No. 5301 January 1, 1953 GEOLOGY AND MINERAL DEPOSITS OF PRE-CAMBRIAN ROCKS OF THE VAN HORN AREA, TEXAS By PHILIP B. KING and PETER T. FLAWN Bureau of Economic Geology John T. Lonsdale, Director Prepared in co-operation with the United States Geological Survey PUBLISHED BY THE UNIVERSITY OF TEXAS AUSTIN Publications of The University of Texas PUBLICATIONS COMMITTEE H. Y. MCCOWN A. MOFFIT C.H. EADS E. C. MossNER C.D.LEAKE F. H. WARDLAW W. P. WEBB Administrative Publications H. Y. MCCOWN F.L.Cox R. C. ANDERSON B. GONZALES, JR. L. L. CLICK B. E. SHORT 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. 5301 is the first publication of the year 1953.) These bulletins comprise the official publications of the University, publications on humanistic and scientific subjects, and bulletins issued from time to time by various divisions 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, Research Laboratory in Ceramics, 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 procured from the Bureau of Economic Geology, The University of Texas, Austin 12, Texas THE UNIVERSITY OF TEXAS PRINTINQ DIVISION ~ No. 5301: January 1, 1953 GEOLOGY AND MINERAL DEPOSITS OF PRE-CAMBRIAN ROCKS OF THE VAN HORN AREA, TEXAS By PHILIP B. KING and PETER T. FLAWN Bur-u of Economic Geolosy John T. Lonsdale, Director Prepared in co-operation with the United States G-losical Survey PUaLISHIED aY THIE UNIVIERSITY TWICE A MONTH. ENTERED AS SECOND• CLA98 MATTIER ON MARCH 12, 1913, AT THE POST OFFICE AT AUSTIN, TEXAS, UNDER THE ACT OF AUGUST 24, 1912 The benefits of education arul oj 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, arul while guided and controlled by virtue, the noble.st attribute of man. It is the only dictator that freemen acknowledge, and the only security which freemen desire. Mirabeau B. Lamar CONTENTS PAGE Abstract ·······--------------------------------------------------------------------------------------------------------------------------------9 INTRODUCTION, by Philip B. King and Peter T. Flawn ___ ---------·---·-··· ....... ---···--··· .. 13 Geologic setting ------------------------------------------------------------------------------------------------------13 Previous work ------------------'········--------------------------------------------------------------------14 Present work --····-··-----·-··-------··-····---·-----·····-··--···-······--·---··-···········--------------··········· --·---·········· ___-----·-· 14 Acknowledgments ------···----------------------------------------------------------------15 Geomorphology ··························-··········· ·············-······---·-·--··-·-···----···-···------------------··--·····················-···· 16 Summary of pre-Cambrian rocks ···-·············----·--·-----------···-················-···-···········-··-····-··-···-····---··--·-·-18 Stratigraphic nomenclature ·····-·····-···--················· ···········-· ---·--··-··--··-···--···-······--------------···-···-··········-··· 20 Petrographic nomenclature ------------------------------------------------------------------------·····-------24 NORTHWEST VAN HoRN MOUNTAINS (MICA MINE AREA), by Peter T. F1awn -------------------------Z1 Pre-Cambrian rocks --------------------------------------------------------------------------···········------------------· 27 Permian rocks ··············· ····------------·-----·······--------------·· -------------······-··-------·---···-··········-·········-········-······ 34 Cretaceous rocks ·············----------------------------------------------------------------·-------------········ 36 Tertiary igneous rocks ············---------------------------------------------------------------------------·-····------------36 Structure -----------------·-···········--------------------------------------------------------------------------·· 36 NORTHEAST VAN HORN MOUNTAINS, by Peter T. F1awn ........... ············--39 Pre-Cambrian rocks ------··········-·------------------·······--·····-·---------------------------------------------------------39 Younger rocks ------···----------------············-------------------------------------------------------------------------40 Structure --------------------'--------------------------------------------------···· -------------------------------------40 WYLIE MOUNTAINS, by Peter T. Flawn ---------------···------------·--·······················----------·········------------41 Pre-Cambrian rocks ----------------------------------------------------------------------------------------------------------------41 Permian rocks -------·-------------------------------------·----------------------------------------------------------------------43 Tertiary rocks ······----------------·······---------------------------------------------------------------------------------------------·· 43 Structure . --········· ....... --·-··········-········-·· . --······-·····--····---······--· 43 EAGLE MOUNTAINS, by Peter T. F1awn ----------------·-----------------------------------------------------------------" 45 Pre-Cambrian rocks ----···········-······· -------------------------------------·-··········------------------------------------------45 Permian rocks -···············-·······-···-·····------------------------····················----------------------------------------------49 Cretaceous rocks ·---·····················------·····················---·-----············-----------------------------------------·------49 Structure ------------------················--·-------·---------···········-·-----·-·-------------------····---··------------·.-------------------49 CARRIZO MOUNTAINS, by Peter T. Flawn .. ······-····-········· ...... ---······ ·············-····· ... ... ........ 51 Pre-Cambrian rocks -----------------------------------------------------------------------------------------········-----51 Metasedimentary rocks of main sequence ---------------------------------······--------------------------·--· 52 Metasedimentary rocks of uncertain stratigraphic relations ___---·-····-·········· ------------···-······· 58 Meta-igneous units ················--------·-···········------------------------------···········-----------------------------60 Quartz veins ---------·····-······-----------------------------·······-----------------------------·············-----------------------··· 64 Pre-Cambrian (?) rocks _ ........................ ·····-. ···--············· . . ---------······-------···········-······ 66 Van Hom sandstone ........... ---···------------------------------·-·······-----------------------------------------66 Permian rocks -----------------··········------------··-·---------------------------------------·-············----------------------67 Hueco limestone -------------·-----------------------'-···············--------------------------------------------------67 C1·etaceous rocks -----·············--···----------·-·······················-----------------------------·------------------------------67 Tertiary (?) intrusive rocks ···········-·-------------------·------------·--·-·-------·-----------------------------------67 Structure -------------------------------------------------------------------------------------------------------------------------------67 SIERRA DIABLO FOOTHILLS, by Philip B. King ·······-_____ ---·· ··--·· . __ ................ . -------····· 71 Topographic setting -----························-························-··---------···················-···--·--····-----------------71 Present investigation ----------·-------------------------······-------------------···········-···········-----------------------71 Pre-Cambrian rocks ---------·-····--------··-------------------------------------------------···---·········--------------------73 Carrizo Mountain group ----------··········-···----------------------------------------------------------------73 Allamoore formation --·············-···-···-··--······················ -----------------···················-··-----·-···· 76 Tumbledown Mountain section -----------------------------·-------------------------------------------------··-····· 79 PAGE Blackshaft mine area --------------------------------.,------··--·----------------·------------------------------79 Hazel formation -------···--·--·----------------------------------------------------------·----------·-------------------------------84 90 Prew-Cambrian (?) rocks ---------·-·---·-·----------·-· -------····--·---·----·---------·-----·-·----------------·--· ------· -­ 90 Van Horn sandstone -------------_ ____ ...... ................... . 97 Paleozoic and younger rock~ ------------·--------·--------------·--·-----------------------------------------------------------------­ 97 Ordovician rocks --------·--···:---------·------------------------··------------------------------·-·--···----·-----·-··-·----------­ tructure contours on top of pre-Cambrian rocks; paleogeology of surface on which Wolfcamp series was deposited; and paleogeology of surface on which Cre­ taceous was deposited. FACING PACE 20. Metaquartzite, meta-arkose, muscovite schist, and pegmatite in Carrizo Mountain group of northwef>t Van Horn Movntains ------------------------------------------------------------------------------182 PLATES- FACING PACE 21. Basal section of Hueco limestone and slabby metaquartzite of Carrizo !\fountain group. 1!11 22. Lineation in metarhyolite and layering in epidote amphibolite of Carrizo Mountain group .... 186 23. Limestone of Allamoore formation . -·-----·------------------·------------·---·----...... -·-··-··--·-····-----····-·-188 24. Phyllite of Allamoore formation and massive conglomerate of Hazel formation--------·---····---···--190 25. Conglomerate of Hazel formation -·-···-·-----·-······------·----··-·---·········-·-·······-·····-·······-·----··-···------·····--····· 192 26. Van Horn and Bliss (?) sandstones _ ... ·------···· ---· ....... ...... . .......... .. . 194 27. Outcrops of Van Horn sandstone ---···--··--·-·-----·-···---------·······-·····-···········--·····--·-·········----·······----······· 196 28. Escarpments. of Beach Mountain and Sierra Diablo ------·-······---············-·-·-····----·--··--·----···-·---···-198 29. The Hazel mine in 1889 and in 1951 ····---·-···--····-----··-·--·····-··-···--······--··········-···--·····----·········--····· P. 201 PHOTOMICROCR.UHS-­ 30. Pre-Cambrian rocks from northwest Van Horn Mountains .... ········--······-········---·-··-·-·------···--------· 202 31. Pre-Cambrian rocks from northwest Van Horn Mountains ............. ·-··--···-······--·--·····---······-··-··· 204 32. Pre-Cambrian rocks from northwest Van Horn Mountains .......................................................... 206 33. Pre-Cambrian rocks from northwest Van Horn Mountains ........... ---·-······-···-··--··-·-----·······-·----·-208 34. Pre-Cambrian rocks from Carrizo Mountains ........ ----·--·-··----··-··-···-··--········-----··-···-··-----·-········ 210 35. Pre-Cambrian rocks from Carrizo Mountains.·----·-·--·········-···············-·-····-·····--------··--···--·······-212 36. Pre-Cambrian rocks from Carrizo and Eagle Mountains ............. ·--···-·····--···-·-···-··-··----·--·--····--·-··--214 TABLES-- PACE 1. Development of stratigraphic nomenclature of pre-Cambrian rocks of Van Horn area 21 2. Estimated modes of representative rocks of the metaquartzite-muscovite schist sequence of the northwest Van Horn Mountains ... --------··-···-··----------·-···--·-······-·--·-·····-··--····-------····· 28 3. Chemical analyses of quartzo-feld~pathic rocks·-----------------····--······--·----·-··---------·-·--·-·-·· 29 4. Estimated mod~ of representative biotite-bearing rocks from the pre-Cambrian of the northwest Van Horn Mountains ········-·-·-------·------·-·--········-·-····-·--·--·-·--·-·---··--·-··· 30 5. Estimated modes of representative amphibole-bearing rocks from the pre-Cambrian of the northwest Van Horn Mc.unt11ins --·---·-----·--··----··-······--··-················-·--·---·-··-----···----··-··--31 6. Chemical analyses of amphibolites and allied rocks·-·····-··-·--·····-·-·····--·-···-----·--·····-·-··----·-··· 32 7. Estimated modes of pre-Cambrian rocks of the northeast Van Horn Mountains..................... 39 8. Estimated modes of representative rocks from the pre-Cambrian of the Wylie Mountains .._ 42 9. Estimated modes of pre-Cambrian rocks from the Eagle Mountains ....... ·-·----··-·······------··-····-·-48 10. Stratigraphic column of the Carrizo Mountain group in the Carrizo Mountains......... 52 11. Estimated modes of representntive metas.edimentary rocks in the Carrizo Mountains ........ -.. 59 12. Estimated modes of metarhyolite in the Carrizo Mountains ... ·-··--·-···--··--·····-·----·-··--··-····-·-········ 62 13. Estimated modes of amphibolite in the Carrizo Mountains_·----·----·-···---··-····------------··-·····--·--· 65 14. Sequence of pre-Cambrian rocks of Sierra Diablo foothills as interpreted in 1931.................... 72 15. Sequence of pre-Cambrian 1ocks of Sierra Diablo foothills as interpreted in 1938 ............ _.. 73 16. Analysis, in percent, of limes.tone from Allamoore formation.·-·-·--·---·-····---····-·--·--·--·----·········-76 17. Stratigraphic section of Allamoore formation on Tumbledown Mountain ..... ·--·-·-··-····-·-··--···· 80 Hl. Estimated thickness of red sandstone of Hazel formation near Pecos mine ... ·---·--·-···-·-----·--87 19. Stratigraphic section of Van Horn sandstone west of old Circle ranch house........................ 94 20. Tectonic history of Sierra. DiabJo foothills .... -···-··-·-·-·---·-·-··-·-·············--··-·······--·------··----······--·-101 21. Estimated modes of pre-Cambrian rhyolite near Hueco Pump Station -·-·--·-······-··--------····--···· 123 22. Sedimentary, metamorphic, and structural history of south part of Van Horn pre-Cam­brian area --········--·-··············-····-·--·····---···-·--·--·-------··-·--·-·-------··--··--·-----·····--·---·-------··-··----··· 126 23. Sedimentary, metamorphic, and structural history of north part of Van Horn pre-Cam­brian area --··-··--·······-······-······--··········----········-----·---·-----···---·-···-··------·-··---------------127 24. Sedimentary and structural history of pre-Cambrian rocks northwest of Van Horn area._______ 128 25. Tentative correlation of structural events in different parts of Van Horn region. .. ·-····--·--· 129 26. Estimated total production of Allamoore-Van Horn copper district ........................................ 149 27. Estimate of ore reserves of Allair.oore-Van Horn district ..... ·-··-·······--·-········--·-·····------··--····-···-151 28. Known production of the Huzel mine ·····-··-------·-·····--·---·····-·········--···-····-···-···-·-----····-------·· 154 29. Average analysis, in percent, of ore from Blackshaft mine ....·------·--·-·---·----··----·-······-------·-------· 163 30. Analyses of samples from "potash mine" near Van Horn, Texas ... ·-··-----·---·-···--·-------········ 173 GEOLOGY AND MINERAL DEPOSITS OF PRE-CAMBRIAN ROCKS OF THE VAN HORN AREA, TEXAS Philip B. King1 and Peter T. Flawn2 ABSTRACT lntroduction.-This publication de­scribes the geology of pre-Cambrian rocks exposed in the vicinity of Van Horn-their principal area of outcrop in Trans-Pecos Texas. The pre-Cambrian rocks of the area had previously been studied in reconnais­sance by Von Streeruwitz for the Dumble Survey ( 1890-1893) , by Richardson for the U. S. Geological Survey (1904, 1914), and by Baker for the Texas Bureau of Eco­nomic Geology (1927). The present report is a joint undertaking of the U. S. Geo­logical Survey and Bureau of Economic Geology. Following a summary of the geo­morphology and pre-Cambrian geology of the area, the stratigraphic and petrographic nomenclature used in the report is set forth. Northwest and northeast Van Horn Mountains, Wylie Mountains, Eagle Moun­tains, Carrizo Mountains, and Sierra Di­ablo foothills.-The geology of the pre­Cambrian rocks in their several areas of occurrence is described in six chapters, five by Flawn on the smaller southern areas and one by King on the large northern area. These are summarized together under the present heading. Pre-Cambrian rocks are exposed in many places within a 20-mile radius of the town of Van Horn, in an area that has some­times been called the "Van Horn dome." Actually, the structure is not domical in the usual sense, but rather, the pre-Cam­brian rocks come to the surface in a num­ber of separate but adjacent mountain up­lifts. The largest area is in the eastern and southern foothills of the Sierra Diablo. Near the line of the Texas and Pacific Rail­road this is separated by a graben of younger rocks from another area in the Carrizo Mountains to the south. Farther south, pre-Cambrian rocks emerge again in smaller areas on the northeast side of the Eagle Mountains, on the west side of 1 Geolociat, Geological Sa"ey, U. S. Department of tho Interior. 1 Ceolociat, Bureau of Economic Geology, The Univcnity of Tesu. the Wylie Mountains, and in two areas in the Van Horn Mountains, one of which is known locally as the Mica Mine area. The most highly metamorphosed and perhaps the oldest pre-Cambrian rocks lie to the south, in the Carrizo, Eagle, Wylie, and Van Horn Mountains, and constitute the Carrizq Mountain group. This is a body of altered sedimentary rocks, includ­ing meta-arkose, metaquartzite, schist, phyllite, and limestone, which has been intruded by large volumes of igneous rocks, originally rhyolite and diorite, now altered to metarhyolite and amphibolite. Extensive exposures of the group in the Carrizo Mountains show a sequence of sedimentary rocks as much as 19,000 feet thick, which does not appear to have been repeated by folding or faulting. The group shows much homogeneity in original character from place to place, suggesting that it is a single sedimentary series rather than several, but it shows considerable variation in degree of meta­morphism. Rocks farthest south, in the Van Horn Mountains, are the most metamor­phosed (medium metamorphic grade, or amphibolite facies), retain few of their original sedimentary structures, and are extensively veined by pegmatite. Rocks farther north, in the Carrizo and Eagle Mountains, are less metamorphosed (low metamorphic grade, or greenschist facies), and preserve many of their original sedi­mentary structures. Their metamorphic his­tory is complex, however, as a retrogressive and cataclastic metamorphism is superim­posed on an earlier progressive regional metamorphism. The associated intrusive metarhyolites are also cataclastically al­tered and in part mylonitized. The cata­clastic metamorphism was perhaps pro­duced by dislocative movements that were associated with the Streeruwitz overthrust, immediately beyond. The Streeruwitz overthrust, whose trace lies in the Sierra Diablo foothills a little north of the Texas and Pacific Railroad, is a line of major discontinuity in the pre­Cambrian rocks. Along it, the dominantly metamorphic and igneous rocks of the Carrizo Mountain group have been thrust northward over the dominantly sedimen­tary Allamoore and Hazel formations. The original stratigraphic relations of the rocks on the two sides of the overthrust are unknown, but presumably those on the north are younger, and they may be much younger. The Allamoore formation, the oldest unit north of the overthrust, consists of interbedded cherty limestones, phyllites, and volcanic rocks, the latter including pyroclastics, flows, and perhaps shallow intrusives. Some of the limestones contain structures that may be of algal origin. Be­cause of the complex structure, the thick­ness of the Allamoore cannot be deter­mined, but it is certainly thousands of feet thick. The Allamoore is succeeded by a very different, but comparably thick, deposit, the Hazel formation, whose basal part is a thick coarse conglomerate, made up almost wholly of angular rock fragments de­rived from the Allamoore formation. The two formations are obviously unconform­able, and were probably separated by a time of orogeny which may still have been in progress when the earlier Hazel deposits were being laid down. The conglomerates are interbedded with, and are succeeded by, a thick, uniform mass of fine-grained, silty red sandstone which constitutes the upper part of the Hazel formation. After deposition of the Hazel formation, both it and the Allamoore were deformed together during a time of orogeny that was probably related to movements on the Streeruwitz overthrust. For some miles north of the trace of the overthrust both formations are thrown into recumbent folds and thrust sheets that were driven north­ ward. Deformation dies out away from the overthrust, and in the northernmost expo­ sures the rocks are little disturbed. The deeply eroded edges of the deformed and partly metamorphosed Hazel and Alla­ moore formations and the Carrizo Moun­ tain group are overlain unconformably by another and coarser red elastic deposit, the Van Horn sandstone, whose maximum thickness is about 800 feet. The Van Horn contains numerous rounded cobbles and boulders, made up not only of rocks from formations immediately beneath, but also of red granite and rhyolite porphyry prob­ably derived from the north, an~ of meta­rhyolite derived from the Carnzo Mou~­tain group to the south. The Van. Horn. is a continental, post-orogenic deposit, .which is tilted and locally faulted, but is not folded thrust or metamorphosed like the rocks beneath. Its age is uncertain; it is unlike any Cambrian deposits in the region and may have been laid down before the Cambrian, in late pre-Cambrian time. On Beach Mountain, a part of the Sierra Diablo foothills, the Van Horn and older formations are overlain unconformably by a remnant of Ordovician rocks whose lowest unit, the Bliss (?) sandstone, was the first fossiliferous marine Paleozoic de­posit laid down in the region. The Ordo­vician and other older Paleozoic systems have been extensively eroded from the pre­Cambrian area as a result of deformation in late Pennsylvanian time, so that in most places the next formation above the pre­Cambrian is the Hueco limestone, of early Permian (Wolfcamp) age. In part of the Sierra Diablo foothills even this was re· moved by erosion, so that Cretaceous rocks lie directly on the pre-Cambrian. Aside from unconsolidated bolson deposits of late Tertiary and Quaternary age, the only other rocks associated with the pre-Cam­brian are lavas and small intrusives, prob­ably of early Tertiary age. The pre-Cambrian and associated rocks are broken in all exposures by normal faults of late Tertiary or later age. In the southern areas of outcrop these are of unsystematic trend, but in the Sierra Di­ablo foothills most of them trend west­northwest, as do the principal joints of the same area. A considerable body of evidence suggests that the west-northwest faults and joints of the Sierra Diablo foot­hills originated early in geologic time, perhaps as transcurrent faults during the dosing stages of the post-Hazel, pre-Van Horn orogeny, and have undergone suc­cessive dislocations during later periods. In one area in the Sierra Diablo foothills the west-northwest structures are crossed by the Hazel fracture zone, a set of en echelon, mineralized fractures of nearly east-west trend, whose age relation to the other structures is uncertain. Pump Station Hills.-A brief descrip­tion is · included of an outlying area of pre-Cambrian rocks, near Hueco Pump Station in the center of the Diablo Plateau, 55 miles northwest of Van Horn. Here, rhyolites project in low hills and are ap­parently overlain unconformably by Per­mian ·and Cretaceous rocks, although ex­posures are too poor to establish the rela­tions conclusively. General problems of pre-Cambrian rocks. -The pre-Cambrian rocks exposed in the Van Horn area are a fragment of the fundamental architecture elsewhere mostly concealed from view, on which the more familiar geologic formations of the south­western United States were laid. The rocks and structures of the pre-Cambrian of the Van Horn area have many of the char­acters of a mobile border of a cratonic area, or stable region, but the outlines of the stable region and the mobile belt which bordered it cannot be determined at this time and must await further research. The pre-Cambrian rocks of the area have been subjected to several periods of meta­morphism, with later metamorphic min­erals and structures superimposed on the earlier. The rocks of the Carrizo Mountain group have been altered by progressive regional metamorphism which increases southward from low to medium grade. In the northern exposures, however, the rocks were later subjected to retrogressive meta­morphism, perhaps related to development of the Streeruwitz overthrust. Intrusion of igneous rocks, now metarhyolite and am­phibolite, took place immediately before and during the time of retrogressive meta­morphism. Foliation resulting from the regional metamorphism is generally paral­lel to the original stratification of the rocks. In some of the rocks to the ncnth this foliation is deformed by later, or S3, metamorphic structures, which are prob­ably about contemporaneous with the retro­gressive metamorphism. Also contempo­raneous was a cataclastic alteration of the metarhyolite intrusives, which produced pronounced foliation and lineation, the latter parallel to the a fabric axis, or di­ rection of transport. Alteration of the Allamoore and Hazel formations appears to have been a simple progressive meta­ morphism during a single cycle, apparently contemporaneous with the deformation of the rocks. It dies out northward and the rocks to the north are not altered. Metamorphic rocks of medium rank, or amphibolite facies, are best developed in the Mica Mine area, where several varieties of equilibrium assemblages are present, which may be grouped mineralogically into a quartzite-muscovite schist sequence and a biotite schist-amphibolite sequence. Several varieties of amphibolites are present in the Carrizo Mountain group. Those to the northwest are altered from original sill-like dioritic intrusives, but those to the southeast are of more obscure origin and may either have originated from igneous rocks or have been altered from impure carbonate sediments. Economic geology.-The most important mineral resource of the pre-Cambrian rocks of the Van Horn area is its copper and silver deposits, which occur as veins con­taining metallic sulfide minerals. These have been prospected at many places and have been worked at seven mines, of which the most prolific is the Hazel, which has been worked intermittently since the 1880's. Total production of the district is difficult to ascertain, because of imperfection of earlier records, but it may have produced approximately 130,000 tons of ore, con­taining 2,600,000 pounds of copper and 4,000,000 ounces of silver. Ores of the district are of relatively low grade, mostly containing 21;2 to 3 percent of copper, but rich pockets have been found. Ores of the district occur in four prin­cipal associations: (1) The Hazel type of deposit, in vertical mineralized fractures such as the Hazel fracture zone, in red sandstones of the Hazel formation. (2) The Blackshaft type of deposit, in a steeply to gently dipping thin bed of crushed and sheared Allamoore formation enclosed in the Hazel formation. (3) Small, poorly productive veins in the Allamoore forma­tion and Carrizo Mountain group. (4) A single deposit (at the Dallas prospect) on a fault of .eost-Van Horn and pre-Bliss (?) age. Fractures containing the mineral de­posits are of diverse ages and origins, but mineralization itself is probably of Ter­tiary age. Estimate of reserves of the district is difficult to make as the deposits are so erratic that they cannot he predicted or projected for any distance. One estimate provides 2,100 tons of indicated ore and 18,500 tons of inferred ore, with an average grade of 21h percent copper. Various areas deserve further prospecting, especially near such proved deposits as the Hazel and Blackshaft mines. Wider ranging ex­ploration of all the fractures of Hazel type might also be desirable, because of the possibility that they may contain deposits at workable depths which do not reach the surface. Rocks of the Carrizo Mountain group in the Mica Mine area south of Van Horn contain 80 bodies of mica-and feldspar­bearing pegmatite more than a foot in diameter. Attempts have been made to pro­duce various grades of mica from the de­posits, but they are not now being worked. The deposits probably have no future as ~ source of mica as a primary product, although it could he produced as a ~y­product in the mining of feldspar, which has greater possibilities. Crushed stone from metarhyolite of the Carrizo Mountain group has been produ?ed since 1926 from one quarry and cr~shmg plant and is used as ballast on the lme of the Texas and Pacific Railroad. Minor de­posits of nitrate and turquoise occur in the pre-Cambrian rocks but a~e .~o.t now being worked and have no poss1b1hties for commercial development. One carload of talc has been produced from a new pros­pect northwest of Eagle Flat and is ~e­ing tested to determine its commercial possibilities. INTRODUCTION Philip B. King and Peter T. Flawn GEOLOGIC SETTING This publication describes the geology of pre-Cambrian rocks in their area of largest exposure in Trans-Pecos Texas, that in the vicinity of the town of Van Horn (fig. 1) . This district has sometimes been referred to as the "Van Horn dome," but it is domical only in the sense that it has tended to be positive through much of geologic time since the pre-Cambrian. Here, Paleozoic and younger deposits either were laid down on the pre-Cambrian to less thickness than in surrounding areas, or if laid down, were partly or wholly eroded before tl\e next body of sediments was de­posited over them. Actually, pre-Cambrian rocks exposed within a 20-mile radius of Van Horn come to the surface in a num­ber of adjacent but disconnected mountain uplifts-the Sierra Diablo, and the Carrizo, Wylie, Eagle, and Van Horn Mountains. To the citizen, the pre-Cambrian rocks of the Van Horn area are of more than ordinary interest, as they contain a variety of useful mineral deposits. Their copper­and silver-bearing veins have yielded the largest amounts of copper produced in the tO 10!":1° t.htr U 5 ~ICGI 5'Jtvty Geotoo"-c tkp ol Tecos Fie. I. Index map of part of Trans-Pecos Texas, showing outcrops of pre-Cambrian rocks. State, and their mica-and feldspar-bearing pegmatites have long been famous. To the geologist, the pre-Cambrian rocks of the Van Horn area are also of interest in affording a glimpse of the fundamental architecture on which the more familar geologic formations of Texas were laid, and in providing a fragment of those chap­ters of geologic history before Cambrian time that are otherwise lost to view. The pre-Cambrian rocks of the Van Horn area, unlike those of many areas, are not a monotonous expanse of metamorphic and plutonic rocks but include considerable bodies of partly altered sedimentary rocks whose depositional record, unconformities, and structural features can be worked out by familiar geologic methods. Elsewhere in Trans-Pecos Texas, out­crops of pre-Cambrian rocks are smaller and more widely scattered (fig. 1). North­west of Van Horn they emerge in a small area at Hueco Pump Station near the center of the Diablo Plateau (pp. 123-124) and in another area in the foothills at the south end of the Hueco Mountains; farther west they are boldly exposed on the east face of the Franklin Mountains north of El Paso. East and southeast of Van Horn, pre-Cam­brian rocks do not come to the surface but are deeply buried beneath great piles of younger rocks. In the west Texas Permian basin and Marathon geosyncline they are covered by Paleozoic sediments, in the Big Bend country and elsewhere by Cretaceous sediments, and in the Davis Mountains by Tertiary volcanics. PREVIOUS wORK Pre-Cambrian rocks of the Van Horn area were first studied by Von Streeru­witz during the course of his work in 'Trans-Pecos Texas for the Dumble Survey, .and prelitninary results were described in various annual reports of that Survey (1890, 1891, 1892, 1893) . Rocks collected by Von Streeruwitz and his associates were described by Osann ( 1893) , and further notes on the geology of the pre-Cambrian area were contributed by Dumble (1902) after the termination of the Survey. Pre-Cambrian rocks of the northern part of the Van Horn area were observed by Richardson during his reconnaissance of Trans-Pecos north of the Texas and Pacific Railroad (1904), and were su~sequently mapped by him in greater detail ~or the Van Horn folio (1914). Reconna1s~a~ce mapping of the southern and remammg part of the Van Horn area was com.Pleted by Baker ( 1927) in the course of his sur­vey of southwestern Trans·Pecos Texas. Besides the general accounts of pre· Cambrian rocks of the area, brief reports have appeared on their mineral depos~ts. Pe<>matites of the Van Horn Mountams we~e described by Sterrett ( 1923, pp. 303­307), Redfield (1946, pp. 29-30), and Flawn (195lb); copper and silver deposits of the Allamoore· Van Horn district were described by Evans (1943) and Sample and Gould (1945); nitrate deposits near Van Horn were described by Mansfield and Boardman (1932, pp. 66-68). PRESENT WoRK This report is a product of the U. S. Geological Survey, represented by Philip B. King, and of the Bureau of Economic Geology, represented by Peter T. Flawn. In 1949, Flawn was directed by Dr.. John T. Lonsdale, State Geologist, to undertake a study 0f pre-Cambrian rocks ·of western Texas. Mapping was begun in the southern part of the Van Horn pre-Cambrian area, and plans were made not only to complete the mapping of the Van Horn area but also to study outlying exposures of pre-Cam­brian rocks such as those in the Franklin Mountains and to investigate pre-Cambrian rocks encountered in wells drilled in west­ern Texas. Work on the Van Horn area has now been completed, and work on pre­Cambrian rocks encountered in wells is · in progress, but study of the Franklin Mountains has been postponed on account of extensive military activity in that area. Previously, between 1931and1939, King had completed mapping of the northern part of the Van Horn pre-Cambrian area as part of a study of the Sierra Diablo region for the U. S. Geological Survey. · Preliminary results of this work have been published (King, 1940; King and Knight, 1944) , but preparation of a final report was delayed as a result of diversion to other work during World War II. Such a report would, however, have lacked de­cision on a number of questions of geologic interpretation because detailed petro­ graphic work had not been done. Such petrographic work was carried out during Flawn's investigation. As the two investigations are mutually supplementary parts of a whole record of the pre-Cambrian geology of the Van Horn area, it is desirable to present them in a single publication. The prese.nt publication has therefore been prepared jointly by the two authors, responsibility for the different parts being indicated in the section head­ ings. The two authors have collaborated on the introduction and on the section on economic geology, and each author has contributed other sections on areas with which he is most familiar. Even in those sections for which a single author is re­ sponsible, he has greatly benefited from exchange of ideas with the other author, in field conferences, in critical reviews of each other's writings, and in correspond­ ence. Part of the information contributed to this publication by Flawn is taken from a dissertation submitted to Yale University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Part of this dissertation has been pub­ lished elsewhere (Flawn, 1950, 195lb). The pre-Cambrian rocks of the southern part of the Van Horn area were mapped by Flawn on a scale of 1,000 feet to the inch, using for field sheets enlargements of the U. S. Geological Survey's air photo· graphs of the region. The pegmatite-bearing pre-Cambrian rocks in the northwest Van Horn Mountains were mapped in further detail on a scale of 200 feet to the inch by means of plane table and telescopic alidade. A suite of 200 thin sections of the m11tamorphic and igneous rocks was studied under the microscope. Field methods employed by King in mapping the northern part of the Van Horn area are described in the chapter on the Sierra Diablo foothills. ACKNOWLEDGMENTS During the survey of the northern part of the Van Horn area, King was ably assisted by Mr. J. Brookes Knight, who in 1931, 1936, and 1938 made detailed studies of the stratigraphy and paleontology of the Ordovician, Permian, and Cretaceous rocks. Although these studies do not re· late directly to the subject of the present report, part of them are summarized briefly herein. In 1938, King accompanied Mr. S. J. Lasky of the Minerals Branch of the U. S. Geological Survey on an inspection of the mines and prospects of the district, and received valuable suggestions and judgments on both mineral deposits and adjacent rock formations. In the same year King also reviewed the rocks of the Carrizo Mountain group in the Sierra Diablo foot­hills in company with Mr. Earl Ingerson, at that time with the Carnegie Geophysical Laboratory, and was aided by him in their structural interpretation. Thin sections of some of the rocks collected in the area have been studied by Mr. C. S. Ross, and other determinations have been made by Messrs. K. J. Murata and Charles Milton, all of the Geochemistry and Petrology Branch of the U. S. Geological Survey. During the survey of the southern part of the Van Horn area, between 1949 and 1951, Flawn was assisted in the field by Messrs. W. E. Tipton, M. E. Dehlinger, D. L. Taylor, T. M. Anderson, and S. M. Awalt. Preparation of his report has been greatly aided by criticism and advice by Dr. John T. Lonsdale and Dr. V. E. Barnes of the Bureau of Economic Geology and Professor Adolph Knopf of Yale Univer­sity. Two independent investigations of the mineral deposits of the area have been made, one by Glen L. Evans for the Bureau of Economic Geology in 1943, and the other by R. D. Sample and E. E. Gould for the U. S. Geological Survey in 1944. Partial results of the first have been pub­lished in a brief report (Evans, 1943), and the results of the second have been released as an open-file report (Sample and Gould, 1945). Mr. Evans has kindly contributed a considerable body of unpublished infor­mation resulting from his investigation for use in the present report, and the U. S. Geological Survey has permitted maps and other data contained in the open-file report to be copied in this one. Much information concerning the min­eral deposits has been obtained by King and Flawn from local residents engaged in prospecting and mining, including Messrs. A. P. Williams, Brent Melton, and Tom Suttlemeyer. Both authors also wish to acknowledge the hospitality of numerous landowners in the area, who were unfail­ingly courte~ms in permitting surveys to be made of their properties. Geologic drafting of the maps, diagrams, and figures accompanying this publication was competently executed by Mr. James W. Macon and Mfil. Ann Connor of the carto­graphic staff of the Bureau of Economic Geology and Mr. Robert L. Traver of the Office of Geologic Cartography, U. S. Geo· logical Survey. GEOMORPHOLOGY The Van Hom area is part of the south­western arid region, with scanty rainfall, short mild winters, and relatively high temperatures during most of the year. Meteorological data are available for Van Horn since 1939, and the total annual pre­cipitation during the succeeding ten-year period is as follows:8 1939 ............ ........... . ..... .... 12.48 1940 . ····· ................ .... ... ······ .. 5.83 1941 ··································· ··········· 27.27 1942 ........................................... 8.85 1943 ······················. ····· ········ ..... 8.67 1944 .. ...... ...................... 11.40 1945 .............. .... ...................... 9.10 1946 ......... ············ ... ...... ..... .......... ..... 10.35 1947 . ...................... 7.09 1948 .............. .............. ........... . ..... .. ..... 4.98 Yearly rainfall during the period has thus varied from less than 5 inches to more than 27 inches, which is probably typical of much of the lower country in the vicin­ity. Somewhat greater precipitation in the higher Sierra Diablo and Eagle Mountains to the northwest and southwest is attested by greater density of plant cover in those areas. As elsewhere in the arid region, continu­ous sod cover is lacking in the Van Horn area, and soils are thin, poor, and cal­careous (Carter and others, 1928, pp. 7-11, 25-48). Limestones project in bold out­crops, non-calcareous rocks weather largely by granular disintegration, and there are wide areas of interior drainage. Mountains and ridges project with harsh outlines, re­sistant rocks are carved into mesas and mural escarpments where flat-lying and a U. S. Weather Bureau. CJimalological data, Te:ras section : vols. ~53, 193~1948. into jagged hogbacks where tilted. Streams, although intermittent, have scored the mountains with steep-walled canyons and have built alluvial fans along the moun· tain bases. Much of the runoff from sud­den rains is unchanneled and spreads as broad sheetfloods on the plains. Drainage of the area of pre-Cambrian rocks near V lln Horn is dominated by the Salt Basin, a long intermontane depression without outlet to the sea, whose lowest part is north of Van Horn (fig. 2; King, 1948a, Pl. 23). Drainage on the east faces of the Sierra Diablo, Carrizo, and Van Horn Mountains, and the west face of the Wylie Mountains, leads directly into the basin. Drainage of parts of the Sierra Diablo, Carrizo, Van Horn, and Eagle Mountains farther west gathers into Eagle Flat another intermontane depression, the east~rn part of which slopes into the Salt Basin through a gap which is followed by the line of the Southern Pacific Railroad. The western part of Eagle Flat is an inde­pendent drainage basin whose lowest part is at Grayton Lake, 25 miles west of Van Horn. Salt Basin and Eagle Flat are of tectonic origin. They are down-faulted segments of the crust that have been deeply filled by Tertiary and Quaternary unconsolidated deposits, washed in from the adjacent mountains. In the Salt Basin, the municipal and Texas and Pacific Railroad water wells at Van Horn have been sunk 600 feet in these deposits, and the Southern Pacific Railroad well at Lobo farther south has been carried to 1,300 feet. In Eagle Flat, the Southern Pacific Railroad well at Hot Wells is more than 1,000 feet deep and obtains from the unconsolidated deposits water which has a temperature of 90° to 100° Fahrenheit. As the Salt Basin is an area ef interior drainage, streams which drain into it are adjusted to a slowly rising base level, and along the edges of the mountains they have built up broad bajada surfaces which slope gently away from the mountains. Erosional forces seem to have been re­freshed from time to time by renewed down-faulting of the central and deeper part of the basin. On the east face of the Sierra Diablo 15 to 30 miles north of Van Horn, opposite this deeper part, steep allu­ vial fans are being built into the Salt Basin along the bases of fresh fault scarps. On the northeast foot of the Eagle Mountains, similar fans have been built into Eagle Flat, apparently over earlier bajada sur­faces, and may mark a similar tectonic renewal of that area. In general, however, erosional and depositional processes are less active in Eagle Flat than in the Salt Basin, partly perhaps because of greater tectonic stability in the recent past, and partly because of the circuitous drainage connection between this area and the main basin. The relatively stable interior drainage system of Salt Basin and Eagle Flat is being vigorously attacked by drainage leading into the lower-lying exterior drain­age system of the Rio Grande. Glenn Creek (also called Green Creek), which drains south to the Rio Grande between the Van Horn and Eagle Mountains (fig. 2) has intricately dissected the earlier basin fill into myriad spurs and intervening valleys. Scale Fie. 2. Map of Van Hom area, showing drainage and outlines of the mountain areas (after U.S. Geological Survey topographic quadrangle maps) . The head of the Glenn Creek drainage is on a low divide between the two ranges about a mile northwest of the Mica Mine pre-Cambrian area of the northwest Van Horn Mountains. The pre-Cambrian rocks of the Carrizo, Eagle, Wylie, and Van Horn Mountains have been carved into contrasting forms according to their resistance to erosion. Hard rocks, such as quartzite, pegmatite, and rhyolite, project in ridges and hog­backs, and less resistant rocks, such as mica schists, phyllites, and slates, have been worn down to valleys. The topog­raphy may, in part at least, be resurrected from that on which the Hueco limestone (Permian) was laid down, because in parts of the Wylie and Van Horn Mountains valleys filled with soft basal Hueco sedi­ments are now being re-excavated. Drain­age in the extensive area of pre-Cambrian exposure of the Carrizo Mountains flows between parallel ridges of hard rocks in a trellis pattern. In the smaller exposures of pre-Cambrian rocks elsewhere, much of the drainage probably originated as consequent streams on scarp faces of the mountain blocks and developed by headward exten­sion; part has been superimposed through a former cover of Hueco on to the pre­Cambrian beneath. The area of pre-Cambrian rocks in the Sierra Diablo foothills is crossed by drain­age which leads down from the heights of the Sierra Diablo on the north into Eagle Flat on the south and Salt Basin on the east. Drainage leading into Eagle Flat across the foothills has been maintained under conditions of prolonged still-stand and has carved broad rock-cut plains, or pediments, now for the most part thinly mantled by alluvial deposits. Much of the pediment area is underlain by red sand­stones of the Hazel formation, which to the north also form the lower slopes of the Sierra Diablo escarpment, beneath the surmounting cliffs of Hueco limestone. Pro­jecting above the pediment some miles south of the Sierra Diablo escarpment are rocky hills several hundred feet high, the Strecruwitz, Bean, and Millican Hills, that are carved from more resistant conglom­erates of the Hazel formation and from limestones and volcanics of the Allamoore formation. In places on the summits of these hills are traces of accordant levels which may mark earlier pediment surfaces (fig. 6, B). In the eastern part of the foothill area, Sulphur and Hackberry Creeks, and other streams draining into the Salt Basin, are dissecting the same pediment surface as that on which Eagle Flat drainage is now flowing. Here, red sandstones of the Hazel formation are scored to depths of a hun­dred feet or more by an intricate network of valleys and ravines. The crests of many of the intervening ridges are capped by gravel deposits (older gravel deposits, Qg, of Pl. 2), which are remnants of a once­continuous, gently sloping, alluvial cover of a pediment that is now nearly destroyed by erosion. Westward, the alluvium of the valley bottoms below rises and merges with the gravel cap above, so that the two sets of deposits cannot be differentiated in the Eagle Flat drainage area. The more vigor­ous dissection by Salt Basin drainage, as compared with Eagle Flat drainage, may have resulted from renewed downfaulting of the Salt Basin area after the pediments were cut. SUMMARY OF PRE-CAMBRIAN ROCKS Pre-Cambrian rocks are exposed in six general areas in the vicinity of Van Horn (figs. 1, 3). The largest is in the Sierra Diablo foothills, extending northward along the east side of the Sierra Diablo nearly to Victorio Peak, and westward along