 1 Carter: 224 No. 1 Chesher: 224 No. 1 Clarke: 1S9 No. 1 Cox: 121 No. 1 Glenn: 22S No. 3 Glenn: 22S No. I Harwood: 230 No. I Livingston: 224 No. 3 Miers (Core Test No. 3): 198 No. 1 Nelson: 121 No. 1-A Nelson: 121 No. 1 Nichols: 128 No. 1 Pittman: 42, 138 No. I Saunders: 231 No. l·E Scar horough : 121 No..3 State: 224 No. 4 State: 224 No. S State: 224 No. 6 State: 224 No. 8 State: 223 No. I Taylor Glenn: 22.5 No. 3 Taylor Gl enn: 22S No. 4 Turner: 225 No. II Turner: 22S No. 14 Turner: 22S No. 15 Turner: 225 Shell and Sinclair No. I Purcell: 30, 202 Shell and Texas No. 1 Collins: 122 Shell (Humphreys) No. 1University:189 Shell Oil Company, thin sections from: 119 Shelton No. 2, Humble: 16S, 166 No. l , Kerr-McGee: 166 Sherman County: 41, 48, 49 petrographic rt>ports on wells in : 197 Shildneck No. 1, Black: 206 Shipp No. I, Phillips: 223 Sidwell No. I Bowers: 161 Signal No. I Bell-Federal: 231 Siluro-Devonian rocks: SS Sims No. I, Stanolind: 122, 12.3 Sindair No. 1 Barton: 28, 22S No. 4 Brunson: 225 No. S Brunson: 22S No. I Massie: 37, IS7 No. 2 State 367: 226 No. 6-A Walton: 204 Sinclair-Prairie No. I Bush: 193, 194 No. 1 Grisham-Hunter: 122 Sixmile granites: 26 Skelly No. 5-B Baker: 226 No. 1 Heitholt : 161 No. 1 Stichter: 226 Skelly and Lion No. I Lanning : lSS Skinner No. 44, Texas: 202 Slaughter No. 1, Stanolind: 138 Slick-Urschel No. 1 Standefer: 199 Smith No. 2 Farren: 49, SO, 62, 200, 201 No. l, Humble: 187 No. 1-E Johnson: 162 No. 1, Magnolia: 230 Sneed Heirs No. l, Pure: 148 Sorely No. I, Jones: lSS South Basin No. 1 Good: 29, 214 Southern Union No. I Lucas: 231 Southern Union and Magnolia No. 1 Elliott: 216 Spartan No. 1-36 State: 231, 232 specific gravity determinations: lS Spencer No. 1, Humble: 196 Spiller No. 1, Phillips : 171 Spires No. 14, Humble: 170 Standard of Texas No. 1 Bush: 194 No. 1 Daniel: 1S7 No. 1 Heard-Federal: 28, 447 No. 1 Johnson: 198 No. 1 Keliehor: 163 No. 1-3 MacDer: 189 No. t-4 Mac Der: 190 No. I Owen: 132 No. I-A Palm: 12S No. 1 Scarp Unit: 228, 2·29 Standefer No. I, Slick Urschel: 199 Stanford No. 1, Humble: 196 Stanolind No. I Beck: 166 No. I Bird : 1S9 No. I Broome: 15'3 No. I Conry-Davis: 190 No. I Corbin: 125, 126 No. S Edwards: 138 No. 2 Fisher: 163, 164 No. 1 Fuller : 41, 229 No. 1 Green: 183, 184 No. 1 Griffin: 133 No. 1 Hegi: 164 No. 1-A Hin yard: 29, 190 No. I Hopping: 174 No. I Jarrell: 185., 186 No. I Jones : 226 No. I Lewis: 15.3, IS4 No. I McCrea: 122 No. I Pkacho Unit: 29, 227 No. I Reed: 138 No. 1 Sims: 122, 123 No. 1 Slaughter: 138 No. 11-X State: 226 No. 1 Stiles: 123 No. 3-AE University: 123 No. 4-AE University: 123 No. 6-PP University: 123 No. I W. H. Jones: 226 Bureau of Economic Geology, The University of Texas Stanolind (Superior and lntex) No. l Jordan: 196 Stanolind Oil & Gas Company, thin sections from: 119 State BTA No. 1, Amerada: 42, 217, 218 No. 1-RA, Amerada: 206 No. 1-A, Barnsdall: 206 No. 3-S, Cities Service: 219 No. 1-N, Humble: 210 No. 1-U, Humble: 210 No. 1-Y, Humble : 211 No. 3-V, Humble: 222 No. 5-V, Humble : 222 No. 6-V, Humble: 222 No. 8-V, Humble: 222 No. 9-V, Humble: 222 No. 1-W, Magnolia: 215, 216 No. 1-Z, Magnolia: 211 No. 3, Shell: 224 No. 4, Shell, 224 No. 5, Shell: 224 No. 6, Shell: 224 No. 8, Shell : 224 367 No. 2, Sinclair : 226 No. 1-36, Spartan: 231, 232 No. 11-X, Stnnolind: 226 No. l, Union and Dekalb : 213, 214 State-Chaves U No. l, Gulf: 208 State-Graham No. 