the south side beyond Eagle Flat section house (Pis. 2 and 3). Near the line of the Texas and Pacific Railroad this pre­Cambrian area is separated by a graben of Paleozoic and Cretaceous rocks from an­other in the Carrizo Mountains (Pl. 1). Farther south, pre-Cambrian rocks emerge in smaller areas in other mountain up­lifts-on the northeast side of the Eagle Mountains (Pl. 7), on the west side of the Wylie Mountains (Pl. 6), and in two areas in the Van Horn Mountains (Pis. 4, 5). The most highly metamorpho5ed and perhaps the oldest pre-Cambrian rocks lie to the south, in the Carrizo, Eagle, Wylie, and Van Horn Mountains, and constitute the Carrizo Mo~ntain group. This is a body of altered sedimentary rocks, including meta-arkose, metaquartzite, schist, phyllite, N CO. PACIFIC HORN I' I I I \WY;IE MTN ~REA \ \ \ \ \ I MTN AREA ' \ \ NE VAN HORN MTN AREA MICA MINE AREA SCALE 0 5 10 MILES (AFTER U.S.G.S. GEOLOGIC MAP OF TEXAS) slate, and limestone, which has been in­truded by large volumes of igneous rocks, now metarhyolite and amphibolite. Ex­tensive exposures of the group in the Carrizo Mountains show a sequence of sedimentary rocks as much as 19,000 feet thick, which does not appear to have been repeated by folding or faulting. The group shows much homogeneity in original char· acter from place to place, suggesting that it is a single sedimentary series rather than several, but it shows considerable varia· tion in degree of metamorphism. Rocks farthest south, in the Van Horn Mountains, are the most metamorphosed (medium metamorphic grade, or amphibolite facies), retain few of their original sedimentary structures, and are extensive! y veined by pegmatite. Rocks farther north, in the Car· rizo and Eagle Mountains, are less meta· FIG. 3. Map of Van Horn area, showing outcrops of Carrizo Mountain group. morphosed (low metamorphic grade, or greenschist facies) and preserve many of taeir original sedimentary structures. Their metamorphic history is complex, however, as a retrogressive cataclastic metamor· phism is superimposed on an earlier pro­gressive metamorphism. The associated in· trusive metarhyolites are also cataclasti· cally altered and in part mylonitized. The cataclastic metamorphism was perhaps pro· duced by the same forces as brought about the Streeruwitz overthrust, immediately beyond. The Streeruwitz overthrust, whose trace lies in the Sierra Diablo foothills a little north of the line of the Texas and Pacific Railroad, is a line of major discontinuity in the pre-Cambrian rocks. Along it, the dominantly metamorphic and igneous rocks of the Carrizo Mountain group have been thrust northward over the dominantly sedi· mentary Allamoore and Hazel formations. The original stratigraphic relations of the rocks of the two sides are unknown, hut presumably those on the north are younger, and they might he much younger. The Allamoore formation, the oldest unit north of the overthrust, consists of inter· bedded cherty limestones, phyllites, and volcanic rocks, the latter including pyro· elastics, flows, and perhaps shallow in· trusives. Some of the limestones contain structures that may be of algal origin. Because of the complex structure, the thick· ness of the Allamoore cannot he deter· mined, hut it is certainly thousands of feet thick. The Allamoore is succeeded by a very different, but comparably thick, deposit, the Hazel formation, whose basal part is a thick, coarse conglomerate, made up almost entirely of angular rock fragments derived from the Allamoore formation. The two formations are obviously uncon· formable and were probably separated by a time of orogeny which may still have been in progress when the earlier Hazel deposits were being laid down. The con· glomerates are interhedded with, and are succeeded by, a thick, uniform mass of fine-grained, silty red sandstone which con· stitutes the upper part of the Hazel for· mation. After deposition of the Hazel formation, both it and the Allamoore were deformed together during a time of orogeny that was probably related to emplacement of the­Carrizo Mountain group on the Streeru· witz overthrust. For some miles north of the trace of the overthrust both formations are thrown into recumbent folds and thrust sheets that were driven northward. Defor­mation dies out in this direction, and in the northernmost exposures the rocks are little disturbed. The deeply eroded edges of the deformed and partly metamorphosed Hazel and Alla· moore formations and Carrizo Mountain group are overlain unconformably by an­other and coarser red elastic deposit, the Van Horn sandstone, whose maximum thickness is about 800 feet. The Van Horn contains numerous rounded cobbles and boulders, made up not only of rocks from formations immediately beneath hut also of red granite and rhyolite porphyry, prob­ably derived from the north, and of meta­rhyolite derived from the Carrizo Mountain group to the south. The Van Horn is a continental, post-orogenic deposit, which is tilted and locally faulted hut not folded, thrust, or metamorphosed like the rocks beneath. Its age is uncertain; it is unlike any Cambrian deposits and may have been laid down before the Cambrian, in late pre· Cambrian time. The tilted and faulted Van Horn is truncated and overlain unconfonnably by the Bliss (?) sandstone, of early Ordo· vician age, the first fossiliferous marine Paleozoic deposit laid down in the Van Horn area. STRATIGRAPHIC NOMENCLATURE The stratigraphic names used for the pre­Camhrian rocks of the Van Horn area have undergone progressive modification from the time of their first study by Von Streeru­witz, as illustrated in Table 1. The name "Carrizo schist" was first used by Von Streeruwitz (1891) for the meta­morphic rocks of the Van Horn area be­cause of their typical occurrence in' the Carrizo Mountains; the same rocks were called the "Carrizo formation" by Richard· son (1914). Unfortunately, the name Car­rizo has also been used for the well-known Carrizo sand of the Eocene series in the Texas Coastal Plain, and in 1933 Sellards Table 1. Developmen.i o/ siratigraphic nomenclaiure of pre-Cambrian rocks of Yan Hom area. - i f ~ 0 a­ ..!a. -.::: ~ ~ ~ _a w ~ t-" ( 1933, p. 38), with the concurrence of the U. S. Geological Survey, changed the name of the unit in the Van Horn area to "Carrizo Mountain formation." On the geologic map of Texas (Darton, Stephen­son, and Gardner, 1937) and in subsequent publications (King, 1940; King and Knight, 1944) the unit was termed "Car­rizo Mountain schist"-the term schist be­ing applied not in a strict petrographic sense but to denote a body of foliated metamorphic rocks whose subdivisions had not been worked out. Flawn's detailed work on the Carrizo Mountain demonstrates that it is a body of metamorphic rocks of great thickness, which in individual areas can be sub­divided into a considerable variety of rock types, most of which are mappable as separate entities. Many of these entities are well-defined sedimentary units a thousand feet or more in thickness, which would be formations in their own right in a sequence less disturbed and more widely exposed. It seems inadvisable to apply formal strati­graphic names to these subdivisions, be­cause of the paucity of available geo­graphic names, the limited area of ex­posure, and the impossibility of correlating units in one area of outcrop with those of another. The sedimentary rocks of the Carrizo Mountain in all exposures were originally feldspathic and arkosic sands and interbedded argillaceous sediments. They may, therefore, have been a single sedimentary series. Nevertheless, in view of the disconnected nature of the exposures and the considerable variation in degree of metamorphism from place to place, the possibility has not been eliminate? that the unit is heterogeneous and comprises more than one series of rocks. Flawn's work indicates the inadvisabil­ity of continuing further use of either "schist" or "formation" as a designation for the Carrizo Mountain. The term schist, however useful in general reconnaissance mapping, is not applicable to detailed map­ping, where many lithologic types of other than true schists must be differentiated. A formation is, of course, merely a unit of mapping (Ashley and others, 1933, pp. 430-431) and as such may include both small and large bodies of rock ; in prac­tice, however, it is generally used for re.la­tively small, narrowly defined rock umts. The Carrizo Mountain has more the natu~e of a "terrane" as this term was used m some geologic reports of half a century ago but this term has passed out of favor. It ~!so has some of the characters of a "complex," which is a "large mass * * * * composed of diverse rocks of any cla~s or classes * * * * characterized by highly compli~ated structure" (Ashley and others, 1933, p. 445), but to most geologists the word "complex" connotes ~ greater d~gree of metamorphism, plutomsm, and m?e­cipherability than poss~ssed by th~ Carrizo Mountain. If the Carrizo Mountam could be divided into named formations that could be matched from one area of ex­posure to another, it would correspond to the formal definition of a "group" (Ash­ley and others, 1933, p. 429) .. In~ormally, "the term group may he apphed m recon­naissance work, particularly in Alaska, to assemblages of rocks that have some strati­graphic unity but that have riot yet be~n subdivided. It is to be expected that m later work, such groups will be divided into named formations" (Ashley and others, 1933, p. 438). Similar informal use of the term "group" for metamorphic and partly metamorphic units appears in many of the reports of the Canadian Geological Survey. The case of the Carrizo Mountain resem­bles these in some particulars but differs in that the unit has been mapped in detail and has been subdivided into unnamed and uncorrelated units in local areas of ex­posure. The U. S. Geological Survey, through its Committee on Geologic Names, prefers to class the unit as the "Carrizo Mountain formation" for the following reasons :4 The code of rules used by the Survey states that a group is composed of two or more forma­tions. This means that each formation of the group has been formally proposed with a state­ment of the geographic feature from which its name came, a type locality, description of its boundaries, and lithology, and an interpretation of the age and correlation. Descriptions of the Carrizo Mountain indicates that it contains several lithologic units to which no names have been applied. Because it is not intended to formalize these units at this time, it is recommended that the Carrizo Mountain be treated as a formation. In the future, if the units of the Carrizo Mountain are formalized, the Carrizo Mountain can be raised to the rank of a group. "Memorandum from J. B. Reeside, Jr., Chairman, Feb· runry 18, 1952. · The Texas Bureau of Economic Geology believes, on the other hand, that the case of the Carrizo Mountain is sufficiently am· biguous to warrant special treatment. The unit does not correspond with other "for­mations" as these are commonly differen­tiated in Texas, and although it is im­practicable to subdivide it into named di­visions, it deserves informal designation as a "group" in much the same manner as the metamorphic units in Alaska and Canada. The Carrizo Mountain is therefore treated as the "Carrizo Mountain group" in the present report, although without prejudice as to whatever usage may be adopted in subsequent reports of the U. S. Geological Survey. The present Allamoore and Hazel forma· tions were studied in reconnaissance only by Von Streeruwitz, and no complete inter­pretations of them were offered. The lime­stones of the Allamoore formation were noted in so few places that it was not realized that they constituted a distinct stratigraphic unit; they were thought to represent "Carboniferous" limestone tilted up in zones of unusual disturbance. The red sandstones of the Hazel formation were included in his t Diabolo sandstone5 (Von Streeruwitz, 1891, pp. 682-683), which he supposed might be of Devonian age, pos· sibly from analogy with the Old Red sand· stone of Europe. From descriptions of Von Streeruwitz it is evident, however, that he also included in the tDiabolo the Van Horn sandstone and thick red phases of the Powwow member of the Hueco limestone, so that the name has no value in precise work. Dumb le ( 1902) subsequent! y recognized the lithologic, stratigraphic, and structural differences between the red sandstones of the Hazel and Van Horn and applied the name Hazel sandstone to the former. Rich­ardson, in the limited time available for mapping the Van Horn folio, was unable to separate Dumble's Hazel sandstone from the conglomerate, limestone, and other rocks with which it was associated and accordingly grouped all the pre-Cambrian rocks of the Sierra Diablo foothills as the tMillican formation (1914, p. 3). 1 A dagger (t) preceding a geologic name indicates th.:it the name haa been abandoned or rejected for use in clusi· fication in publictttions of tiie U. S. Geological Survey. Subsequently, King (1940, pp. 147-148) demonstrated that the tMillican was di­visible into two well-marked stratigraphic units, separated by a major unconformity, and it was recommended that the name ·j'i\1illican be abandoned as a stratigraphic term. It could not be restricted to a part of the former unit without greatly changing its original meaning. It could be retained as a group term for the two formations, but the writer believes that this is undesirable, since the sediment& in the two formations are so distinct in their nature, and since the unconformity between them in· dicates that they were formed during unrelated times of deposition. For the lower of the two units the new name Allamoore limestone was proposed, for the village of Allamoore. "The village itself stands on alluvium, but outcrops of the limestone rise in prominent hills a few miles north." Two spellings of the name were available-"Allamoore" for the post office and "Allamore" for the railroad station; the spelling given to the post office was adopted. While it is true that lime· stones constitute a prominent part of the Allamoore, nearly half of the unit is made up of volcanic rocks and phyllite, so that in the present report it seems desirable to alter its original title to Allamoore for· ma ti on. For the higher of the two subdivisions of the tMillican formation Dumble's term Hazel sandstone was revived. This was originally applied by Dumble only to the red sandstones which are a prominent part of the unit, but these are so indissolubly linked with conglomerate of almost equal volume, that the term was extended to in­clude this also. In the present report it seems desirable to give greater recognition to the large component of conglomerate in the unit, and its designation is accordingly changed to Hazel formation. The name Van Horn sandstone has not been changed since the term was first pro· posed by Richardson (1904, p. 28), hut its definition and age designation have been emended. It was originally placed with some doubt in the Upper Cambrian, but King (1940, pp. 152-153) subsequently determined that the fossiliferous beds at the top were separated from the main body of the unit by a significant unconformity. The upper fossiliferous beds were corre­lated with the Bliss sandstone of the Frank­lin Mountains, but the age of the main body of the formation beneath was left undeci?ed as between Cambrian or pre­Cambnan. In the present report it seems ~esirable to express the stronger presump­tion that the unit is of pre-Cambrian age by classing it as "pre-Cambrian (?) ". Further discussion of the definition and age of the Van Horn will be found in the section on the Sierra Diablo foothills. PETROGRAPHIC NOMENCLATURE In describing the metamorphic rocks of the Carrizo Mountain group, Flawn has indicated the approximate mineral compo· sition of the rock in each thin section studied by means of tables of estimated modes. These are basic data for the rock units and are vulnerable only to the extent that variations may exist which are not represented in the specimens collected. If the terminology used herein should be dif­ferent from that used by the reader, he can make adjustments to his own termi­nology by referring to the tables. Although mineral compositions of the metamorphic rocks can be determined with relative ease it is much more difficult to arrive at names to be used for them, as there are considerable divergences between terminology used by different geologists. In order that names used in succeeding pages shall have precise meaning, it is desirable to indicate here the particular usage employed in this publication. The common rock terms "schist" and "gneiss" have been very loosely applied. In the Van Horn area, the term "schist" has been used to imply merely the foliated and metamorphic nature of the rocks. In other ~reas, very coarsely layered meta­morphic rocks similar to many in the Van Horn area have been called "gneiss" by some geologists (cf. Moneymaker, 1938, pp. 286-289), even though the layers of different mineral composition were ob­viously original sedimentary beds. "Schist" and "gneiss" are here inter­preted primarily as structural terms for different sorts of foliated metamorphic rocks. The fundamental structure, or folia­tion, comprises any class of mineral orien­tation, such as slaty cleavage, schistosity, or gneissosity, but may be used in context as a synonym of each of them.6 Schist is here applied to coarsely crystalline, strongly foliated, thinly fissile metamor­phic rocks, and gneiss to more massive foliated rocks with au gen and/or mineral banding of metamorphic origin. Professor Knopf (personal communication, 1952) has pointed out that in European literature the term schist has been further restricted to rocks with dominant quartz content. It so happens that most schists do contain a large quartz component, but there are rocks in which quartz is lacking that can appro­priately be called schists by every other qualification. The quartzo-feldspathic rocks which form great thicknesses of the Carrizo Mountain group in die Van Horn area cannot well be termed either schists or gneisses. Their foliation reflects parallel orientation of whatever mica is present, hut they are massive or slahhy rather than schistose, and they lack the augen and metamorphic handing characteristic of gneisses. It is,. moreover, obvious that they are altered elastic sedimentary rocks, and recognition of their original nature is de­sirable. These rocks are therefore termed "meta-arkose" where the feldspar content exceeds 25 percent and "metaquartzite" where quartz is preponderant; intermediate !YP~S ar_e termed "felaspathic metaquartz­1te. This usage follows definitions of un­altered sedimentary rocks recommended by the Committee on Sedimentation (Allen, 1936, p. 44), in which arkose is described as "a sandstone containing 25 or more percent of feldspar, usually derived from the disintegration of acid igneous rocks of granitoid texture." To metamorphosed quartzo-feldspathic rocks such alternative titles as "arkose gneiss" or "granulite" (Harker, 1939, P: 246) might he applied, hut. the~e.expre~s1ons fail to convey either their ongm, their composition, or both. Ideally, the term "meta-arkose" should apply only to those rocks which were e Adolph Knopf, personal communication 1952 • different uaacee have been employed b T' ·( Slightly 275) and Harker (1939, p. 203) Y u_rner 1948, P• "1ynonymoua uae of foliation and • •c~c:or.d1ng to Turner those mecaacopically conipicuous par 1;s1oa~t~ ~o cover all morphic origin which impart a de8nita ~ Ta r1c1 of meta• 881 in ~hicb they occur, perhaps acco:d uy to the tocb t~rmmology;n thut apparently not divor'. best. with c~rrent uon from 6saility. Harker carefull ;.m~ m~neral orient&· ecbi1to11ity (&..ility or cleava e) Y I&Un~1~he1 between orientation and segregation). g and foliation (mineral originally sedimentary arkoses, and whose reaction to metamorphism was simply re­crystallization of original components. The possibility cannot be dismissed that at least some of the rocks termed meta-arkose in the area contain feldspar which was intro­duced during or formed by metamorphism; this possibility is so difficult of proof that in practice it must be ignored. Schists described in this publication are differentiated qualitatively by prefixing one or more mineral adjectives to the noun, or rock name, the most important mineral qualifier being placed next to the noun. Thus, an albite-biotite-muscovite schist would contain a greater percentage of muscovite than biotite and a greater percentage of biotite than albite. Some writers place the qualifiers in opposite order, but as Knopf points out:7 Here one stands on an impregnable principle in English: The most important qualifier stands next to the thing qualified. In a mica-quartz schist quartz predominates over mica; mica-quartz schist is therefore an unnecessary term as it is synony­mous with mica schist [on the principle that all schists are quartzose by definition]. In a quartz· mica schist, mica predominatea over quartz. Obviously all the minerals composing the rock cannot be prefixed to the rock name without making a useless and un­wieldy term. Likewise following a rule that requires prefixing all minerals com­posing more than 5 percent of the rock to the rock name is too rigid a system to be generally applicable. The best solu­tion is to use the important or diagnostic mineral qualifiers to supplement the noun, despite the accompanying introduction of the personal element. Thus, in this report a schist composed of 4 percent sericite, 4 percent biotite, 4 percent chlorite, and the remainder quartz is termed a sericite­biotite-chlorite schist; a schist composed of 4 percent chlorite, 15 percent sericite, and the remainder quartz is termed a sericite schist. In the writer's opinion the chlorite, although an important foliate mineral in the first example, is not necessary in identi­ 'I' Adolph Knopf, penonal comm.unicatien to P. T. Flawn, February 1952. fication of the second example, although, percentagewise, it is the same in both rocks. Some geologists might prefer to call the second example a chloritic sericite schist, but in this report minerals not necessary to characterize the rock are eliminated, and complete mineral composition can be found in the tables of modes. Some authors (Billings, 1937, p. 491; Kruger, 1946, p. 167) have proposed a semi-quantitative terminology for schists based on a system of prefixes that express percentages of quartz and feldspar. Thus, a mica schist would contain less than 60 percent of quartz plus feldspar; a mica· quartz schist would contain 60 to 80 per­cent of quartz plus feldspar; and a quartz­mica schist or quartzite would contain over 80 percent of quartz plus feldspar. This system is clearly useful in classifying rocks which have been studied in thin section, and it may also be useful in the field in areas of homogeneous rock units. It is not useful in the field for rocks such as those of the Van Horn area where there were rapid variations in the arenaceous and argillaceous components of the original sediments, causing the present quartz-mica ratio to vary from place to place in one map unit, one outcrop, or even one thin section. "Amphibolite" is used in this publica­tion for an amphibole-plagioclase rock of metamorphic origin which may or may not have a schistose structure; fer the most part these rocks do not have a well-devel­oped schistosity. For strongly schistose am­phibole-rich rocks, which in this area are restricted in occurrence, the name "am­phibole schist" or "hornblende schist" is employed. These rocks are commonly rich in quartz and contain biotite as a promi­nent varietal mineral. In the discussion of amphibolites on later pages it has been necessary to consider also rocks of low amphibole and high quartz content-not because they are amphibolites by defini­tion, but because they are transitional types which it is necessary to understand before the amphibolites themselves can be in­terpreted. NORTHWEST VAN HORN MOUNTAINS (MICA MINE AREA) Peter T. F1awn SUMMARY termine thicknesses of units. The writer The Van Horn Mountains are a hlock­faulted range that rises about 10 miles south of Van Horn and extends southward to the Sierra Vieja (fig. 1). Pre-Cambrian rocks of the Carrizo Mountain group are exposed in a horst that forms a northwest­ern extension of the main mountain mass, and the area of exposed pre-Cambrian rocks is known as the Mica Mine area. The horst is bounded on the southwest and north­east by normal faults that trend approxi­mately northwest-southeast, and along both faults pre-Cambrian and Permian rocks are raised against Cretaceous sandstone. Pre­Cambrian rocks have been exposed by re­moval of the unconformable Permian cover. PRE-CAMBRIAN ROCKS General features.-The Carrizo Moun· tain group of the Mica Mine area consists of a thick sequence of meta-arkose, feld­spathic metaquartzite, and feldspathic mus· covite schist containing thin beds and lenses of biotite schist and amphibolite. Amphibolite and biotite schist comprise less than 10 percent of the section. The metasedimentary rocks are intruded by peg­matites which have been described in detail by Flawn ( 195 lb) . For the most part these rocks strike east of north and dip steeply southeast (Pl. 4). Four units were mapped in the area: ( 1) feldspathic metaquartzite and meta-arkose, (2) feldspathic musco­vite schist, (3) biotite schist and amphibo­lite, and (4) pegmatite. The massive meta­quartzite and meta-arkose form the center of the overturned southwesterly plunging anticline that is the major structure in the area. The muscovite schist occurs strati­graphically above the metaquartzite-meta­arkose section and is repeated on the north and south limbs of the fold. Biotite schist and amphibolite occur within the musco­vite schist. Pegmatites intrude both the muscovite schist and the massive meta­quartzite-meta-arkose hut are more nu­merous in the schist. Pegmatite intrusion and local folding make it difficult to de­ estimates a minimum thickness of 1,500 feet of muscovite schist and 900 feet of metaquartzite-meta-arkose. The rocks of the Mica Mine area are of medium metamorphic grade and show the highest degree of metamorphism in the Van Horn area. They fall within the am­phibolite facies of the Eskola classification. Alman dine garnet and anthophyllite · oc­cur in amphibolites in the sequence, and the quartzo-feldspathic rocks show com­plete recrystallization and complete recon­stitution of intergranular material to form oriented mica plates. No relict sedimentary structures except large-scale stratification are visible. Foliation is approximately parallel to the original bedding. F elds pathic metaquartzite and meta­arkose (pCCq) (Table 2, modes /, II, Ill, and V; Table 3, chemical andysis /.J­ Feldspathic metaquartzite and meta-arkose with varied content of biotite and musco­vite occur throughout the metasedimentary sequence and comprise 30 to 40 percent of the exposed pre-Cambrian rocks. They oc­cur as thin beds (up to 3 feet thick) within the schist areas and as massive beds (up to 30 feet thick), separated by thin layers of schist, in the quartzite areas. Compared to the associated muscovite schist, these rocks are resistant to weathering and form blocky ledges and rough steep hogbacks that can be followed through the entire exposure of pre-Cambrian rocks (Pl. 20). In general the rock is fine-to medium-grained, hard, and weathers dark brown on the outcrop. In fresh sample it is a pink or buff rock in which mica plates, magnetite grains, and sporadic larger grains of quartz and feldspar are visible. The mica content of the quartzites is varied, and all gradations between quartz­ite and mica schist are present. If the rock possesses a visible schistosity, it is here designated a schist. The mica content of the quartzites averages about 5 percent hut is locally as high as 10 percent. Where the mica content exceeds 10 percent the rock is schistose. Petrography.-In thin section the rock shows a well-developed granoblastic fabric-a mosaic .of quartz, microcline, and plagioclase, usually alb1te (twinned or untwinned, average An•-10). The average grain size of the mosaic is 0.2 to 0.5 ~m. with some grains reaching 3 to 5 mm. The mica plates show a preferred orientation, but no mineral segregation into bands, that is, develop­ment of gneissic structure, has taken place. Table 2 gives the modes of some representative meta­quartzites and meta-arkoses. Mo!>t of the feldspar occurs as an integral part of the mosaic, but some small round inclusions of feldspar are present within quartz. This may indicate a tendency to­ward development of a "sieve" or diablastic fabric. The high feldspar content of these rocks sug­gests there may have been an introduction of feldspar or a feldspathization, perhaps in con­nection with pegmatite emplacement. Further, sporadic feldspar grains show evidence of growth and are poikilitic. However, ~ost.of the !elds~ar is present in a simple mosa1~ with stra1ght·hf!e boundaries and recrystallization of feldspath1c sandstone 'or ark06e provides a satisfactory ex­planation for the origin of these ~ocks. . Mica, either biotite or muscoVIte, or both, 1s well orieonted. The biotite is green-brown and is variably altered to bleached biotite &r bauerite. The bleached biotite takes on a "sickly" faded look but retains high birefringence. Zircon, in quantity less than 1 percent, is pres­ent in all the quartzo-feldspathic rocks. It shows evidence of attrition but retains the crystal form. Magnetite or ilmenite occurs in scattered grains and in some rocks is surrounded by red iron oxide. In some rocks the opaque mineral occurs in thin plates within the biotite cleavage. This suggests that iron or iron and titanium in excees of that needed by biotite during growth was excluded -or rejected. Leucoxene is present in some thin sections, as are a few grains of apatite. Feldspathic muscovite schist (pCCms) {Table 2, mode IV; Table 3, chemical analyses II, III) .-Feldspathic muscovite schist, frequently biotitic, makes up 50 to .()0 percent of the exposed pre-Cambrian section. It forms glittering white outcrops of soft rock that crumble into micaceous sand under the hammer. The schist crops out in two irregularly shaped areas that roughly comprise the northern and south­ern thirds of the exposed pre-Cambrian rocks and are separated by a high ridge of massive metaquartzite. To the north, east, and south the schist disappears be­neath Permian and Cretaceous strata; to the west alluvial deposits mask its faulted boundary. Petrography.-Except for the higher mica con­tent and schistosity, this rock is in every way similar to the quartzo-feldspathic rocks discussed previously. The muscovite content is rarely over 30 percent and averages 15 to 25 percent. Feld­spar, magnetite or ilmenite, and zircon are present. The average grain size of the mosaic is 0:2 to 0.5 mm., with mica laths (in section) averaging 1 by 0.2mm. Table 2. Esti~tedS modes of ~eprese~tative rocks of the metaquartzite-muscovite schzs_t se­quence of the northwest Van Horn Mountains. II Ill IV v• Quartz .................. 70 50 64 60 46.2 Microcline ............ 9 37 15 10 31.4 Plagioclase ............ 15 5 15 5 9.7 Muscovite ··········-·-· 6 12.7 5 25 Biotite .......... 2 5 Magnetite or ilmenite ..... 1 1 Zircon .................... tr tr tr tr tr Totals ................100 100 100 100 100.0 •Mode determined by Ro1iwal analysis. I, Muscovitic feldspathic metaquartzite; aver· age grain size 0.1 to 0.2 mm. (Maximum grain size 4 mm.) . II, Biotitic-muscovitic meta·arkose; average grain size 0.2 to 0.4 mm. III, Biotitic meta-arkose; average grain size 0.2 mm. IV, Feldspathic muscovite schist; average grain size 0.5 mm. V, Muscovitic meta-arkose (slightly schistose); grain size 0.2 mm. (photomicrograph in PI. 30, A). Biotite schist (pCCbsa) .-Intercalated with the quartzite and muscovite schist are thin lenses and beds of biotite schist, com­monly associated with amphibolite. The contacts of these units are obscure. In some places seemingly discontinuous lenses of biotite schist or amphibolite are successive outcrops of a continuous bed. Elsewhere a number of biotite schist or amphibolite lenses occur along the same horizon; these are perhaps remnants of a once continuous layer that has been squeezed into lenses by tectonic action. Biotite schist occurs in layers that range from less than 1 foot to 25 feet thick. The maximum thickness of 25 feet of biotite schist is in the biotite 8 Modes . were determined by visual estimation, observing (a) the ahde as a whole under low-power macnification and (b) randomly selected portiona of the slide under higher power magnification. The personal error in these eatimated modes is a function of the amount of mineral preaent and is a maximum in elidea compoaed more or leH equally of three or m?re minerala. Maximum error ia probably about plus or minus 5 .percent, and error of this magnitude must be expected 10 the values for albite and potauium feldepar wher~ both make up significant portions of the rock. The ~•hmated modea are preaented to give the reader a general picture of the rocke 1tudied. Modea used in con• ncctio? with c_hemic~l analyses were determined by Rosiwal analy111 (aeter11ked 10 tables). Roaiwal modes nre given to the nearest tenth, but it is doubtful if any figures to the right of the decimal are significant within the limits of accuracy of the method. schist-amphibolite sequence in the south· ern Mica Mine area (fig. 18, b). The biotite-bearing rocks of the Mica Mine area have a varied mineralogy (Table 4) and seem to represent a ·transition he· tween the potash-rich muscovite-microcline· quartz rocks on the one hand and the hornblende-plagioclase rocks on the other. For purpose of petrographic description three major varieties of biotite-bearing rocks are distinguished: ( 1) biotite schists associated with muscovite-bearing rocks; (2) biotite schists associated with am· phibolite; and (3) altered biotite schists. Rocks of the third category are of minor importance and are derived from (1) or (2). ( 1) Biotite schists associated with musco· vite-bearing rocks: The biotite content of the normal mus· covite schist may increase locally to form a biotitic muscovite schist or a biotite-muscovite schist. This biotite is a green-brown variety that occurs with muscovite as parallel plates in a mosaic of quartz and feldspar. Rarely, biotite is the sole micaceous constit· uent (Pl. 30, B). (2) Biotite schists associated with am· phibolite: The common association of biotite schist and amphibolite in beds and lenses in the quartzo-feldspathic rock sequence has a definite bearing on the problem of origin of the amphibolites of this area. Not uncommonly an am· phibolite bed 5 or 10 feet thick is separated from muscovite schist by several feet of biotite schist or biotite· albite schist. Biotite schists associated with amphibolites fall into two groups: (a) biotite-quartz schist and (b) biotite-plagioclase rocks with or without amphibole. (a) Biotite-quartz schist in beds 6 inches to 25 feet thick forms glit· tering black outcrops. Biotite plates several millimeters in di· ameter, red-stained quartz, and white feldspar are visible in the hand specimen. Table 3. Chemical analyses 11/ quartzo·feldspathic rocks (R. M. Wheeler, analyst).' II III IV v SiO, ...... ................ ..... 78.15 77.12 78.37 81.89 75.71 79.00 AI10a 11.09 13.69 12.57 9.10 11.4 11.38 ·········-·····-··········· Fe,O. 1.41° 0.80° 1.52° 0.17 2.4 0.68° FeO ... ::.::::::::::::::: : nd nd nd 0.16 nd MgO .... .. ······•······ ···· 0.25 1.45 0.97 0.02 0.1 0.25 CaO ........... 0.42 0.37 0.35 0.64 1.6 0.80 ·······-·······-····· ·······-· Na,O ... 1.64 1.91 1.85 0.11 2.0 2.45 K,O 5.56 3.00 3.35 7.11 5.6 3.45 H.0-....... ... · •·· nd nd nd 0.21 nd 0.6 } H.0+ ............. nd nd nd 0.03 nd Ignition loss ......... 0.95 1.38 1.56 nd nd 1.08 TiO, 0.11 0.13 0.17 0.06 0.14 P,O. ...... 0.02 0.02 0.01 0.07 tr MnO -· . ·········· ··•···· 0.01 0.02 0,03 0.26 0.2 nd co nd nd nd 0.28 0.4 nd Bao ..... ···•······ nd nd nd 0.03 nd nd Totals 99.61 99.89 100.77 100.14 99.8 99.23 •Total iron reported aa Fe,9a. nd =not determined. I, Meta-arkose, Mica Mine area. II, Feldspathic biotitic muscovite schist, Mica Mine area. III, Feldspathic muscovite schist, Mica Mine area. IV, Quartz orthoclase (microcline) granulite with a little muscovite (metamorphosed arkose) , from Lewisan (Richey and Thomas, 1930, p. 8). V, Average of three analyses of arkose (Pettijohn, 1949, p. 259). VI, Represent~tive pegmatite, Mica Mine area (30 percent pink microcline perthite; 40 percent __white plag1oclase, An,; 20 percent quartz; 10 percent muscovite). 1 Tbl1 table hu been publiahed in 1lightl7 different form in an e.rlier paper on the pesmatites of the Mica Mine area {Flawa, 195lb). Petrography.-Thin section shows parallel plates of biotite in a mosaic of quartz and albite. There is a wide variation in size among the grains of the mosaic (0.5 mm. is a fair average) . The rock is composed of 40 to 60 percent quartz, 10 to 20 percent albite, and 30 to 40 percent biotite (olive· brown variety). Microcline is present in some sections. One section shows 3 per­cent garnet restricted to a narrow zone parallel to the foliation. Apatite, sphene, leucoxene, magne­tite or ilmenite, rutile, and carbonate occur in minor quantities. (b) The biotite of the biotite-plagio­clase schist reflects a potassic phase of the general amphibo­lite assemblage. Hornblende and/ or anthophyllite, epidote, and sphene may accompany biotite in this phase (Pl. 30, C and D). The biotite occurs in small flakes ( 1 mm. or less and makes up less of the total mineral assem­blage than in (a) above. A strik­ing example of the biotite-antho­phyllite-bearing rock is at F.5­18.2, Plate 4. This rock occurs in a bed 5 feet thick and contains fan-shaped groups of anthophyl­lite blades up to 10 cm. in length in a ground-mass of biotite, oligo­clase, and quartz. Petrography.-Tbin section shows a mosaic of albite-oligoclase averaging 0.5 mm. in grain size and making up 50 to 75 percent .of the rock .. 1:he plagioclase is more or less altered to sen~1te. Biotite, an olive-brown to red-brown vanety, makes up 10 to 30 percent of th~ ~ock and shows parallel orientation. The. b1olite laths average 0.2 by 1 mm. Some specimens were ob­served to contain blue-green hornble~de or .a~t~?· phyllite. The hornblende occurs m po1k1ht1c crystals or as subhedral prisms aligned with the biotite. The anthophyllite occurs in bladed or fan-shaped porphyroblasts as much as 10 to 20 ~m. in length. Minor quartz, ilmenite or magneUte, sphene, leucoxene, apatite, and carbonate are present. Platy ilmenite (associated with leu­coxene) occurs in biotite cleavages in some sections. (3) Altered biotite rocks: The biotite of the previously discussed rocks shows a varied amount of altera­tion. When the alteration is advanced a fibrous pale green mineral is visible in the hand specimen. Petrography.-Under the microscope this altera· tion is seen to involve color fading, d~elopment of more fibrous habit, and development of variable birefringence (although the birefringence remains high). The end-product of this process is bleached biotite, or bauerite, and in many rocks only relicts of the original biotite are present. Rutile and sphene appear in these altered bio­tite rocks. Amphibolite (pCCbsa) .-There are four main varieties of amphibolite in the Mica Mine area: (1) hornhlende-plagioclase Table 4. Estimated modes of representative biotite-bearing rocks from the pre-Cambrian of the northwest Van Hom Mountains. II Ill IV v Quartz............................................ 55 Biotite ..... 30 oba Plagioclase ............ .............. ...... . . ....... 12 (albite) Microcline .. ....... . . ...... .................... ....... ... . Almandine ..... . ........ ............................. 3 Zircon .................................................... tr Apatite ................................................. . Magnetite or ilmenite ....................... Sphene .................................................. ... . Hornblende ......................... . Anthophyllite ...................................... Epidote ....... ......................................... . Leucoxene ........................................... . Carbonate .. ................................... . Totals ............................................... 100 17 20 rbb 15 ( oligoclase) 48 tr tr 100 10 ob 55 ( oligoclase) 1 4 30 100 35 15 ob 20 (altered) 1 2 20 7 100 5 30rb 55 (albite) 4 2 3 1 100 •Olive-brown. bRed -brown. I, Garnetiferous albite-biotite schist; average grain size 0.4 mm. (irregular). II, Quartz-biotite-microcline schist; average grain size 0.2 mm. (photomicrograph in Pl 30 B) III, B!ot!te-anthophyllite a.mphibo.lite; average grai.n siz.e 0.5 mm., porphyroblasts 4 cm. · ' · IV, B1oute-homblende-alb1te schist; average gram size 0.2 to 0.5 mm. (photomicrograph in Pl. 30, C). V, Quartz-biotite-albite schist; average grain size 0.2 to 0.3 mm. rock, (2) almandine-hornblende-plagio­clase rock, (3) anthophyllite-hornblende­plagioclase rock, and (4) epidote-horn­blende-plagioclase rock, commonly con­taining streaks and layers of pure epidote rock or epidotite. The distribution of these rocks is shown in figure 18. The amphibo­lites are in beds that range from less than 5 to 60 feet thick. Maximum thickness of a single amphibolite layer is reached in the biotite schist-amphibolite section of the southern part of the area (fig. 18, b) . (1) The mineralogically simple horn­blende-plagioclase amphibolite is a massive to slabby green-black rock in which minute prisms of hornblende are visible. In some rocks the hornblende shows a fair linea­tion and in others it occurs in a mat of non-lineated prisms parallel to the general foliation of the area. The average length of the hornblende prisms is 1 to 3 mm., but local coarsenings are common. Horn­blende may comprise as much as 70 per­cent of the rock and plagioclase is the only other major mineral. Petrography.-Under the microscope this rock shows poikilitic hornblende prisms in a mosaic of untwinned plagioclase. The hornblende is a blue­green variety with Z/\C =16 to 18 degrees, fJ = 1.665 to 1.672, and negative optic sign. These determinations, unless otherwise stated, hold true for blue-green hornblende of all amphibolite types in the Mica Mine area. The hornblende averages 30 to 50 percent of the rock but in places is as high as 70 percent. The plagioclase (oligoclase-andesine) has the same percentage range as the hornblende (average, 30 to 50, maxi­mum, 70) . It invariably shows some degree of alteration to sericite. An inverse zoning can be seen in most sections. Quartz, if present at all, is usually in amounts less than 10 percent, but Table 5. Estimated modes of representative amphibole-bearing rocks from the pre-Cambrian of the northwest Van Horn Mountains. II III• IV v• VI• VII VIII IX x Plagioclase --40 30 35.5 50 10.6 8.7 50 20 (ande­ (oligo­ (ande­ (ande­ (oligo­ (oligo­ (badly sine) clase­ sine) sine) clase) (albite) clase) altered) ande- Hornblende ___ 52 sine) 45 38.8 --­ 7.2 tr 25 53 25 Anthophyllite _ Cummingtonite __ _ 21.6 40 8 Clinochlore 4 Biotite 20 Epidot_e __ 51.4 86.8 20 8 Almandine 15 tr 5 Sphene ___ 2.8 4.5 tr 2 Leucoxene ___ tr Magnetite or ilmenite ·-­ 5 7 3.1 4 0.6 3 20 2 tr (ilmenite) (ilmenite) Apatite 3 3 0.9 2 --­ 2 1 .... tr Quartz 27.3 25 85 25 Calcite tr Totals __ __ 100.0 100.0 99.9 100.0 99.9 100.0 100.0 100.0 100.0 100.0 •Mode determined by Roeiwal analy1i1. I, Amphibolite; average grain size 0.3 mm. (photomicrograph in Pl. 31, B). II, Almandine amphibolite; average grain size 0.1 to 0.2 mm.; porphyroblasts 5 mm. to l cm. (chemical analysis in Table 6). III, Anthophyllite amphibolite; average grain size 0.1 to 0.2 mm.; porphyroblasts 2 cm. (photomicrograph in Pl. 31, A; chemical analysis in Table 6). IV, Anthophyllite amphibolite; average grain size 0.3 mm.; porphyroblasts l cm. V, Epidote amphibolite; average grain size 0.1 to 0.2 mm. (photomicrographs in Pl. 31, D ; chemical analysis in Table 6). VI, Epidotite; average grain size 0.1 mm. (photomicrograph in PI. 31, C; chemical analysis in Table 6). VII, Epidote amphibolite; average grain size 0.1 to 0.2 mm. VIII, Magnetite-hornblende gneiss; average grain size 0.1 mm. (photomicrograph in Pl. 33, C and D). IX, Almandine cummingtonite metaquartzite; average grain size 0.3 mm. (photomicrograph in Pl. 33, B). X, Epidote-biotite-albite-hornblende schist; average grain size 0.1 to 0.2 mm. (photomicro­graph in Pl. 30, C). quartz.rich layers containing as much as 25 or 30 percent quartz occur in some thin sections. The quartz is easily distinguished from the altered plagioclas~. Magnetite or ilmenite (4 to 5 percent) and apatite (1 to 2 percent) are invariably present. The average grain size of the mosaic is 0.2 to 0.4 mm. and the hornblende prisms average 1 to 2 mm. in length. (See Table 5, I, for mode.) Several interesting deviations from the normal amphibolite occur in the biotite schist-amphibolite sequence of the southern Mica Mine area. A thin bed (less than 1 foot thick) of magnetite-hornblende gneiss occurs in association with biotite schist in this area. Megascopically the rock is a fine-grained heavy dark green slabby gneiss. Petrography.-Thin section (Table 5, mode VIII, and Pl. 33, C and D) shows a mosaic of quartz containing oriented prisms of poikilitic hornblende and layers of magnetite grains. The prismatic shape of the hornblende is not well developed, but the orientation is definite. This hornblende, a green variety with Z (\ C =18 de­grees and f3 =1.659, makes up to 40 to 60 percent of the rock and is distributed in layers. It is of special interest because it shows a well-developed 001 parting (Pl. 33, D). The magnetite occ~rs in discrete rounded grains and masses of grams concentrated in layers. Magnetite makes up 10 to 15 percent of the rock. The quartz grains show alignment of their long axes and undulatory c:x· tinction. Apatite is present in small quantity in some thin sections. Associated with the hornblende-magne· tite gneiss is another unusual rock-an almandine-cummingtonite quartzite. The rock occurs in a 2-foot layer of thin-bedded (2 to 3 inches) brown quartzite with gar· nets 1 to 2 mm. in diameter protruding above the bedding plane surfaces. Petrography.-Quartz, showing undulatory ex­tinction, forms a mosaic which makes up 85 per­cent of the rock. Skeletal garnet crystals (Pl. 33, B) occur throughout the mosaic and comprise about 5 percent. Also scattered through the quartz mosaic are small prisms of colorless cum­mingtonite showing fair lineation, Z/\C = 18 degrees, f3 =1.663, positive optic sign, 6. = 0.021, 2v,......,ao 0 ; it makes up about 8 percent of the rock. Magnetite grains make up as much as 2 percent of the slide. (2) Almandine amphibolite was ob­served in one outcrop (F.0-19.3, Pl. 4). Table 6. Chemical analyses of amphibolites and allied rocks (R. M. Wheeler, analyst). II III IV v VI Vil VIII SiO, ............._ ___ AI.Os ......__ __ ________ 60.13 12.98 42.73 23.67 45.55 12.89 61.92 14.93 41.08 18.23 48.12 12.80 47.51 16.87 48.38 12.76 Fe.Oa ----------------FeO ...... _.._________ 12.43* 4.43 5.75 13.70 5.83 7.60 0.57 13.28 0.51 1.60 3.25 4.41 7.99 8.91 4.43 MgO ··--­--·· ····-· 5.10 7.20 9.45 1.52 1.29 2.55 4.27 6.29 CaO ·­- 0.56 9.29 6.35 11.52 19.38 10.77 8.o7 7.65 Na.o 1.30 1.76 0.14 0.05 0.60 3.26 1.13 K.O ···-­------·····­--­ 5.85 0.37 0.32 0.29 0.32 3.60 3.16 1.67 H.O­·--···-­----·-­ nd nd nd nd nd 1.75 H.O+ ·--··-­--·--­--­Ignition loss ____ nd 1.44 nd 1.00 nd 0.70 nd 0.98 nd 2.48 3.20 nd 0.99 nd 6.00 nd TiO, ··-··············­ 1.34 2.56 2.72 0.83 l.89 0.78 2.65 2.07 MnO ···--········· ···· 0.22 0.39 0.27 nd nd 0.09 tr P.O. .................. 0.01 0.66 0.62 0.28 0.64 0.65 0.55 0.64 Cr20a ···-······ ·-···­ nd nd NiO .................. ·····-­ nd nd co.·-····­-·-­-----·--­ nd nd nd nd nd 9.19 nd nd so. ···················· s ........................ BaO ·---· ­-·· SrO ··········· nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 0.24 0.88 0.01 0.24 nd nd nd nd nd nd nd nd Totals -.. 100.06 99.93 100.16 100.58 99.15 100.36 100.16 100.27 •Total iron reported u FeaOs. nd = not determ.ioed. I, Gametiferous biotite schist, Mica Mine area (see Table 4 for mode). II, Almandine amphibolite, Mica Mine area (see Table 5 for mode). Ill, Anthophyllite amphibolite, Mica Mine area (see Table 5 for mode). IV, Epidote amphibolite (containing 27.3 percent quartz), Mica Mine area (see Table 5 for mode). V, Epidotite, Mica Mine area (see Table 5 for mode) . VI, Marine marlstone (Pettijohn, 1949, p. 287). VII, Dolerite (Washington, 1917, p. 476, No. 70). VIII, Basalt tufl (Washington, 1917, p. 894, No. 75). Here the normal green-black amphibolite contains porphyroblasts of garnet averag· ing between 5 mm. and 1 cm. in diameter and making up 10 to 15 percent of the rock. They weather out in perfect dodeca­hedrons. The garnet is deep red and has a specific gravity of 4.14. Petrography.-About 40 to 50 percent of the rock consists of lineated prisms of blue-green hornblende in a mosaic of plagioclase ( oligoclase­andesine). There is no distortion or bending of the hornblende prisms where they abut the garnet. Inclusions of magnetite in the garnet show a lineation at an angle to the lineation of the rock as a whole and may be the result of a crystallographic control or a rotation of the garnet. The plagioclase makes up about 30 percent of the slide and is partly altered to sericite. Magnetite or ilmenite makes up 5 to 8 percent of the rock, and apatite, in rounded grains, com­prises as much as 3 percent (Table 5). In several slides the hornblende is optically anomalous. A single prism abruptly changes from blue-green to colorless along a transverse line. This change in color is accompanied by an in­crease in Z!\C from 17 to 21 degrees and a change in optic sign from negative to positive (Pl. 32, C). The writer has no satisfactory ex­planation of this phenomenon. (3) In contact with the almandine am­phibolite is an anthophyllite amphibolite, different from the biotite-anthophyllite rock previously described. In hand speci­men the rock is massive, green-black, with bladed porphyroblasts of anthophyllite, on the order of 1 mm. wide by 15 mm. long, showing fair lineation. Petrography.-Tbin section shows a mosaic (0.1 to 0.2 mm.) of andesine containing oriented prisms of blue-green hornblende. The plagioclase and hornblende each make up about 40 percent of the slide. Large poikilitic porphyroblasts of anthophyllite constitute the remaining 20 percent of the rock. The anthophyllite blades, averaging about 1 cm. in length, show no particular orienta­tion and cut across the lineation of the horn­blende. The high {J index (1.660) of the antho­phyllite indicates that it is a high iron variety (Winchell, 1951, p. 426). The optic sign is posi­tive. (For a chemical analysis of this rock, see Table 6, analysis Ill.) A pleochroic green core was observed in one anthophyllite crystal (Pl. 32 D). Apparently this represents a period of growth of the mineral during which the coloring element (perhaps ferrous or ferric iron and alum­ina or some combination thereof) was incorpo­rated in the lattice. Magnetite or ilmenite and apatite are the important accessory minerals in this rock (Table 5, mode III). In contact with the rock just described is another anthophyllite rock which is com­posed of 50 percent andesine and 40 per· cent anthophyllite and contains no horn­blende. Tiny garnets ( 1 mm.) make up about 1 percent of this rock (Table 5, mode IV). It is interesting to note here that within the same exposure of amphibolite there are three distinct varieties: almandine am­phibolite, anthophyllite-hornblende am­phibolite, and anthophyllite amphibolite (without hornblende). The relations are obscured by colluvium, but the three rock types are apparently in contact along planes parallel to the general foliation of the area. Individual thicknesses are diffi­cult to determine here. The aggregate thickness is about 30 feet. (4) Discontinuous layers and lenses of epidote-bearing amphibolite are distributed through a more or less linear zone in the northern schist outcrop (fig. 18, a). With the appearance of a substantial epidote con­tent the normal green-black amphibolite takes on a waxy green cast. Locally thin bands of more hornblende-rich, plagio­clase-rich, or epidote-rich rock give this epidote amphibolite a layered appearance. Petrography.-The epidote amphibolite consists of a mosaic of quartz and oligoclase containing blue-green poikilitic hornblende prisms, grains and granular masses of epidote, and grains of sphene. There is some tendency for the hornblende and quartz to be concentrated in layers. The rock is composed of 10 to 30 percent well­lineated prisms of blue-green hornblende, 10 to 50 percent plagioclase, and a variable amount of epidote and quartz. The oligoclase may be twinned and, as in the other amphibolites, partly altered to sericite. Quartz, if present, is in amounts less than 10 percent, although in one section (Table 5, mode V) it reaches as high as 30 per· cent. Epidote, distinctly yellow in thin section, ranges from 10 to 70 percent, at which point the rock may be classed as an epidotite. The epidote occurs as small discrete equant grains (0.05 to 0.1 mm.) and groups of grains in a mosaic and clearly originated through metamorphism rather than hydrothermal alteration. In one section masses of epidote surround plagioclase (Pl. 33, A) and seem to have grown at the expense of the plagioclase. Sphene makes up 1 to 3 percent of the rock. One section showed aggregates of sphene surrounding ilmenite and flooded with leucoxene (Pl. 32, A and B). Ilmenite is a com­mon accessory and in some samples makes up as much as 5 percent of the total composition. The modes and photomicrographs of epidote amphib­olites are given in Table 5, modes V and VII, and Plate 31, D, and Plate 32, A and B). Yellow-green streaks show up in the darker hornblende-rich rocks, and these streaks develop locally into layers of waxy yellow-green rock consisting almost en­tirely of epidote. These layers are half an inch to 2 inches thick and commonly discontinuous. The rock conforms to the description of an epidotite (Flawn, 195la). Petrography.-The epidoi:ites of the Mica Mine are~ contain 80 to 90 percent granular epidote in grams 0.04 to 0.06 mm., in diameter and 8 to 10 percent albite or albite-oligoclase, commonly al­tered. Sphene makes up 2 to 4 percent of the rock. Dark green hornblende is present in amounts up to 5 percent. The fabric is crystalloblastic (Table 5, mode VI, and Pl. 31, C) . It is interesting to note that in the Mica Mine area, rocks containing a large amount of epidote have a more sodic plagioclase than associated rocks with low epidote con­tent. A discussion of the mineralogy and origin . of the amphibolites in the pre­Cambnan rocks of the Van Horn area is given in a separate section (pp. 136-147). Pegmatite (Table 3, analysis Vl}.-The writer has described the pegmatites of the Van Horn Mountains in detail in an earlier paper (Flawn, 195lb). Zoned and unzoned perthite-quartz-plagioclase-muscovite peg· matites in the form of tabular bodies irregular-branching tabular bodies, elon'. gate lenses, irregular masses without defi­nite shape, lit-par-lit zones, small augen, and stringers are distributed throughout the. m~tasedimentary sequence. The great ma1onty of the pegmatites are tabular or lens-like bodies that conform to the folia­tion of the host rock. Zoned bodies have a core of perthite and quartz and a plagio­clase-quartz-perthite-muscovite wall zone. The pegmatites contain numerous schist in­clusions, and some show evidence of con­t~mination by biotite schist and amphibo­hte. About 80 pegmatites more than 1 foot thick were mapped. The field, petrographic, and chemical evidence indicates that these pegmatites were emplaced in an essentially closed system, in part by forcing aside the ~ost rock (dilation) and in part by diges­tion of the host rock (see Flawn, 195lb). Qu':'r~z vei11;5.-Veins of white quartz contammg variable amounts of biotite and hematite occur within the Mica Mine area. They are mostly restricted to a north-south ~on~ e~st of Mica Mine No. 1 (Pl. 4), md1catmg an old locus of fissuring, and seldom exceed a thickness of 2 to 4 feet. The location of minor quartz veins is in places controlled by the hanging wall or footwall of pegmatites, and the veins are seemingly later than the pegmatites. The veins probably are the product of high temperature hydrothermal solutions carrying iron and silica. The reaction of these iron-bearing solutions with the potash and alumina of the wall rocks formed bio· tite, commonly in coarse sheaf-like masses. Locally coarse leaves of biotite have been wrapped around quartz augen. These dis­continuous and distorted masses of quartz and biotite conform to the structure of the metamorphic rocks and may be interpreted as the product of pre-metamorphic or syn­metamorphic hydrothermal solutions. Plates of barite up to 2 inches wide occur in the west fault of the Mica Mine horst and are well shown where the fault is exposed in the quarried face of a pegma­tite along the road about 1 mile southeast of the mill. Barile was not observed to occur elsewhere in the area. The age of the barite mineralization is probably Ter· tiary (younger than the fault zone in which it occurs). PERMIAN RocKs (Php AND Ph) Permian rocks rest on the pre-Cambrian rocks of the Mica Mine area with marked angular unconformity. On the basis of lithol?gic character and megafauna, the Permian section of the Mica Mine area is assigned to the Hueco limestone and corre­lated with the Wolfcamp series. Two litho­logic units can be distinguished within the Hu~o limestone in this area: a conglom­erahc sandstone that will hereafter. be called the Powwow member by analogy with the basal Permian strata of that name ~n the Hueco Mountains10 and an overly­mg compact aphanitic gray cherty lime· stone. ~he Powwow member is a transgressive fac1es extreme! y varied in thickness. The thickness is controlled by the topography of. the ~re-Permian surface. On pre-Cam­brian hills the succeeding limestone mem· her rests directly on the pre-Cambrian surface, while in low places on the old s~rface as much as 250 feet of conglomer· ahc sandstone and conglomerate inter­venes. The Powwow is a fine t -o coarse­ 10 DiacuHion of use of the n . Horn area i1 given in the sect ' ame Powwow in the Van hills (p. 98). ion on the Sierra Diablo foot· grained red and brown micaceous felds­pathic sandstone containing sporadic peb­bles of quartz, feldspar and pegmatite. In the southern part of the Mica Mine area, 20 to 30 feet of boulder conglomerate is present at the base of the Powwow. Boul­ders of schist, quartzite, and pegmatite reach diameters of 3 to 4 feet. The Pow­wow member grades into the overlying limestone through a 20 to 30-foot zone of interbedded sandstone and silty limestone. Conformably overlying the conglom­eratic sandstone is a compact aphanitic gray cherty limestone in beds 6 inches to -;;; -~ ., "' 0. E 0 ~ 0 3: z ::! ~ a: w Cl.. ., c: 0 -.;; ., E 0 (.)., :::> I Compact, ophonitic, gray, cherty limestone. Contains brown chert in nodules, stringers, ond irregular mosses; crystalline calcite in veinlets and lining cavities; and mongonese dioxide in veinlets ond lining cavities . Beds I to 3 feel thick. Strong fetid smell on breaking. lntercolated brown conglamerotic sandstone and red and brown silty limestone . Beds less !hon I foot thick. Red micaceous conglomeratic sandstone containing pebbles of quartz, perth ite, and pegmatite. Locally bleached. Boulder conglomerate . Angular bould.ers of schist and peg mo ti te up to 3 feet in diome !er in o matrix of red micoceous sandstone. UNCONFORMITY Metosedimentary rocks. Fie. 4. Columnar section, showing representative sequence of basal Permian rocks (Powwow member of Hueco limestone, and overlying beds) in Mica Mine area, northwest Van Hom Mountains. 6 feet thick which forms bold cliffs around the exposed pre-Cambrian rocks. The lime­sto~e contains abundant brown chert in strmger.s, nodu~es, and irregular masses. Crystallme calcite occurs in veinlets and c~vit.y linings, and nodules of manganese d10~1de are common in cavities. Silicified echmoid spines and plates are numerous throughout the section. Baker (1927, p. 10) refers to this limestone as "the lower great limestone member of the Eagle, Van Horn, and Wiley mountains" and states that its "full thickness is perhaps about 1000 feet." However, in the immediate vicinity of the Mica Mine area the thick­ness of the limestone does not exceed 250 feet. A section measured east from a point located by coordinates K.6-9.3 Plate 4 . ' ' is representative of the Permian rocks in the Mica Mine area (fig. 4). CRETACEOUS ROCKS (Kcx) In the Mica Mine area Cretaceous rocks are for the most part in contact with older rocks only along normal faults. Permian Hueco limestone has been raised against Cretaceous Cox sandstone along many nor­mal faults, and pre-Cambrian rocks have been raised against Cretaceous rocks along parts of the two main normal faults that form the horst. The oldest Cretaceous for­mation in the area is the Cox sandstone ~hose upper part is probably of late Trin­ity age. The rock is a medium-to coarse­~r~ined sandstone; well indurated (locally ~t is an orthoquartzite) and well sorted. It is composed mostly of quartz with less than 5 percent altered feldspar and less than 1 percent magnetite. The grains are sub-round. Bands of chert pebbles and cross-bedding are common. Thin beds of s.ilty limestone and of conglomerate with limestone and chert pebbles occur within the sandstone, and masses of oyster shells occur locally in the limestone beds. Just east of the Mica Mine area the Cox sand­stone is cut by a series of closely spaced n<;>rmal !aults and is present as a jumble of shckens1ded blocks. A complete section of Cox sandstone is not present in the Mica Mine area, and therefore it is difficult to estimate the thickness of the formation in this area. Baker (1927, p. 12) states that there is 1,500 feet of Cox sandstone in the eastern Van Horn Mountains, a distance of about 5 miles from the Mica Mine area. The Cox sandstone is overlain by Finlay lime­stone (Fredericksburg) east of the Mica Mine area. TERTIARY IGNEOUS RocKs Trachyte and analcite-bearing diabase dikes ranging from 1 to 4 feet thick occur in the Mica Mine area. From the field evi­dence these intrusives cannot be assigned a definite age. The trachytes cut Permian rocks. On the basis of similarity to the rocks described by Lonsdale (1940) and Baker (1927) , these dikes are classed as Tertiary (?). Tertiary volcanic rocks, tuffs, and flows occur in the Van Horn Moun­tains east of the Mica Mine. STRUCTURE Pre-Cambrian structures.-The largest structural feature in the pre-Cambrian rocks of the area is an overturned anti­cline whose axis strikes northeast-south­west and plunges southwest (G.0-14.0 to K.0-15.5, Pl. 4). Both limbs dip 35° to 45° southeast. This structure conforms to the regional northeastward "grain" of the pre-Cambrian rocks. It may be that the amphibolite belts in the north and south parts of the Mica Mine area represent the sa~e beds repeated on opposite flanks of this fold; however, there are significant mineralogical differences between the am­phibolites on the north and south. Small open folds (amplitudes of less than 30 feet) are present but not common. Severe conto~ion is present locally but is not a promment feature. The more micaceous rocks commonly show a small-scale fold­i~g or !uc~ing, with the amplitude of the tmy phcations less than 1 cm. This ruck­ing is an S3 structure, because it deforms pre-existing bedding ( S1 ) and bedding plane-foliatio~ (S2) (cf. Turner, 1948, pp. 177-179). D1storhon of foliation is also foun~ in the vicinity of pegmatite contacts and is probably related to pegmatite em­placement. You~ger structures.-The Mica Mine horst is ~ounded by two parallel north­ west-trendmg normal faults about three­ fo?rths ?f a mile apart. Southeast of the Mica Mme area the east fault d T tiary volcanic rocks aaainst F' rlopsl' er­ o may 1me­ stone (Fredericksburg), and the fault is therefore younger than the volcanic rocks. In the immediate vicinity of the Mica Mine area Cox sandstone (Trinity) has been thrown down against Hueco limestone (Wolfcamp) and pre-Cambrian rocks. Per­mian rocks in the horst have been raised high above the surrounding Cretaceous rocks, and at the northern limit of the pre­Cambrian exposure the highest point on the Permian-pre-Cambrian contact (eleva­tion 4,838 feet) is about 350 feet above the Cretaceous sandstone against the fault about 1,000 feet to the east (elevation 4,481 feet). To this figure must be added the thickness of the entire Permian section (a minimum of 250 feet) and an unknown thickness of Cretaceous rocks. Displace· ment on the faults bounding the horst, therefore, exceeds 700 feet and is prob­ably in the neighborhood of 1,000 feet. At the north end of the horst the structure is terminated by a north-south fault that in­tersects both northwest-trending faults, and those faults change direction to a more northerly trend. The down-faulted Cre­taceous rocks north, east, and south of the horst are broken by a number of normal faults of small displacement. NORTHEAST VAN HORN MOUNTAINS Peter T. Flawn SUMMARY brown) . Quartz ranges from 40 to 55 percent and The northeast Van Hdrn Mountains rise in a line of rugged hills about 10 miles south of Van Horn and about 1 mile due west of Lobo. This area is about 5 miles northeast of the Mica Mine area previously described. This part of the Van Horn Mountains is a block-faulted complex of pre-Cambrian, Permian, Cretaceous, and Tertiary rocks. Pre-Cambrian rocks of the Carrizo Mountain group are exposed, mostly in deep valleys, beneath uncon­formably overlying Permian rocks on up­thrown fault blocks. This area was mapped by D. L. Taylor in 1950. PRE-CAMBRIAN ROCKS Two lithologic units, meta-arkose and pegniatite, were mapped in the Carrizo Mountain group of the northeastern Van Horn Mountains. These crop out in a num­ber of discontinuous exposures along faults, where the cover of Hueco limestone has been stripped back (Pl. 5). Foliation is more or less parallel to bedding in the few places where bedding can be seen. In gen­eral, the rocks strike east of north and dip south, but in the westernmost exposure (Pl. 5) the strike is varied, probably as a result of local folding. Metamorphic grade probably corresponds to that in the Mica Mine area. Meta-arkose (p£Cma) (Table 7, modes I, II).-Meta-arkose is the only metasedi­mentary unit exposed in the northeastern Van Horn Mountains and is similar to that found in the northwest Van Horn Moun­tains and the Wylie Mountains to the south­west and northeast. It is a massive fine­to coarse-grained brown to buff rock with sporadic grains of quartz and feldspar reaching 4 to 5 mm. in diameter. Locally the rock is slightly schistose, but foliation has not developed to a point where the rock may be termed a gneiss. Thin beds of bio­tite schist are included within the unit. Petrography.-Under the microscope the rock shows a poorly sized mosaic of quartz and micro­cline. The mosaic has straight-line boundaries, and all the original interstitial material has recrystallized to form oriented plates of musco­vite and hiotite (green-brown to dark olive­microcline ranges from 35 to 50 percent of the rock. Small amounts of albite, magnetite or ilmen­ite, zircon, and apatite are also present. Grain size ranges from 0.05 to 2 mm., and the fabric is granoblastic. This rock is a recrystallized arkose. A thin layer of amphibolite, too small to map, crops out at B.0--3.6, Plate 5. Whether this rock is of sedimentary or igneous origin has not been determined. No trace of an igneous fabric remains, if one was ever present, and field relations are obscure. The quartz content is appre­ciable (at least 10 percent). Petrography.-This rock contains oriented laths of dark brown biotite and blue-green hornblende (Z!\C =17°, {J =1.689 ± .002, negative optic sign) in a mosaic of altered plagioclase (kaolin­ized ) and quartz. Small grains of ~ phene occur in masses and strings. Apatite, magnetite or ilmenite, and chlorite are present (Table 7, mode IV). Pegmatite.-Two pegmatites intrude the meta-arkose in this area. These bodies are conformable elongate lenses and are in all respects similar to the perthite-quartz­plagioclase-muscovite pegmatites in the Mica Mine area of the northwest Van Horn Mountains. No internal zoning was distin­guished. Several small quartz-tourmaline masses are present in the area. Table 7. Estimated modes of pre-Cambrian rocks of the northeast Van Horn Mountains. II III Quartz ···­·-·-----·-···· 55 Microcline .......... 36 43 50 10 Plagioclase ··-·-·---· 3 48 ( oligoclase) Muscovite 5 5 Magnetite or ilmenite ............ 1 tr 1 Zircon ···­·············· tr tr Biotite ----­--------­--·­ tr 2 5 Apatite Chlorite ---------·-····· ···­----·· ····--­--· .... 1 3 Hornblende -·-··-·· .... 30 Sphene ······· ·-·····-· ··-· 2 Epidote ---­···­···----­---­ Totals ........... 100 100 100 I, Muscovitic meta-arkose; grain size 0.04 to 2 mm. II, Muscovitic meta-arkose; grain size 0.05 to 0.5mm. III, Amphibolite; grain size 0.1 to 0.2 mm. The University of Texas Publication No. 5301 YOUNGER ROCKS The Permian and Cretaceous rocks in the northeast Van Horn Mountains are similar to those in the northwest Van Horn Mountains described in the preceding sec­tion. In addition there are tuffs and flows of the Tertiary volcanic sequence, the details of which were not worked out by Taylor. STRUCTURE Pre-Cambrian rocks in this area main­tain, for the most part, their regional north­east strike and southeast dip. Foliation is approximately parallel to the original stratification, althoug}i this stratification is evident only where schistose beds occur within the massive rocks. Three north-south faults, downthrown to the east, have thrown Permian against pre­Cambrian, and Tertiary against Permian and pre-Cambrian (Pl. 5). There is also an east-west structural trend marked by two faults of "dog-leg" plan downthrown to the north; these have thrown Tertiary and Cretaceous rocks against Permian and pre­Cambrian rocks with displacements on the order of hundreds of feet. Detailed map­ping was restricted to the area of exposed pre-Cambrian rocks, and no attempt is made to present a broader structural anal­ysis of this very complex area. WYLIE MOUNTAINS Peter T. Flawn SUMMARY The west scarp of the Wylie Mountains rises from the basin fill 4 miles southeast of the town of Van Horn. Pre-Cambrian rocks are exposed in a series of spurs along the base of the scarp on the upthrown side of a fault (Pl. 6). Pre-Cambrian rocks of the Carrizo Mountain group and uncon­formahly overlying Hueco limestone have been raised along two intersecting faults, one striking north-northwest along the west scarp of the range and the other striking east-west along the southern limit of the range. PRE·CAMBRIAN RocKs General features.-Three rock units can be distinguished in the Carrizo Mountain group of the Wylie Mountains, namely {in order of decreasing age) mica schist, meta­ arkose, and amphibolite. They are exposed in a long, narrow arcuate outcrop along the west and south scarp of the mountains, between the faults and the unconformahly overlying Hueco limestone. ,The mica schist has a small outcrop and occurs strati­ graphically beneath the meta·arkose which makes up the bulk of the pre·Camhrian ex­ posure. Amphiholite occurs in small masses that intrude11 the schist and meta-arkose. Schistosity is restricted to the thin outcrop of mica schist and to thin layers within the massive meta-arkose. Where present, it is approximately parallel to the bedding. Metamorphism in the Carrizo Mountain group of the Wylie Mountains is probably of slightly lower grade than in the Mica Mine area, although there is no precise mineralogical indication of the metamor­phic grade. There are indications of incip­ient retrogressive metamorphism in this area. The over-all grain size of the rocks is smaller than that in the Mica Mine area, and the meta-arkose shows cataclastic ef­fects not observed in similar rocks south­westward along the strike in the Van Horn Mountains. In the mica schist of the Wylie U Amphibolites of igneous origin are metamo)phic rocks and were not intruded in their pre1ent form. However. for brevity these meta·igneou1 rocks will be referred to as intrueiTe to avoid repeated referencee to "the original igneous rock." Mountains chlorite has formed at the ex­pense of hiotite. The amphiholite has a re­lict igneous fabric and is similar to that of the Carrizo Mountains rather than that of the Mica Mine area. Biotite -muscovite schist (pCCbms) (Table 8, mode !).-Mica schist in the Wylie Mountains crops out only in valleys at the western base of the scarp, and even there it is largely concealed by uncon­solidated basin fill. Exposed thickness does not exceed 30 feet. The rock is a fine­grained light·colored glittering schist with the mica unevenly distributed in layers. Mica plates reach 1 mm. in diameter. The mica schist grades into the overlying meta­arkose through a zone of schistose, mica· ceous meta-arkose. Petrography.-Under the microscope the x:ock shows a mosaic of quartz containing oriented plates of muscovite, biotite (olive-brown to green­brown varieties), and chlorite (clinochlore). About 85 percent of the rock is quartz. The chlo­rite has formed at the expense of biotite and the muscovite is partly altered to sericite. Untwinned feldspar, sphene, apatite, rutile, leucoxene, and zircon are present in minor quantities. The grain size of the quartz mosaic ranges from 0.02 to 0.20 mm., and the mica laths average 0.05 by 0.2 mm. The fabric is lepidobla&tic. Layers of varied i::rain size and mica content are interpreted as relict bedding. Meta-arkose {pCCma) (Table 8, mode /J).-Meta-arkose forms projecting spurs beneath the scarp of Hueco limestone and makes up over 90 percent of the pre·Cam­brian rocks exposed; in most of the area it is the only unit present. The rock is medium to coarse grained; buff, brown, or red; schistose, slahhy, thin bedded, or massive, and is plainly bedded. The rock is more massive toward the top of the exposed sec· tion. Schistosity, where developed, is par­allel to the bedding within the unit and to the foliation of the underlying schist. Within the meta-arkose, concentrations of biotite are common along bedding planes and result in thin layers of dark glittering hiotite schist, usually less than 6 inches thick. Thin layers of amphibolite occur conformahly within the meta-arkose sec­tion. Petrography.-In thin section the rock is a mosaic of quartz and feldspar that shows a wide range in size of constituent grains (0.01 to 0.25 mm.) and contnins sporadic grains of feld· spar, quartz, and fragments of larger-grained quartz mosaics up to 2 mm. in diameter. In the general mosaic the feldspar is mostly microcline with minor plagioclase. The sporadic large feld­spar grains are microcline-microperthite and al­bite. The rock is composed of about 50 perecent feldspar, 45 percent quartz and less than 5 percent of biotite (green-brown variety, partly bleached), muscovite, ilmenite or magnetite, leucoxene, and zircon. Table 8. Estimated modes of representative rocks from the pre-Cambrian of the Wylie Moun­tains. u• III IV v Quartz -­ -­ 85 45.0 37 15 Microcline Plagioclase -­ 5 } 50.5 52 25 Muscovite 2 Biotite ----·------· 4 4.5 3 30 4 Chlinoclore -· 3 1 tr Sphene --­Apatite --­ tr tr 2 2 tr 1 l Zircon ..... tr tr Ru tile tr tr Leucoxene tr tr Magnetite or ilmenite tr 1 1 tr Hornblende -­--­ 30 50 Epidote 10 30 4 Totals _____ _100 100 100 100 100 •Mode determined by Ro1iwal analy1ia. I, Muscovite-chlorite-biotite schist; average grain size 0.02 to 0.2 mm.; chlorite laths up to 0.1 by 4 mm. II, Biotitic meta-arkose; grain size 0.01 to l mm. III, Epidote amphibolite; grain size 0.01 to 2 mm. IV, Biotite-epidote-quartz rock; grain size to 2 mm. V, Sc.histose amphibolite; grain size 0.01 to l mm. This rock superficially resembles the metarhyolite of the Carrizo Mountains to the west and may be confused with sheared rhyolite even in thin section, but it is here interpreted as a meta-arkose for the follow­ing reasons: ( 1) It shows bedding, and round grains of probable detrital origin can be seen in the hand specimen. (2) In thin section it shows a wide and virtually continuous range of grain size. In the metarhyolite of the Carrizo Moun­tains there is a sharp break between the fine-grained, sheared ground-mass and the broken and sheared phenocrysts. (3) It is similar in appearance a~d mineral composition to meta-arkoses m the northeast and northwest Van Horn Mountains and is on strike with them. The latter are metamorphosed elastic sediments which range from meta-arkose to feld­spathic metaquartzite. ( 4) There are no indications that strong shearing stresses have affected the ro.ck. The rhyolite is characterized by rotation and crushing of the phenocrysts, conversion of microcline to sericite, banding and line­ation. These are not criteria of origin, but they serve to emphasize the present dif­ferences between the two rocks in structure and texture. Amphibolite (p~Ca) (Table 8, modes III, V).-Four mappable bodies of mas­sive fine-to coarse-grained dark green­black rock intrude the schist and meta­arkose of the Carrizo Mountain group in the Wylie Mountains. This amphibolite occurs in four small bodies that, as ex­posed, have a roughly equarit shape (Pl. 6) . The bodies are probably short, thick lenses and apparently cut across the bedding­foliation structure in the meta-arkose which they intrude. The rock is generally massive with non-oriented hornblende prisms visible to the naked eye; locally, in particular around the margins of the bodies, a poor foliation has developed. A number of thin sills (generally less than 2 feet thick) of schistose green-black rock (too small to map) intrude the meta-arkose along bed­ding planes. Petrography.-A thin section of the mas~i"'' rock shows a mosaic of andesine containinJ! matted masses of blue-green hornblende. The plagioclase is cloudy with incipent sericite and i> characterized by the indistinct zoning common in the plagioclase of metamorphic rocks. Sporadic large grains of plagioclase show a ghostly poly­synthetic twinning that is interpreted as relict from an original igneous faLric. Andesine makes up about 55 percent of the rock. Masses of small non-lineated prisms of blue-green hornblende (Z/\C =20 to 21°, f3 =1.664 ± .002, negath·e optic sign) constitute aLout 35 percent of the rock and have a shredded or matted appearance. Sporadic larger poikilit.ic grains of hornblende with good clea~age and a yellow-green to grass­gr~n pleochro1sm are prominent. These larger grams of hornblende have a shredded periphery and may also be interpreted as relict from an earlier igneous fa~ric. Epidote occurs in grains and groups of grams throughout. Biotite (yellow­brown to dark. olive-brown), ilmenite, sphene, le~coxene, a\>~llte, and chlorite are present in mmor quantities. The grain size of this rock ran11:es from 0.01 to 2 mm., plap;ioclase and epi· dote constituting the finer grains. The chlorite, a clinochlore, occurs in plates and sheaves and was formed at the expense of the hornblende. The fabric is dominantly crystalloblastic but shows minor cataclastic effects (crushing, shredding). The rock is an epidote amphibolite that origi­nated through metamorphism of a basic igneous rock, probably of the diorite or gabbro family. A dark green fine-grained schistose amphib­olite from a 2-foot sill in the meta·arkose was examined under the microscope. The section shows a fine·grained mosaic (0.03 to 0.05 mm.) of andesine (25 percent) and quartz (15 percent) containing masses of small prisms of blue.green hornblende unevenly distributed in layers. Al­though the hornblende in this rock possesses a fair lineation, it also exhibits the shredded ap­pearance noted in the massive amphibolite. Spo· radic larger hornblende crystals showing shredded borders probably are relicts from a previous igneous fabric. Hornblende makes up about 50 percent of this rock. Dark brown biotite, epidote, apatite, ilmenite, and sphene are present in minor quantities. The fabric is mainly crystalloblastic but crushing and shredding of grains indicates subordinate cataclasis. Quartz veins.-Quartz-tourmaline veins and masses of quartz-tourmaline are com­mon throughout the pre-Cambrian of the Wylie Mountains. The tourmaline, a bril­liant black variety with E =pale pink­brown and 0 = deep gray-black, occurs in veinlets and masses of fine needles within the quartz. ,Tourmaline needles also are found on bedding plane and joint surfaces. Masses of quartz-tourmaline reach 4 feet in diameter. Smaller stringers without tourmaline are also present and commonly are contorted. Biotite has formed along the contact between the quartz veins and the country rock. This probably results from reaction between the potash and alumina of the country rock and the iron and silica in the vein.forming solutions. PERMIAN ROCKS (Php AND Ph) Hueco limestone composes a large part of the Wylie Mountains and forms a steep cliff on the western face of the mountains. As in the Van Horn Mountains, the basal Powwow member and an overlying gray cherty limestone are present. The Powwow member (Php) in the Wylie Mountains is a medium-to coarse­grained soft red feldspathic sandstone, mostly conglomeratic, which probably rep· resents a reworked regolith. Thickness, con­trolled by the pre-Hueco topography, is varied. An old pre-Hueco valley is well shown on the map (B.9-7.8, Pl. 6) by the wedge of this elastic rock that occupies it. The sandstone grades upward into the overlying limestone through a zone of inter­bedded red to brown sandstone and brown silty limestone. Baker (1927, map) mapped the Powwow member of the area as Van Horn sandstone, which it closely resembles. The limestone (Ph) overlying the Pow­wow member is a compact aphanitic gray limestone containing stringers, nodules, and irregular masses of brown chert; crys­talline calcite in veinlets and as lining of cavities, manganese dioxide in veinlets and lining cavities. Echinoid spines and plates are common. Freshly broken lime­stone is dark gray to black with a strong fetid odor. It occurs in beds 6 inches to 6 feet thick, forms steep cliffs, and is in all respects similar to the limestone in the northwest Van Horn Mountains (pp. 34­36). Two University of Texas master's degree candidates, E. F. McGee and H. Hay-Roe, mapped in the Wylie Mountains during the summer of 1951 and were kind enough to permit th~ writer to make advance ~se of some of their information. Accordmg to their measurements the Powwow mem­ber of the Hueco limestone ranges from 15 to 150 feet in thickness and averages about 60 feet in this area. They divided the overlying limestone into two members: (1) a lower gray-brown limestone, 660 feet thick, and (2) a black limestone, 80 feet thick. Aggregate thickness of the Hueco limestone in the western Wylie Mountains is 736 feet plus or minus 30 feet. A water well (Harper Mountain well) drilled about 1 mile east of the northern limit of the pre-Cambrian exposure encountered meta­arkose at 655 feet. TERTIARY RocKS (Tv) Tertiary volcanic rocks (not 2700 p£Cq. Metaquartzite and meta-arkose: in direct contact with p£Cm,; partly ~ min. concealed by Paleozoic rocks and bolson deposits; forms light-gray hills"'z p£Cm, Mixed unit : directly overlies p€Cq,; intruded by metarhyolite, am­ ..... 400± < phibolite, granodiorite, and Tertiary diabase, in part concealed bv z ~ Paleozoic rocks and bolson deposits :::> 300± p£Cq, Metaquartzite and meta-arkose: in part directly overlying p£Cm1, in 0 part separated from it by amphibolite intrusion; forms ridges to east, ::g becomes more micaceous and less resistant to west 0 N 2000± p£Cm, Mixed unit: separated from p£Cm, by metarhyolite sill ; intruded by ~ thin amphibolite sills - ~ 500± p£Cm. Mixed unit: intimately penetrated and sloped by amphibolite intrusion; < loses identity as a unit to west; lies between metarhyolite and am·u phibolite intrusions 600± p£Cp Phyllite: discontinuous outcrop between p£Cq, and amphibolite in­trusion; intimately penetrated and stoped by amphibolite 1700± p€Cq, M~ta-arkose: _in part _directly overlying. p€~m1, in part separated from it by amph1bohte; mtruded by amph1bohte; forms prominent ridge 1700± p€Cm, Mixed unit: directly overlies p€Cq1; intruded by thin amphibolite sills: forms broad lowland · 1800± p€Cq2 Meta-arkose and metaquartzite: intruded by amphibolite along lower contact; forms prominent white ridge -?-?-?-?-?-?-? 3700± p£Ccms Chlorite-mica schist: surrounded by intrusive amphibolite and metarhy­ max. olite; stratigraphic relations uncertain -?-?-?-?-?-?-? ? p€Cq, Meta-arkose: occurs mostly as roof pendants in a large amphibolite in­ trusiYe; stratigraphic relations uncertain lap of bolson deposits. Another metasedi­m en t a r y unit, chlorite-mica schist (p£Ccms) crops out north and southeast of the meta-arkose unit. Toward the southeast, at two places where original relations have not been disrupted by intrusions (G.~ 11.0 and F.~10.0, Pl. 1), the meta-arkose dips beneath the chlorite-mica schist with apparent conformity and is evidently older. To the north the meta-arkose is so sepa­rated from the chlorite-mica schist by in­trusive rocks, or so concealed by alluvium, that the relations of the two units are not clear. For the most part the meta-arkose is fine grained, light gray, and slightly schis­tose, but it includes thin layers of sericite schist and phyllite. The sericite content of the rock imparts a foliation that is parallel to the bedding, resulting in a slabby, platy outcrop. Petrography.-In thin section the meta-arkose shows a granoblastic mosaic of quartz and feld­spar. Cataclasis is indicated by areas of crushing and shearing. In general the feldspar is albite, accompanied by microcline in some sections. It is difficult to identify and correctly estimate percentages of feldspar in these rocks because of fine grain size, crushed areas, and finely divided sericite. Sericite makes up as much as 15 percent of the rock and occurs as layers of fine fibers or as fine intergranular fibers that appear as a braided network. Chlorite (clinochlore), magnetite or ilmenite, leucoxene, apatite, zircon, rutile, and tourmaline are present in minor amounts. Grain size ranges from less than 0.01 mm. to 0.5 mm. and averages 0.1 to 0.2 mm. Estimated modes of representative rocks are given in Table II. Chlorite-mica schist (p£Ccms) .-Chlo­ rite-mica schist forms a low hilly area of dark-colored rocks in the northwest Car· rizo Mountains (E.~13.0 to F.~10.0, Pl. 1) . The outcrops are surrounded by metarhyolite hills to the north, east, and south and by amphibolite hills to the south­ west. Profuse intrusion has separated the chlorite-mica schist into five isolated masses surrounded, for the most part, by intrusive metarhyolite and amphibolite, and stratigraphic relations are not clear. Maximum thickness included between the bordering intrusive rocks is roughly 3, 700 feet. This unit corresponds to the "chlorite schist" of King and Knight (1944) . The unit is composed of a wide variety of metasedimentary types. The most abun­dant is fine-grained lustrous dark sericite­chlorite-biotite schist, which forms most of the low hilly ground within the outcrop area. Thin beds of dark slate and phyllite occur within the schist, and thin limestone beds (less than 5 feet thick) are rarely present. On a prominent ridge in the south­ern half of the principal exposure are more resistant coarser meta-arkose beds with intercalated highly crinkled phyllite lay­ers. Flattened pebbles in some of the meta­arkose beds indicate their original con­glomeratic nature. The phyllitic rocks were probably shales or tuffs. Bedding (S1 ) is particularly well pre­served in the more competent arkosic mem· hers, but it is obscure in the phyllitic and highly schistose members. Schistosity (S2 ) is broadly parallel to the bedding, but locally it transects bedding at small angles. Where the unit abuts more competent meta­rhyolite, it is strongly crinkled and con­torted, apparently by a later structure (S3 ) resulting from mashing of incompetent schist against the metarhyolite, during the northward thrusting. Lineation, interpreted here as a lineation, was measured at one place (E.3-13.5, Pl. 1) and is parallel to lineation within the metarhyolite. The nature and origin of the lineation are dis­cussed in the section on structure. Petrography.-Under the microscope the schist shows elements of lepidoblastic and cataclastic fabrics. Oriented plates and smeared out masses of biotite, chlorite, and sericite occur in a strained and crushed mosaic of quartz and feld­spar. In some sections the green-brown biotite is partly bleached to a red-brown variety and in other sections has gone over to chlorite (penni· nite) . The feldspar is chiefly albite, but microcline was observed in one section. Zircon, magnetite or ilmenite, leucoxene, and tourmaline are present as accessory minerals. Micro-folding and micro­thrust-faulting were observed in association with the S. structures. Average grain size ranges from less than 0.01 to 0.5 mm., although sporadic larger pebbles are present in the meta-arkose beds. Estimated modes of representative rocks of the unit are given in Table 11. The fine grain size and the general smeared and crushed appearance of the rock, the faded and "sickly" character of the hiotite, and the disequilibrium indicated by its partial conversion to chlorite indicates retrogressive metamorphism. Meta-arkose and feldspathic metaquartz­ite (p£Cq2 ) .-This unit forms a promi­nent white ridge that trends northeast­southwest through the entire mountain mass and is offset by a fault at the north­east end (0.~14.5 to G.0-8.2, Pl. 1). Beds are vertical or dip steeply southeast. The unit is separated from older sedi­mentary rocks of the sequence by a thick amphibolite sill intruded along its lower contact. The thickness of the unit varies as measured across the outcrop because of the somewhat irregular trace of the intru­sive contact, but 1,800 to 2,000 feet is a fair maximum thickness. Rocks of the unit are fine-grained thin-bedded light gray feldspathic metaquartzite and meta-arkose. Bedding and cross-bedding are visible and indicate that the top of the unit is toward the southeast. Foliation parallel to the bedding causes the rock to break into thin slabs and plates, on the surfaces of which is a sericitic sheen. Petrography.-Thin sections of representative rocks from the unit show a cataclastically altered granoblastic mosaic of quartz and feldspar, con­taining a variable amount of sericite. Partly bleached biotite, chlorite (penninite), magnetite or ilmenite, leucoxene, apatite, zircon, carbonate, and rutile are present as minor constituents. The feldspar is albite or albite and microcline and makes up 20 to 57 percent of the bulk mineral composition. The fine grain size, fine sericite, and cataclastic alteration make it difficult to separate the two feldspars. The grid twinning of the microcline is vague and obscure. Estimated modes are given in Table 11. Mixed unit (pCCm1 ) .-This unit forms a northeast-southwest-trending lowland that extends through the entire mountain mass and is known locally as Cat Draw (0.0­ 14.0 to H.0-8.0, Pl. 1). The rocks di­rectly overlie the quartzo-feldspathic unit previously described (pCCq2 ) and consist of a sequence of intercalated thin beds of fine-grained gray and blue quartzite, light­gray sericite schist, dark phyllite, dark lustrous slate, thin-bedded blue chert, thin­bedded to laminated brown and black cherty limestone, and thin layers of am­phibolite. The abundant limestone beds, commonly showing intricate folding, dis­tinguish this mixed unit from others in the sequence. Intrusion of amphibolite, complicated small-scale folding, and con­tortion have greatly increased the thick­ness of the original sedimentary unit which now has a thickness of about 1,700 feet. In the unit as a whole foliation is parallel to the bedding. The unit corresponds to the "limestone and slate" of King and Knight (1944). Petrography.-These rocks are very fine grained to cryptocrystalline and yield little additional information under the microscope. Estimated modes of three specimens from the unit are given in Table 11. Two thin sections of limestone from the unit were examined. The rocks are composed chiefly of calcite in a twinned mosaic. Grains range from 0.05 to 0.2 mm. to 1 to 2 mm. Quartz occurs in scattered grains and patches of grains and in places makes up as much as 15 percent of the slide. Albite, sericite, and iron oxide are present in minor amounts. Meta-arkose (pCCq8 ) .-In the north­eastern part of the area, meta-arkose di­rectly overlies the mixed unit previously described (0.0-13.0, Pl. 1); in the south­west part of the area the meta-arkose is separated from the mixed unit by an am­phibolite sill about 700 feet thick which intrudes both the mixed unit and the meta­arkose (J.0-9.0, Pl. 1). Two other major amphibolite intrusions occur within the meta-arkose. The unit is composed of two distinct lithologic types, a gray meta-arkose and a reddish brown meta-arkose (not differentiated on Pl. 1) , and has a thick­ness of about 1,700 feet, although measure­ments vary in different parts of the area because of the intrusions. The gray meta-arkose comprises approxi' mately the lower 800 feet of the unit and is fine-grained, thin-to medium-bedded sericitic rock with platy or slabby out­crop. Pebbles, stratification, and cross­bedding are prominent, and cross-bedding indicates that the top of the unit is to the southeast, in normal order. Lying with sharp contact on the gray meta-arkose is about 250 feet of hard, red-brown meta· arkose in beds up to 2 feet thick, which form a blocky resistant outcrop on the crest of a ridge. Bedding and cross-bedding are present but are less well developed than in the gray meta-arkose. During field work, this ridge-making member was sus· pected o_f .being a meta-igneous unit, per­haps ongmally a soda aplite. This was disproved by discovery of relict sedimen· tary structures. The red-brown meta-arkose memb~r grades up into the top member of the. ~~1t, a softer, less brittle, gray to buff senc1tic meta-arkose that is similar to the l?~er member of the unit. The gray seri­c1t1c ~eta-arkose is plainly foliated when ex~m~ned. under th~ microscope, but the ~ohat10n ~s no.t ob~1ous in the field except m sporadic thm mica-rich layers. The red­brown meta-arkose is not foliated. P. B. King (personal communication 1951) informed the writer that in an ex'. posure once present in a road cut along U. S. highway No. 80 (0.5-12.9, Pl. 1) the meta-arkose of this unit showed ripple­marks which may have been relict sedi­mentary structures. When the writer viewed this surface in 1950 it had been damaged (possibly by the eager hammers of itin­erant geologists), and no reliable deduc­tions could be made as to the nature of the ripples (whether sedimentary struc­tures or tectonic pseudo-ripples). However, cross-bedding is distinct, particularly in smooth outcrops in streams south of the road and in a series of observations indi­cated a normal section. Petrography.