5-F, Amerada: 218 Steele No. I, Hunter & Hunter: 39, 170 Stephens No. 14-A, Barkley-Meadows: 48, 202 No. 1-C, Texas: 156 No. 7, Texas: 156 Stevens No. 6, Humble: 202 No. 1-A, Phillips: 127 Sticher No. 2, Gulf: 221 No. l, Skelly: 226 Stine No. l, Perkins: 137 Stipp, T. F.: 14 Stiles No. I, Stanolind: 123 Stone No. I, Naylor: 140 Stonewall County: 8, 39 petrographic reports on wells in: 197 Stribling No. 1, Amerada: 174 Strickland No. I, l\lid-Continent: 230 Strickler No. l, Chiles: 156 Streeruwitz thrust fault: 34 structures of- Panhandle volcanic terrane : 43 Red River mobile belt : 38 Van Horn mobile belt: 34 Texas craton: 31, 32 Wichita igneous pro,·ince: 51 structural history of Swisher i:abhroic terrane: 46 structural nomenclature: 20-22 structure contours : 53 subdivision of Precambrian time: 69 Sun No. I Haberer: 45, 134 No. I Herrin~: 45, 134, 135, 136 No. 2 Elwood: 177, 178 No. I Pinion: 213 Sun and Ohio No. I Helms: 197 Sunray No. I Kimbrough: 45, 186 Superior No. I Cameron: 1.39 No. I Cordova Union: 190 No. 54-9 Gray Ranch: 184 No. I Howard : 184 No. 1 llfatador 184 No. 2 Matador 184 No. 3 Matador 185 No. 4 Matador 185 No. I Mr.Dowell: 30, 40, 19S No. I Pitchfork: 172 No. 2 Pitchfork: 172 No. 8 Wood "194": 170 Superior of Cal. No. I Brown: 141 Sutton County: S4 petrographic reports on wells in : 198 Swenson No. 13-1, Gen eral Crude: 144 No. 33-1, General Crude: 146 No. 43-1, General Crude: 181 No. 1-B, Gulf: 158 Swisher County : 9, 23, 43 petrographic reports on wells in: 198 Swisher ~abbroic terrane: 7, 9, 23, 42, 43-46, Sl, 57, 68 ~ravity anomalies: 67 Sztykgold No. l Charles: 37, 51, 176, 179 Tapper No. l , Seaboard and Shamrock: 146 Tarrant County: 63 Taylor County: 39, 41 petrographic reports on wells in: 198-199 Taylor No. l, Shamrock: 161 Taylor Glenn No. 1, Shell : 225 No. 3, Shell : 225 tectonic land: 61 terrane, definition of: 21 Terrell County: 61 Terry County: 41 petrographic reports on wells in: 199 Terry No. I, Gallagher & Lawson: 37, 141 Tertiary igneous activity: 6'2 Texas American Syndicate No. I, Dodson (Hin­ton): 128 Texas arch: 12, 31, 53 Texas craton: 7, 9, 12, 22, 23, 25--32, 40, SO, SI, S6, 66, 68, 69, 70 gravity anomalies of: 63 margin of: 7, 8, 10, 2S, 32, 34, 63, 66, 67, 68 Texas Lineament: 3S Texas Peninsula : 12, 31, SS Texas Railroad Commission: 14 Texas No. I Blinebry: 226 No. 4 Blinebry: 226 No. 41 Byers: 137 No. I Capitol Freehold Land Trust: 148 No. 1 Cobb: 179 No. 6 Cowden: 29, 154 No. 1 Jen kins : 158 No. 3 Johnson: 158 No. l Lemons: 179 No. 2 Lockhart : 227 No. l Main : 202 No. 414 Skinner: 202 No. 1-C Stephens: 156 No. 7 Stephens: 156 No. l Yeatts : 149 Texas Gulf No. l Matador: 166 Texoleum Trust No. l White: 174 textural terms, definition of: US-116 texture, defined : llS Theison No. 1, California: 169 Thomas No. 1White:177 Thomas & Mr.Farland No. 1 Kachelhoffer: 201 Thompson No. l, Shamrock: 132, 133 Thousand Islands No. I Hayden: 159 Throckmorton County : 66 Thurman-Federal No. I, Continental: 21S Tidewater No. 1 Boone: 232 No. I Grady Best: 232 Timbered Hills area: 47 Basem1•nt Rocks. Texas-New Mexico Tindall No. I, Best : 200 Tipton & Waggoner No. I McConnell : 133, 134 Tishomingo granite: 47 Tom Green County, petrographic reports on wells in: 199 Town Mountain granites : 26, 27 Trigg No. l·A, Richfield: 212, 213 Troy granite : 47 Tubbs No. 3, Loffiand: 147 Turner No. 4, Shell : 225 No. II, Shell : 225 