-Estimated modes of representa­ tive rocks of this unit are given in Table 11. The gray meta-arkose is composed of grains of feld­ spar, quartz, and quartz mosaic (0.2 to 0.5 mm.) in a matrix of fine quartz (less than 0.02 mm.) and finely divided sericite. The finely comminuted quartz and sericite have ftowed around the larger grains as a result of incipient mylonitization. Sericite comprises as much as 15 percent of the rock. The feldspar, albite and microcline, is diffi­ cult to assess quantitatively in the slides examined. Albite and microcline together constitute about 30 percent of the rock. Magnetite or ilmenite, zircon, and rutile are present in small quanti­ ties. The fabric is cataclastic. The hard brown or red-brown meta·arkose as­ sociated with the gray rock is also cataclastic (photomicrograph, Pl. 34, A). Strained grains of quartz with sutured edges, albite, and perthite occur in a fine crush matrix. Partly bleached biotite, magnetite or ilmenite, zircon, and apatite also are present. Grain size ranges from less than 0.01 mm. for the crush material to 0.5 mm. for the unreduced grains. This rock, like the above, shows the mortar structure of incipient mylonitiza­tion. Where crushed material is at a minimum, the rock shows a sutured mosaic of albite and quartz. Phyllite (pCCp) .-Directly over the meta-arkose (pCCq3 ) is a blue-gray phyl­ lite about 600 feet thick. This forms a narrow lowland between the meta-arkose ridge and a massive amphibolite ridge (P.0-13.0 to M.0-10.0, Pl. 1). Am­phibolite intrusions occur along both hang­ing and footwall contacts and within the phyllite, where there are also thin layers of chlorite-epidote schist (probably sheared amphibolite sills). Locally the rock is slaty. The slaty cleavage must be approximately parallel to the bedding as it is parallel to the plane of the contact with the underlying meta-arkose; however, bedding laminae are not evident in the phyllite itself. Petrography.-ln thin section the phyllite is seen to be a very fine lepidoblastic masa of quartz, finely divided opaque mineral (probably magne­tite), biotite, and sericite. Traces of apatite and leucoxene are also present. The biotite is a faded green-brown variety occurring in masses and scattered plates. It makes up approximately 25 percent of the rock. Grain size is less than 0.02 mm. The phyllite contains unusual spheri­cal masses a few millimeters in diameter, which protrude like porphyroblasts on cleavage surfaces. Thin sections reveal that the masses are poor in mica and rich in magnetite; they may be relicts of former porphyroblasts destroyed during re­trogressive metamorphism. Mixed unit (pCCm2 ) .-This unit is map­pable only in a small area in the north­east part of the mountains, where it occurs between a thick amphibolite sill on the footwall contact and a thick rhyolite sill on the hanging wall contact (P.0-11.0 and Q.0-12.0, Pl. 1). It is obscured by remnants of unconformably overlying Van Horn sandstone north of U. S. highway No. 80 and by recent alluvium south of the highway. To the southwest the unit is intimately penetrated by amphibolite and loses its identity. Some of the beds crop out within the amphibolite and are trace­able for short distances as thin partitions within the meta-igneous rock. On aerial photographs these partitions, lighter in color than the amphibolite, stand out re­markably and give the amphibolite a striped appearance. The mixed unit is composed of about 500 feet of thin-bedded fine-grained gray metaquartzite, brown cherty limestone, gray sericite schist, and blue to green slaty rocks which in places have been thrown into small folds. Apparently these rocks were first intruded by rhyolite along the hanging wall contact and then by am­phibolite along both contacts and through­out the unit. The unit is probably a part of the mixed unit stratigraphically above and to the southeast (pCCm3 ) and was separated from it by the two intrusions. Petrography.-A thin section of a limestone member of this unit was examined. The rock io composed of 95 percent calcite with small amounts of chlorite, muscovite, quartz, and iro:1 oxide in an uneven-sized mosaic ranging from less than 0.02 mm. to 0.5 mm. (Table 11). The fabric is granohlastic. Mixed unit (pCCm:J .-Southeast of the rugged metarhyolite ridge that culminates in Hackett Peak is a lowland developed on another mixed unit (P.0-10.0 to 1.0­6.0, Pl. I) . This consists of fine-grained sericitic quartzite, fine-grained biotite­sericite schist, and phyllite. One bed of brown cherty limestone 5 feet thick is present about midway in the unit, and some of the schists contain calcite. Thin sills of amphiholite occur throughout, and a rhyolite dike 4 feet thick cuts across the metasediments in the southwest part of the area (J.0-7.0, Pl. I). Present thick­ness of the unit is about 2,000 feet hut it has been thickened by local folding and contortion. Foliation is approximately parallel to the original bedding, but it is locally distorted by 53 structures (crenu· lations), particularly near the contact with the metarhyolite. Petrography.-Estimated modes of represent&· tive rocks of this unit are given in Table 11. The schistose rocks are composed of oriented plates and fibers of biotite, chlorite, and sericite in a mosaic of quartz and feldspar with or without calcite. In some sections biotite has a "digested" appearance and in others seems to have gone over to chlorite. Except where partly bleached, the biotite is a dark brown variety. Partial bleaching results in a faded fibrous red-brown variety. The chlorite is penninite and occurs as discrete plates or as fibers and masses with biotite. Sericite oc­curs in fibers, streaks, and matted layers. In most sections the feldspar is albite. Microcline and microcline-microperthite accompany albite in one section. Where cataclastic phenomena are pronounced the feldspar occurs as lenticular crushed areas of low relief. Quartz makes up a large proportion of the fine-grained mosaic but als~ occu~s in . coarser "eyes" and layers. Mag­netite .or ~memte, brown iron oxide, and apatite occur m mmor amounts. All the rocks of the unit are micro-layered, with numerous mica-rich, quartz-rich and carbonate· rich layers. This layering is either the result of a tectonic unmixing associated with metamor­phism or is an original sedimentary feature. In the schistose rocks small rucking and chevron folding of the more micaceous layers are com­mon, and in the quartzo.feldspathic rocks crush· ing, straining, and development of augen are characteristic. These fabrics are a combination of lepidoblastic and cataclastic elements and may be classed as compound fabrics. Grain size ranges from less than 0.02 mm. in phyllitic rocks to 0.05 to 0.1 mm. in the schists and quartzites. One thin section of the limestone from this unit was examined. The rock is composed equally of twinned calcite and quartz with the minerals distributed in layers. Sericite, magnetite or il· menite, and red iron oxide are also present. Grain size ranges from 0.05 to 0.20 mm. (photo· micrograph, Pl. 36, C). Feldspathic metaquartzite and meta­arkose (pCCq4 ) .-This unit directly over· lies the mixed unit previously described (Q.0-10.0 to J.0-6.0, Pl. I). It is made up of light-gray sericitic feldspathic meta· quartzite that forms a prominent ridge in the northeastern part of the outcrop. To the southwest the mica content increases and the unit becomes more schistose, losing its ridge-making character, probably be­cause of an increase in content of argil­laceous material in the original sediment. Bedding and cross-bedding are visible and the latter indicates a normal southeast dip; the unit is about 300 feet thick. Foliation has developed that is about parallel to the bedding hut which locally cuts the bedding at small angles. To the northeast a north· east-trending fault horizontally displaces the quartzite ridge. Petrography.-In thin section the rock shows a granoblastic-cataclastic fabric. Quartz comprises most of the slide and occurs as grains within the mo.saic, in crushed areas, and as secondary quartz m veinlets parallel to the schistosity. Albite is the principal feldspar, although a few grains of microcline were noted. Sericite is present as ~ne ~brous intergranular material. Magnetite or 1lmemte, leucoxene, tourmaline, apatite, and zir. con are present as accessory minerals. The tour· maline appears to be a secondary mineral intro· duced by the hydrothermal agencies that operated throughout the area. The grain size ranges from less than 0.01 mm. in the crush areas to 0.3 mm. (Table 11). Mixed unit (pCCm4) .-This unit is sep· ar~ted from the meta-arkose previously de· scnbed (pCCq4) by intrusive bodies of metarhyolite and amphiholite (P.0-9.0 to K.O-?.O, Pl. I) . The area of exposure is complicated by metarhyolite, amphibolite, an? granod~orite intrusions (pre-Cam· bnan), Tertiary diabase intrusions and faulting, and is obscured by unconform­ably overlying Van Horn sandstone Hueco limestone, and bolson deposits. l~tricate small-scale folding is common, and the beds are contorted locally. The unit is roughly 4,000 feet thick. The rocks are chiefly fine-~rained schists and phyllites, but fine-gr~m".d schistose quartzites are present. Thm sills of amphibolite are com· ~on! a~d ma~sive irregular-shaped grano· d10nte mtrus1ons occur just north of Bass Canyon. These bodies have acted as irregu· lar competent buttresses within the incom· p~tent he~s of the unit, and this, together with f~ultmg, has brought about a strong c?nto~hon of t_he schists. Elsewhere folia· t10n is approximately parallel to the bed· ding. Throughout roost of the unit the strike is east of north, but at the southern limit of the exposure the strike swings to west of north, conforming with the succeed­ing unit. In the vicinity of M.0-5.9, Plate l, is a thin conglomerate member containing flat­tened chert pebbles with a long axis of about 2 inches and a short axis of about half an inch. The outcrop of this member is largely obscured by colluvium. Petrography.-The sericite schists are fine­grained light-gray rocks that have a lustrous or silvery sheen in outcrop. They are composed of <;ericite and quartz, or sericite, quartz, and albite. The sericite occurs in fibers, layers, and fibrous masses, commonly showing micro-folding and thrust-faulting (Pl. 35, A) . Magnetite or ilmenite, chlorite, partly bleached green-brown biotite, epidote, apatite, carbonate, and rutile are present in small quantities. Grain sire is for the most pan less than 0.1 mm. Fabric is lepidoblastic-cata­clastic. Interlayered with the sericite schists and phyl­lites are fine-grained green schistose rocks that may be cataclastically altered amphibolite sills. They are composed largely of epidote, albite, and quartz with small needles of amphibole that is probably actinolite. Biotite, a partly bleached olive-brown variety, apatite, sphene, magnetite or ilmenite, and carbonate are present in minor amounts. The fabric is crystalloblastic-cataclastic, and the grain size ranges from less than 0.02 to i>.2 mm. Flowage structures and comminuted grains are the principal catclastic features. Es­timated modes of representative rocks of this unit are given in Table 11. A hard brown rock crops out at K.6-5.5, Plate 1, and forms a ridge extending northeast. In the field this rock appears to be a fine-grained quartz­ite, but thin section shows a sutured granoblastic mosaic of albite (65 percent) and quartz (35 percent). Traces of apatite, muscovite, and mag­netite or ilmenite are present. Grain size ranges from 0.05 to 0.5 mm. lntergranular crushing is marked. Good sizing of grains and essentially hi· mineralic composition indicate that this rock may be cataclastically altered soda aplite, per­haps a phase of the granodiorite with which it is in contact. If this rock is meta-arkose, the original sediment was well sorted in contrast to the typical meta-arkose of the area. F elds pat,hic metaquartzite and meta­arkose (p£Cq5 ) .-This unit forms a series of low gray hills at the southeast end of the pre-Cambrian exposure (N.0--4.0, Pl. 1) . In Bass Canyon the rocks are in con­tact with the schists of the mixed unit just described (p£Cm4 ) but to the northeast the contact is offset, distorted, and ob­scured by down-faulted Van Horn sand­stone and Hueco limestone, intrusions of Tertiary diabase, and unconsolidated bol­ son deposits. Southward, these rocks dis­appear beneath unconformably overlying Hueco limestone. As exposed, the unit has a minimum thickness of about 2,700 feet. To the northeast it strikes east of north, but to the south the strike swings through north·south to west of north and indicates the first major deviation encountered in the usual northeast strike and southeast dip of the rocks. The greater part of the unit is composed of fine-grained light gray sericitic feldspathic roetaquartzite and seri­citic meta-arkose. On the outcrop the rock breaks into thin slabs that gleam with seri­cite. The foliation is parallel to the con­tacts of the unit and is essentially a bed­ding plane foliation. Relict sedimentary structures are not apparent. Intercalated with the sericitic meta-arkose and roeta­quartzite are beds less than 1 foot thick of very fine-grained white, blue, and gray quartzite or chert and thin layers of seri­cite schist. The number of schist layers increases westward toward the contact with the underlying mixed unit. Near this contact is a 3 to 5-foot layer of almandine­mica schist with small unaltered idio­morphic garnets 1 to 2 mm. in diameter. This is the only garnet-bearing schist ob­served in the Carrizo Mountains. The layer, extremely contorted, is a thin incompetent body between competent quartzites. Petrography.-Estimated modes of two rocks of the unit are given in Table 11. In thin section the quartzo-feldspathic rocks show a granoblastic mosaic of quartz and feldspar. Sericite occurs as oriented flakes and as fine intergranular material. The feldspar is albite, microcline, and microcline­microperthite. Biotite, a fibrous partly bleached red-brown variety, zircon, apatite, magnetite or ilmenite, leucoxene, rutile, and calcite are present in small quantities. The almandine-mica schist is composed of about 85 percent sericite and biotite showing extreme micro-folding and thrust.faulting. The mica flows around the garnet porphyroblasts (photomicro­graph, PL 36, A) . The garnets occur as idiomor­phic crystals and show no diaphthoritic effects. Long laths of chlorite (penninite) have grown in the pressure lees of the garnet porphyroblasts, hut this chlorite does not appear to have been de­rived from the garnet. Small amounts of magne­tite or ilmenite, quartz, and feldspar are present. This almandine-mica schist presents cer­tain contradictions. The biotite is a "sickly" faded mineral typical of diaphthoritic rocks, but the idiomorphic garnets show no evidence of retrograde phenomena and in­dicate a higher metamorphic grade than is characteristic of the Carrizo Mountain se­quence in general. It is possible that the soft incompetent mass provided by the very highly micaceous matrix (which shows extreme deformation) has absorbed the crushing and shearing stresses which seem to have been responsible for the retrogressive phenomena elsewhere in the sequence and thus has protected the garnets from cataclasis, much as conglomerate pebbles are protected or cushioned in a soft clay matrix. METASEDIMENTARY ROCKS OF UNCERTAIN STRATIGRAPHIC RELATIONS In outlying parts of the Carrizo Moun­tains, mainly to the north, east, and south, are relatively small outcrops of additional pre-Cambrian rock units which, because of a variety of reasons, chiefly aberrant struc­ture, cannot be fitted into the homoclinal sequence just described. Limestone (pCCl) .-A fine-grained hard brown calcareous rock crops out at the easternmost salient of the mountains (Q.0-9.0, Pl. 1). It is bordered on three sides by unconsolidated bolson deposits and is bounded by intrusive metarhyolite on the fourth; it occurs in an area of metarhyolite and amphibolite intrusions, quartz-tourmaline veins, and contorted beds. Southeastward along the strike, the outcrop is obscured by bolson deposits, down-faulted younger rocks, and Tertiary intrusions. The rock is unique in the se­ quence and its stratigraphic relations have not been established. Petrography.-In thin section the rock shows a granoblastic mosaic of albite and calcite, contain­ing poikilitic porphyroblasts of pink chlorite identified as delessite. The carbonate and albite occur in patches. Biotite, apatite, muscovite, rutile, leucoxene, and tourmaline are present in small quantities. The mosaic averages 0.05 to 0.2 mm. in grain size with the porphyroblasts attaining a maximum diameter of 2 mm. (Table 11). F eldspathic metaquartzite and feld­spathic sericite schist (pCCq6 ) .-This unit forms an area of light-gray hills in west­ern Bass Canyon at the extreme southwest limit of the pre-Cambrian exposure (L.0­4.0, Pl. 1). It is overlapped to the west and south by Hueco limestone and uncon­solidated bolson deposits. The rock is a fine-grained light-gray sericitic meta­quartzite, with distinct schistosity, that weathers into thin plates and slabs. Relict sedimentary structures are not distinct, al­though banding in the rock that generally parallels the schistosity may be interpreted as bedding. The unit may be a structurally dislocated part of metaquartzite and meta­arkose (pCCq0), which it resembles litho­logically. Structure of the unit differs from that of the adjacent metasedimentary rocks to the north, as it does not share their pre­vailing southeast dip and northeast strike; instead its rocks strike northwest and stand vertical or dip northeast. Relations with the adjacent rocks of the sequence are ob­scured by intrusions and contortion of the beds. The simplest and most satisfactory interpretation seems to be that it is sepa­rated from mixed unit pCCm4 to the northeast by a fault, along which it has been moved horizontally northwestward. Partial confirmation of this interpreta­tion is afforded by the unit's petrographic character. Unlike the quartzo-feldspathic rocks near by to the north, it lacks indica­tions of cataclastic alteration and contains plates of muscovite rather than fibrous in­tergranular sericite. It thus possesses a higher metamorphic grade than the rocks to the north, which is more like that of the rocks to the southeast in the Van Horn Mountains. The fault, if present, may be younger than the metamorphism and may have brought rocks of two metamorphic grades into contact. Such displacement need not have been large, as the rocks of the main sequence in the Carrizo Moun­tains themselves show southeastward in­crease in metamorphic grade and decrease in retrogressive metamorphism. Petrography.-Under the microscope the rock shows a mosaic of quartz and albite (partly altered to sericite), containing oriented fibers of sericite and muscovite plates. The mica is unevenly dis­tributed in layers. Magnetite or ilmenite, zircon, and apatite are present in small amounts. The fabric is granoblastic to lepidoblastic, depending on the amount of muscovite. Grain size ranges from 0.05 to 0.2 mm. (Table 11). Allamoore formation(?) (pCAls(?).­In the northern Carrizo Mountains, be­tween the Texas and Pacific Railroad and the Hillside fault, are a number of out­crops of associated metarhyolite and lime· stone. Most of the outcrops are small and. surrounded by alluvium so that a con· ESTIMATED MODES OF REPRESENTATIVE METASEOIMENTARY ROCKS IN THE CARRIZO MOUNTAINS ? ~ 0­ .., ~· ::i:i Q ~ ~ ~ ;:s .., ~ ;:s ::i:... .., ry, lustrous, sericitic schist. A rhyolite porphyry which is coarser and more massive than the rest occurs in three low hills that project from the alluvium 41h miles northwest of the village of Allamoore (P.5-6.5 and Q.5-6.5, Pl. 3). The dominant rock consists of pink feld­spar phenocrysts as much as half an inch in diameter set in a dark gray or blue­~ray, fine-g~ained ground-mass.. Foliation 1s expressed only by rude layering, on the surface of which lineation is faintly de· veloped; in some places the phen~crysts have been broken and strung out m the direction of lineation. The porphyries are associated with fine-grained massive rocks, consisting largely of ground-mass and con­taining very few phenocrysts, and with thinly fissile schist. Specimens of various phases of the meta­rhyolite from the Sierra Diahlo foothills have been examined under the microscope by Flawn, who reports as follows (memo­randum of February 1952) : A specimen of the normal phase of the meta· rhyolite from directly over the Streeruwitz over­thrust in outcrops 2 miles northeast of Allamoore contains 80 percent fine-grained (less than 0.02 mm.) crushed ground-mass of feldspar and quartz, showing flow structure and possessing a cata· elastic fabric. Fourteen percent of the rock is relict phenocrysts, of which 5 percent are albite (chessboard twins cut by veinlets of the same feldspar), 9 percent microperthite (albite in irregular veins in potassium feldspar; may be microcline with vague distorted twinning), and 5 percent quartz (severely strained, in streaks, eyes, and elongate grains). Minor constituents are calcite, zircon, and red iron oxide. Secondary quartz occurs in veinlets. A specimen of the coarse phase of the meta· rhyolite from 4~ miles northwest of Allamoore consists of 70 percent ground-mass, formed of fine-grained ( 0.01 to 0.05 mm.) feldspar and quartz. Both indices of refraction of the feldspar are consistently less than 1.530, indicating that it is potassium feldspar. Broken and crushed phenocrysts with diameters up to 5 mm. make up 20 percent of the rock. Identification of the feld­spar of the phenocrysts is difficult. Some show an anomalous twinning that may he a vague and distorted microcline quadrille; others show chessboard twinning of the sort that has been interpreted as indicating replacement of micro· cline by albite; still others are microcline-micro· perthite. D_eterminations on two phenocrysts gave the lower mdex of refraction as about 1.530 in· dic~ting that at least part of the phenocrysts' are a!b1te. Coa_rser eyes of quartz and grains of cal­cite occur in pressure lees of the phenocrysts. The quartz of the eyes and of the grains in the ground­mass .makes up about 7 percent of the rock. Minor ?onsll.tuents of the rock include magnetite or ilmemte (small scattered grains), sericite (fibers com~only wrapped about the phenocrysts), chlonte _(ass?ciated with an opaque mineral from. which iron was derived), and traces of apatite and sphene. A specimen of the fine-grained brown carbonate rock, from an outcrop surrounded by metarhyo­lite 2 miles northeast of Allamoore shows a com­pound fabric which includes cataclastic, crystal­loblastic, and diablastic (hornfels) elements. Grain size is 0.02 to 2 mm. Albite mak~ up 57 percent (grains in a mosaic with calcite showing bent twin lamellae; also as fresh twinned grains in veinlets with tourmaline and calcite) ; quartz 20 percent (severely strained grains and in larger masses showing relict crush-suture fabric) ; tourmaline 8 percent (zoned prisms and groups of prisms). Minor constituents are red­dish iron oxide and apatite. The rock may have formed by high-temperature hydrothermal ac­tivity near the fault zone, with replacement of original limestone by sodic solutions during the same period as that which albitized the pheno­crysts in the metarhyolite. Amphibolite (pCCa) .-The metarhyolite is interbedded with thin to thick bodies of amphibolite, which form sills lying parallel to the foliation. Most of these, too small to show on the map, are thin beds or streaks a few feet thick, with well-marked foliation and lineation parallel to that in the enclos· ing metarhyolite. Among the thicker bodies, two in the southern Streeruwitz Hills are differentiated on the map. The largest (D.5-8.5, Pl. 3) is actually a complex of many smaller sills, separated by sheets and leaves of meta· rhyolite. Close to the metarhyolite the amphibolite is as strongly foliated as in the smaller bodies, hut the interior parts are more massive. Specimens from the sill were studied under the microscope by C. S. Ross, who reports as follows (memorandum of June 21, 1938) : One specimen is a highly ferro-magnesian rock, composed dominantly of plagioclase and horn­blende. The ground-mass is moderately coarse­grained, and a few plagioclase phenocrysts are present. The rock seems to be a hypabyssal type, of andesitic character. Abundant secondary min­erals are epidote, calcite, chlorite, and part of the hornblende. Alteration is marked, but less extreme than in the next specimen. In the latter, alteration is so thorough that the original charac­ter is not clearly determinable, although it may have been similar to the first specimen. It is now composed of some primary hornblende, sec­ondary hornblende, clinozoisite, quartz, and chlorite. Sphene is very abundant. The alteration is hydrothermal rather than dynamic. Quartz and quartz-feldspar veins.-Both metarhyolite and amphibolite contain vein material, generally in narrow stringers and irregular hlehs a few inches thick but partly in more massive bodies several feet thick. The dominant mineral is milky or vitreous quartz, but in places this is asso· ciated with pink feldspar, muscovite, spec­ular hematite, and martite. Some of the veins and adjacent host rocks contain un­deformed cubes of pyrite. Narrower quartz veins also traverse the adjacent altered limestones of the Allamoore formation. Structural relations of the veins are com· plex. Although they occur to some extent in the amphibolite, they appear to be most common in the metarhyolite, perhaps be­cause this rock was more brittle and sus­ceptible to fracture. Many veins follow the foliation of the enclosing rocks or spread through them irregularly; some of these are slickensided parallel to the prevailing lineation. Other veins lie on joints that cross the foliation, and some form the ce­ment of brecciated metarhyolite. Still other cross-joints break and offset the veins and show no mineralization except for scant coatings of specular hematite or chlorite. Interpretation of Carrizo Mountain group.-According to observations of Flawn, the metarhyolite of the Carrizo Mountains intrudes a thick sequence of sedimentary rocks, also a part of the Car· rizo Mountain group; no remnants of this earlier host rock remain in the metarhyo­lite of the Sierra Diahlo foothills. Succeeding rocks and structures de­veloped under the influence of dynamic forces, in part at least related to develop­ment of the Streeruwitz overthrust. Differ­ential movement within the metarhyolite produced foliation and cataclasis; pheno­crysts and other mineral aggregates were drawn out in the direction of transport, or a structural axis, to create the all-pervading lineation. Less competent zones were con­verted to mylonite and schist. Close to the sole of the overthrust, continued move­ments contorted the previously formed foliation and skewed the lineation into aberrant trends. After the beginning of the dynamic period, amphibolite was injected in the metarhyolite as sills parallel to the folia­tion. Further movements crushed and foliated the thinner bodies between the en· closing, more competent masses of meta­rhyolite hut produced little change in the thicker bodies. Introduction of vein material succeeded that of the amphibolite and took place dur­ing the waning stages of the dynamic epoch. The earliest veins are probably those paral­lel to the foliation, as their slickensides attest continuation of differential move· ments like those which produced the linea· tion in the enclosing rock. Later veins fol­low the cross joints, but joints continued to open after the period of vein formation, as many of the veins are broken and offset by the latest, little·mineralized joints. ALLAMOORE FORMATION I ntroduction.-The Allamoore formation crops out mainly well south of the Sierra Diablo scarp, near Eagle Flat, where it occurs in a belt about 5 miles wide that trends west-northwest. The formation pro· jects in low, barren, sub-parallel ridges. The most prominently exposed parts of the Allamoore are its limestone beds and units, but these are interlayered with great masses of volcanic rocks, including flows, pyroclastics, and perhaps shallow intru· sives. There are also many thin units of argillaceous rocks, now altered to phyllite. As the Allamoore is exposed mainly to the south where deformation of the pre­Camhrian rocks is greatest, the formation is nearly everywhere tilted at high angles and complexly folded and faulted (fig. 10). Determination of stratigraphic sequence is therefore difficult, and estimation of total thickness almost impossible. On Tumble· down Mountain (R.5-8.5, PI. 2), where relations are unusually well shown, about 1,630 feet of the formation is exposed but neither the top nor base is present, and the sequence is at least partly duplicated by thrusting. Any traverse across the strike of the formation will cross a wide expanse of steeply dipping beds that can hardly have been produced by repetition of a few thin layers; the formation is probably as much as several thousand feet thick. Limestone (pCAls) .-Limestones of the Allamoore formation stand in jagged ledges and ridges. They are characteristically seamed by bands of chert a quarter to half an inch thick, lying parallel to the bedding and disposed at intervals of a few inches (Pl. 23, A). Differential weathering of chert and limestone gives the ledges a striking ribbed appearance. The chert ap· pears to be a primary or diagenetic fea­ture; it at least antedates the pre-Cambrian orogeny, for where the rocks are strongly deformed, chert seams are sliced and broken while the less brittle intervening limesto'nes are deformed by flowage. The limestones themselves, where least altered, are mainly thin bedded and com· pact, with occasional more granula~ or more crystalline layers. Most of the lime­stone is blue, gray, or brown, but some is reddish or purplish. Some limestone beds are very thinly and evenly laminated by light and dark seams; others up to 8 feet thick are massive and contain no chert. At a. few places the limestones are interbedded with intraformational limestone-pebble conglomerate. A specimen of dark gray, fine-grained, little altered limestone of the Allamoore formation from Tumbledown Mountain (bed 5 of Tumbledown Mountain section) was analysed by K. J. Murata in the chemi· cal laboratory of the U. S. Geological Survey with the following results (mem­orandum of March 4, 1937) : Table 16. Analysis, in percent, of limestone from Allamoore formation (K. ]. Murata, analyst). Inorganic insoluble .................................. 6.69 Organic insoluble ...................................... 0.05 Soluble R.Oa ................................................ 0.97 CaCOa ......................................................... 83.82 MgCOa ............................. ................... 9.32 MnCOa ........................................................ 0.06 Cas(P0,)1 .................................................. none Soluble CaO other than carbonate ........ none Soluble SiO, .............................................. none Total ....... . ······ .100.91 The analysis indicates that the limestone is only slightly magnesian. According to Richardson (1904, p. 25) "A partial anal­ysis, by Mr. George Steiger, of a sample collected 3 miles northeast of Eagle Flat, shows considerable magnesian content." So far as the present analysis shows, there is nothing to distinguish this specimen from similarly analysed Paleozoic limestones of the same region, except perhaps the appre· ciable content of manganese. According to Mr. Murata, the organic matter indicated by the analysis is apparently similar to the bituminous material which darkens the Paleozoic limestones. The organic matter in the limestones seems to prove existence of life during for­mation of these ancient rocks yet searches for fossils that have so far been made re­veal nothing hut possible algal remains. Many of the chert-banded or laminated limestones have a crinkled, wavy, or dome­like structure similar to the Cryptozoon or stromatoporoid growths in the older Pale­ozoic rocks, and in places they rise into low mounds or reefs (Pl. 23, B). Other beds resemble the mottled limestones of the older Paleozoic rocks which were perhaps formed by algal growth. Many of the ob­served structures in the limestones of the Allamoore formation closely resemble those from the Belt series of Montana that have been described and figured as fossil algae by C. L. and M. A. Fenton (1937, pp. 1941-1965). Mention should also he made of various obscure objects which occur here and there in the limestones, that might once have been fossils. They lie in rock now so altered and silicified that if organic structure once existed it has since been destroyed. Part of the material may once have been shells, shell fragments, or broken algal crusts, hut so far as can he determined, most of the objects are of inorganic origin. On a high knob in the northern Millican Hills 1~ miles east of the Garren ranch house and south of the Anaconda no. 2 prospects (1.5­7 .5, Pl. 2) are limestones of surpassing interest, for they show many features that suggest an organic and possibly algal origin. The limestones of the knob dip north, away from the Hazel for· mation and toward volcanic rocks. Near the north base of the knob the limestone is blue-gray or brown and seamed by chert, much as it is elsewhere. About halfway up the slope to the south is a brown limestone which encloses angular fragments of blue-black, crinkled limestone, ap· parently of organic origin. Near by are some blue, thin-bedded, mottled limestones, made up of winding bluish bodies enclosed in a sparse yellow matrix. On the northwest tip of the hill are structures that look like Cryptozoon and stromatoporoid reefs. The rock contains great reefy heads 10 feet or more across, composed of dark, bituminous, papery, laminated limestone. The laminae run in waves a foot or so across, whose crests are rounded and troughs acute. Be­tween the reefy masses, and ending abruptly against them, are layers of straight-bedded, thinly laminated, siliceous rock. Above them are brec­cias or conglomerates, full of fragments of the laminated reef limestones. Similar reefy structures of probable organic origin are described below, in bed 5 of the Tumbledown Mountain section and are illustrated in Plate 23, B. lntraformational conglomerates occur in the lime&tones at several localities a mile or two southeast of the Garren ranch house. They are composed of limestone pebbles and flags up to several inches across, set in a limestone matrix. The limestone conglomerates are easily deformed by pressure, and in places the pebbles have been mashed. Their outcrops are not far south of the contact between the Allamoore and Hazel forma· tions, and the adjacent conglomerates of the latter contain boulders of the older conglomerate. Structures which have the superficial appear· ance of fossils were observed in the limestone in the hills immediately west of the Canning ranch house (P.5-11.5, Pl. 3) and on a ridge crest near a prominent synclinal fold 1% miles south­east of the Garren ranch house (H.5-4.5, Pl. 2). At the first locality the structures are globular, siliceous masses from the size of a pea to the size of a walnut ( % to 1h inch in diameter). At the second they are i;,pherical bodies of about the same size, composed of white, finely crystalline calcite, in part with a thin siliceous shell. The structures at both localities are probably inor­ganic, although some of them have tantalizing sugge&tions of shell structure. Volcanics (pCAv) .-Volcanic rocks probably make up from one-fourth to one­half of the total volume of the Allamoore formation, but they are much less conspic­uous than the limestones, as they have been worn down generally to soil-covered slopes, sags, and valleys. Their best exposures are in creek banks and prospect holes. In a few places, more massive parts of the vol­canics project in rugged black hills, cov­ered by ledges and boulders. The volcanics are interbeded with the limestone in units a few feet to hundreds of feet thick and in places also contain thin limestone beds. Volcanic activity and limestone deposition were thus essentially contemporaneous. Some of the volcanic rocks are very mas­sive and weather to brown or black, bould­ery surfaces; when fresh, such rocks are dark green or red. Diabasic structure is prominent in places, and in others amyg­dules are abundant. Superficially these massive igneous rocks resemble the intru­sive amphibolites in the Carrizo Mountain group that are described by Flawn else­where in this report. They differ in the oc­currence of amygdules, which indicate a surface or near-surface rather than a deep­seated origin; and in the fact that they are diahasic rather than dioritic. Specimens examined by Flawn contain pyroxene rather than amphibole. A considerable part of the dark massive igneous rocks were no doubt originally spread out as subaerial or subaqueous lava flows, but little remains of primary flow structures except for the rather common amygdules. Other parts may have orig­inated as welded pyroclastics, and some of the thicker, more massive bodies may have been shallow, sill-like intrusives. In the openings at the Anaconda no. 2 prospect in the Millican Hills (J.5-7.5, Pl. 2), dia­basic rocks are crossed by aphanitic bands that may be dikes. Some of the other, less massive volcanic rocks are probably of pyroclastic origin, for they show well-developed sedimentary structures hut are made up in large part of igneous detritus. Coarser varieties include dark red, brown, or green sandstones with pebbly seams of igneous fragments and in­terbedded layers of volcanic conglomer­ate. Finer beds include brown or green siliceous shales, thinly laminated pink sili­ceous rocks, and light green schistose la­minated rocks, all probably derived from different sorts of tuffs. lnterbedded with the pyroclastics are thin beds of reddish limestone that contain angular igneous fragments. Specimens of the volcanic rocks have been examined under the microscope by C. S. Ross (memorandum of June 21, 1938) and P. T. Flawn (memorandum of Febru­ary, 1952), whose reports are as follows: One specimen examined by Flawn from about 2Y2 miles northwest of Eagle Flat section house (Pl. 3) "is an altered diabase composed of 74 percent plagioclase (considerably altered hut index of refraction indicates it is probably ande­ &ine or lahradorite), 7 percent augite, 5 percent magnetite or ilmenite and iron oxide, 3 percent carbonate, 2 percent chlorite, 7 percent fibrous radiating mineral that may he chlorite, 2 per­ cent of an unidentified low-birefringent, high re­ lief fibrous radiating mineral that, except for structure, resembles apatite, and traces of epidote and zeolite. Fabric is suh-ophitic or diahasic; grain size is 0.2 to 0.4 mm. Rock is probably a shallow intrusive." Another rock from the same locality examined by Flawn "is very hard, green, vesicular, and aphanitic, consisting of about 5 percent quartz (as angular fragments, veinlets, and spongy ca_vity fillings), 5 percent carbonate (masses of dirty, brown-stained grains), and 90 percent of gray­ green cryptocrystalline ground-mass. The rock is an altered and indurated pyroclastic." A considerably more altered rock from the vicinity, examined by Ross, is of _less certain origin. It is a "very strongly lammated rock, with abundant augen-like inclusions that them­ selves have a fine schistose structure. The ground­ mass is very fine-grained, with abundant ferro­ magnesian minerals and dark pigmenting material that forms wavy parallel stringers. The augen are in part calcitic and in pan a material that is probably a light-colored chlorite aggregate. It is not clear how much of this lamination is due to shearing. The structure of the gro~nd-mass material could he produced by compaction of a vesicular pyroclastic aggregate, followed by thorough recrystallization. On th~ _other h9:nd, . a distinct parallelism of the chlontlc material m the augen indicates a rather defi,~ite shearing. Probably both factors played a i:iart. . Still another specimen studied by Ross is a pyroclastic rock from 2 miles northwest o~ tee Canning ranch house (C.5-14.5, Pl. 3). I~ is a volcanic tuff of rhyolitic character. The. mmerala in the rock fragments are very ~ne-gramed, and the only recognizable primary mmerals are feld­spar and possibly some quartz. However, much of the quartz is in aggregates that seem to represent sandstone and perhaps novaculite fragments that have been incorporated with the tuff. Secondary calcite is abundant. The rock is much altered." Typical outcrops of the v?lca~ics mal'. ~e. seen in the northern Millican Hills m the v1c1ruty of the Anaconda no. 2 prospect a mile east of the Garren ranch house (H.5-7.5, Pl. 2). The prospect is on low, rounded, greenish hills of igneous rock. The pits expose the following r?ck types: (1) Massive, diahasic rock, dark greerush­gray when fresh, weathering dark reddish brown. (2) Similar rock, hut containing amygdules, some of which are filled with green chloritic mineral. (3) Augite rock of light olive-green color, with dark brown weathered surface and schistose chfo· rite on joints. ( 4) Olivine rock of dark green color. (5) Aphanitic hands, which may be dikes, in the more granular rock. In the vicinity of the pits the volcanics are interhedded with thin lenses of limestone, some of which are of orange-red color. North of the volcanics at the prospect is a thin bed of white, laminated, calcareous phyllite, fol­lowed by 150-foot unit of limestone in 2-foot beds, interhedded with pink, thinly laminated siliceous rock, perhaps an altered tufl. Between this and the succeeding Hazel formation is dark red, gritty sandstone, probably a pyroclastic and believed to he part of the Allamoore formation. The conglomerates of the Hazel formation in this vicinity contain fragments of many of the rock types just described. Half a mile west of the Anaconda no. 2 pros· pect are other prominent exposures of the volcan· ics. To the north, in contact with the Hazel formation, is massive, bouldery igneous rock which weathers maroon-red. A little to the south are interhedded lenses of limestone 2 to 3 feet thick; these are highly tuffaceous, of deep red color, with seams of igneous grits. Near by are beds of huff siliceous laminated rock. The volcanics are also well exposed at Buck Spring, 3Y2 miles southeast of the Garren ranch house (J.f>.--3.5, Pl. 2), which issues in a gorge carved in the limestones and volcanics. At the spring is a belt of volcanics 300 feet wide, stand­ ing nearly vertical and in sharp contact with lime· stones on the south. Part is a massive amygda· loidal igneous rock, hut there are interhedded greenish pebbly and gritty layers, made up of igneous detritals, and thin limestone lenses. In places, the more massive volcanics have a deceptively youthful appearance. In the western Millican Hills 214 miles west of the Garren ranch house (A.f>--7.5, Pl. 3) is a black, bouldery knob which looks very much like a recent basalt plug. Most of the rock on the knob is dark amygda· loidal il!:neous rock, showing no trace of deform&• tion. Howev~r, toward the edges of the body the same rock 1s strongly sheared and jointed, and mapping discloses that the rock on the knob is merely a massive phase of a more extensive hand of outcrop of a volcanic unit of the Allamoore formation. In the southern Streeruwitz Hills northwest of Eagle Flat section house, the geologic map (Pl. 3) shows several wide bands of volcanics inter­bedded in limestones of the Allamoore formation immediately north of the trace of the Streeruwitz overthrust. There is some question whether these are interbedded volcanic units or whether they are intrusive sills related to the amphibolites that intrude the metarhyolite of the Carrizo Mountain group a short distance to the south. The thickest igneous body shows no evidence of cataclastic alteration like that in the adjacent amphibolite, but on a conical hill near the west end of the exposure it is foliated and phyllitic near its contact with the limestone on the south. In the interior of the body the rock is massive fine-grained greenstone. The specimen examined under the microscope by C. S. Ross, which has the appearance of an extrusive igneous rock, seems to have come from one of these igneous bands. Phyllite (pCAp) .-Phyllite is almost as widely distributed as limestone and vol­canic rocks in the Allamoore formation but has considerably less volume; it generally forms units less than a hundred feet thick. Like the volcanics, its beds are poorly re­sistant to erosion and are commonly worn down to low ground between the limestone ledges. In many places the phyllite and volcanics are closely associated, and a single interval between the limestones may consist in one place of phyllite, in another of volcanics, and in others may contain both ·rocks. The original argillaceous constituents of the phyllite have been thoroughly altered to foliated, sericitic or talcose minerals. Most of the rock is gray to dark gray, but parts are black and apparently graphitic, and others are calcareous. Bedding is indi­cated by alternation of the gray, black, and calcareous varieties, generally in layers a few inches thick. Bedding is intensely contorted and is crossed by strongly marked slaty cleavage roughly parallel to the axial planes of the folds (Pl. 24, A). The rock weathers to white or ashen colors and splits into flakes, plates, or papery sheets parallel to the cleavage. The phyllites were originally shale units of the Allamoore formation and except for greater alteration are not unlike the calca­reous shales and thin interbedded lime­stones of the early Ordovician formations of the Marathon region. The rather inti­mate association of phyllite and volcanic units in the Allamoore is curious and per­haps significant; the argillaceous sediments may in part have had a volcanic source. Tumbledown Mountain section.-Some of the finest exposures of the Allamoore formation in the region are on Tumble­down Mountain, a high western spur of Beach Mountain 11/2 miles northeast of the Yates ranch house (R.5-8.5, Pl. 2; Pl. 11) . The Allamoore of the mountain has a synclinal structure and lies on the Hazel formation, probably as a result of thrusting on a "surface of movement"; another "sur­face of movement" within the Allamoore divides it into at least two parts. ,The syncline is bordered by later high-angle faults on the east and south, and the folded rocks are overlain unconformably on the east by the Bliss ( ? ) sandstone (Lower Ordovician) . The Allamoore formation of Tumble­down Mountain is much less altered than farther south, and in the upper beds the structure is simple and plain. The mountain is therefore one of the few places where a stratigraphic section of the Allamoore can be described and measured (fig. 5). Although this section is at least partly du­plicated by faulting, and although it has no top or base, it is of interest in showing a representative succession of the rock types of the formation (Table 17, p. 80). Blackshaft mine area.-Northeast of the Millican Hills in the midst of the Hazel formation, a thin bed of Allamoore forma­tion crops out from the Blackshaft mine (M.5-9.5, PI 2) for a mile and a half westward and northwestward past the St. Elmo and Sancho Panza mines. The forma­tion in this vicinity is of interest, as it is the host of productive ore deposits. In an earlier publication the writer (King and Knight, 1944) termed the bed a tuffaceous member of the Hazel forma­tion, hut Sample and Gould ( 1945) in­terpreted it, probably correctly, as part of the Allamoore formation on the sole of a thrust sheet. The bed of Allamoore formation lies on and is overlain by Hazel formation and is probably separated from the Hazel be­neath by a "surface of movement." The upper contact with the Hazel may approxi­mate the original unconformable surface between the two formations, hut it has also been subjected to some movements, as in mine workings the Hazel forms a smooth At the open cuts of the Sanch? P11:n~ mine, where the bed is about IO feet thick, it is gray, strong surface and rests on contorted and finely granular, siliceous rock, perhaps SJ? altered schistose Allamoore. The bed shows great limestone or tuff, marked by many wavy, ~rre~lar contortion, shearing, and pinching and laminae which are evidently not stratification. swelling, and is much more metamorphosed Similar siliceous rocks appear in pits a shoi:t distance to the northwest, where they are associ­ than the enclosing more competent Hazel ated with greenish tuffaceous elastics. About .a formation. It varies considerably in make­ quarter of a mile northwest of th~ open ?uts ~s up along its course and is a complex of a considerable body of coarse-gramed, d1abas1c limestone, carbonaceous shale, tuff, and igneous rock. Sample and Gould (1945) suggest that this might be a young intrusive, perhaps of igneous rock. Tertiary age, but it seems more likely that it is one of the volcanic members of the AllamooreAt the Blackshaft mine, where the bed is about formation. 5 feet thick, it consists of soft, contorted, schis­tose rock, probably originally tuffaceous, of lime­ Rock alteration.-Most of the rocks of stone, and of platy, rotten, black graphitic schist, in places resembling anthracite coal. The schist the Allamoore formation have been more is veined by calcite and contains sulfides. or less changed from their original charac­ On the ridge west of the St. Elmo mine the bed ter, perhaps mainly as a result of the de­ is highly mineralized cherty limestone, with some formation to which they have been sub­ associated fine-grained igneous rocks, possibly of extrusive origin. jected, but probably with the aid of hydro- Estimated Bliss ( ?) sandstone at top of section thickness Unconformity (in feet) Allamoore formation : Table 17. Stratigraphic section of Allamoore formation on Tumbledown Mountain. (10) Brown, thinly laminated limestone, with siliceous bands, forming massive ledges. Ex· posed only in small patches in core of syncline, beneath Bliss (?) sandstone................ 100 (9) (8) Dark greenisli "r dark reddish, amygdaloidal igneous rock, probably a lava flow or flows; in part interbedded with underlying sandstone.........­------·····----­--------­-----­-----------­--­Medium to coarse-grained, maroon-red sandstone, in part filled with dark grains a 200 few millimeters across, perhaps igneous detritals. The sandstone is well laminated, the (7) gritty beds alternating with the finer grained beds. The sandstone is darker and less reddish on freshly broken surfaces........----­-----------------------·-----------·--------­---· -···-­-------­--­--·-·-·--­Very massive, jagged-surfaced brown limestone, perhaps dolomitic. Forms top of re· 300 (6) sistant beds of mountain, the beds above being carved into a canoe-shaped basin be­tween the enclosing limestone ridges....---···-----·--­·-----­---------·----­--­---·---­------­--·------­---­--­---------­Silice9us beds, strikingly banded by light buff siliceous layers, and red-brown sandy­ 30 (5) calcareous layers, the respective layers being a fraction of an inch to 6 inches thick. Some bedding surfaces show faint small-scale ripple marks....---------------------­--­-----------­------­Main limestone body of the mountain. Pale gray or pale brown limestone, with some 200 dark gray or blue-gray limestone, the latter thinly laminated and possibly bituminous; the chemical analysis by K. J. Murata was from such rock. Chert is common in many beds but particularly in the more massive layers. Some of the chert bands in the massive layers follow a series of zig-zags, the points of which are hardly rounded; these seem not to be due to crumpling and may be original in the deposit, and perhaps caused by organic growth. At one place the top bed consists of massive pinkish limestone with some laminated structure that resembles Cryptozoon, whose upper surface rises into knobs and points that are overlapped by the succeeding (4) siliceous beds of member (6) (Pl. 23, B>----­------------------­-----·---------­---··---·­----------·---·­··­-·······--­Blue-black, fissile phyllite, sericitic and graphitic, with some blue-black homstone 250 and gray calcareous layers. Bare exposures at southwest end of mountain show in­ tensely contorted bedding caused by prominent slaty cleavage. Exposed only on south and southwest sides of mountain; pinches out between member (5) and underlying Hazel formation on north....·--------­-------------­------­-·------­----­---------­--------------­--­·-·····---­---····--------···-­ 100 (3) Greenstone, either an intrusi:"'e or a massive flow. Dense, fine-grained, tough rock, blue­ black where fresh, dark ohve·green where weathered; some fragments in float are amygdaloidal. Crops out only on south side of mountain__ ____ _____________________ _____________ _____ _____ _ 200 Surface of movement (2) Reddish sandstones of pyroclastic origin, with some interbedded lava. The member is !de~tical in appearance to bed (B) and quite different from bed (3), which overlaps (1) 1t m places........­---­-----·-­-------·-·· ···-···--·· -···---------­-------· ····-·-­·--­·-···----­--------­----------·-··---------­---­-------­Lower limestone, similar to bed (5); exposed only on lower western slopes of 250 mountain --------·-----------------------------------------------·-------·········---··········--·-·····----···-····--·-····-··--·--···-··--···-· 50-150 Surface of movement Hazel formation at base of section; overlain by bed (3) on south side of mountain· by bed (1) on west end ; and by bed (5) on north side. ' 5 NORTH A 4750 ' B 4750' c 1000 2000 Feet Fie. 5. Profiles showing stratigraphic section of Allamoore formation on Tumbledown Mountain ( Q.5---8.5, Pl. 2). A, South side of mountain; B, west end of mountain; C, north side of moun­tain. Numbers refer to units in section as described in Table 17. p€H =Hazel formation; p€v = Van Hom sandstone: Ob = Bliss (?) sandstone. Note "surface of movement" between Allamoore formation and underlying Hazel formation in all profiles, and that between beds 2 and 4 of profile B. thermal solutions. Alteration is least toward the north, as on Tumbledown Mountain, and increases southward to a maximum near the outcrops of metarhyolite of the Carrizo Mountain group that forms the overriding body of the Streeruwitz over­thrust. The limestones exhibit various types of alteration. Perhaps the most striking is marmorization, which occurs in small, er­ratically placed areas, perhaps related to intense flowage or squeezing, or to hydro· thermal activity. The limestones are re· crystallized to marbles of sugary texture, mostly white, but with reddish and bluish streaks, possibly inherited from original bedding. Striking specimens of "red, white, and blue" banded rock may be collected. Where the limestone was originally impure or shaly, it has been rendered schistose, and the original bedding laminae have been thrown into "jackknife" folds. Over a wide area immediately north of the trace of the Streeruwitz overthrust the limestones have been extensively silicified into rocks resembling those classed as jas­peroid in other areas. ,Thoroughly silicified limestones have lost all trace of bedding and are traversed by many quartz veinlets. Their weathered surfaces are brown and felty and are recessed around quartz veins and siliceous knots like other calcareous or carbonate-bearing rocks. However, they probably actually contain little lime, for they break with conchoidal fracture into dark gray, steel-hard chips. Where cherty limestones have been sheared, the chert layers are sliced, offset, and broken along innumerable planes of fracture cleavage, while the intervening less brittle limestone has accommodated itself by flowage around the chert. Where shearing is greatest, such rocks have been converted into a rubble of angular chert fragments, set in a reconstituted limestone matrix. The more massive diabasic volcanics may have undergone considerable internal min­eralogical change, but little of tl_iis is ap­parent to the unaided eye, and even the amygdules show little indication of com· pression. By contrast, the fine-grained tuff­aceous volcanics and the phyllite units are highly contorted and have been rendered thoroughly schistose. Silicification of the limestone near the Streeru· witz overthrust is well displayed near the ~eta­rhyolite outcrops 2 miles northeast of the village of Allamoore (E.5-2.5 and F.5-2.5, PI. ~). Here, the limestones are silicified for. half a mile north of the metarhyolite, but alteration seems to be greatest on the hill summits, presumably nearest the eroded sole of the overthrust, and is less in the valleys, where there are occasional outcrops of blue-gray limestone of more nearly normal aspect. The silicified rock is dense and very hard when broken but weathers to a brown, felty, calcareous­appearing surface, with an irregular network of more siliceous ridges and occasional quartz veins. Bedding is nearly destroyed, and the rock weath· ers into irregular ledges and craggy blocks. Some of the altered rock is distinctly granular and might have been derived from sandstone rather than limestone; there are a few patches of schist, probably derived from volcanics. Close to the metarhyolite is much cream·colored, streaky, massive marble, which at one locality is interbedded with blue, blue-green, or blue·gray calcareous schist, containing blebs of bright blue mineral. In places the blue schist weathers out in "pencils" and "logs" that are probably of tectonic origin. The blue mineral has been identified as an amphibole by Horace Winchell of Yale Univer· sity, who made the following tests on it: COa present by acid test. Fe•• and Fe*** present in HCl solution. X-ray pattern shows most of lines of tremo· lite-actinolite, with remarkably good agreement indicated. Fibrous habit typical of amphibole. No tests for Na, hence for glaucophane; could be this. Two specimens of altered limestone from near the Streeruwitz overthrust in the hills 2 miles northeast of Allamoore were examined under the microscope by F1awn (memorandum of February 1952), who reports as follows: "One specimen, which looks like layered petri· fied wood in hand specimen, and weathers to a gray, rough, slaggy surface, is a tourmaline­biotite-albite limestone. It has a granoblastic to crystalloblastic fabric, with grains 0.04 to 0.2 mm. in diameter. Carbonate comprises 84 percent and forms a fine mosaic. There is also 8 percent albite (scattered grains showing secondary over· growths), 3 percent green-brown bioti~ (patches), 2 percent tourmaline (green, brown spongy crystals, 2 percent of opaque mineral (spongy, poor-reflecting grains), 1 percent seri· cite, and traces of apatite and leucoxene." "The other specimen is a fine-grained banded pure calcite marble of light buff or pink color, with a granoblastic fabric. Calcite comprises nearly the whole rock and is a mosaic of twinned and untwinned grains between 0.04 and 0.1 mm. in diameter; there are tr!lces of sericite." Interpretation of Allamoore formation. -The limestones of the Allamoore forma· tion were probably laid down in a marine environment. They differ little in chemical composition from marine limestones of Paleozoic age that occur in the same region and like them contain appreciable amounts of bitumious material, as well as reefy structures of possible algal origin. The in· traformational conglomerates suggest, how­ ever, that the water was relatively shallow. The phyllites and the finer-grained, well­ bedded pyroclastics also have the appear­ ance of water-laid sediments, and the phyl­ lites are comparable to the less altered marine shales of the Paleozoic rocks. There is, however, no evidence that the lava flows were erupted under water. Pillow struc­ tures have not been observed, although it is possible that other features characteristic of subaqueous eruption might be discov­ ered by further study. As the water in which the nearly contemporaneous inter· bedded sediments were laid down was prob­ ably relatively shallow, the flows may have been built up on the sea floor to such an extent that they projected above the sur­ face. The rocks of the Allamoore formation have many of the characters of eugeosyn­ clinal deposits as described and defined by Kay (1951, pp. 4-5, 69-77). Large vol­ umes of lavas, pyroclastics, and other vol­ canic rocks are diagnostic of the eqgeosyn­ clinal facies. Limestones, it is true, are more characteristic of the miogeosynclinal facies, but those of the Allamoore are highly cherty and are interlayered with bedded siliceous rocks like those common in eugeo­ synclinal deposits. Stratigraphic relations.-The base of the Allamoore is nowhere exposed. It is in con­tact on the south with the possibly older metarhyolite of the Carrizo Mountain group, but this has been emplaced by move­ment on the Streeruwitz overthrust and overlies rather than underlies the Alla­moore. Farther southeast, in the Carrizo Mountains, the metarhyolite intrudes a thick sedimentary sequence, described by Flawn elsewhere in this report. The relation of the sedimentary rocks of the Carrizo Mountain group to those of the Allamoore formation have not been determined; the first may represent a downward continua­tion o~ the second, or the two might be far apart m age. The Allamoore formation is also in con­tact with the Hazel formation. From struc­tural relations alone it is not possible to de­termine the original nature of the succes­sion, for either formation may overlie the other. However, the conglomerates of the Hazel indicate clearly that this is the younger formation, as they consist in large part of fragments of limestone, volcanics, and other rocks identical with those in the Allamoore and derived from its erosion. The conglomerates show, further, that the Hazel lies unconformably on the Allamoore, and their great thickness and the coarseness and angularity of their fragments suggest that the Allamoore depositional epoch was closed by orogeny, which probably con­tinued into the initial phases of Hazel dep­osition. This is further attested by the fact that some of the limestone fragments in the conglomerates are marmorized in the same manner as parts of the limestone still in place in the Allamoore. The nature of the deformation caused by the pre-Hazel orogeny is somewhat obscure, as a considerable part of the structure now visible in the Allamoore is shared with the adjacent Hazel and is the result of a later, post-Hazel orogeny. The contact between the two formations is strongly discordant in most places, but it is generally sheared and faulted along a "surface of movement," so that it is difficult to determine the original degree of angular divergence between them, or the amount of irregularity of the surface on which the conglomerate was deposited. At several localities there is a suggestion that the angular discordance between the Allamoore and Hazel is not entirely caused by the "surface of movement" which separates them but results in part from original angular unconformity. North of the Anaconda no. 2 prospect a mile east of the Garren ranch house (H.5--7.5, PI. 2) conglomerates of the Hazel formation are in con­tact within short distances with reddish tuffa­ceous sandstone, limestone, and massive, boul­dery lava. The conglomerate includes fragments of these rocks, as well as of others seen in place deeper in the Allamoore in the same vicinity. Southeast of the Garren ranch house the conglom­erates of the Hazel contain boulders of the lime­stone-pebble conglomerate that is interbedded in the limestones of the Allamoore formation near by, as though they were derived from uptilted ledges of this rock. In the southern Streeruwitz Hills northwest of Eagle F1at section house the Allamoore is bor­dered on the north by coarse, massive conglom­erate of the Hazel formation which projects in a high? dark ridge. Strikes and dips of the two format10ns are nearly parallel but successive limestone, volcanic, and phyllit; members of the Allarnoore are truncated by the conglomerate toward the west ( C.5--10.5, PI. 3) , and the conglomerate itself wedges out against the Alla­moore toward the east (F.5--10.5, Pl. 3). These relations may be the result of faulting along a "surface of movement" but they may in part be caused by original angular discordance between the Allamoore and Hazel and by irregular deposi­tion of the conglomerate against a surface of con­i;oiderable relief. HAZEL FORMATION lntroduction.-The Hazel formation crops out in many parts of the deformed area immediately north of the trace of the Streeruwitz overthrust, where its rocks are intimately infolded with the Allamoore for­mation. Unlike the Allamoore, however, it is also exposed much farther north, in areas of less deformation. Outcrops extend around the south and east sides of the scarps of the Sierra Diablo, where they reach to within a few miles of Victorio Peak, and a large inlier also occurs in the interior of the range near Sheep Peak. Not all these northern outcrops are shown on the geologic map (Pl. 2) ; for the whole ex­tent of the formation the reader should re­fer to the general geologic map of the Sierra Diablo region (King and Knight, 1944). The Hazel formation consists of two sorts of rock, red sandstone which characterizes the upper and northern part, and con­glomerate which characterizes the lower and southern part. The two rocks are in­timately interbedded. Adjacent to the Alla­moore formation are thick, massive layers of solid conglomerate, but farther away and higher in the sequence conglomerate beds become thinner and more sandy and are separated by increasingly thicker beds of red sandstone, which finally dominate altogether. The conglomerate may also wedge out northward in the sandstone, away from the uplifted masses of Alla­moore formation from which it was derived, although this relation is difficult to prove because of the complex structure. The Hazel is clearly a very thick forma­tion, but structural relations are such that only incomplete estimates of thickness can be made. Extending northward from the Al­lamoore formation in the central Streeru­witz Hills there appears to be a continuous, steeply dipping succession at least 5,000 feet thick; of this, more than half is con­glomerate. On the eastern scarps of the Sierra Diablo near the Pecos mine, north of the map area, at least 2,250 feet of gently dipping sandstone is exposed. Conglomerate (pCHc). The conglomer­ate of the Hazel formation is one of the more resistant rock units of the pre-Cam­brian succession and in places forms ridges as high as, or higher than, those . of the limestones of the Allamoore format10n. In the southern belts of outcrop in the Streeru­witz and Millican Hills it projects in high, irregular ridges, covered by deep brown or black-surfaced, craggy ledges or blocks. Farther north it is less consolidated and forms fewer ledges, although it still rises in high hills. The conglomerate consists almost en­tirely of fragments derived from the Alla­moore formation, and particularly of its limestones. All types of limestone are pres­ent, including the chert-seamed, laminated, and carbonaceous varieties, as well as a lesser number of marmorized pieces. There are also occasional cobbles of bright red, jaspery limestone or chert. Almost as abun­dant as the limestone fragments are those of volcanic rocks, including massive, dia­basic or amygdaloidal lavas, and coarse to fine pyroclastics. The finer pyroclastics in some of the fragments appear to have been rendered schistose before having been in­corporated in the conglomerate. In a few places the conglomerate also contains cobbles and boulders of red gran­ite and coarse rhyolite porphyry. These are especially abundant between Carrizo Spring and Tumbledown Mountain in the eastern Millican Hills (N.5-3.5 and P.5-8.5, Pl. 2), but a few were seen in the northwestern Streeruwitz Hills (N.5-14.5, PI. 3). The granite and porphyry are unlike the ig­neous rocks in the Carrizo Mountain group but closely resemble those in boulders of the succeeding Van Horn sandstone. Like those in the Van Horn they resemble pre­Cambrian granite and porphyry exposed some distance to the northwest, in the Pump Station Hills and southern Hueco Mountains. The known distribution pattern of boulders in the Hazel does not, how­ever, suggest a northwestward source, and they may have been derived from other areas, as yet unknown, where they are now concealed by younger sediments. Fragments in the conglomerates are of all sizes, ranging from fine grits and peb· bles up to large blocks, mainly of lime­stone, 6 feet or more across, The large blocks are most abundant in the lower part, near the contact with the Allamoore forma­tion, but they are surprisingly common hundreds or even thousands of feet above the base. The fragments are generally poorly rounded and many are angular, in this respect differing notably from those in the overlying Van Horn sandstone; the dif­ference in rounding provides a ready means of distinguishing the Hazel from the Van Horn in the field. The conglomerate matrix is variable. That in the lower and southern conglomer­ate beds, which are most resistant to ero­sion, may originally have been highly cal­careous, but it generally has been silicified and impregnated with iron on weathered surfaces (Pl. 24, B). In the higher and more northern conglomerate beds the ma­trix contains a larger proportion of sand and arkose, causing the rock to be softer and less coherent (Pl. 25) . Most of the fragments lie helter-skelter in the conglomerate, pieces of all sizes, shapes, and composition being mingled without sorting. Little or no grain gradation is visi­ ble from base to top of a single layer, and in the more massive phases bedding itself is difficult to see. In the higher conglomer­ ates there are occasional finely gritty seams and lenses or thin interbedded layers of red sandstone. It is rather surprising, however, that sediments transitional from conglomer­ ate to sandstone are small in volume. There are no coarse sandstones or pebbly sandstones of any thickness near the con­ glomerates. Instead, many of the coarse conglomerate beds are inserted abruptly between red sandstones that are as fine and silty as any elsewhere in the area. The thickness of the conglomeratic part of the Hazel formation seems to vary from place to place along the outcrop, but it is uncertain how much of this is the result of original differences in amount of conglom­ erate deposited, and how much the result of structural complication. In the central Streeruwitz Hills, next to the southern out­ crop belt of the Allamoore formation, con­ glomeratic beds are at least 3,000 feet thick, of which the lower half is solid conglomer­ ate and the upper half is interbedded con­ glomerate and sandstone. Conglomerate of similar structural relations in the southern Millican Hills is about 2,000 feet thick. Adjacent to other belts of Allamoore formation farther north, conglomeratic beds of the Hazel formation are more variable and in places much thinner. Such variations may be seen on the map along the edges of the belt of Allamoore which extends east and west from the Garren ranch house in the northern Millican Hills. In these north­ern belts the conglomerate may have been partly cut out along the "surface of move­ment" between the two formations, hut at least some of the variations may be original. Conglomerate occurs in two areas that are sur· rounded by outcrops of the Allamoore forma­tion, one lying north and east of the Dwees ranch house in the southern Millican Hills (B.5­ 4.5 to E.5-4.5, Pl. 2) and another 2 miles north· west of the Canning ranch house in the north­eastern Bean Hills (N.5-13.5, Pl. 3). These conglomerates are probably part of the Hazel, although they differ somewhat from the usual con· glomerates of that formation; they seem to be syn· clinal remnants deeply infolded in the Allamoore, on which they rest unconformably and without an intervening "surface of movement." Near the Dwees ranch house the matrix is a brown-weath­ered limestone, which encloses fragments of lime­stone a few inches to a foot across, and a few fragments of brown-red volcanics. The rock is greatly sheared and bedding is almost obliterated. In two places the southern outcrops of Alla­moore formation are succeeded on the north by thick, massive conglomerates. One is in the cen­tral Streeruwitz Hills northwest of Eagle Flat section house (C.5-10.5 to F.5-10.4, Pl. 3), the other in the southern Millican Hills between the Garren ranch house and Carrizo Spring (E.5-5.5 to N.5-3.5, Pl. 2); the latter ou.t­crops are crossed by the county road 2~ miles north of the village of Allamoore. The conglomerate of both areas is presumably the basal unit of the Hazel formation, although it is so discordant with the Allamoore on the south as to suggest that the two formations are separated in both places by a marked uncon­formity or "surface of movement." In the central Streeruwitz Hills the lower massive conglomerate is about 1,500 feet thick and is succeeded by an equal thickness of interbedded conglomerate and sandstone. In the southern Millican Hills the massive conglomerate is about 2,000 feet thick and is succeeded by sandstone, with only a little interbedding at -the contact. In both areas, the matrix is now thoroughly silicified and deeply crusted by iron, so that the rock projects in mas­sive ledges in which bedding is scarcely apparent (Pl. 24, B). Most of the fragments are a few inches to a foot in diameter, but there are some larger angular blocks. In the Streeruwitz Hills the pieces are mainly blue and gray cherty limestone like those in the Allamoore, but there are some of bright red siliceous jasper and of a schistose rock that may have been derived from altered pyro­clastics. The pieces in the Millican Hills are simi­lar but include some boulders of limeston~ pebble conglomerate. Granite porphyry occurs in I-foot boulders near Carrizo Spring, but nowhere else in the two areas.. Near the middle of the unit in the Streeruwitz Hills are lenses of thin­bedded brown sandstone and toward the top are thicker layers of red pebbly sandstone. The suc­ceeding conglomerates interbedded with red sandstone have a sandy, less coherent matrix than t~at bel.ow; even these higher beds contain occa­ s10nal limestone blocks 2 to 4 feet across. The conglomerate of one of the more northern belts is well exposed in the northeastern Milli­ can Hills, east of the Anaconda no. 1 prospect and south of the Blackshaft mine (J.5--7.5 to N.5--7.5, Pl. 2), where it is deeply dissected by Hackberry Creek and its tributaries. One of the most str!king outcrops is on Hackberry Creek half a mile southeast of the Blackshaft mine ( N .5 -8.5, Pl. 2), where the channel drops abruptly 50 feet or more over a vertical bed of conglom­ erate 275 feet thick, lying in red sandstone. In the northeastern Millican Hills about 65 percent of the fragments are limestone, 30 per­ce!1t are lav~s and pyroclastics, and 5 percent are mmor constituents. The limestone and volcanic fragments are mostly pebbles and cobbles but there are occasional boulders and blocks of 'lime­stone as much as 4 feet across. Large blocks are not confined to the conglomerate nearest the Allamoore, but occur far distant from it and pre­sumably high above the base of the Hazel forma­tion. The volcanics include red diabasic lavas dark aphanitic basalts, and red tuffaceous sand: stones. The minor constituents include red granite porphyry and marmorized limestone. The por­phyry fragments are conspicuous but never dominant constitutents; some fragments reach 2 feet in diameter and unlike comparable pieces in the Van Horn sandstone are only moderately rounded. The fragments of white, crystalline marmorized limestone contrast notably with thos~ of the l~ss altered limestones with which they are assocmted. In the northeastern Millican Hills th~ fragments lie in a sandy or gritty matrix, con­tammg numerous small chips or angular grains of a composition similar to that of the larger pieces. During field work, numerous attempts were made to employ graded bedding for de­termination of tops and bottoms of beds but usually with indifferent results. Many grad~tions from coarse to fine and fine to coarse were ob­served, but they provided conflicting evidence even within the same outcrop. ' Red sandstone (pCHs) .-Red sandstone forms the most conspicuous and extensive outcrops of the Hazel formation. It crops ~ut on smoothly rounded red surfaces, only hghtly masked by vegetation. These extend to considerable heights on the scarps of the south and east sides of the Sierra Diahlo where they are surmounted by light gra; cliffs of the Hueco limestone (PI. 28, B). At the bases of the scarps the slopes flatten into broad pediments, capped in many places by benches of Quaternary gravels, but mostly carved into an intricate network of hills and valleys, such as those between Beach Mountain and the Hazel mine (0.5 -13.5, PI. 2). The hills are dome-like, with rounded crests that descend into steep slopes near the incising streams. The dis­sected hills have an especially pleasing as­pect toward sunset, when lights and shad. ows bring out details of their modeling. The red color of the sandstones is uni­versal, ranging from brick-red in the north­ern exposures to a darker maroon-red far­ther south. Most of the sandstones are very fine grained, verging on or grading into siltstones. They coarsen somewhat toward the south, where there is more variation in texture from bed to bed; here some of the coarser layers are traceable in long strike ridges. Part of the rock is rather loosely consolidated, hut most is fairly hard and well-cemented, so that it has a conchoidal fracture. The following reports on micro­scopic study of specimens of the sandstone are available. (1) G. B. Richardson (1914, p. 4), locality not stated: "The texture is characteristically sedi· mentary and the rock is composed chiefly of subangular or rounded grains of quartz and sub· ordinate feldspar, with some flakes of muscovite and rather abundant interstitial calcite, the min· erals bein~ coa!ed with a. thin film of red pig· ment-fernc oxide-that gives the uniform color to the rock." (2) S. J. Lasky (memorandum of October 28, 193!l), from near Hazel mine: "A fine-grained, arg1llaceous, calcareous sandstone, slightly feld· spathic, with angular grains and a red iron-oxide pigment in the matrix." (3) Charles Milton (memorandum of Sep­tember 26, 1951), from hillside northwest of Garren ranch house: "Consists of evenly-sized angular to sub-angular particles of quartz and alkalic plagioclase, with a small amount of potassic microcline feldspar and colorless mica in worn wisps, and a few opaque grains of hematite. Calcite, which makes up 5 to 10 percent of the rock, is interstitial." Bedding of the sandstone is indicated in most places by thin, closely spaced dark laminae. These are commonly cross-bedded on a small scale, and in places there are rip­ple marks on the bedding surfaces. In the structurally complex areas attempts were made during field work to use cross-bed­ding and grain gradation as evidence for tops and bottoms of beds, but uniformly successful results could not be obtained in the time available. Over most of the area the rock does not split along bedding planes, f~r these a~e thoroughly welded to­gether; mstei:d, 1t breaks into angular bl~cks .al?ng mnumerable vertical or in· chne? J omts, so that in many places the bedd~ng m~y be determined only by close scrutmy. Richardson (1914, p. 4) reported that the sandstone. beds near the Hazel mine stood nearly vertical, whereas outcrops in this vicinity show faint but persistent bed· ding which lies nearly flat. The total thickness of the red sandstone has not been determined but probably reaches thou­sands of feet. The thickness locally exposed on the east-facing escarpment of the Sierra Diablo from the Pecos mine northward (nonh of Pl. 2) can be estimated, as south-dipping lines of bed­ding show up faintly but distinctly on the red sandstone slopes below the cliffs of Hueco lime­stone. Measurements in this area yield the fol­lowing results: also extends in ramifying stringers through the rock. Specimens of this material from near the Garren ranch house were examined under the microscope by Charles Milton, who reports as follows (memorandum of September 26, 1951) : The black material on high magnification re­solves itself into aggregates of tourmaline. The tourmaline is stubby prismatic, brownish-green, weakly pleochroic. It is unquestionably authi­genic. With the tourmaline is fine black dust of Table 18. Estimated thickness of red sandstone of Hazel formation near Pecos mine Thickness (in feet) Hueco limestone, with Powwow member at base, forming cliffs at top of escarpment. Unconformity Red sandstone of Hazel formation: (4) Youngest and highest beds, which come in southward, between Pecos mine and Hazel mine; thickness undetermined ···---------·------·-·--···----------------------------------··----····-·-·--------------------··· ? (3) Beds exposed on escarpment above Pecos mine, disregarding those below and to east on pediment. Tracing of bedding indicates that base of this unit rises to base of Hueco a little beyond Bat Cave, 2 miles to the north·---······-----··-···-·-----··········-···-········-··-··········--· 750 (2) Thickness of beds on escarpment north of Bat Cave·---------···-----------·····-------········----·-······---· 1,000 (I) Older and lower beds, exposed mainly on pediment north of last locality; much more than ·-·······-·----·-·····--------------·---·······---··········--············----·-·········-·---··--···-----·---·····---··--·········---········ 500 Total thickness, more than_________ Rock alteration.-Rock alteration is gen· erally less severe in the Hazel formation than in the Allamoore formation. This is partly because the formation generally lies farther from the center of strong deforma­tion, and partly because its sandstones and conglomerates are both much more com· petent than any of the rocks in the Alla­moore. In many places in the strongly deformed areas to the south the conglomerate has been sliced along a multitude of closely spaced shear or cleavage planes. These are most prominent in the matrix but are likely to cut through the fragments as well. Flat· tening of the fragments is rare and is con­fined to the most strongly deformed areas. The sandstones are relatively little al­tered, and pressures were evidently dis­persed along innumerable clean-cut vertical and inclined joints. Because the sandstone has not been readily sheared or squeezed in the strongly deformed areas, it has prob­ably been sliced by many minor thrust planes. These are especially abundant in the belt of sandstone in the Millican Hills that extends eastward from the Garren ranch house (F.5-6.5, Pl. 2), where they are marked by slickensided surfaces, com­monly coated with a black highly polished substance a few millimeters thick, which -------·····---··--··--····--------·-·----·············-----········-----------2,250 hematite, identified as &uch from its appearance in polished section. It is believed that the tourmaline­hematite aggregate accounts for the blackness in hand specimen and the opacity in thin section. Graphite and carbonaceous matter were consid­ered as a possibility, but were not found by chemi­cal or microscopic study. Some veinlets of quartz and calcite also traverse the rock. It is of interest to note that Flawn has observed quartz-tourmaline veins in the Carrizo Mountain group of the Carrizo Mountains. The relation between the intro­duction of tourmaline in the Carrizo Moun­tains and the Millican Hills is uncertain. Flawn interprets the quartz-tourmaline veins as probably caused by pre-or syn­metamorphic hydrothermal activity, where­as the tourmalinized slickensided surfaces appear to have formed during the late stages of the orogenic epoch. Near some of the large, high-angle faults north of the Millican Hills, notably the Grapevine fault (N.5-9.5, Pl. 2), the sandstone appears to have been mashed and reconstituted. Near such faults it loses its flinty, conchoidal aspect and becomes massive, soft, and earthy. Near many faults, joints, and fractures in the northern area the sandstone is de­colorized. For several inches or feet away from the fracture the red rock has been bleached to buff or yellow. The bleaching may be related to ascent of mineralizing solutions, as it is best developed near pros· pective or productive veins; it is consider ad as a favorable indication of mineralization by the local prospectors. Bleaching of the red sandstone was also noted immediately beneatQ. the unconformity at the base of the Hueco limestone in the buttes 3 miles south-southeast of the old Circle ranch house (E.5-12.5 to G.5-12.5, Pl. 2); this either took place during the pre-Hueco erosion interval or during later circulation of ground water. Interpretation of Hazel formation.­ From the foregoing description, it is evi­dent that the Hazel formation is an unusual deposit. The thick, coarse, poorly rounded and sorted conglomerates, lying in or wedging into fine-grained or silty red sand­stones, certainly must be "tectonic sedi­ments," formed not merely from destruc· tion of a deformed terrane after the close of a period of orogeny but from active ero­sion of rising folds and fault blocks while orogeny was still in progress. They are re· markably similar to late Cretaceous and Tertiary conglomerates and sedimentary breccias in southern Nevada and the Mo­jave Desert region of California, which the writer has seen under the guidance of C. R. Longwell, D. F. Hewett, and L. F. Noble. One of these, the Overton fanglomerate of the Muddy Mountains of Nevada, is re· lated by Longwell (1949, pp. 933-940, 963-964) to the advance of the Glendale thrust, and in fact contains outlying klip· pen of the thrust, buried in their own debris. Like the conglomerate of the Hazel, the fanglomerate of the Overton is inter­bedded with, and wedges into, fine silty sediments. The conglomerate of the Hazel also strongly resembles the Etholen con· glomerate (Huffington, 1943, pp. 1007­1009; Smith and Albritton, 1949, p. 1921), lying in the midst of the Cretaceous in the Sierra Blanca area, not far to the west in Trans-Pecos Texas. The coarse, angular limestone fragments of the Etholen have a different source from those of the Hazel, as many of them contain Permian fusu­linids. The origin of the fine-grained, silty red sandstones of the Hazel formation is per· haps more perplexing than the conglom­erates. Their well-marked laminae, and associated ripple marks and cross-bedding, suggest subaqueous deposition, hut if they are marine, the environment was wholly different from the supposed marine deposits of the preceding Allamoore. They some­what resemble the poorly fossiliferous red siltstones and fine sandstones of the Lower Cambrian Rome formation of the Southern Appalachians, which have been interpreted as marine deposits derived from weather· ing of the regolith of the continental in· terior. They also somewhat resemble, except in color, the fine-grained Tertiary inter· montane deposits which occur in many places in the Cordilleran province, some of which are associated with "tectonic sedi· ments" of the sort already noted. ,The origin of the red color of the sand· stones of the Hazel formation is uncertain. The writer has heard the origin of "red beds" debated since his student days, but to his knowledge no completely satisfactory explanation has yet been offered. It is certainly remarkable that the two red formations of the section in the Van Hom region-the Hazel and Van Hom-should lie one on the other, even though the en· vironments in which the two were deposited were not entirely the same. Stratigraphic relations.-The Hazel for· mation is overlain by various much younger strata, including those of Permian and Cretaceous age, but the next youngest formation which succeeds it is the Van Horn sandstone. Even with the Van Horn, its relations are strongly unconformable, and the Hazel and Van Hom epochs are separated by a major period of orogeny, by which the Hazel and Allamoore formations were strongly deformed, and by a pro· longed period of erosion, during which they were deeply cut. Emplacement of the Car· rizo Mountain group along the Streeru· witz overthrust probably took place during this orogeny, as the conglomerates of the Van Horn are the first in the sequence to contain fragments of lineated and mylonit· ized metarhyolite. The Van Hom of the Sierra Diablo foothills lies not only on the Hazel hut in places on the Allamoore; in the eastern Carrizo Mountains, as shown by Flawn, it lies also on the Carrizo Moun· tain group. Despite the marked unconformity be· tween the Hazel and Van Horn, their sedi· ments are surprisingly alike. There are, of course, differences in their characters 0 500 1000 Feet A t1 ..._ ..._ B -----.... , ----------B -..._ -......A I WEST 0 I Mile ­ B Fie. 6. Sections showing unconformable relations between Van Horn sandstone ( p£v) and Hazel formation (p£H) northwest of Yates ranch house. A, Hill of Hazel formation buried by Van Horn sandstone and partly resurrected, half a mile northwest of Yates ranch house (0.5-6.5, Pl. 2); B, sketch section westward from Yates ranch house and valley of Hackberry Creek, to summit of Millican Hills, showing resurrected surface on which Van Hom sandstone was deposited (A-A'), truncated by a later surface at the summit of the hills (B---B'), which is probably of Tertiary age. in detail, but both include red, unfossilif­erous elastics and coarse conglomerates made up of fragments of the older rocks. Both were probably laid down in conti­nental environments, near areas of con­siderable relief. Some of the best exposures of the contact be­tween the Hazel and Van Horn may be seen along Hackberry Creek near the Yates ranch house (0.5-6.5, Pl 2), between the Millican Hills and Beach Mountain (Pl. 11). In this vicinity, Rich· ardson (1914, p. 4) reported that the gently dipping Van Horn lies in almost vertical beds of the Hazel, but this is true only locally; dips are generally lower than vertical, although the struc· lure is highly complex. The contact between the two formations is well exposed on the north side of a ravine leading into Hackberry Creek from the west, near the road to the Yates ranch house and half a mile to the south ( 0.5-5.5, Pl. 2) . Here, the beds of the Hazel dip 60° north and are truncated by an undulatory erosion surface. The overlying Van Horn lies nearly flat and contains at the base rounded boulders of the Hazel a foot in diameter. As shown on the map (Pl. 2), the contact along Hackberry Creek has a sinuous course, the sinuosities being the result of partial stripping of a moderately hilly surface of Hazel on which the Van Hom was deposited. In places, hills of the Hazel project 50 feet into the Van Horn, which dips away from them at angles as much as 10° (fig. 6, A). The Van Hom is not con· spicuously more conglomeratic near the hills than elsewhere, and its cross-beds maintain their usual southward dip. The terrain west of the Hazel-Van Hom contact seems to be an exhumed part of the pre-Van Horn landscape. Here, a long, rugged slope rises westward from Hackberry Creek to the summits of the Millican Hills, at about the same angle as the dip of the Van Hom to the east (fig. 6, B). The slope is truncated at the summit of the hills by a nearly level surface, which may represent a much later peneplain, perhaps of Tertiary age. PRE-CAMBRIAN (?) ROCKS VAN HORN SANDSTONE (pCv) lntroduction.-The Van Horn sandstone is exposed in relatively small areas in the Sierra Diablo foothills, the wide dispersal and isolation of the outcrops being due in no small measure to its unconformable re­lations with various formations that overlie it. All its outcrops are shown on the ac­companying maps (Pis. 1, 2, 3) , except for a few small areas in the Baylor Moun­tains, which are shown on the general geologic map of the Sierra Diahlo (King and Knight, 1944). The most extensive out­crops are in the Red Valley northwest of the town of Van Horn and southwest of Beach Mountain (K.5-2.5 to R.5-2.5, Pl. 2), which Richardson (1904, p. 28) designated as the type area of the forma­tion. It is also exposed on the northwest corner of Beach Mountain south of the B­Bar ranch house (S.5-10.5, T.5-11.5, PI. 2; PI. 28, A) ; on the southeast corner of the Sierra Diablo between the Hazel and Pecos mines (N.5-18.5, 0.5-16.5, PI. 2), and on the south-facing scarp of the Sierra Diablo 4 to 7 miles west of the old Circle ranch house (M.5-18.5 to R.5) -18.5, Pl. 3; PI. 27, A; fig. 7). Flawn has found some outliers of the formation in the eastern Carrizo Mountains, south of the Texas and Pacific Railroad. The Van Horn is characteristically a coarse, red, arkosic sandstone in thick or massive beds, mostly friable and poorly consolidated, cross-bedded in many places, and containing occasional scattered peb­bles. No fossils are known. The sandstones are interhedded with and are underlain by thin to thick beds of conglomerate, made up of rounded pebbles, cobbles, and even boulders, that are made up not only of the older rocks of the immediate area but also of granite and rhyolite porphyry un­like any exposed in the vicinity. The great· est thickness of the formation preserved at any one place is about 800 feet, but it is generally much thinner. The red, massive, elastic rocks of the Van Horn form some of the most striking outcrops in the region (Pis. 11, 27, and 28, A). Its sandstones project in great rounded ledges, largely barren of vegeta­tion, and on the faces of escarpments rise in picturesque towers, prows, and battle­ments. Tables, pedestals, and other fantas­tic erosion forms are common. The massive ledge-making beds are commonly separated by others less resistant to erosion, gener­ally more conglomeratic, which form in. tervening shelves, in places overhanging. Widely spaced joints, commonly set at right angles, are worn into creases or crev­ices that extend across the outcrops and are prominently visible from adjacent mountain tops or in air photographs. Stream channels across the outcrops form rounded swales on the shelves but descend across the ledges, with many swirls and potholes, in narrow slots that follow one of the prevailing joints. Conglomerate.