No. 14, Shell : 225 No. 15, Shell: 225 Tumey No. I, McCandless: 188 Turney.Federal No. I, Magnolia: 211 TXL No. 1-J, Phillips: 154 Union and Cities Service No. l·A Davies : 159 Union and Dekalb No. I State: 213, 214 Union of California No. I Elliott : 151 No. 1-C Heiner : 190 University No. 2-A, Atlantic: 146 "Z" No. 9-E, Gulf: 119 No. l·L, Humble: 187 No. 1-B, Los Nietos: 188 No. I, MrCandless: 188 No. 5-M, Phillips: 121 No. 38, Phillips: 120 No. 50, Phillips: 120 No. 57, Phillips: 120 No. 58, Phillips: 121 "C" No. 1; Seaboard and Shamrock: 169 No. I, Shell (Humphreys): 189 No. 3-AE, Stanolind: 123 No. 4-AE, Stanolind: 123 No. 6-PP, Stanolind : 123 U pion County: 63 petrographic reports on wells in : 199 URB No. 6, Phillips: 42, 160, 161 U.S. Geological Survey: 12 U.S. Smelting & Refining No. 1-A Osborner: 186 Valley Spring gneiss: 26, 27, 29 Val Verde County : 60,61 Van Horn area : 8, 23, 25, .31, 33. 34, 53, 55, 60, 68 dome: 35 mobile belt: i, 8, 9. 22, 23, 25, 31, 32-36, 43, 56, 59, 60, 6.3, 68, iO gravity anomalies of: 66 Mountains: 34, 59, 70 sandstone: 34, 53 Varnell, W. R.: 14, 23.3 Virginia No. 1, Phillips: 197 Waide No. I, Jenkins, Kelsey, Jones & Eubanks: 149 Walden No. I, Amerada: 218 No..3, Amerada: 218 No. 4, Amerada: 218 Wallis No. I, Phillips: 198 Walton No. 4, Phillips: 204 No. 5, Phillips: 29, 204 No. 6-A , Sinclair: 204 No. 10-E, Richardson & Bass : 204 Warner No. I Monroe : 180 Warren A-29 No. I, Continental : 26, 220 R-29 No. 2. Continental: 220 weathering, basement surface : 53 Weaver No. I, Humble : 165 Webb No. 1, Jamison : 198, 199 Welch No. I Espy : 3-1, 70, 195 No. I Lazy RG Handt: 154 wells, petrographic reports on: 119-232 statistics: 15 Western Lampasas Oil No. 1Whittenburg:174 Westheimer No. I, Humble: 138 west Texas basin : 32 West Texas Mortgage and Loan No. 1, El Paso Natural Gas: 45, 126, 127 Whaley No. 1, Continental: 141 No. I, McElreath & Suµ:µ:ett: 142 Wheeler County: 48, 49, 50, 62 petrographic reports on wells in: 200-201 White No. I, Dekalb and l\fo~nolia: 207 No. 1-3, Richfield: 213 No. 1, Texoleum Trust: 174 No. 1, Thomas: 177 Whittaker No. 2, Honolulu: 181, 182 No. 3, Honolulu: 182 No. 4, Honolulu: 182 Whittemore No. I, Cities Service: 132 Whittenburg No. 1, Western Lampasas Oil: 174 W. H. Jones No. I, Stanolind : 226 Wichita County: 36 petrographic reports on wells in : 201-202 Wichita-geosyncline: 38 igneous province: 7, 9, 21, 23, 24, 46-52, 57, 62, 68 gravity anomalies: 67 Mountains: 37, 38, 413, 45, 46, 47, 49, 50, 51 , 53, 55, 62, 68 petrography of : 48 uplift : 52 system: 38, 39, 52, 66, 70, 71 uplift: 39, 51, 67 Wilbarger County: 36, 48, 67 petrographic reports on wells in: 202 Wilberns formation: 54 Wiley No. I, Jones & Stasney: 145., 146 No. 1, Magnolia: 150, 151 Williamson County: 30 petrographic reporl• on wells in: 202-203 Williamson No. I, Woodward: 151 Wilson County: 60 Wilson, J. L.: 62 Wilson No. 1, Humble: 60, 188 Winkler area: 28 Winkler County: 28, 29, 30 petro~raphic reports on wells in: 203-204 Wood No. 5, Amerada: 218 No. 6, Amerada : 218 No. 9, Amerada: 218 No. 10, Amerada: 218 "194" No. 8, Superior: 170 Woods, Raymond D.: 14 Woodward No. l Williamson: 151 Woolworth &Hawkins No. 1 Myrick : 214, 215 Worley No. I, Phillips: 161 Wurzback No. I, Moore: 60 Wylie Mountains: 3'4 Yeatts No. 1, Texas: 149 Yoakum County : 41 petrographic reports on wells in : 204-205 Zelle No. I, Prairie: 177