-The conglomerates of the Van Horn form beds a few feet to several hundred feet thick, which are com· monly thickest and coarsest at the base and thinner and finer grained higher up. Basal conglomerates are of variable thickness. In the section 4 miles west of the old Circle ranch house (Q.5-18.5, PI.3), described below, they are more than 300 feet thick (beds 1 to 5), hut they thin both eastward 5000 Feet 0 Horizontal .scale 2000 Feet 0 Vdrficol .scale .FIG. 7. Profile sho:-ving rocks exposed on south-facing escarpment of the Sierra Diablo, 21h to 6 miles west of old Circle ranch house (T.5-17.5 to N.5-18.5, Pl. 3) . Numbers indicate beds in measured section of Van Horn sandstone which is described in text. p€Av =volcanic member o.! Allamoore formation; p€Als =lime1;tone member of Allamoore formation; p€H = red sandstoneti of Hazel formation; p€v = Van Horn sandstone; Ph = Hueco limestone, with Php Powwow ~~~~ . and westward-to the east and up the dip apparently against a buried hill of lime­stone of the Allamoore formation (fig. 7). In some other places, as near the Yates ranch house, there is no well-defined basal conglomerate, although conglomerate beds and dispersed pebbles are common through­out the section. Fragments in the conglomerate are made up of pebbles, cobbles, and boulders, those in the basal beds 4 miles west of the old Circle ranch house (bed 1 of section) be­ing as much as 2 feet in diameter (Pl. 26, A). The great majority are smoothly and perfectly rounded, and some have almost polished surfaces; a few of obvious local derivation show somewhat less rounding. Many of the fragments have been shattered, sheared, and offset by subsequent deforma­tion, but many of these were afterward recemented. The fragments are set in an arkosic sandy matrix, ordinarily somewhat less consolidated than the adjacent sand­stones, in some being closely packed, in others widely dispersed in the matrix. Weathering of the poorly consolidated matrix sets free vast quantities of the rounded fragments, which strew the out­crops of the formation. The fragments in the conglomerate con­sist of a wide variety of older rocks, but the dominant ones, especially in the north­ern and northwestern exposures, are red granite and red rhyolite porphyry. They are prominently displayed in outcrops 4 miles west of the old Circle ranch house (Pl. 26, A), where a suite of specimens was collected from boulders near the base (bed 1 of section). These rocks were examined under the microscope by C. S. Ross, who reports as follows (memorandum of June 21, 1938): (A) Strongly porphyritic rock. Phenocrysts are sodic plagioclase, microcline, and quartz. Both feldspar and quartz have been somewhat rounded by resorption. Magnetite fonns large, irregular grains. The ground-mass is a very fine, equi­granular mass of interlocking grains of quartz and feldr.par. On the whole, the rock is fresh, but a little sericite has developed, especially in the plag­ioclase. (B) Related to preceding, but phenocrysts are very sparse. The magnetite grains are altered to hematite, and abundant hematite pigment has developed in the ground-mass of nearly equi­granular feldspar and quartz grains. (C) Dominantly phenocrysts of microcline and minor amounts of sodic plagioclase. A small pro­portion of interstitial material is quartz and feld­ spar. This tends to be micrographic in habit, with the late feldspar continuous with that of the phenocrysts. Feldspar somewhat altered to seri­cite; hematite pigment present; no ferromag­nesian minerals. (D) Probably belongs to same group as first three, but feldspar phenocrysts are surrounded by very abundant micrographic intergrowth of feldspar and quartz. The only other primary min­erals are magnetite and a green biotite. The rock is more altered than others of the group, and secondary chlorite, serpentine-like mineral, and abundant pigment have developed. (E) Very coarse-grained granitic rock, com­posed of microcline, quartz, and sodic plagioclase. No interstitial fini>-grained material. Rock fresh except for hematite, some of which seems to be introduced. These rhyolites and granites differ greatly from any igneous rocks exposed in place in the Van Horn region. They are unlike the metarhyolites of the Carrizo Mountain group, except possibly the coarse phase 41h miles northwest of Allamoore. Moreover, fragments clearly derived from the metarhyolites are also present in the Van Horn. The rhyolites and granites most closely resemble pre-Cambrian rocks ex­posed some distance northwest of the Van Horn region, such as the red rhyolite por­phyry of the Pump Station Hills (described elsewhere in this report) and the coarse red granite at the south end of the Hueco Mountains (King, King, and Knight, 1945, sheet 1). Mr. Ross was asked whether there was petrographic evidence to justify compari­son of the fragments in the Van Horn sand­stone with rocks exposed to the northwest. He comments that it is very probable that specimens examined by him from the Van Horn sandstone, the Pump Station Hills, and the Hueco Mountains are related. The rock from the Pump Station Hills "shows alteration similar to that in specimen (D), and traces of the same micrographic struc­ture that characterizes specimens (C) and (D). The rocks from both the Van Horn sandstone and the Pump Station Hills were originally very low in dark minerals." Metarhyolite derived from the Carrizo Mountain group is a significant but never dominant constituent of the conglomerates of the Van Horn. This fact was first noted by Dumble (1902) who mentions "frag­ments of material resembling silicified wood * * * identified by Dr. Osann as belonging to the quartz porphyries." The fragments show the same lineation and cataclastic structure as the rock in the parent ledges. The metarhyolite has been observed in nearly every outcrop of the formation but is perhaps commonest in the southern exposures. Other fragments include fine to coarse­grained basic igneous rocks, probably de­rived from the volcanics and amphibolite intrusives of the Allamoore formation and Carrizo Mountain group; limestone, chert, and jasper from the Allamoore formation; red sandstone from the Hazel formation; and vein quartz. The followin g notes, arranged from northwest to southeast, record the composition of the conglom· erates in different outcrops of the formation: ( 1) Seven miles west of old Circle ranch house near tank at summit of Sierra Diablo escarpment (north of Pl. 3), conglomerate beds 5 feet thick in upper part of formation: 90 percent of frag· ments are red sandstones of Hazel formation, the remainder being chert and limestone of Alla· moore formation and red granite and rhyolite por· phyry; the latter are rounder than the others. (2) Five and one-half miles west of old Circle ranch house on Sierra Diablo scarp (N.5-18.5, PL 3), 100-foot bed of conglomerate at base: Fragments include some of lineated metarhyolite. (3) Four miles west of old Circle ranch house, at measured section (Q.5-18.5, PI. 3): Lower thick conglomerates (beds l to 5) largely made up of red granite and rhyolite porphyry, in rounded boulders as much as 2 feet across (Pl. 26, A). Higher conglomerates (beds 7 to 9) con· tain same rocks and also basic igneous rocks, vein quartz, and limestone of Allamoore forma· ti on. (4) Spur of Sierra Diablo south of Pecos mine (N.5-18.5, PL 2), conglomerate at base: Rounded and polished cobbles as much as l foot in diameter, dominantly of red rhyolite por· phyry but including quartzite, basic igneous rock, and chert. · (5) Buttes northwest of Baylor Mountains (north of PL 2) : Conglomerate at base consists of well-rounded pebbles and cobbles of chert, red sandstone, limestone, vein quartz, basic igneous rock, and schist. In higher beds, besides these, are pebbles of chert, red rhyolite porphyry, and lineated metarhyolite. (6) South end of Baylor Mountains, 3 miles east of B-Bar ranch house (east of PL 2), thin conglomerates in upper part of formation: Frag· ments include red sands.tone of Hazel formation and lineated metarhyolite. (7) Northwest corner of Beach Mountain, south of B-Bar ranch house (T.5-11.5, PI. 2); no thick conglomerate at base, hut many thin beds throughout the sandstone, as well as isolated pebbles and e.mall cobbles: Fragments consist of chert and limestone of Allamoore formation, red sandstone of Hazel formation, vein quartz, basic igneous rock, some red rhyolite porphyry, and lineated metarhyolite. (8) North of Yates ranch house and one-fourth mile south of Grapevine Spring (P.5-7.5, PI. 2) ; occurrence of conglomerate similar to that of (7) : Fragments are dominantly basic igneous rocks and vein quartz, but there are also pebbles of red sandstone and buff chert. (9) Near old nitrate prospect at west end of Red Valley (K.5-18.5, Pl. 3), probably high in formation, scattered pebbles generally less than an inch in diameter: Black, gray, and red chert, vein quartz, and igneous rocks. (10) North of Hillside siding and southwest of Threemile Mountain (P.5-14.5, PI. 1), S. foot conglomerate bed, probably high in fonna. tion: Pebbles as much as 2 inches in diameter of vein quartz, igneous rock, and chert, some of the latter green-colored. Sandstone.-The sandstones of the Van Horn are dominantly red but are generally of a slightly darker hue than those of the Hazel-most are maroon-red, some even purplish-red. The strong tints seem to die out upward, and the highest beds of the formation, where present, are orange-red or red-brown. The sandstones are coarse­grained, grains commonly being a milli­meter in diameter, with scattered still larger grits and small pebbles; they con­trast with the fine silty red sandstones of the Hazel. The grains consist of quartz and feldspar, apparently in about equal proportions, with at some localities an ap· preciable amount of mica. In most of the rock, beds are 3 to 10 feet thick, but some are even thicker. Within many layers are well-marked cross-beds, upon which sys­tematic observations were made during field work (fig. 8). It was found that the greater number of these slope southward, the remainder sloping southeastward or southwestward but never northward. These dips are maintained even where the en­closing strata are inclined in opposing directions. In some of the higher parts of the for· mation are interbedded members of buff or brown sandstone in layers a foot or two thick, which appear to be more cleanly washed than the rest and to contain more quartz than feldspar. On the south-facing scarp of the Sierra Diablo west of the old Circle ranch house, one of these units near the middle of the formation (bed 8 of section) can be traced for more than 2 miles (fig. 7). Others are common in the outcrops south of Threemile Mountain and east of Hillside siding. In outcrops near a tank at the summit of the Sierra Diablo escarpment 7 miles west of the old Circle ranch house (north of PI. 3), the massive arkosic sandstones are interbedded with 5-foot layers of soft flaggy sandstone and greenish clay shale, which are nearly un­consolidated and contain apparent fucoid markings. The Van Horn sandstone in outcrops eaot of Hillside siding and south of Threemile Mountain (Q.&-14.5 to S.&-12.5, Pl. 1) deserves special notice, as it is of somewhat different aspect from that characteriHic of the formation. It may either be a higher part of the formation than preserved elsewhere or a southern facies of the formation. In this area, the sandstones lie in straight, blocky ledges a few feet thick, rather than great, rounded ledges as elsewhere; they are orange­brown or red-brown rather than red. The rock in most beds is coarse and gritty hut does not appear to be as feldspathic as elsewhere; some beds are rather quartzitic. Conglomerate is absent, except for a single 5-foot bed near Hillside siding. in which the pebbles are less than 2 inches in diameter. Sections of Van Horn sandstone.-Only one section of the Van Horn sandstone was measured, at a locality on the south-facing escarpment of the Sierra Diablo 4 miles west of the old Circle ranch house (Q.~ 18.5, Pl. 3). The section begins at the base of the scarp and proceeds northward up a valley on the west side of a prominent mesa of Hueco limestone (Table 19, p. 94). At the northwest corner of Beach Moun­tain (T.5-11.5, PI. 2), 300 or 400 feet of Van Horn sandstone is exposed on the steep slopes, below cliffs of Bliss (?) sand­ r:::l --~~jj~ L::.::J Dip of cross-Von Horn sandstone Other bedrock Alluvium bedding in .formations Von Horn sandstone 0 5 Miles Frc. 8. Map of Sierra Diablo foothills, showing outcrops of Van Hom sandstone and observed directions of dip of its cross-bedding. Note absence of northward dips. stone and El Paso limestone and above red sandstones of the Hazel formation. The Van Horn is, however, discordant with the Bliss ( ? ) and its beds are truncated eastward (fig. 9, C). In a reentrant valley on the north side of the mountain a mile to the east, it is little more than 75 feet thick. suggests that these lands were no longer in the process of active uplift. The Van Horn is almost certainly of continental origin. The present distribution of the Van Horn in widely scattered, basin-like remnants might suggest that it was laid down in several disconnected areas, were it not that Table 19. Stratigraphic section of Van Hom sandstone west of old Circle ranch house. Thickness (in feet) Hueco limestone at top of section, with Powwow member at base. Unconformity Van Horn sandstone: (9) Friable, arkosic sandstone, vermilion or brownish, in part cross-bedded, containing ferruginous nodules. There are some thin conglomeratic beds containing rounded peb­bles of rhyolite, granite, and limestone of Allam9ore formation........................................ 200 (8) White or buff sandstone, spotted with limonite flecks, in 1 to 3-foot beds, resembling the near-by Cox sandstone (Cretaceous) . Cut by two sets of joints at nearly right angles, which impart to outcrops of member an angular or sawtoothed appearance. Bed is traceable for at least 2 miles westward along escarpment........................................ 20 (7) Friable, arkosic sandstone, similar to bed (9) but redder colored (Pl. 27, A) ................ 200 (6) Thinly and regularly bedded brown sandstone........................................................................ 4 (5) Conglomerate of igneous pebbles, interbedded with layers of red arkose several feet thick. Most of cobbles are red granite and rhyolite porphyry, but some are basic igneous rocks and vein quartz...................................................................................................... 300 (4) Arkosic and pebbly red sandstone.............................................................................................. 20 (3) Similar to bed (1)........................................................................................................................ 3 (2) Pebbly arkose.................................................................................................................................. 4 (1) Coarse conglomerate of ovoid and spherical, well-rounded boulders as much as 2 feet across, closely packed, but with some arkosic matrix in the interstices (Pl. 26, A). Boulders are mostly red granite and rhyolite porphyry (see petrographic report by C. S. Ross)................................................................................................................................ 8 Unconformity Allamoore formation at base of section: Massive greenish igneous rock at foot of scarp, either thick flow or intrusive sill. To east, the Van Horn lies on limestones of the Allamoore, and beyond on red sandstones of the Hazel formation (fig. 7). Total thickness of Van Horn sandstone...................................... ............................................................ 759 The thickness of the Van Horn can he its conglomerate fragments are chiefly not estimated fairly closely on the west side of local origin, and its cross-beds maintain of Beach Mountain three-fourths mile south their southward dip regardless of the dip of the Yates ranch house (P.5-5.5, Pl. 2). of the beds into the basins (fig .. 8). The Here, 250 feet is exposed below the present basins are thus probably remnants Bliss (?) on the steep slope east of Hack­of an originally more extensive deposit, berry Creek. The base of the formation which was fragmented by subsequent crops out across the creek not far to the down-folding, down-faulting, and erosion. west, so that the total thickness cannot The original depositional area was prob· exceed 325 feet. Farther south in the Red ably a trough extending east and west be­Valley the formation may be much thicker, tween deformed and uplifted Allamoore as it crops out over wide areas and in and Carrizo Mountain rocks on the south places dips as steeply as 45°. and highlands of granite and rhyolite Interpretation of Van Horn sandstone.­porphyry on the north. The dip of the The Van Horn sandstone is a post-orogenic cross-beds, and the dominant granite and deposit, rather than a syn-orogenic deposit porphyry fragments, suggests that the basin like the Hazel formation. Its base lies on was filled mainly by sediments brought in a deformed and deeply eroded terrane of from the north, with only minor contribu­ the older rocks ; its sediments were derived tions from other directions. from lands of considerable relief, hut the The Van Horn sediments closely re­ perfect rounding of its included fragments semble those of the Triassic Newark group of the eastern states, and the Pennsylvanian Fountain formation of Colorado, which likewise are red, thick-bedded, arkosic, conglomeratic deposits. They are also strikingly like the Miocene deposits along the San Andreas fault zone from Cajon Pass westward, in southern California (Noble, 1933, pp. 12-13), which are thick­ bedded, arkosic, and conglomeratic but flesh-colored rather than red. Many of these comparable deposits contain vertebrate bones, plants, and other fossils indicative of a continental environment. Stratigraphic relations.-ln most places the Van Horn sandstone is followed by the very much younger Hueco limestone, of Permian age, hut in a few places, as on Beach Mountain, it is overlain by early Paleozoic rocks, the Bliss (?) sandstone, of Ordovician age. The Van Horn and Bliss (?) were not differentiated in earlier reports (Dumble, 1902; Richardson, 1904, 1914) and were thought to be gradational deposits, hut they are actually quite distinct and the contact between them can he located with confidence in all exposures. A marked change in sedimentation took place from Van Horn to Bliss (?) time-from coarse, arkosic, continental, unfossiliferous de­ posits in the Van Horn, to fine-grained, cleanly washed, quartzose, marine, fossilif­ erous deposits in the Bliss (?). The Bliss (?) lies unconformahly on the Van Horn and in nearly all exposures can be seen to truncate the tilted beds of the underlying formation at a low angle. Typi­cal examples are shown on the accompany­ing figure 9. Relations are well shown on the north face of Beach Mountain east of its northwest corner (T.5--11.5, Pl. 2), where there is an angular divergence of 20° between the bedding of the two units, causing more than 200 feet of beds of the Van Horn to be cut out in a distance of a mile along the scarp (fig. 9, C). Sand­stones of the Van Horn for a few feet below the base of the Bliss (?) are commonly bleached from red to yellow, either by pre-Bliss weathering or by later circulation of ground water along the contact. The most striking manifestations of un­conformity are on Tumbledown Mountain (R.5-8.5 and S.5--9.5, Pl. 2) where the Van Horn is dropped against the Alla­moore along two faults, the Dallas and Grapevine, which moved between Van Horn and Bliss (?) time. The fau,Ited ter­rane is truncated by Bliss (?) deposits, which lie on the Van Horn on the down­thrown sides and on the Allamoore on the upthrown sides, in the angle between the two faults. The Dallas fault has not moved since Bliss (?) time (figs. 9, B; and 15), but the Grapevine fault was displaced again in the same direction later. Age.-Dumhle (1902, pp. 1-3) and Richardson (1914, p. 4) have previously interpreted the Van Horn as being of "Potsdam" or "Upper Cambrian (?)" age, a conclusion based partly on the marked unconformity between it and the under­lying pre-Cambrian rocks and on the oc­currence of Scolithus, or worm tubes, in the supposedly conformahly overlying Bliss (?). With the discovery of a notable unconformity between the Van Horn and the Bliss (? ) a Cambrian age for the Van Horn became less plausible, as it has little resemblance to rocks of known Cambrian age elsewhere. The writer (King, 1940, p. 153) has therefore termed it "Cambrian or pre-Cambrian" in an earlier publication, hut he now prefers to term it "pre-Cam­brian (?) ," to express the stronger pre­sumption that it is of late pre-Cambrian age. Arguments favoring a later age for the Van Horn are relative! y insubstantial. It is true that it lies with marked uncon­formity on the Hazel and Allamoore for­mations, which expresses a time of orogeny and deep erosion, hut these events need not necessarily mark the end of pre­Camhrian time. It is poorly consolidated, hut so also is the still older Hazel forma­tion in regions of slight deformation. It is the next formation below the fossilifer­ous Bliss ( ? ) , hut the unconformity be­tween them indicates that there was a large time hiatus between Van Horn and Ordo­vician time. Arguments favoring an earlier age are stronger. There is a well-marked uncon­formity between the Van Horn and Bliss (?), involving tilting, faulting, ero­sion, and changes in sedimentation. The Van Horn is unfossiliferous. It was laid down in a continental environment in a region of considerable relief, unlike any EAST c ~ Oe D 0 1000 2000 3000 Feet WEST 500 Frc. 9. Sections showing unconformity between Van Horn sandstone and Bliss (?) sandstone. A, West side of Beach Mountain, l~ miles southeast of Yates ranch house (Q.5--3.5, Pl 2); B, south of Dallas prospect on Tumbledown Mountain (S.5--8.5, Pl. 2); C, eastward from northwest corner of Beach Mountain (T.5-11.5, Pl. 2); D, in southern Baylor Mountains, 3 miles east of B·Bar ranch house (east of Pl. 2). p€H = Hazel formation; p€A = Allamoore formation ; p€v = Van Horn sandstone; Ob=Bliss (?)sandstone; Oe=El Paso limestone; Ph=Hueco limestone. near-by Cambrian deposits. Its deposits are much more like those of the Hazel than those of the Paleozoic, and they seem to mark a resumption of conditions very simi­lar to those of Hazel time after an inter­vening period of orogeny. It is admitted that these arguments are more weighty against the Van Horn being of Late Cambrian age, than against its be­ing a continental facies of the Early Cam­brian, if such should have been laid down in the region. The dilemma is comparable to that in the Lake Superior region, where coarse, red, unfossiliferous elastics which lie next beneath the fossiliferous Cambrian have generally been assigned to the upper Keweenawan, or late pre-Cambrian, but which, in their terminal parts at least, may be continental Early Cambrian (Raasch, 1950, pp. 148-150). PALEOZOIC AND YOUNGER ROCKS INTRODUCTION After Van Horn time, various younger deposits, of Paleozoic and Mesozoic age, were spread over the pre-Cambrian rocks herein described. Apparently the area be­came increasingly positive with time, as successive transgressive units overstep those beneath, each in turn resting on the pre­Cambrian. There are three transgressive ~ystems-the Ordovician, with the Bliss ( ? ) sandstone at the base; the Permian with the Powwow member of the Huec~ limestone at the base; and the Cretaceous with the Campo Grande16 limestone at th~ base. As descriptions of the Paleozoic and Cretaceous rocks have appeared in other publications, or will be published later, it 1s unnecessary to describe them in detail. Nevertheless, some remarks are desirable rega-rding the different units which come in contact with the pre-Cambrian formations. ORDOVICIAN ROCKS Bliss (?) sandstone (Ob) .-The Bliss ( ?) sandstone contains the oldest identi­fiabl~ fossils in the Sierra Diablo region, and 1s the next youngest formation above the V~n.Horn sand~tone. Within the map area, it 1s exposed m a semicircle around the north and west sides of Beach Moun­ 18 At the time Platea 1 and 3 of this paper were sent to tbe engru·er it wu thou~ht that the approved 1pelliog bad !teen c~nged to Campograode. Unfonunately the writer• were m11talcen; the correct apellinc ii Campo Grande, two word1, and the 1pellin1 on the map1 ia In error. tain, where it forms shelves and thin ledges about halfway up the face of the escarp­ment. A few small outcrops also occur in the Baylor Mountains, farther northeast (King and Knight, 1944). Sections measured by the writer in Beach Mountain show that the Bliss ( ? ) main­tains a nearly constant thickness of 115 to 120 feet. The main part of the formation consists of white or light brown, quartzitic, quartzose sandstones, in beds a few inches to a foot thick, which are commonly lami­nated, cross-bedded, and ripple-marked. Many of the sandstone beds contain vertical worm tubes, or Scolithus, and in the upper part the tubes are abundant in nearly every layer (Pl. 26, B). Between the sandstone layers, especially in the upper part, are partings of softer, more marly sandstone, of gray, brown, purplish, or greenish color. The greenish material was suspected to contain glauconite, but C. S. Ross reports as follows on a specimen examined by him (memorandum of November 19, 1940): No glauconite was observed. The green color appears to be caused by a high iron clay that is interstitial to sand grains. Clays rich in non­tronite may be green. To the writer, the Bliss (?) sandstone of Beach Mountain appeared not to be cal­careous, although he noted calcareous beds in the upper part in the Baylor Mountains. On the other hand, Cloud and Barnes (1948, p. 67) state that "tests with dilute HCl show that most beds are slightly cal­careous, and one IO-inch bed of dolomite was seen near the middle at the north end of Beach Mountain." The basal few feet of the Bliss ( ? ) , lying unconformably on the Van Horn sandstone, is a conglomerate consisting of rounded pebbles less than an inch to as much as 3 inches in diameter, mostly of vein quartz but with a few of chert, quartzite, and schist. At some places, the matrix is red­dish, probably because it contains reworked red detritus from the Van Horn. The Bliss ( ? ) appears to be sharply separated from the El Paso limestone above. The thin-bedded, non-calcareous or poorly calcareous sandstones of the Bliss ( ? ) are succeeded abruptly by thick-bedded dolo­mitic limestones and dolomitic sandstones of the El Paso. The El Paso may lie discon­formably on the Bliss ( ? ) , a possibility confirmed by Cloud and Barnes, who found detritus reworked from the Bliss ( ? ) in the basal El Paso. Various collections of fossils have been made in the Bliss (?) of the Beach Moun· tain area, the most extensive bein(; those of Cloud and Barnes (1948, p. 68). In the upper 4 feet of the formatio~ these autho~s cite Clarkoceras sp., Lytospira sp., Ophil· eta sp., Helicotoma sp. cf. H. uniangulat'! (Hall), various other gastropods, Hystri· curus sp., and archaeostracan crustaceans. These are of Early Ordovician aspect. Nineteen feet lower the beds contain nu· merous unidentifiable gastropods and Lin· gulepis, generally conside~ed to be an index fossil of the Upper Cambrian. Cloud and Barnes (1948, pp. 68-69) tentatively conclude that all the Bliss (?) of the Beach Mountain area is of Early Ordo· vician age and is probably equivalent to the Tanyard formation of the Ellenburger group in the Llano area. Alt~ough they recognize differences ~etween it. and the type Bliss of the Franklm Mountams to the west, they state that "the burden of .Proof perhaps rests with those who would. d1sput.e the correlation of these two stratlgraphi· cally commensurate basal sands." How· ever, Bridge (Kelley, 1951, p. 2205), after reviewing other and later evidence, con­cludes that "the Bliss sandstone at the type locality in the Franklin Mountains and north and west of this locality is Upper Cambrian." A separate name for the Early Ordovician sandstones on Beach Mountain seems unwarranted as both it and the type Bliss are probably part of a single trans· gressive series and are not far apart in age. It therefore seems appropriate to term the beds of Beach Mountain the Bliss ( ? ) sandstone, a conclusion with which Mr. Bridge agrees. · El Paso and Montoya limestones (Oe and Om) .-The El Paso limestone, which suc­ceeds the Bliss ( ? ) on Beach Mountain, is a mass 1,115 feet thick of calcitic and dolomitic limestone, with several thick beds of calcareous or dolomitic sandstone in the lower part. Its fossils indicate that it is of Early Ordovician (Beekmantown) age and equivalent to various formations of the Ellenburger group of central Texas. The EI Paso of Beach Mountain has been described in great detail by Cloud and Barnes ( 1948, pp. 66-71, 352-361), and it is unnecessary to add further observations. On the summit of Beach Mountain, at the extreme east edge of the map area (T.5-6.5 and T.5-7.5, Pl. 2), the El Paso limestone is overlain by massive dolomitic limestone and cherty limestone belonging to the Montoya limestone, of Late Ordovician (Cincinnatian) age. PERMIAN17 ROCKS Hueco limestone (Ph and Php) .-In most of the Sierra Diablo foothills, the next formation above the pre-Cambrian rocks is the Hueco limestone, of Wolf camp (early Permian) age (Pl. 28, B). In the Sierra Diablo region the Hueco lies with major unconformity on all the older rocks of the section, including not only the pre-Cam· brian but also the Ordovician, Silurian, and Devonian systems and the Mississippian and Pennsylvanian series (Pl. 19, C) ~ ,The general features and map relations of the Hueco in this region have been set forth elsewhere (King, 1942, pp. 556-562; King and Knight, 1944), hut some further de­tails pertaining to the Sierra Diablo foot· hills are here given. Sections measured by J. B. Knight indi· Cate that the formation is 565 feet thick on Threemile Mountain (T.5-15.5, Pl. 1.), 370 feet thick in the southern Streeruwitz Hills northwest of Eagle Flat section house (G.5-6.5, Pl. 3), and 430 feet thick in the southeastern part of the Sierra Diablo north of the Hazel mine (P.5-14.5, Pl. 2). This is not the full thickness of the fonna· tion, as the Hueco in this area is either the highest formation present or is overlain unconformably by Cretaceous rocks. At the base of the Hueco is a elastic member, which forms slopes and thin ledges below the surmounting limestone cliffs. For this, the name Powwow member is used, on recommendation of Prof. R. K. DeFord of The University of Texas, sup· planting the term "basal elastic member" used in previous reports. The name Pow· wow is derived from the Hueco Mountains (King and King, 1929, p. 911; King, 1934, p. 743; King, King and Knight, .1945, sheet 2), where the member is a thin, dis· continuous body of conglomerate and red beds that occupies the same position at the base of the Hueco limestone as do the elastics in the Sierra Diablo. The Powwow member is thickest in the south, as on the ridges between Threemile Mountain and the Gifford-Hill rock crusher 17 The U. S. Geological Survey claHe1 the Wolfcam• 1erie1, of which the Hueco limestone is a part, 11 of Permlu ( ?) age. (T.5-15.5 and H.5-16.5, Pl. 1) and the southern Streeruwitz Hills (G.5-6.5 to 0.5-9.5, Pl. 3), where sections 103 to 184 feet thick have been measured: in pJaces in the southwest part of the Red Valley it may be as much as 250 feet thick (K.5-1.5, Pl. 2). In the central Streeru­witz Hills the member is erratically de­veloped, as a result of overlap on a hilly surface of the pre-Cambrian rocks, and in places it is absent. Along the south-fac­ing escarpment of the Sierra Diablo, at the north edge of the map area, it is less than 100 feet thick. ,The member consists of conglomerate, arkose, red and buff sandy shale, and thin varicolored argillaceous or earthy lime­stones, passing up into interbedded gray limestones and fossilferous marls. Con­glomerate fragments are mostly poorly rounded fragments derived from the im­mediately subjacent pre-Cambrian rocks, but where the member overlies the Van Hom sandstone it contains many rounded cobbles reworked from that formation. Fossils in the upper part include echinoid spines, euomphalid and bellerophontid gastropods, Composua, and productids; many of them weather free on the slopes. An unusual facies of the Powwow mem­ber occurs between Threemile Mountain and the Gifford-Hill rock crusher, immedi­ately north of the Hillside fault. Just north of the fault and lllz miles east of the rock crusher (K.5-16.5, Pl. 1) it is a coarse, bouldery conglomerate, made up of angular fragments of metarhyolite of the Carrizo Mountain group. Similar fragments occur in the member for half a mile north of the fault. In this vicinity the Powwow member lies, not on metarhyolite, but on Van Hom sandstone, but the metarhyolite is exten­sively exposed immediately south of the fault. Evidently the metarhyolite fragments were eroded from a scarp along the Hill­side fault that had been raised during the pre-Hueco deformation. The main body of the Hueco limestone above the Powwow member projects in light-colored cliffs and mural escarpments that are prominent along the Texas and Pacific Railroad west of Threemile Moun­tain and northwest of Eagle Flat section house, as well as on the southern scarps of the Sierra Diablo from the Hazel mine westward. Away from the cliffs, it forms step-like ledges or the surface of flat-topped table-lands. In most of the Sierra Diablo foothills the limestone is gray, fine-textured, and calcitic, in part cherty, with a few dolo­mitic beds and marly units. Northeastward, however, in the angle of the Sierra Diablo between the old Circle ranch house and the Hazel mine (Pl. 2), it passes into a monotonous sequence of thin-bedded, dolo­mitic limestone. The calcitic limestones contain an assemblage of fossils similar to that in the upper part of the Powwow mem­ber. Foraminifera are nearly absent, except for the minute Staffella, which abundantly dots the surface of some of the limestones. Northeastward in the dolomitic facies, fusulinids become abundant, almost to the exclusion of any other fossil. Some of these collected near the old Circle ranch house have been identified by Dunbar and Skin­ner (1937, p. 722, localities 94-%). CRETACEOUS ROCKS lntroduction.-Cretaceous rocks of the Sierra Diablo region, like the Permian rocks (Hueco limestone) lie unconform­ably on the preceding formations. In the Sierra Diablo foothills they rest in part on the Hueco but in part overstep it and rest on the pre-Cambrian. Their most ex­tensive outcrops are in mesas along the south edge of the Sierra Diablo escarpment which extend northwestward, beyond the map area, from the vicinity of the Keene ranch house (J.0-16.0, Pl. 3). Small, widely dispersed outliers occur in the Streeruwitz and Bean Hills, and there are several larger ones east of the Gifford-Hill rock crusher near the Texas and Pacific Railroad (l.5-16.5 to L.5-16.5, Pl. 2). Two formations are present in the Sierra Diablo foothills, the Campo Grande lime­stone below and the Cox sandstone above; they are part of the Comanche series and are probably late Trinity in age. Campo Grande limestone (Keg) .-Most of the Cretaceous rocks in the map area belong to the Campo Grande limestone, made up of several beds of massive le::i 0 ~ "" ~ ~ ;:s ::x:: 0 .., ;:s ~ .., .fl ~ ...., ~­~ ::l "' 0 ~ ~ The University of Texas Publication No. 5301 In the descriptions which follow, the structural features will be taken up under the headings indicated in the first column of the table, which are largely in chrono­logical order. STREERUWITZ OVERTHRUST A major feature, and probably the mas­ter structure of the pre-Cambrian rocks of the Sierra Diablo foothills, is the Streeru­witz overthrust, which emerges along the south edge of the foothills and forms the contact between the Carrizo Mountain group on the south and the Allamoore formation on the north. The Streeruwitz overthrust is named for the Streeruwitz Hills, in the south part of which it crops out for a distance of 21h miles (C.5-8.5 to G.5-9.5, Pl. 3; P. 9, section A-A') ; in turn the hills are named in honor of W. H. von Streeruwitz who made the first geological observations there (1891, pp. 681-682; 1892, section 0-P, Pl. 26) . East of the Streeruwitz Hills the overthrust is largely covered by the allu­vium of Eagle Flat, but it emerges at the base of the Millican Hills at two places northwest and northeast of Allamoore (U.5-4.5, Pl. 3; E.5-2.5 and F.5-2.5, Pl. 2; Pl. 9, sections D-D' and E-E'). Farther north in the Millican Hills are several patches of metarhyolite of the Car· rizo Mountain group lying amidst the Alla­moore formation (U.5-7.5, Pl. 3; C.5­6.5, Pl. 2; Pl. 9, section D-D'), which are interpreted as klippen of the overthrust. Features similar to those along the over­thrust in the Sierra Diablo foothills occur at one locality in the northern Carrizo Mountains (L.5-15.5 and M.5-15.5, Pl. 1) and may mark another place of emergence of the overthrust. Many lines of evidence, from both major and minor features, indicate that the con­tact between the Carrizo Mountain group and Allamoore formation is one of over­thrust: (1) The contact separates two different sequences, that on the south (Carrizo Mountain) being dominantly metamorphic and igneous and that on the north (Alla­moore and Hazel) being dominantly sedi­mentary. These differences are not as abso­lute as they appeared in Richardson's time (1914, pp. 3-4), as the rocks to the north are partly igneous and metamorphic and the rocks to the south are partly sedimen· tary. Considerable differences in both habit and facies nevertheless exist. (2) The rocks of the Allamoore and Hazel formations north of the contact strike west-northwest parallel with the trace of the contact, whereas the rocks of the Carrizo Mountain group south of the contact strike northeast over wide areas. The northeast-striking rocks of the group in the Carrizo Mountains are separated from outcrops of the contact by a mile or more of younger rocks, so that exact rela· tions cannot be established, but their pat· tern suggests that they strike up to and are truncated by the contact. The contact thus apparently brings rocks of unlike structure into juxtaposition. (3) The contact involves great strati· graphic displacement. The rocks next to the Allamoore are metarhyolites that are intrusive in the lower part of the sedi· mentary sequence of the Carrizo Mountain group. In the Carrizo Mountains, Flawn has found that this sequence is 19,000 feet thick and dips away from the contact, so that the highest beds are exposed farthest away toward the southeast. Somewhat tenu· ous evidence suggests that even the highest beds are stratigraphically beneath the Alla· moo re. ( 4) The southern rocks (Carrizo Moun· tain) are superimposed on the northern (Allamoore). The contact between them dips south at a low angle, and klippen occur a mile or more to the north. ( 5) The Allamoore and Hazel forma· tions for 3 miles or more north of the con· tact are violently deformed, apparently under the influence of stresses directed from the south; farther north the deforma· tion fades out. ( 6) Indications of large-scale transport of the Carrizo Mountain group are afforded by its minor structures. Its rocks for some distance south of the thrust are cata· elastically altered, with strongly developed lineation and streaks of mylonite. Linea· tion trends generally southward, normal to the trace of the thrust and apparently parallel to the a fabric axis, or direction of transport. (7) Greatest alteration of the Alla· moore formation is close to the fault con· tact, where its limestones are extensively silicified and marmorized, probably by a combination of dynamic and hydrothermal processes. The actual contact between the Carrizo Mountain group and Allamoore formation is visible on ravine banks and hillsides at a number of places in the southern Streeru· witz Hills and northeast of Allamoore but affords little conclusive evidence for or against the overthrust interpretation. There is no sharply marked, clean-cut surface of movement, such as one would find on a younger fault or one of less complex his­tory. Near the contact the metarhyolite is strongly silicified and lineated, and the limestone is contorted, schistose, and mar­morized, but the contact itself is likely to be masked by schistose amphibolite or by quartz veins. In places, hydrothermal alter­ation of both metarhyolite and limestone is so great that they are difficult to differen­tiate. In others, the metarhyolite contains inclusions of brown, highly silicified car­bonate rocks, perhaps in part incorporated tectonically and derived from limestones of the Allamoore formation below the con­tact. Aberrant dips of foliation and trend of lineation in the metarhyolite close to the contact attest further movement after the formation of the cataclastic structures. The most interesting manifestations of thrusting are the klippen in the western Millican Hills, within the area of Alla­moore formation (Pl. 9, section D-D'). These range from patches a few acres or less in extent, to hills as much as half a mile long, all composed of metarhyolite like that in the main area of the Carrizo Mountain group, a mile or more to the south. The largest mass is 134 miles west of the Garren ranch house (B.5-6.5, Pl. 2). The klippen apparently mark a belt of major downfolding of the thrust sheet that trends east and west. Super­ imposition of metarhyolite on Allamoore is plain in the smaller klippen but is some­what more obscure in the larger, as a re­sult of complex folding and confusing dips. However, trends of ledges of the Allamoore around the largest klippe, as revealed by ground surveys and air photographs, sug­gest that it lies in a sharply depressed syncline, probably overturned northward on the south side. In another large klippe a mile to the west ( U .5-7 .5, Pl. 3) , the Allamoore is overturned southward on the metarhyolite. Direct evidence indicates that the Streer­uwitz overthrust is older than the Hueco limestone, as an outlier of the Hueco lies across its trace in the southern Streeruwitz Hills (E.5-8.5, Pl. 3). Indirect evidence demonstrates that it is also older than the Van Horn sandstone, as the Van Horn lies unconformably on all the older formations, truncates structures associated with the overthrust, and contains fragments of meta­rhyolite that was lineated and mylonitized during the thrusting. FOLDS AND THRUSTS OF ALLAMOORE AND HAZEL FORMATIONS lntroduction.-The Allamoore and Ha­zel formations of the Streeruwitz, Bean, and Millican Hills, for 3 miles or more north of the trace of the Streeruwitz over­thrust, are intensely folded and faulted, apparently by stresses directed from the south (fig. 10). Farther north, in the Hazel formation, the disturbance decreases, and on the eastern and southern scarps of the Sierra Diablo the sandstones of the Hazel dip at low angles. As indicated by the coarse, thick con­glomerates of the lower part of the Hazel formation, made up almost entirely of angular fragments of the Allamoore for­mation, the Allamoore was considerably deformed immediately before and during the early part of Hazel time. Part of the complex structure now visible in the Alla­moore formation of the disturbed belt was probably produced by this deformation but cannot now be unraveled from the dominant structure, shared by both Alla­moore and Hazel formations, which is of post-Hazel and pre-Van Horn age. The two sets of structures will therefore he described together. Interpretation of the structure of the disturbed belt is rendered difficult by lack of guide fossils and by uncertainty as to the true nature of the stratigraphic suc­cession. One stratigraphic fact on which much interpretation is dependent is proved by the conglomerates of the Hazel forma­tion, namely, that the Hazel is younger than and originally overlay the Allamoore. This determines the nature of at least the larger features, for where Allamoore now overlies Hazel it must have attained this position by overthrusting or overfolding. Nevertheless, numerous details are diffi­cult to interpret. In places, but particularly in outcrops of the Allamoore, strikes and dips are almost meaningless, for if one so desires he can discover nearly every angle or direction within short distances. Plunges of folds are also not especially helpful. Where a contact curves across the axis of a fold, instead of a consistent low plunge in one direction, one is likely to encounter high, irregular dips. Moreover, the map pattern indicates that plunges are not con­sistent throughout the area but reverse themselves within short distances along the strike. Observations on cleavage and graded bedding might provide helpful clues; in this respect the field work on which this report is based is admittedly weak. Where such observations were made, results were so conflicting or inconclusive that more time would have had to be de­voted to them than was available for the field work. over, there is reason to suppose that over wide areas bedding has been rotated more than 180°, and in such places use of an overturned symbol would be devoid of meaning. (2) Throughout the disturbed belt there are many indications of folds of anticlinal and synclinal habit, but there is good evi­dence that some of these have been rotated more than 180°, so that an apparent syn­cline is actually the inverted nose of a re· cumbent anticline. Without creation of spe· cial symbols and making possibly unwar· ranted interpretations, it appears imprac­ticable to indicate such possibilities on th!'! map. All folds with apparent anticlinal and synclinal structure are therefore shown as anticlines and synclines, regardless of their possible true character and prior his· tory. Some hint of the probable nature of these structures is given on the structure sections (Pl. 9). ( 3) As indicated in discussing the strati· graphic relations of the Allamoore and Hazel formations, the contact between them is complex. In places there may be still SOUTH 0 5 Miles Frc. 10. Synoptic section, showing general structure of pre-Cambrian rocks in Millican Hills. The master structure is the Streeruwitz overthrust, which has carried rocks of the Carrizo Mountain group over the Allamoore and Hazel formations and has intensely folded them for several miles north of the contact. The section is based on features in various parts of the outcrop area, some of which are also shown on sections D-D' and E-E' of Plate 9. p€Ci = igneous rocks of Carrizo Mountain group; p€Cs = sedimentary rocks of Carrizo Mountain group; p€A = Allamoore forma· tion; p€Hc =conglomerate of Hazel formation; p€Hs =sandstone of Hazel formation. The complexities of the structure have necessitated the adoption of several con­ventions on the geologic maps: (1) No overturned beds are differenti­ated. Indication of beds believed to be overturned would introduce a large sub­jective element, and it therefore appeared best to indicate strikes and dips as they were actually observed in the field. More-preserved the original irregular uncon· formable surface of the Allamoore on which the Hazel was deposited, but where the contact is exposed there is generally evidence of movement between them of undetermined magnitude, no doubt result· ing from differences in competency of the rocks of the two formations. In some places, where the Allamoore overlies th' upper part of the Hazel formation, the two are undoubtedly separated by a low­angle fault of large displacement. Differ­entiation of these possibilities from place to place along the contact would be so subjective that it seems undesirable to at­tempt it on the map. To indicate the special nature of the contact it is differentiated from others as a "surface of movement," shown by a special symbol. The closest field study of the disturbed belt was made in the Millican Hills, and although even here some details still defy explanation it is thought sufficient to pro· vide interpretation of at least the larger features. This area will therefore be de­scribed in some detail in the pages that follow, after which interpretations will be suggested for other areas where less de­cisive information has been obtained. Millican Hills.-The Millican Hills are formed of limestone, volcanics, and phyl­lite of the Allamoore formation, and of conglomerate and red sandstone of the Hazel formation, which crop out in suc­cessive belts striking generally west-north­west, in which the beds dip steeply, at diverse angles, and in various directions. The rocks of the hills are considerably altered by dynamic and hydrothermal processes, in the manner indicated in the descriptions of the respective formations, but little of the original sedimentary struc­ the Carrizo Mountain group immediately to the south. Cleavage and schistosity ap­ parently were formed by progressive meta­ morphism during a, single cycle. In the south part of the hills, imme­diately north of the Streeruwitz overthrust, is a belt of Allamoore formation 1 to 2 miles wide, which extends from a point south of the Canning ranch house, 8 miles east-southeast to Buck Spring (Q.5-8.5, Pl. 3, to K.5-3.5, Pl. 2). Within the southern belt of Allamoore formation, sharply depressed synclines pre­serve outliers of conglomerate of the Hazel formation north of the Dwees ranch house (C.5-4.5, Pl. 2) and klippen of metarhyo­lite of the Carrizo Mountain group west of the Garren ranch house ( B.5---6.5, Pl. 2) . Elsewhere, intense folding is indicated by winding outcrops of the limestone, volcanic, and phyllite units of the Allamoore and in places is visible in cross section on the sides of the ridges, as 2 miles southeast of the Garren ranch house (fig. 11). Never­theless, relations are generally so complex that the broader pattern is obscure. Several high-angle faults a mile or more in length break the rocks of the belt (A.5-6.5 and G.5-3.5, Pl. 3), trending west-northwest nearly parallel to the regional strike. These are not Tertiary normal faults, as adjacent steeply folded beds conform to them; more probably they were created by strike-slip movements late in the orogenic epoch. SOUTH 0 500 1000 Feet Fie. 11. Profile, based on field sketch, showing folding in limestones of Allamoore formation 2 miles south-southeast of Garren ranch house (G.5-4.5, Pl. 2). p€Als = limestone unit of Alla­moore formation; p€Av =volcanic unit of Allamoore formation; p€Hc =conglomerate of Hazel formation. ture is lost except near the Streeruwitz overthrust on the south. Tourmaline-bear­ing, slickensided surfaces are common in the sandstones and conglomerates, and in places in the limestones, suggesting that the rocks were deformed under high tem­perature. There is, nevertheless, no discern­ible trace of the cataclastic metamorphism that is prominent in the metarhyolites of External relations of the southern belt of Allamoore formation are more informative than the internal. Between Buck Spring and Carrizo Spring at its southeast end (L.5­3.5, Pl. 2) the Allamoore formation stands in several anticlines, whose flanks dip steeply and whose crests plunge eastward beneath the basal conglomerates of the Hazel formation. Here, the Allamoore is rooted in place beneath the Hazel, and there is no intervening "surface of move· ment." West-northwestward along the con· tact, relations between the two formations become more discordant. Two miles west· northwest of Buck Spring at the Cooper Hill prospects (F.5---4.5, Pl. 2) the contact dips south hut the adjacent conglomerate dips north. The presence of an intervening "surface of movement" is indicated by fracture cleavage in the conglomerate and schistosity in the marmorized limestones, both of which dip south parallel to the con­tact. Movement cannot have been great, as the conglomerate contains boulders of lime­stone pebble conglomerate like that seen in place in the adjacent Allamoore. Relations are much more discordant be. yond. Northwest of the Cooper Hill pros· pects the Allamoore formation extends for nearly a mile over the Hazel formation in a recumbent anticline, or "subsidiary nappe" (fig. 12; Pl. 9, section D-D'), plunging to the west, every detail of which is well ex. posed in the hills west of the Garren ranch house (D.5-6.5, Pl. 2). This feature is on a scale small enough to be apprehended readily yet is apparently comparable to SOUTH A Is larger and more complex features elsewhere in the area, described below. The "sub­sidiary nappe" has a carapace of limestone which is traceable, with some interruptions caused by piercement structure, from the lower limb on the east to the upper limb on the west. Its core is formed of volcanics and phyllite. It is enveloped by conglomer· ate of the Hazel formation which appears to be accordant above and discordant be­low. Accordance above is indicated by con­formity of dip between limestone and conglomerate and by the presence in the conglomerate of limestone and volcanic fragments like those in place in the "sub­sidiary nappe." Discordance below is indi­cated by marked differences in attitude of bedding of the limestone and conglomerate, although within a few feet below the con· tact the discordant conglomerate beds are sharply bent into parallelism with the over· lying limestone. At the place of bending they contain numerous tourmalinized slick­ensided surfaces. The cause of the recum­bent folding in the "subsidiary nappe" is readily apparent, for on its back, a short distance to the west, reposes one of the larger klippen of the Streeruwitz overthrust (Pl. 9, section D-D'). A northern belt of Allamoore formation 3¥2 miles in length extends east and north­west of the Garren ranch house (D.5-­ 9.5 to J.5--7.5, Pl. 2). The rocks of the northern belt are like the southern, and the belt belongs to the same formation as it is physically attached to its parent by an "um­bilical cord"-a narrow curved belt of limestone, plunging westward, 1% miles northwest of the Garren ranch house (B.5 --8.5, Pl. 2). Internally, the northern belt of Alla· moore fomation consists of sinuous out­crops of various limestone, volcanic, and phyllite members, of most complex struc­ture, which north of the Garren ranch house (F.5-8.5, Pl. 2) are split by seven or more north-trending tear faults with considerable strike-slip displacement. Externally, the northern belt has the form of a syncline, which is nested in a syncline of conglomerate of the Hazel formation that comes to the surface up the plunge to the east, beyond the Anaconda no. 1 pros­pect (Pl. 9, section E-E'). This apparent structure is questionable as the conglomer­ate is stratigraphically higher than the Alla­moore. Proof of this succession is afforded locally by fragments in the conglomerate like those which occur in place in the north­ern belt of the Allamoore and by graded bedding in the conglomerates on the north flank which indicate that tops of the beds are away from the Allamoore. Moreover, the apparent syncline in the conglomerate has vertical dips on its north flank and low dips on its south flank (K.5-7.5, Pl. 2; Pl. 9, section E-E'), or the opposite of what should be expected from the gen· eral northward asymmetry of the disturbed belt. The northern belt of Allamoore for· mation is thus not truly a syncline, as it appears to be, but is the inverted nose of a recumbent anticline, or nappe, on a much larger scale than the "subsidiary nappe" but rooted in the same manner in the southern belt of the formation. The enveloping Hazel formation is dis· cordant with the Allamoore formation of the northern belt, but it is uncertain how much of this resulted from original angu· lar unconformity and how much from later dislocation; the whole contact is indicated on the map (Pl. 2) as a "surface of move­ment." Near the two Anaconda prospects at the eastern end of the belt, the disloca­tion is probably slight, as conglomerates at the base of the Hazel contain fragments of rock types that occur in the near-by Allamoore. Westward along both north and south sides of the belt the conglom· erates thin out, and a mile north of the Garren ranch house (F.5--9.5, Pl. 2) the Allamoore is in contact with red sand­stones. Here, marked differences in dip between the two formations indicate a large degree of structural discordance. An unusual complication exists on the hill slope above the county road at the Garren ranch house, where limestones at the top of the Allamoore formation dip 20° northwest beneath the conglomerate of the Hazel formation. Relations are much more complex than the local outcrop would indicate, as the contact can be traced in a spiral, first northwestward, then southwestward along the "umbilical cord" and southeastward through the "sub­sidiary nappe," to the anticlines of the southern belt at Buck Spring. The lime· stones at the Garren ranch house, to at· tain their present position below the con­glomerate, and their low dip, must have been rotated nearly 360°. In this vicinity the south flank of the nose of the recumbent anticline of the northern belt must have been rolled back beneath itself, in the manner suggested in figure 10 (right cen· ter) . This may have resulted from forward thrust of the "subsidiary nappe" after the larger and higher fold had been created. Another, narrower belt of Allamoore formation, 11/2 to 4 miles northwest of the Garren ranch house (C.5-10.5, Pl. 2, to T.5-11.5, Pl. 3) has no visible attach­ment to the southern belt. Its steeply dip­ping rocks afford no indication of its true nature, but it may well be the nose of another recumbent anticline that origi­nally lay higher than that of the belt just described. The conglomerates and red sandstones of the Hazel formation which enclose the Allamoore formation of the Millican Hills share its complex deformation. One belt lies north of Buck Spring and the Cooper Hill prospects and extends southwest from the Garren ranch house (E.5-6.5 to L.5­5.5, Pl. 2), between the southern and northern belts of the Allamoore. Next to the Allamoore on the south is a solid mass of conglomerate 2,000 feet thick, which dips northward beneath a wide band of red sandstone; this in turn dips north­ward at angles of about 30° beneath con­glomerates which border the northern belt of Allamoore formation (Pl. 9, section E-E') . The whole suggests a homoclinal sequence, and it contains no hint of any reversal or indications of great internal stress, save for some tourmalinized slicken­sided surfaces in the red sandstone imme­diately above the southern conglomerate. Nevertheless, the northern conglomerate occupies the same stratigraphic position as the southern, immediately adjacent to the Allamoore, and it is presumably likewise a basal deposit of the Hazel, but now inverted. Explanation of the structure of the Hazel in this belt is elusive, but the tourmalinized slickensided surfaces may mark a zone of reversal or thrusting. Determination of tops and bottoms of beds in the belt would be helpful, but observations which have been made of graded bedding in the sandstones and conglomerates did not yield conclu­sive results. As already noted, the conglomerates to the north, east of the northern belt of Alla­moore, form an apparent syncline which, from its relations to the Allamoore, ap­pears actually to be the nose of a recum­bent anticline. This plunges westward he· neath the Allamoore at the Anaconda no. 1 prospect (J.5-7.5, PI. 2), hut farther east it plunges eastward toward Hackberry Creek (N.5-7.5, Pl. 2) and passes un­formably beneath the Van Horn sandstone at the foot of Beach Mountain. Along Hackberry Creek near the Yates ranch house it spreads out into a series of low, broad folds 1 % miles wide. 1£ these struc­tures conform to those already described, all the beds contained in them must be inverted (Pl. 9, section F-F'). Area north of Millican Hills.-On the north flank of the northern conglomerate belt of the Millican Hills, bedding of the Hazel formation stands nearly vertical for half a mile or more across the strike. Tops of beds are to the north, away from the false synclinal axis, as indicated by occa­sional observations on graded bedding and by the gradual diminution of the conglom· erate component at the expense of the red sandstone component, so that this finally dominates altogether toward the north. South of the Blackshaft and St. Elmo mines there are, however, a number of tight isoclinal folds (K.5-8.5, Pl. 2). North of the mines the red sandstones and inter· bedded conglomerates flatten abruptly to nearly horizontal attitudes, forming the "north edge of the steep dip zone" of the map (Pl. 2). Near the "north edge of the steep dip zone" at the Blackshaft, St. Elmo, and Sancho Panza mines a thin layer of Alla­moore is tectonically interlayered in the Hazel formation (J.5-10.5 to L.5-9.5, Pl. 2). Tq the southeast this dips steeply northward or locally is overturned and dips southward, but to the northwest it dips at low angles eastward or north· eastward, with many wrinkles and offsets. The layer of Allamoore is no more than 10 feet thick and is highly sheared and metamorphosed. In places, as between the Blackshaft and St. Elmo mines, it dis­appears altogether, leaving only a de­colorized plane of shearing in the red sandstone. In mine workings, the hanging wall of Hazel formation, overlying the Allamoore, is shown to be a strong, firm, smooth surface. The footwall is less defi· nite and includes horses of underlying sandstone; in places it is marked by grooves or steps parallel to the strike. Apparently the Allamoore and that part of the Hazel lying above it has been car· ried over the Hazel beneath along a thrust plane, or "surface of movement" (Sample and Gould, 1945). The smooth hanging wall of Hazel formation above the Alla· moore suggests that it is likewise a "sur­face of movement," but displacement on it must have been less than on the one below, as the overlying Hazel contains coarse con­glomerates such as might have been de­posited on or not far above the top of the Allamoore formation. As the "surface of movement" involves at least several miles of horizontal displacement, it must extend for long distances in the Hazel formation beyond the points of disappear· ance of the Allamoore formation, but the red sandstones contain no structures that hint of dislocations of such magnitude. Farther east, the Allamoore formation is exposed on Tumbledown Mountain (Q.5----8.5 to R.5-8.5, Pl. 2), where it again overlies red sandstones and thin interbedded conglomerates of the upper part of the Hazel formation. The Alla­moore and underlying Hazel are folded into an eastward-plunging syncline (Pl. 9, section G-G'), which is more open and regular than most Allamoore structures. The Allamoore is, however, separated from the Hazel by a thrust plane, or "surface of movement," as different units overlie the Hazel froni place to place, and as there is an intervening slice of Allamoore at the west end of the mountain (beds 1 and 2 of Table 17) which is discordant with the Hazel below and main body of the Alla­moore above. The broader relations of the structure on ,Tumbledown Mountain are obscured by later high-angle faults and by overlap of the Van Horn sandstone and younger for· mations. Correlation with structures in the Millican Hills to the west is uncertain. The syncline of Tumbledown Mountain lies on the approximate prolongation of the false syncline of the northern belt of conglom· erate and Allamoore formation of the Mil­lican Hills, hut it differs from the false syncline in that neither the Allamoore nor the Hazel which compose it seem to be inverted. North of the "north edge of the steep dip zone" only red sandstones are visible, to the farthest exposures of Hazel forma· tion on the east face of the Sierra Diablo 10 miles to the north. The sandstones at first lie nearly horizontal, but beyond the Hazel mine they rise northward at angles of 5° to 30°, with the uniform southward dip of the bedding faintly but distinctly visible on the slopes of the Sierra Diablo escarpment. Streeruwitz Hills.-The Allamoore and Hazel formations were not studied in as much detail in the Streeruwitz and Bean Hills as in the Millican HiHs, and their structure therefore cannot be as certainly interpreted. However, many of the same structural units described in the Millican Hills seem to exist also in this area. Immediately north of the trace of the Streeruwitz overthrust in the central Streeruwitz Hills is a southern belt of Alla­moore formation about a mile wide which is bordered on the north by massive con­glomerate of the Hazel formation (B.5­ 11.5 to G.5-10.5, Pl. 3), like that north of the southern belt of the Millican Hills near Buck Spring and the Cooper Hill prospects. Both conglomerates and Allamoore dip 45° to 60° south, but the contact between them is discordant, causing units of the Alla· moore to be cut out against the conglomer· ate on the west and conglomerate to be cut out against the Allamoore on the east. It is uncertain whether this is the result of an original angular and erosional unconform­ity or is due to a "surface of movement" between the two formations, but it has been tentatively mapped as the latter. Northward in the Streeruwitz Hills the conglomerate is followed by red sandstone which contains several thick interbedded conglomerate members. Dips are north· ward, flattening to nearly horizontal at the north edge of the hills. One of the conglom· erate bands in the northwest part of the hills (A.5-14.5, Pl. 3) encloses a narrow mass of limestone half a mile or more long, partly concealed by Permian and Cretaceous rocks on the west. In 1931, the limestone mass and its flanking conglomer· ate were interpreted as the projecting crest of a sharp anticline. This interpretation is The University of Texas Publication No. 5301 retained on the present map, although re­view of air photographs failed to show any curving of beds over the crest of the anticline, down its assumed plunge to the east. Other interpretations are therefore possible: (1) There may be a continuous upward sequence in the Hazel formation from the southern belt of Allamoore formation to the north edge of the Streeruwitz Hills. The conglomerate that contains the limestone mass may be a sedimentary lens high in the sequence. The limestone mass itself may be a klippe of a contemporaneous advanc­ing thrust sheet, buried in its own debris, after the fashion of the limestone klippen in the Overton fanglomerate of southern Nevada (Longwell, 1949, p. 935). (2) A more likely possibility is that the conglomerate and the limestone mass are the inverted nose of a recumbent anticline, in the same manner as the northern lime­stone belts of the Millican Hills. If this be the case, there must be undetected reversals or thrusts in the sandstone and conglom­erate south of it, between it and the south­ern belt of Allamoore formation. As in­dicated in the discussion of the Millican Hills, such reversals would be difficult to detect, even after detailed field work. Bean Hills.-The Bean Hills, which lie between the Streeruwitz and Millican Hills, are a depressed part of the structure, as folds plunge toward the hills from the east and west. Many structural elements characteristic of the Millican and Streeru­witz Hills are not visible here and are per­haps buried, whereas novel and probably higher structural elements make their ap­pearance. In the northeast part of the Streeruwitz Hills along the road north of the Dwees ranch house (N.5-12.5 to H.5-14.5, Pl. 3) the Allamoore formation lies on the Hazel formation in two broadly folded syn­clines that plunge eastward toward the Bean Hills. Structurally these bodies of Allamoore differ from the northern belts of Allamoore in the Millican Hills, as they do not lie in recumbent folds on in­verted conglomerate of the lower part of the Hazel. They resemble the body of Alla­moore on Tumbledown Mountain, as they are a transported mass lying on red sand­stones of the upper part of the Hazel, from which they are separated by a "surface of movement" along which there has been great horizontal displacement. The synclinal bodies of Allamoore for­mation in the northeastern Streeruwitz Hills are the western edge of a large body of Allamoore formation which occupies most of the northern Bean Hills, with a width of a mile or more and a length of 5 miles along the strike (H.5-14.5 to Q.5-12.5, Pl. 3; Pl. 9, sections B-B' and C-C') . Along its northern side contorted beds of the Allamoore lie discordantly on gently dipping red sandstones of the Hazel. North of the large transported mass of Allamoore formation are smaller and more isolated masses. One is in the northern Streeruwitz Hills (E.5-14.5 to G.5-14.5, Pl. 3) , another east of the Keene ranch house (L.5-16.5 to 0.5-17.5, Pl. 3), and a third on the Sierra Diablo scarp west of the old Circle ranch house (R.5-17.5, Pl. 3) . In each, limestones and volcanics of the Allamoore formation are intricately folded and contorted, yet they overlie or adjoin red sandstones of the Hazel forma­tion that dip gently or lie nearly horizontal (Pl. 9, sections A-A', B-B', and C-C'). Presumably these masses were derived by thrusting from the suntain group on the up· thrown side. About half a mile east of the rock crusher, however, Flawn has dis­covered a small patch of Powwow member of the Hueco formation on the south side of the fault, lying on Carrizo Mountain group. This is either part of a wedge in a broad fault zone, or the post-Cretaceous movements on the fault are relatively small, the greater part of the displacement having taken place in pre-Hueco time. Farther east on the Hillside fault, Van Hom sandstone is exposed on both sides, and the displacement is probably less than 600 feet. On some of the faults in the interior of the Sierra Diablo the rocks on the down­thrown side are dragged up steeply against the plane, whereas the adjacent rocks on the upthrown side are little disturbed. Within the map area, drag was observed on the Sheep Peak fault 214 miles east of the old Circle ranch house (I.5-17.5, Pl. 2), on the Circle Ranch fault 1 mile south-southeast of the old Circle ranch house (E.5-16.5, Pl. 2), and on the South Diablo fault 134 miles northeast of the Keene ranch house (M.5-18.5, Pl. 3). Similar drag on the South Diablo fault continues beyond the map area to the northwest (fig. 13). On the Circle Ranch fault, the southward lowering of the Hueco limestone caused by drag is actually much greater than the displacement caused by breakage on the fault itself. The planes of the faults, where visible, stand nearly vertical or dip steeply toward the downthrow, and they are thus normal faults. Steep dips toward the downthrow have been observed on the Sheep Peak fault south of Sheep Peak (north of Pl. 2), on the South Diablo fault north of the Keene ranch house (north of Pl. 3), and on the Hillside fault east of the Gifford-Hill rock crusher (H.5-16.5, Pl. 1). Steep or vertical dips on the Grapevine, Carrizo Spring, and related faults may be inferred by the manner in which they cross the steep western scarp of Beach Mountain (R.5­ 7.5 and R.5-4.5, Pl. 2). The only excep­tion is a fault on the apparent southeast­ward prolongation of the Sheep Peak fault; where this is exposed on the Sierra Diablo escarpment 114 miles northwest of the Hazel mine (M.5-14.5, Pl. 2) its plane dips 60° toward the northern, or upthrown, side (fig. 14, B) . Age of. faulting.-The large, obvious dis­placements on the faults in the Sierra Diablo foothills took place in post-Creta­ceous time, as Cretaceous formations are present in places on the downthrown sides. The deep erosion of their scarps and the presence in places of obsequent fault-line scarps indicate that they are by no means recent features. Their movements thus ante­dated the latest movements of the faults along the edges of the Salt Basin, although they may be nearly contemporaneous with the initial movements of these faults and hence perhaps of late Tertiary age. There is, however, evidence that the faults of the Sierra Diablo foothills pos­sessed a considerable antecedent history and that at least some of them were dis­placed in Mesozoic, Paleozoic, or even pre­Cambrian time. On one fault, the Dallas, faulted rocks are overlain unconformably by younger formations which are not faulted, but in most places earlier move­ments are suggested merely by strati­graphic anomalies near the faults. These anomalies might be coincidental and might in part have resulted from intersection by the younger faults of earlier and different structural features, but the cumulative effect of the anomalies is such as to sug­gest that both younger and older features had common origins. Evidence of earlier movements on or near the faults of the Sierra Diablo foot­ hills is as follows: (1) Along the South Diablo fault, pre­Cretaceous and post-Permian structures oc­cur west of the old Circle ranch house. Re­lations are strikingly displayed 31/z miles to the west (R.5-15.5 to R.5-18.5, Pl. 3; Pl. 9, section C-C'), where the scarp on the north is formed by 900 feet of Van Horn sandstone and overlying Hueco lime­stone, whereas to the south Campo Grande limestone forms a low mesa and lies di­rectly on red sandstones of the Hazel for­mation. Although the latest displacement on the fault is downward to the south, earlier down-faulting or downflexing to the north is indicated. The earlier structure did not, however, exactly coincide with the present fault in all places, as 21/z miles farther west (M.5-18.5, Pl. 3; Pl. 9, sec­tion B-B') the Hueco and Van Horn extend a short distance south 0£it. A similar anomaly occurs in the south­ern Streeruwitz Hills, in the butte north of the Texas and Pacific Railroad. On its north side, the Hueco limestone is broken by a fault trending east and west, on the northern or downthrown side of which, 2* miles northwest of Eagle Flat section house (D.5-7.5, Pl. 3), is a small remnant patch of basal conglomerate of the Campo Grande limestone. The Hueco beneath the remnant is less than 150 feet thick, yet it is more than 350 feet thick immediately south of the fault; the fault must have been down­thrown to the south in pre-Cretaceous time. (2) Pre-Hueco, and probably late Penn­sylvanian, structures occur near many of the faults of the area. Where the Sulphur Creek fault crosses the Sierra Diablo escarpment 1 mile north of the Hazel mine (N.5-15.5, Pl. 2) the Hueco limestone of the northern or up­thrown side is underlain by 750 feet or more of Van Horn sandstone, hut on the downthrown side it lies on red sandstones of the Hazel formation (fig. 14, A; Pl. 9, section F-F'; Richardson, 1914, section E-E'). The Van Horn is folded into an east­plunging syncline, on the south flank of SOUTH 0 500 1000 Feet B FIG. 14. Field sketches of Sulphur Creek and Sheep Peak faults. A, Sulphur Creek fault on promontory of Sierra Diablo llf:? miles north of Hazel mine (0.5-16.5, PI. 2); note apparent absence of Van Horn sandstone beneath Hueco limestone on downthrown side, suggesting an oppo­site downthrow on fault in pre-Hueco time. B, Sheep Peak fault and a minor fault to south on escarpment of Sierra Diablo l~ miles northwest of Hazel mine (M.5-14.5, PI. 2); note north dip of Sheep Peak fault, a departure from usual high angle of dip of faults in this area. p€H =red sandstone of Hazel formation: p€v =Van Horn sandstone; Ph= Hneco limestone, with Php, Powwow member, at base. wlµch, close to the fault, the beds are steeply upturned. Locally at least, the south flank of the syncline appears to have been displaced by movements on the Sulphur Creek fault which were the opposite of the latest movements. Eastward, however, the paleogeologic map (Pl. 19, C) indicates that the syncline diverges northward from the course of the Sulphur Creek fault. Pre-Hueco movements may have taken place along the South Diablo fault in the same area as the pre-Cretaceous movements described above, as the Hueco on the es· carpment to the north lies on Van Horn sandstone, yet some miles to the south and southwest lies on the Hazel and Alla­moore formations. There is no clear evi­dence to indicate that this structure was any more than a broad flexure. Along the western part of the Hillside fault, the Powwow member at the base of the Hueco limestone on the northern or downthrown side contains large quanti­ties of coarse angular fragments of meta­rhyolite of the Carrizo Mountain group. It does not lie on the metarhyolite but on Van Horn sandstone, although it is faulted against the metarhyolite on the south. ,The fragments in the Powwow mem­ber indicate that the metarhyolite projected as a scarp along the Hillside fault during early Hueco time and was perhaps raised by faulting immediately before. The local pre-Hueco structure on the Hillside fault appears to be part of a much broader feature (Pl. 19, C). To the north, the Hueco lies on the Van Horn sandstone and not far away on the Ordovician. To the south, in the Carrizo, Wylie, Eagle, and Van Horn Mountains it lies, with a few exceptions, on the Carrizo Mountain group, indicating a general uplift of this southern area in pre-Hueco time. The truncation of the earlier formations south of the Hillside fault was not complete, however, as Flawn has found a few thin remnants of Van Horn sandstone in the eastern part of the Carrizo Mountains. (3) Faulting before Bliss (?) and after Van Horn time is clearly shown on the Dallas and Grapevine faults near Tumble­down Mountain (S.5-9.5 and Q.5-7.5, Pl. 2). The Dallas fault, which trends north-northwest, drops Van Horn sand· stone down on the east against Allamoore formation on the west, with a displacement of at least 500 feet, yet when the fault is traced southward it passes beneath Bliss (? ) sandstone which is not displaced (fig. 15) . Slight recurrent movements are sug· gested by strong joints along the same trend which extend upward into the Bliss (?) and El Paso; during a visit in 1951 the author saw what appeared to be a few feet of displacement in the Bliss (?) near the workings of the Dallas prospect north of the point of overlap. The Grapevine fault on the south side of Tumbledown Moun­tain, which trends nearly east-west, was displaced in a like manner in pre-Bliss (?) time, so that the Bliss (? ) lies on Van Horn sandstone on the south and Alla· moore formation on the north. Here, how· ever, the earlier faulting has been obscured by later displacements in the same direc· tion. Farther south on the west face of Beach Mountain, near the Carrizo Spring fault, the Van Horn is locally steeply upturned and truncated by the Bliss(?) (fig. 9, A), and similar steep dips in the Van Horn, but without a cover of Bliss (? ) , occur near the fault farther west. These structures suggest that movement may have taken place during pre-Bliss (?) time on the Carrizo Spring fault also. (4) Besides the early structures just de­scribed, which are of post-Van Horn and pre-Bliss (?) age, there are others of ap· parent early date near the high-angle faults in the pre-Cambrian area whose age is less certain. Along the Grapevine fault ha1£ a mile northeast of the Blackshaft mine (M.5­9.5, Pl. 2), the "north edge of the steep dip zone" in the Hazel formation is offset northwestward on the north side by about 2,200 feet, so that for a short distance verti· cal beds on the north lie against horizontal beds on the south (Pl. 9, section F-F'). This could not have been produced by nearly vertical downdrop on the south, such as has displaced the Van Horn and younger formations on the same fault farther east, but must have resulted from a sinistral18 strike-slip, or transcurrent, movement. ts As pointed out by E. M. Anderson (1942. p. 55) "from whichever aide the observer views a fault-plane, motion on the dietant aide will appear in the one class to be towarde the right, and in the other cla111 towards the left. ••••• The two cla11e1 of fault planet may well be termed 'dextral' and 'sinistral•.u Other west-northwest-trending faults in the pre-Cambrian area seem to show evi­dence of transcurrent movement. A minor fault a mile northeast of the Grapevine fault also offsets the "north edge of the steep dip zone" but by a smaller amount and in a dextral direction. In the southern Millican Hills two faults in the Allamoore formation on the western prolongation of the Carrizo Spring fault zone (A.5-5.5 and G.5-3.5, Pl. 2) involve highly folded beds in such a manner that they cannot A 0 5 Feet B FIG. 15. Field sketches of Dallas fault in outcrops south of Dallas prospect (S.5-8.5, Pl. 2), where it is overlain unconformably by the Bliss (?) sandstone. A, General view of locality; note joints without displacement which continue up into Bliss (?) sandstone. B, detailed view of contact. p£A = Allamoore formation; p£v =Van Hom sandstone; Ob= Bliss (?) sandstone; Oe =El Paso limestone. have formed by mere vertical displacement. Similar faults are also indicated on the map in the Bean and Streeruwitz Hills (K.5-12.5 and F.5-10.5, Pl. 3), but their relations are less clear. ;fhe transcurrent movements on the west­northwest-trending faults may have taken place during the post-Van Hom, pre-Bliss (?) dislocations just described. This seems to he hardly likely, as large-scale trans­current movements are produced during times of orogeny by compressive forces. The Van Hom is a post-orogenic deposit which has been tilted and block-faulted hut apparently not strongly compressed. It is therefore possible that the transcurrent movements took place before Van Horn time, in the waning stages of the post­Hazel orogeny. We have thus traced the history of the west-northwest-trending faults of the Sierra Diahlo foothills well hack into pre­Cambrian time. The ancient origin of these features would seem to account for the west-northwest "grain" which is possessed by all the rocks of the area. The long his· tory of this "grain" should give pause to anyone who should attempt to interpret the mechanism of the faulting in the area in terms of pattern alone. Stresses at an early period created the pattern, yet the pattern was renewed in younger rocks during suc­cessive periods, under the influence of quite different stresses. loints.-The writer has made systematic observations on joints in the Sierra Diablo and southern Guadalupe Mountains, some of the results of which have already been published (King and Knight, 1944, inset map; King, 1948a, pp. 114-117, 119-120, 123-124, Pis. 20 and 21) . The Sierra Diahlo foothills were included in the area thus studied, and details of the results ob­tained here are shown on Plate 10. All the rocks of the Sierra Diablo foot­hills are cut by joints. Those in the highly deformed Carrizo Mountain group, Alla· moore formation, and Hazel formation trend in diverse directions, dip at all angles from horizontal to vertical, and are so com­plex that it was not considered worth while to observe or analyse them. Those in the Van Hom and younger formations stand about vertically and are arranged in sys· tematic patterns that are susceptible of measurement and study. Similar joints also occur in the gently dipping part of the Hazel formation, along with other joints in­clined at lower angles. The steeply dipping joints in this part of the Hazel formation were also included in the study. Most of the joints in all the rocks are straight and smooth, even where they cut through irregularly bedded rocks, or alter­nations of hard and soft layers. At the sur­face, some of the joints are open fissures, but these have probably been widened by weathering, and most of them are prob· ably tight and narrow at depth. Single joints commonly extend across the entire breadth of any exposure, and where several sets are present they commonly cross one another without deflection. Spacing of the joints depends on the nature of the rock; they are widely spaced in massive rocks, such as the Van Hom sandstone, and are closely spaced in thin-bedded brittle rocks, such as the Ordovician and Permian limestones. Nearly all the joints dip steeply or are nearly vertical. During the course of the work, conspicuous deviations of the joints from vertical were conscientiously noted, but it was found that such deviations were so slight and so uncommon that they could well be ignored. The most significant fea­ture of the joints appeared to be their trend in horizontal plan, and on this the largest number of observations was made. In the Sierra Diablo foothills, trends of 1,265 joints were measured, at 156 stations. These observations are shown on Plate 10, where the trends of the joints are sum· marized by two methods: (1) Joints at individual stations are shown by lines of various lengths, radiating from the point of observation. Records made at the stations commonly consisted of measurements at two or more points in the immediate vicinity, at each of which all joints were recorded, and notations made on the prominence of each; observations at a station may include as many as 20 indi­vidual measurements. It is not possible to show all these measurements on the map; the joints at each station have accordingly been summarized, and the most abundant five or six have been plotted, with the lengths of the lines indicating relative prominence of each. On the compilation sheet from which Plate 10 was prepared, the joints were further differentiated ac­cording to the formations in which they oc­cur, in an effort to discover whether there The University of Texas Publication No. 5301 were any significant differences in pat­tern in rocks of successive ages. So little discernible difference was found that it did not appear to be worth while to include this differentiation on the final map. The user of the map can observe to some extent the joint patterns in rocks of different ages by noting the formations in which the respective stations are located. (2) Joints in different units are further summarized statistically in the form of rosettes, which are overprinted at various places on the map. Units are differentiated by age and locality. Separate statistical summaries were made for the Hazel forma­tion, the Ordovician rocks, the Hueco lime­stone, and the Cretaceous rocks. The oc­curence of joints in the different strati· graphic units was further subdivided ac· cording to locality. Thus, different rosettes were prepared for the Hueco limestone in the southeast part of the Sierra Diablo, the Streeruwitz Hills, and the ridges south of the Red Valley. Each rosette shows the relative abun­dance and prominence of the different joints measured in each unit. Tallies were made (a) of the number of joints observed in each 5 ° of arc and (b) of the relative prominence of the joints observed. The two tallies were combined, giving greater weight to the first factor than the second; actually, the two corresponded closely, for the most abundant joints also proved to be the most prominent joints. The combined tallies were converted into percentages of joints in each 5° of arc, in terms of the total number recorded in the unit. As originally worked out, wide variations were found between percentages at some of the adjacent points on the arcs, which appar­ently resulted from a personal equation in making the observations. The percentages were therefore evened by means of moving averages. Each figure used in plotting the rosettes is thus the average of the original percentage for the direction shown and the percentage of the two directions lying 5 ° on each side of it. Observations on the trends of the joints amplify and fill in the "grain" of the rocks which is otherwise partly suggested by the fault pattern. Like the faults, the dominant joints trend west-northwest, both near the faults and in areas where few or no faults are present. In the eastern part of the area, where the faults change their trend to east-west or east-northeast, the dominant joints likewise change their direction. In the west· ern part of the Red Valley, where minor faults trending north and south are present, joints of similar trend are also prominent. After the dominant west-northwest set, the next most abundant is a set which trends nearly at right angles, or north· northeast. Ordinarily this set is not £oJ. lowed by any faults. Of the remaining joints, the rosettes indicate that the greater number commonly lie about midway in the angles between the two dominant sets. The characteristic joint pattern of the area thus consists of two dominant joint sets at nearly right angles and two subordinate sets which bisect the angle between them. A similar pattern occurs in the southern Guadalupe Mountains where, however, the dominant joint set trends north-northwest rather than west-northwest. In the southern Guadalupe Mountains much of the jointing seems to have originated during the Ceno· zoic uplift which produced the present mountains, and the joint pattern seems to be related to the strains thereby set up (King, 1948a,pp. 123-124). The origin of the joint pattern in the Sierra Diablo foothills is less obvious. Pat­terns in rocks of different ages, which are separated by significant times of disturb· ance or orogeny, seem to be essentially alike. This may be because the dominant pattern in all the rocks is less closely re· lated to the earlier orogenies than it is to the latest upheaval of the area. This does not seem likely, because the latest upheaval was primarily along faults of the northerly· trending system, with which the joints are not parallel. Moreover, evidence along the. faults of the foothill area indicates that the dominant west-northwest "grain" of the rocks originated early in their history, and perhaps as early as late pre-Cambrian time. It would thus appear that joints re· lated to this "grain," and perhaps others, were formed long before the latest up· heaval of the area and were renewed in successive younger rocks as they were laid down unconformably on the older. The joints may he related to regional structure. The map (Pl. 19, B) showing contours on the top of the pre-Cambrian rocks indicates that in a belt extending west-northwest or northwest through the Van Horn area these rocks lie much higher than on either side. The continuity of this high-standing belt is much disordered by the northerly-trending faults which cross it, but it may have been a single long, broad uplift or positive area before the faulting and during Mesozoic and Paleozoic time. It is possible that the dominant west­northwest-trending joints and faults of the Sierra Diablo foothills were controlled by this uplift and were formed parallel to its axis during its several periods of move­ment. If this interpretation is correct, the joints and faults have the same mechanical relation to the early west-northwest­trending pre-Cambrian uplift of the Van Horn area as do those of the Guadalupe Mountains to its later north-northwest­trending uplift. Fractures.-The geologic maps (Pis. 2 and 3) differentiate, besides the faults, an­other class of structures that are designated as "fractures." These have been traced largely from air photographs where they are expressed in the same manner as the faults-by alignment of minor valleys and ridges. They differ from faults in that they show no clear evidence of displacement. They may be faults of small displacement or strong lines of jointing on which there has been no displacement. The fractures as mapped are therefore probably heterogeneous, but they include one well·characterized group of structures which seem to be different from any others in the area-the "fractures of Hazel type." Fractures of Hazel type are vertical min­ eralized fractures or fissure veins in which the ore bodies of the Hazel mine and those of similar mines and prospects are located (Pl. 12 and fig. 19). They cut gently dip­ ping red sandstones of the Hazel formation not far from the bases of the escarpments of the Sierra Diablo. In plan, the fractures are clustered in zones, in which they lie paral­ lel or en echelon, or diverge from each other at acute angles; in places, minor frac­ tures branch off at wider angles. Surface outcrops of the fractures are commonly indicated by bleaching of the red sand­ stones immediately adjacent. In vertical section they are zones a few feet to 40 feet wide of bleached, brecciated, and sliced country rock, interlaced with a gangue of barite, calcite, and quartz, which contains sulfides of copper, silver, and other metals. Areal mapping discloses that fractures of Hazel type form two· zones-one a sys· tern of en echelon fractures 31/z miles in length, trending nearly east and west, ex­tending from the Mohawk mine to the Hazel mine (H.5-12.5 to 0.5-13.5, Pl. 2), and the other a set of similar fractures about 2 miles in length, trending north­northeast, on which the Pecos mine and Eureka prospect are located (north of Pl. 2; see fig. 19 and map of King and Knight, 1944). The relation of fractures of Hazel type to the high-angle faults and joints of the same area is uncertain. They run across the dominant west-northwest "grain" of the country rock, apparently without offset­ting it or being offset by it, yet the domi­nant "grain" seems to have originated in late pre-Cambrian time and to have been subjected to intermittent movement until the late Tertiary. Thus, one of the Hazel fractures is crossed without offset by a west-northwest-trending fault near the Marvin-Judson prospect (L.5-12.5, Pl. 2), and another is crossed without offset by the west-northwest-trending Cox Mountain fault near the Pecos mine. First movements on the west-northwest "grain" produced transcurrent faults with considerable strike­slip displacement, and the fractures of Hazel type must be younger than these movements. Later movements on faults of the west-northwest "grain" were domi­nantly vertical and might have displaced the vertical fractures of Hazel type without offsetting them laterally. In the present discussion, the fractures of Hazel type are treated as structural fea­tures and not as ore bodies. The mineral deposits which they contain are described in the section on "Economic Geology." A distinction must be made between the time of formation of the fractures and the time of emplacement of the mineral deposits; the deposits were introduced into zones of weakness already formed and are younger -perhaps much younger. The faults and joints of the west-north­west "grain" must be different from the fractures of Hazel type, as few of them show any significant mineralization. Evi­dently they were less susceptible to the circulation of mineralizing solutions. Pos­sibly they are tighter, straighter, cleaner breaks than the fractures of Hazel type and were accompanied by less slicing and brecciation of the country rock. PUMP STATION HILLS Philip B. King and Peter T. Flawn INTRODUCTION the northern Sierra Diablo where it has Rhyolite of probable pre-Cambrian age is exposed in an area of about 12 square miles in the Pump Station Hills, near the center of the Diablo Plateau, 55 miles northwest of Van Horn and 40 miles north of Sierra Blanca (Darton, Stephenson, and Gardner, 1937; King, 19~a). Hueco Pump Station on the Pasotex pipe-line lies near the center of the hills, and they are crossed by the county road leading north from Sierra Blanca. The pre-Cambrian nature of the rocks of this area was first recognized by N. H. Darton, during his reconnaissance work for the geologic map of Texas. Previously published geologic maps had grouped the igneous rocks of the Pump Station Hills with the near-by intrusives of Tertiary age (Richardson, 1904; Beede, 1920). RELATIONS TO SURROUNDING ROCKS The rhyolite projects in low rounded hills, on which there are no prominent ledges. Much of the surface of the hills is masked by colluvium made up of rhyolite fragments, and the intervening and sur­rounding valleys are covered by alluvium. Outcrops are insufficient to determine de­cisively the relation of the rhyolite to the surrounding sedimentary rocks. About 2 miles to the south on the Sierra Blanca road are indistinct ledges of Cretaceous limestone and sandstone, and the same rock forms low hills south of the pipe-line about a mile east of the pump station. Several miles to the north and east are outcrops of Permian limestone. These rocks have not been seen in contact with the rhyolite and are separated from it by areas of alluvium. The rhyolite hills are very different in appearance from the jagged buttes and peaks formed by the near-by Tertiary al­kalic intrusives, and they have the appear­ance of a long-eroded surface, perhaps the exhumed summit of a pre-Cambrian ridge. Structurally this may be related to Paleo­zoic deformation, as it lies at the upper end of a considerable flexure in the Per­mian rocks which extends southeast into been called the Babb flexure. Deep drill­ing in this part of the Diablo Plateau would undoubtedly bring to light many interesting features, but at present the wells that have been drilled are spaced many miles apart, and subsurface data are scanty. A well drilled 7 miles northeast (A. and E. Jones No. 1 E. C. Mowry, T. D. 3,84.0 feet) encountered Devonian, thick Silurian, Mon­toya, and topmost El Paso. LITHOLOGY The rock is a dark red rhyolite porphyry. Phenocrysts of pink feldspar and clear glassy quartz occur in an irresolvable ma­roon ground-mass. Phenocrysts range in size from 2 mm. to 1 cm. and range in amount from less than 5 to 40 percent of the rock. Dark green rocks containing red feldspar phenocrysts in an aphanitic green ground-mass are locally found within the outcrop. In one place this green rock crops out along a linear fissure zone. It is prob­ably an altered phase of the normal red rock. The rocks are well jointed but show no discernible attitude or structure. The rhyolite is very similar to the rhyolite found as cobbles in the Van Horn sandstone (Table 21, mode IV) and to Table 21. Estimated modes of pre-Cambrian rhyolite near Hueco Pump Station. II Ill IV Ground-mass ................ 80 Quartz ··········· ··········-··· · .... Albite ···········-···-······-··· .... Potassium feldspar .... 15 Apatite ·····-··············· ··· tr Zircon ·················-···-···· ··-­Opaque ··-···--·-··-··········· 1 Chlo rite ·-······················ 1 Sericite ·· ·················----­tr Sphene tr Calcite 2 80 15 1 4 82 10 7 tr 1 tr 75 10 14 tr tr 1 Totals ...................... 100 100 100 100 I, Near Hueco Pump Station west of road. II, Near Hueco Pump Station west of road. III, Near Hueco Pump Station east of road. IV, Cobble from Van Horn sandstone, east edge Carrizo Mountains. the cataclastically altered rhyolite of the Carrizo Mountain group in the northwest part of the Carrizo Mountains. Petrography.-In thin section the rhyolite shows altered phenocrysts of feldspar in a ground-mass of altered feldspar microlites (maximum size 0.04 mm.). Sericite and chlorite are the principal alteration products. Calcite replaces feldspar to a limited extent. Quartz comprises as much as 10 percent of the rock hut is not present in all samples. Magnetite, apatite, and sphene are present in minor amounts. The altered character of the rock makes it difficult to determine the feldspar. The greater part of the feldspar in the phenocrysts is albite that shows an erratic and disrupted "chessboard" twinning, suggesting a replacement of micro­cline by alhite. The low index (N.) is about 1.530. The feldspar constituting the ground-mass is too altered to give a satisfactory determination. Perthitic feldspar was observed in one slide. Moat of the rock is alhite rhyolite. GENERAL PROBLEMS OF PRE-CAMBRIAN ROCKS STRUCTURAL AND STRATIGRAPHIC PROBLEMS Philip B. King STRUCTURAL HISTORY lntroduction.-In chapters which have gone before, the structural history of the pre-Cambrian rocks has been set forth in piecemeal fashion under discussion of the different rock units; part has been sum­marized in Table 20 (p. 101). Under the present heading the structural history in the different areas will be reviewed, and an at­tempt will be made to coordinate them into a single sequence. Structural history of south part of Van Horn pre-Cambrian area.-Flawn's study of the Carrizo Mountain group in the ~outhern part of the Van Horn pre-Cam­brian area indicates the following sedi­mentary, metamorphic, and structural history (Table 22, p. 126). Structural history of north part of Van Horn pre-Cambrian area.-King's study of the Sierra Diablo foothills in the northern part of the Van Horn pre-Cambrian area indicates the following sedimentary, meta­morphic, and structural history (Table 23, p. 127). Structural history of pre-Cambrian rocks northwest of Van Hom area.-Northwest of the Van Horn area in Trans-Pecos Texas, pre-Cambrian rocks crop out in the Pump Station Hills, the south end of the Hueco Mountains, and in the Franklin Mountains (fig. 1). These are considerably different from those of the Van Horn area, and out­crops are so small and scattered that it is difficult to make out a structural history. The sequence in the principal area of the Franklin Mountains is, moreover, in need of modern detailed study, which might alter published conclusions. The follow­ing very tentative sequence (Table 24, p. 128) is compiled from Richardson (1909, pp. 3, 6-7) and King (1935, pp. 226-227) and from notes given to King by N. H. Darton in 1930. Correlation of structural events in the different areas.-Correlation of events be­tween the three different areas is beset with uncertainties. Few connections can he traced between them, because the south­ern and northern parts of the Van Hom area are separated by the Streeruwitz over­thrust, and the two parts of the Van Horn area are in turn separated from the pre­Cambrian areas to the northwest by a cover of younger rocks. There are, more­over, only slight physical resemblances between the three sequences, yet the physi­cal features are the only ones on which a correlation can now be based. A tentative correlation between the structural histories of the different areas based on such physi­cal features is given in Table 25 (p. 129). Reference to Table 25 indicates that the last major structural event in the south part of the Van Horn area was emplace­ment of the Streeruwitz overthrust, and that this can be dated in the north part of the Van Horn area as of post-Hazel and pre-Van Horn age. Cataclastic and retro­gressive metamorphism of the rocks of the Carrizo Mountain group took place at this time, and detrital fragments of rocks thus altered appear in the succeeding Van Horn sandstone. Lower in the columns, another possible comparison can be made between the rhyo­lite intrusives of the south part of the Van Horn area and the rhyolite eruptives and possible intrusives northwest of the Van Horn area. Flawn has pointed out a petro­graphic resemblance between rhyolites of the Carrizo Mountain group in the Car­rizo Mountains and rhyolites in the Pump Station Hills. The rhyolites to the north­west are older than the Van Horn sand­stone, as it contains their detrital frag­ments. Tenuous evidence suggests that they may also he older than the Hazel forma­tion, as the Hazel contains detrital frag­ments of red granite, and red granite in­trudes the rhyolite of the Franklin Moun­tains.19 The Hazel is a non-igneous forma­ a Richardoon (1909, p . 7) cla..ed the granite of the Franklin Mountains as "post-Carboniferous." King, how­ever, observed the Bliae lying unconformahly on red granite at the head of McKilligan Canyon. Accordine to N. H. Darton (note1 of 1930) there appear to be two granitea jn the Franklin Mountains : a red granite of pre-Cambrian age, presumably younger than the rhyolite and Lanotia quartzite, and a granite porphyry of late Cretaceous or Tertiary •se, which invadee the Paleozoic as well a1 the pre-Cambrian recb. Table 22. Sedimentary, metamorphic, and structural history of south part of Van Hom pre-Cambrian area, by Peter T. Flawn. I-' ~ l1rneoua activity Sedimentation Metamorphism Deformation Van Hom sandstone (local) Unconformity Quartz veins during late stages of Late stages of cataclastic metamor­ Climax of movements on Streeruwitz cataclasis, and aiter. phism; diorite altered to amphib­ overtbrust. olite. Intrusion of diorite in sills and ir­regular bodies along pre-existing fo11ation. No Cataclastic and retrogressive meta­morphism; development of foliation, Initiation of movements on Streeru­ sedimentary lineation, and mylonite in metarhyo­ witz overthrust. lite; retrogression of metamorphic record facies in sedimentary rocks in north­west part of area. Intrusion of rhyolite, mainly in sills. Injection of pegmatite to southeast, Regional progressive metamorphism Tilting of sedimentary rocks into possibly related to granitic batholith homoclinal sequence of northeast at depth. of low to medium grade, increasing ·strike and southeast dip; overturned folds in southeast. in intensity to southeast. Carrizo Mountain group (19,000 feet plus) : Quartzo-feldspathic are­naceous sediments, with interbedded Volcanics in lower part ( chlorite­argillites and some carbonates; vol­mica schist of Carrizo Mountains). canics near base of exposed section in Carrizo Mountains ( chlorite-mica schist). ~ .,,... ~ ~ ~· ~ ;;: ~· ~ ~ e ;p O" .... ~· c;· ;:s ~ ~ ~ Table 23. Sedimentary, metamorphic, and structural history of north part of Jlan Horn pre-Cambrian area, by Philip B. King. - Sedimentation Unconformity (with Bliss (?) sandstone) Van Horn sandstone (800 feet plus): Continental, post-orogenic; frag· ments of cataclastically altered meta· rhyolite from south and rhyolite and granite from north. I l\[ a j or un co nformit y I Hazel formation (many thousands of feet): Subaqueous, perhaps inter­montane red sandstone; syn-orogenic conglomerate, with lragments from Allamoore formation, and a few of granite, probably from north. Igneous activity Metamorphism Deformation Quartz veins Tilting and block faulting. ~ ~ ;; CJ­ ... Low-grade progressive metamor· Major orogeny. Strong folding and phism, dying out to north; axial thrusting of Allamoore and Hazel plane cleavage in phyllites; marmori· formations, dying out to north; em· zation in limestone; tourmaline veins placement of Streeruwitz overthrust; in sandstone. transcurrent faults in late phases. £i• ;::i ~ ~ ~ ~ ;::i ~ ., ;::i ... ., ~ .F ~ I=> Metamorphism at least locally; mar-Strong deformation of unknown "' Unconformity hie and possibly schistose pyroclas-character, masked by post-Hazel tics in conglomerate of Hazel for-orogeny. ma ti on. Allamoore formation (many thou­sands of feet) : Cherly limestones, phyllites, volcanics; marine, eugeo­synclinal facies. Volcanics interbedded in Allamoore formation: Pyroclastics, flows, and shallow intrusives. Diabase a promi­nent component. ...... N) ...... t-:> co Table 24. Sedimentary and structural history of pre-Cambrian rocks northwest of Van Horn area (compiled from Richardson (1909) and King (1935) ). Sedimentation Iirneoua activity Granite, and probably rhyolite, are Erosional older than Hazel formation as gran­unconformity ite fragments occur in its conglomer­(with Bliss sandstone) ate. Intrusions of red granite (uncon­formable below Bliss in Franklin and Hueco Mountains). Rhyolite eruptions; thick flows with Rhyolite Hows (Franklin Mountains) basal agglomerate in Franklin Moun­ tains; rhyolites elsewhere not de­ termined as extrusive or intrusive. ·-----­ Erosional unconformity I I Lanoria quartzite (1500 feet plus) : Gray, quartzose, fine grained, me­dium bedded. Known only in Franklin Mountains. Metamorphism No metamorphic history recorded Orogeny Distinct angular unconformity be­tween Bliss sandstone and underly­ing rhyolite and Lanoria quartzite (observations of N. H. Darton, 1930). In parts of Franklin Mountains and in Hueco Mountains Bliss lies on granite which presumably intrudes these formations. Unconformity between rhyolite flow and Lanoria quartzite is of undeter­mined significance; apparently slight where observed. "'-3 ;:,-.. "' ~ ;:i ;:;· ~ "' ~­ ~ ~ El "ti ~ 0­ ...... ~· c· ;:s '.<: ~ ~ South part of Van Horn area Van Horn sandstone Major orogeny; emplacement of lreeruwitz overthrust; catacla tic and retrogressive metamorphi m. Diorite intrusions Beginning of orogeny Northwest of Van Horn area North part of Van Horn area Tilting and faulting Van Horn sandstone Major orogeny; emplacement of Streeruwitz overthrust; low-rank progressive metamorphism. Hazel formation Orogeny; local metamorphism Allamoore formation Rhyolite intrusions Major orogeny; progressive regional metamorphism Carrizo Mountain group tion, and it seems unlikely that rhyolites were forming south and northwest of it while it was being deposited. Unfortunately, no rhyolite is present in the sequence in the north part of the Van Horn area, so that it cannot be determined by what measure the rhyolite precedes the Hazel formation. The underlying Alla­moore formation contains volcanic units but no massive rhyolite bodies. The rhyo­lites must either be equivalent to the Alla­moore, or be older. Accepting the rhyolites as a datum throws new light on the relation of the Allamoore formation and Carrizo Moun­tain group. The writer (King, 1940, p. 146) suggested earlier that: "The rocks of sedimentary origin in the Carrizo Moun­tain schist may not be very different in age from those of the Allamoore lime­stone • • •. They may have formed dur- Granite intrusions Rhyolite eruptions Slight unconformity Lanoria quartzite ing the same cycle of deposition, with the sedimentary rocks of the Carrizo Moun­tain representing the initial deposits, and those of the Allamoore limestone the later deposits. • * * The types of rocks pres­ent, and the degree of metamorphism and deformation are similar." It was therefore inferred that the two units were laid down nearly contemporaneously in different en­vironments that were widely separated be­fore the telescoping movements on the Streeruwitz overthrust; also that the post­Allamoore and pre-Hazel orogeny and ac­companying metamorphism was the same as the orogeny which tilted and regionally metamorphosed the Carrizo Mountain group. This interpretation has been greatly weakened by Flawn's detailed work in the Carrizo Mountains. Metasedimentary rocks (second and third meta-arkoses and first mixed unit) are not synclinally infolded in an older extrusive meta·igneous se­quence (metarhyolite and amphibolite) as was supposed in 1940; instead the meta­igneous rocks are intrusive, the structure is essentially homoclinal, and the meta­morphic history is complex. Moreover, if the rhyolites are contempo· raneous with or older than the Allamoore, the post-Allamoore and pre-Hazel orogeny cannot be the same as that which tilted and regionally metamori;>hosed the Carrizo Mountain group. The rhyolite of the Car­rizo Mountain group was injected after the tilting and regional metamorphism of the sedimentary part of the group. The Allamoore and Carrizo Mountain were therefore laid down during separate cycles of deposition, and the orogeny at the end of Allamoore time is different from and younger than the orogeny at the end of deposition of the Carrizo Mountain sediments. There were, therefore,. three orogenies in the Van Horn area, rather than two. It is perhaps odd that the Car­ rizo Mountain should show the structural and metamorphic effects of only the oldest and youngest orogenies, but the Carrizo Mountain was not in juxtaposition with the Hazel and Allamoore before over­ thrusting took place, and the second orogeny may not have extended into the Carrizo Mountain area, or was blended with the initial effects of the third orogeny. Some confirmation of the conclusion that the Carrizo Mountain is older than the Allamoore may be had in the differ­ences between the sediments of the two units. The Allamoore consists largely of limestone and volcanic rocks, both of which are unimportant in the Carrizo Mountain. It contains no sand other than detrital material of volcanic origin, whereas the Carrizo Mountain contains thick bodies of quartzo-feldspathic sand­stone. These differences indicate either that the two units were formed at the same time in areas widely separated geographically, or that they are of different ages. Correlation between the Van Horn area and the Franklin Mountains involves greater hazards than those just discussed because of the distance and lack of con­tinuity. Accepting the rhyolite as a datum suggests the possibility that the Lanoria quartzite is nearly equivalent to the Car. rizo Mountain group. The Lanoria is sep­arated from the succeeding rhyolite flows by an unconformity, and the Carrizo Moun­tain was tilted before being intruded by rhyolite. The structural break between the Lanoria and rhyolite appears to be less than that between the Carrizo Mountain and rhyolite, but this may be because the Carrizo Mountain developed in a more mobile area. Richardson (1909, p. 3) describes the Lanoria quartzite as a gray, medium­bedded, quartzose sandstone, and Darton (notes of 1930) describes it as a thinly laminated white or pinkish sandstone with some interbedded reddish shaly layers. It appears to be much grayer and much less red than the sandstones of the Hazel for. mation, but it is seemingly less feldspathic than the sandstones of the Carrizo Moun· tain. The correlations just outlined are subject to so many imponderables that they will no doubt be extensively revised as further in· formation is obtained. More information is especially desirable on the relative ages of the different rocks. Specimens of limestone from the Allamoore formation and Carrizo Mountain group have been submitted by Flawn to Dr. J. Lawrence Kulp of Lamont Geological Observatory, Columbia Univer· sity, for age determinations by the stron· tium method, but work on these is still in orogress. Perhaps other means of age de· termination by radioactive methods will be devised in the future that can also he applied to the rocks of the Van Horn area and adjacent regions. STRUCTURAL PATTERN The Streeruwitz ove1thrust and features associated with it have every appearance of a major structure on the border between a stable region or cratonic area, on the north, and a mobile belt on the south (Pl. 19, A). In the southern Appalachians, mylonites and cataclastic structures like those on the Streeruwitz overthrust occur in crystalline rocks near the great thrusts of the Blue Ridge and Piedmont provinces ( Crickrnay, 1933; Oriel, 1950, p. 44) . These are sepa· rated from the flat-lying sedimentary rocks of the Cumberland Plateau in the stable region on the northwest by a belt 50 miles" or more in width, the Valley and Ridge province, made up of strongly folded and faulted sedimentary rocks. In the Van Horn area the comparable belt of strongly folded and faulted Allamoore and Hazel forma· tions is, rather surprisingly, only a few miles in width. In the southern Appalachians, as in the Van Horn area, there is doubt as to corre· lation between the highly metamorphosed and injected crystalline rocks within the mobile belt and the sedimentary rocks which border them on the northwest. The crystalline rocks are generally considered to be older, although there is a possibility -as in the case of the Carrizo Mountain and Allamoore-that they are at least in part equivalent. REGIONAL RELA'l'IONS A comprehensive · review of the pre­ Cambrian rocks in the region surrounding the Van Horn area appears to be some· what futile at this time, as it would furnish only disconnected items whose relations to other items is not evident. Some of these uncertainties will no doubt be cleared up with further research, although many fea­ tures will probably forever remain obscure. Further clues as to regional relations will no doubt be obtained by study of other areas of pre-Cambrian outcrop, although all the outcrops of the region are now known in a general way, and it appears unlikely that any others will yield features as varied as those in the Van Horn area. Clues as to relations between the outcrops may be obtained when more deep wells penetrate the pre-Cambrian, and geophysi­cal surveys may reveal gravity and mag· netic anomalies that cannot be caused by Paleozoic and younger rocks alone but must express structures in the pre-Cambrian beneath. Greatest progress will be made, however, when more age determinations by various radioactive methods become avail­able, as these will knit together items al­ready known, as well as those remaining to be discovered. Physical features which are apparently similar and equivalent may turn out to have been formed under like conditions at quite different times, and dis­similar physical features may turn out to have formed at the same time in different environments. The pre-Cambrian rocks of New Mexico and Arizona have been summarized by Darton (1925, pp. 15-37; 1928, pp. 3-5), and further details of the older pre-Cam· brian rocks of Arizona have recently been given by Anderson (1951, pp. 1331-1346). An area in central New Mexico which con· tains ancient metasedimentary and meta· volcanic rocks intruded by granite has been described bv Stark and Dapples (1946, pp. 1127-1143)' and by Reiche (1949, pp. 1186-1198). In Arizona, a complex of older pre-Cambrian metasedimentary and metavolcanic rocks, widely intruded by. granite, is overlain unconformably by younger pre-Cambrian sedimentary rocks of the Grand Canyon series and Apache group. The Grand Canyon series and Apache group have many resemblances to the Allamoore and Hazel formations, al~ though they differ in being little deformed, or at most tilted and faulted, rather than strongly folded. In particular, the chert· banded Mescal limestone of the Apache group, as described and figured by Darton (1925, p. 32), strikingly resembles the lime­stones of the Allamoore formation; like the limestones of the Allamoore it is at least locally associated with lavas. If the Car· rizo Mountain group is older than the Hazel and Allamoore, as it appears to be, it may be equivalent to the complex of older pre­Cambrian rocks of Arizona. The larger pattern of pre-Cambrian structure in the region adjacent to the Van Horn area is still elusive. An attempt to portray the pre-Cambrian "grain" of Texas was made by Rettger (1932), but this was based on the assumption that pre-Cambrian structures closely parallel those in the over· lying Paleozoic rocks, which may or may not be correct. Pre-Cambrian structures might cross the Paleozoic structures that have been superimposed on them at any angle, and even if Paleozoic structures were broadly controlled by pre-Cambrian structures there are probably significant local deviations. If the Van Horn area marks the border between a pre-Cambrian stable region and mobile belt, it would be of interest to de­termine more of the outlines and extensions of each. Judging from the low degree of metamorphism and the local nature of the deformation in the rocks of the north part of the Van Horn area, they are of rela­tively late pre-Cambrian age, and the stable region and mobile belt indicated in the Van Horn area may also be primarily a feature of the later pre-Cambrian. The pre-Cambrian rocks of the Llano region of central Texas might be a part of the stable region, as radioactive determinations made at Barringer Hill indicate an age of 1,040 million years, or about the same as that of the Laurentian intrusives in the Grenville province of the Canadian shield (Holmes, 1937, pp. 204-206); rocks of similar age have also been determined in the Pikes Peak area of Colorado. The old notion that the North American continent grew outward from a nuclear stable area by peripheral accretion of suc­cessive mobile belts along its borders has recently been revived by Wilson (1948, p. 721): The symmetry of the North American continent is well known. It consists of a large and central shield area, over much of which pre-Cambrian rocks are exposed. There are four systems of mountains around the shield, cut out of later sedimentary rocks, in each case folded tangenti­ally or parallel with the margins of the shield. The Appalachian Mountains, folded in late Paleozoic time, and the Cordillera, of Mesozoic and Tertiary age, are well known. Their inner margins are thrust over the covered shield. The fact that these mountains form a V or U·shaped margin to the shield has often been mentioned, as has the similar arrangement of the foliation within the shield. Less studied, but comparable in size with the Appalachians, are the East Green­land Mountains and those that cross North Green­land and Ellesmere Island. • • • • • When these northern mountains are considered, it can be seen that the continent has the shape of an irregular polygon rather than a U or a V. The orogenic forces that produced these successive additions around the margins of the continent must pre­sumably have acted inwards towards its center. It is here suggested that the same radial and inward forces acted to form tangentially folded ranges of mountains during pre-Cambrian time as have acted since. The Van Horn pre-Cambrian area, and the supposed stable region that possibly embraces the Llano and Pikes Peak areas, lies far south of the Canadian shield for which the above theory was proposed by Wilson. If the continent developed by out­ward growth from the Canadian shield alone, the existence in pre-Cambrian time of a stable region farther south would be unlikely. If the theory of peripheral accre­tion is accepted it is more probable that the continent developed from several separate nuclei, as suggested by Kay ( 1951, p. 97), which were welded by orogenies into a single large stable region before Paleozoic time. STRUCTURE OF THE PRE-CAMBRIAN SURFACE The structure of the surface of the pre. Cambrian in the region surrounding the Van Horn area has been contoured in gen· eralized form by Moss (1936, pl. 4, p. 948), and later in more detail by J?hn Emery Adams as part of a research proJet!t for the Standard Oil Company of Texas. Contours made by Adams on the top of the pre-Cambrian in northwestern ,Trans· Pecos Texas are reproduced in Plate 19, B, of the present paper with his kind con· sent. Adams' work shows that the Van Horn pre-Cambrian area lies at the crest of a broad domical area, oriented northwest­southeast, on the summit of which the top of the pre-Cambrian rises to about 6,000 feet above sea level. On the northeast flank the top of the pre-Cambrian slopes beneath the Paleozoic rocks of the Delaware basin, and on the southwest it slopes more abruptly beneath the thick geosynclinal Mesozoic sediments of the deformed belt of the Eagle and Quitman Mountains. This is the Van Horn "dome" or "uplift" of Baker (1927, p. 41; 1935, p. 182), who has termed it "the highest uplift in Texas." The area of high-standing pre-Cambrian rocks is domical only in the broadest sense. In detail, the pre-Cambrian rocks come to the surface in a number of closely adja· cent, but distinct, mountain uplifts, sepa· rated by down-faulted areas. Detailed con· tours on the pre-Cambrian would show complex offsetting of the surface from one fault block to the next. Adams (1944) has properly criticised the concept of the Van Horn area as "the highest structural point in Texas," pointing out the different re· suits that can be obtained by application of several criteria and concluding that the Franklin Mountains, a much narrower and more localized uplift farther west, contains the "highest structural point." The Guadalupe Mountains now stand several thousand feet higher than the Sierra Diablo, yet they expose only later Paleozoic rocks, whereas the latter exposes older Permian, older Paleozoic, and pre-Cambrian rocks. Structure contoun drawn on the top of the pre-Cambrian indicate that the pre-Cambrian in the south part of the Sierra Diablo stands higher than in any other area in trans-Pecos Texas. Most of this uplift re· suited from the greater structural height of the Sierra Diablo in early Mesozoic time, for the range was not uplifted as much as the Guadalupe Mountains in Cenozoic time (King, 1948a, p. 108.) The area of high-standing pre-Cambrian rocks may have had a much more domical form before it was disrupted by the later Cenozoic faulting; during parts of Pale­ozoic, Mesozoic, and early Tertiary time it may have been the dominant structural feature of this part of Texas. It evidently originated after pre-Cambrian time, as it includes part of the pre-Cambrian mobile area on the southeast and part of the pre­Cambrian stable area on the northwest. Its origin was probably complex. Much of its development may have been relatively passive, as Paleozoic and later sediments were deposited over it to less thickness than elsewhere, or if deposited, were partly or wholly removed by erosion (Pl. 19, C and D). On the broader feature more active but lesser features were superimposed-the folds of late Paleozoic time, others of early Tertiary time, and finally the normal faults of later Cenozoic time. PROBLEMS OF ~ETAMORPHISM Peter T. Flawn SUMMARY OF METAMORPHISM The pre-Cambrian rocks of the Van Horn area present different metamorphic facies from one area to another-one in the Car­rizo Mountain group in exposures in the northwest and northeast Van Horn Moun­tains and the Wylie Mountains, another in the Carrizo Mountain group of the Eagle and Carrizo Mountains, and a third in the Allamoore and Hazel formations of the Sierra Diablo foothills. Southeastern metamorphic /acies of Car­rizo Mountain group.-Outcrops in the northwest and northeast Van Horn Moun­tains and the Wylie Mountains lie in a northeast-southwest line, nearly parallel with the strike of the rocks. At each locality the rocks are chiefly meta-arkose, feld­spathic metaquartzite, and feldspathic mus­covite schist-essentially a quartzo-feld­spathic sequence. These have been thor· oughly recrystallized, with complete recon­stitution of intergranular material to form plates of biotite and muscovite. Associated with these rocks in the northwest Van Horn Mountains are almandine-and anthophyl­lite-bearing para-amphibolites. The facies is made up of medium grade metamorphic rocks, in the almandine zone of regional metamorphism, or amphibolite facies of Es­kola. Fabrics are crystalloblastic, and the rocks consist for the most part of mineral assemblages that are in equilibrium. The rocks have been so thoroughly al­tered that original bedding (sl) is no longer visible, although the larger stratifi­cation is indicated by interbedding of min­eralogically contrasting layers. Foliation (S2 ) is more or less parallel to the stratifi­cation. Northwestern metamorphic /acies of Car­rizo Mountain group.-Outcrops in the Eagle and Carrizo Mountains lie 12 to 15 miles northwest of the line of strike which connects the preceding exposures and are themselves essentially along the regional strike. Slate, phyllite, chlorite schist, and limestone occur between thick quartzo­feldspathic members in a metasedimentary sequence. The metaquartzite and meta­arkose units show some intergranular chloritic and sericitic material, not recon­stituted into mica plates. All the rocks of the sequence are fine grained, and cata­clastic fabrics dominate. Microscopic ex­amination shows disequilibrium relations and an over-all smeared, comminuted, and faded appearance. Helicitic garnets in phyl­lite in the Eagle Mountains are almost com­pletely converted to chlorite. Biotite partly converted to chlorite is seen in most rocks. A perceptible northwestward increase in cataclastic features is seen within the breadth of the Carrizo Mountains. This facies is made up of diapthoritic or retro­grade rocks, which had previously pos­sessed a higher grade of regional meta­morphism. The earlier metamorphism is probably correlative with the metamor­phism of the preceding facies but is not as intense and seldom exceeds the stage of biotite formation. Relict sedimentary structures (S,), such as bedding laminae, cross-bedding, and pebbly seams, are well preserved in many of the quartzo-feldspathic rocks of the se­quence. Foliation (S2 ) is more or less paral­lel to the original stratification of the rocks, except for a few local deviations. In some of the schist or phyllite units, foliation is distorted by rucking, crinkling, and chev­ron folding (S3), the axes of which lie in planes that cross the planes of the foliation at wide angles. Such structures occur lo­cally in the Van Horn Mountains, but they The University of Texas Publication No. 5301 are most abundant in those parts 0£ the Eagle and Carrizo Mountains where retro­gressive metamorphism is most intense, and especially where incompetent units abut against competent and massive metarhyo­lite intrusions. Intrusive rhyolite and diorite, now al­tered to metarhyolite and amphibolite, show strong cataclastic structures. The metarhyo­lite has a pronounced foliation, in the plane 0£ which is a conspicuous lineation that plunges southeast, formed by streaking and drawing out 0£ relict phenocrysts and other mineral aggregates. Intense shearing and dislocative movements have converted parts of the metarhyolite into sericite schist, or to mylonite. The amphibolite, which is younger than the metarhyolite, in places contains similar but less pronounced cata­clastic structures. The cataclastic metamor­phism 0£ the metarhyolite and amphibolite is clearly contemporaneous with the retro­gressive and cataclastic metamorphism of the adjacent metasedimentary rocks. Whether the metarhyolite, like the adjacent metasedimentary rocks, had earlier passed through a progressive regional metamor­phism is uncertain; if its effects were once present they have now been erased. More probably the rhyolite was intruded into the metasedimentary rocks after they had undergone their first metamorphism. Metamorphic facies of Allamoore and Hazel formations. The Allamoore and Hazel formations 0£ the Sierra Diablo foothills, although strongly deformed, do not show as obvious a development of metamorphic minerals and structures as do the rocks of the Carrizo Mountain group. The most striking manifestations of metamorphism are in the phyllites and fine-grained pyro­clastic sediments, whose original constitu­ents have been converted into strongly foli­ated minerals such as sericite and chlorite. Locally, also, the limestones have been re­crystallized into banded marbles. However, microscopic examination of other rocks from the southern outcrops of the two for­mations indicates that many of them have had a more complex history than would be suspected from superficial examination, and that they have been subjected to alteration at relatively high temperatures. Limestones of the Allamoore formation are found to contain biotite, muscovite, and albite as metamorphic products, and the Hazel for­mation shows black, slickensided surfaces which consist largely of tourmaline. Petro­graphic study of the Allamoore and Hazel formations is incomplete, and the extent of their metamorphic features, either geo­graphically or stratigraphically, has not been determined. In exposures to the north, the two formations appear to have been little altered. Foliation in the phyllites, pyroclastics, and limestones of the Allamoore formation is primarily an axial-plane cleavage, which crosses stratification at wide angles. No cataclastic structures appear to be present, although they are prominent in the meta­rhyolite 0£ the Carrizo Mountain group im­mediately to the south. The occurrence of fragments of marmorized limestone and possibly of schistose pyroclastics, derived from the Allamoore formation, in conglom­erates of the succeeding Hazel formation suggests that there may have been two pe­riods of metamorphism, one preceding and one following Hazel time. Nevertheless, no superimposed metamorphic structures are visible in the Allamoore formation itself, and so far as one can determine, its struc­tures were created by low-rank progressive metamorphism, during a single cycle. Albitization.-Petrographic study sug­gests that there was an introduction of sodium, manifested as the mineral albite, into the rocks in the general vicinity 0£ the Streeruwitz overthrust. Albite is a promi­nent constituent 0£ Allamoore limestone near the overthrust and occurs (a) in a mosaic with and apparently replacing cal­cite and (b) in cross-cutting veinlets. Also suggestive of albitization is the presence of "chessboard" albite in the metarhyolite near the overthrust (and its absence in metarhyolite in the southeastern Carrizo Mountains). This sodium metasoniatism is probably a late feature 0£ the cataclastic metamorphism and perhaps belongs to the same period of hydrothermal activity re· sponsible for the introduction of tourmaline in this same area. Albite rhyolite in the Pump Station Hills also shows indications of albitization, but correlation of sodium metasomatism in the two widely separated areas is tenuous at best. Interpretation of metamorphic structures. -Parallelism 0£ foliation and stratification has been considered anomalous, but it is actually widespread in metamorphic ter· ranes. Axial plane cleavage and other marked deviations from parallelism are likely to occur mainly on the borders of a deformed belt, as in the Allamoore forma­tion of the Van Horn area. In the Carrizo Mountain group foliation and stratification .are generally parallel in both the higher­rank metamorphic rocks to the southeast and in the lower-rank metamorphic rocks to the northwest, where two sets of meta­morphic structures have been superim· posed. It has been thought that bedding plane foliation indicates geothermal meta· morphism, or recrystallization caused by deep burial, which depressed the rocks into higher temperature levels of the crust and subjected them to pressure from a thick load of overlying rocks, without ac· companying tangential shearing stresses. Turner ( 1948, pp. 176, 278), following Sander, suggests that a pre-existing set of S-planes such as stratification renders the rock sufficiently anisotropic as to exert con­trol on subsequent development of slip sur­faces. In the Carrizo Mountain group, original stratification may thus have con­trolled not only the formation of schistosity during the regional metamorphism but also the development of slip surfaces during the later cataclastic metamorphism. It may appear anomalous that original sedimentary structures are best preserved in the Carrizo Mountain group in its north­western exposures, where retrogressive metamorphism is superimposed on pro­gressive metamorphism. It is inconceivable that a rock which has been transformed into a thoroughly crystalloblastic schist and then retrograded to a fine-grained phyllite should show any trace of its orig­inal structure. However, nothing as extreme as this has taken place in the area. Regional progressive metamorphism decreased in in­tensity northwestward, and in the area of greatest retrogressive metamorphism it scarcely passed the stage of biotite forma­tion. Moreover, the sedimentary structures are best preserved in the quartzo-feld­spathic rocks, which reacted by simple mass recrystallization. The structure and meta­morphism of the interbedded schist-phyllite­slate units is much more complex, and a great deal of the applied stress during the retrogressive metamorphism may, in fact, have been dissipated in them. One of the manifestations of rock failure in the schist-phyllite-slate units is the de­velopme~t of rucking,_ crinkling, and chev­ron foldmg (S3 ), which deforms the foJi. ation. In other areas similar structures have sometimes been interpreted as formed dur­ ing a phase closely succeeding the develop· ment of the foliation, or even to be a mere "fracture cleavage" formed by a component of the same force which produced the fo}i. ation. In the Van Horn area, at least, the S3 structures appear to be much younger than the foliation, and to be contemporaneous with the retrogressive metamorphism. Re­ gionally, they are best developed toward the northwest where retrogressive meta· morphism is greatest, and specifically they are best developed where incompetent units are crowded against competent metarhyo­ lite intrusions. The structures are therefore younger than the metarhyolite, and the metarhyolite, as here interpreted, is itself younger than the regional metamorphism. The lineation in the metarhyolite of the northwest part of the Carrizo Mountains ap­pears to be the result of strong northwest­ward dislocative movements that were con­temporaneous with the retrogressive meta­morphism of the near-by metasedimentary rocks. The lineation is interpreted as an a lineation, parallel to the a fabric axis, or direction of transport. According to Turner (1948, p. 180): In mylonites and similar rocks which originate locally by intense cataclastic deformation in planar zones such as slickensides, where con­siderable displacement is accomplished by rela­tively rapid movement within a small thickness of deformed rock, a linear structure (Rillung) parallel to the direction of movement, i.e., to the a axis of the fabric, is typically conspicuous. As to the ultimate cause of the two meta­morphisms: The profuse intrusion of pegmatites in the northwest Van Horn Mountains indi-· cates the possibility of a subjacent batholith in the southeastern part of the area, and this batholith, if present, may have pro· vided the heat necessary for the regional metamorphism. Whether the regional meta­morphism was directly or only indirectly related to the tilting and other deformation of the rocks of the Carrizo Mountain group has not been established. The retrogressive and cataclastic meta­morphism has a striking areal relation to the Streeruwitz overthrust which bounds the Carrizo Mountain group on the north and increases in intensity toward it. The lineation in the metarhyolite, a cataclastic structure, plunges southward or southeast­ward, approximately at right angles to the The University of Texas Publication No. 5301 trace of the overthrust, and perhaps paral­lel to its plane. It would seem that the cataclastic metamorphism took place at the same time as the emplacement of the Carrizo Mountain group on the Streeruwitz overthrust and was caused by the same forces. The amount of time which elapsed be­tween the regional metamorphism and the later cataclastic metamorphism is uncer­tain. The two may be related parts of a con· tinuous kinetic-thermal system, during op­eration of which there was a thermally dominated environment in the southeast part of the area, nearest the hypothetical batholith, while the stress component of regional metamorphism increased north­westward, where it culminated in cataclastic metamorphism and overthrusting. However, intrusive rhyolite and diorite in the north­west part of the area experienced the cata­clastic metamorphism but are apparently younger that the regional metamorphism. Also, the Streeruwitz overthrust, to which the cataclastic metamorphism is related, can be dated as younger than the unmeta­morphosed Hazel formation. The cataclastic metamorphism thus seems to be a late fea­ture in the structural and metamorphic history of the area and may also have been of relatively local extent. MINERAL RELATIONS IN THE MICA MINE AREA The Mica Mine area is unique among exposures of pre-Cambrian rocks in the Van Horn area in that it presents a se· quence of rocks of varied mineralogy, re­gionally metamorphosed to the amphibolite facies and not complicated by subsequent retrogressive metamorphic reactions. Eight chemical analyses (Tables 3, 6) were made in connection with the detailed study of pegmatites and amphibolites in the area, and a discussion of the mineral relations is in order. The study of metamorphic rocks has been facilitated in recent years by use of the facies classification developed by Es­kola ( 1915, p. 114) . The classification of metamorphic rocks into metamorphic facies is based on the thesis that: In any rock of a metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pres­sure conditions, the mineral composition is con· trolled only by the chemical composition. Thus a metamorphic facies includes rocks that have reached equilibrium during meta­morphism under a particular set of physi­cal conditions. Whether or not a particular assemblage of minerals constitutes an equilibrium as· semhlage is a difficult problem. A partial solution to this problem is found by apply­ing the mineralogical phase rule. 20 Turner (1948, p. 50) says: A general agreement . . . between the number of observed associated minerals in a series of rocks and the number required by the mineralogi· cal phase rule may • . • he interpreted with some assurance as indicating a general approach toward equilibrium. The chief difficulty in applying the min­eralogical phase rule lies in determination of the number of participating components. Substitution of one component for another may decrease the number of phases. Other difficulties arise from consideration of the mobility of certain components since, ideally, the maximum number of phases in equilibrium is equal to the number of inert components. Because the ideal mobile con­dition is rarely attained, the principal min­eral phases may be accompanied by small amounts of other phases due to the limited mobility of the corresponding components (Turner, 1948, p. 53). It should he noted that in application of the phase rule only the chief mineral constituents are con­sidered, and minor constituents are a factor only in so far as they contain components in common with chief minerals; for ex· ample, if calcite is present as a minor con· stituent an amount of CaO equivalent to the C02 must be subtracted in calculations from the total CaO. Water, as one component, is considered to be everywhere present, and the existing phases are those which can exist in the pres· ence of water. The number of solid phases, then, is one less than the number of com­ponents. Eskola (1915, p. 138) treats the rocks of the Orij iirvi region as members of a six-component system but states that lim­ ited isomorphous substitution of Fe and Mg compounds may result in a seven-com· ponent system with six stable minerals. 2> Goldschmidt'e mineralogical phase rule (a re1tatement of Willard Gibb's phase rule to apply to metamorphic rocb) is given by Turner (1948, p. 48) : "The maximum number of crystalline minerals that can coexist in stable equilibrium is equal to the number of individual components that are con· tained in the minerala (provided the singular transition pointl are omitted from consideration)." Turner (1948, p. 78) following Eskola, dis· cusses the rocks of the amphibolite facies as members of a six-component system. Turner (1948, pp. 77-88) includes four subfacies under the amphibolite facies: (1) Cordierite-Anthophyllite Subfacies ( essen· tially a contact-metamorphic facies formed under a low shearing stress) (2) Staurolite-Kyanite Subfacies (rocks formed by medium to high-grade metamorphism involving strong deformation under high pressure and shearing stress---the product of regional metamorphism) (3) Sillimanite-Almandine Subfacies (a prod­uct of high-grade regional metamorphism characterized by the presence of silli­manite) (4) Almandine-Diopside-Hornblende Subfacies (a subfacies probably developed under high pressure at great depth) It should be noted that these subfacies of Turner (1948) have genetic implications, such as that of depth, not present in the original facies classification of Eskola. Al­though the critical minerals staurolite and kyanite are not present in the Mica Mine area, the rocks are probably isogradic with the Staurolite-Kyanite Subfacies of Turner. The absence of these critical minerals is a reflection of the chemical composition of the rocks. Mineral as$emblages in the Mica Mine area.-The major metamorphic mineral as­semblages in the Mica Mine area are as follows: I. Quartzitl}-muscovite schist sequence 1. Quartz-microcline-albite-muscovite­biotite II. Biotite schist-amphibolite sequence 1. Quartz-biotite-"albite-oligoclase"-21 almandine (Almandine is not every· where present.) 2. Quartz-biotite-oligoclase-anthophyllite 3. "Oligoclase-andesine"-biotite-horn­blende-anthophyllite (Biotite, horn· bl en de, anthophyllite; biotite and an· thophyllite; or biotite and hornblende may be absent. Quartz is present in some specimens.) 4. Andesine-hornblende-almandine 5. Oligoclase-hornblende-epidote-quartz (Quartz is extremely variable in amount and may be absent.) 6. Epidote-albite-sphene (This assemblage is an extreme of number 5. Sphene has increased to almost 5 percent and can be considered a major mineral. Horn­blende is present only in amounts near 1 percent. Epidote makes up about 90 percent of the rock and albite makes up about 5 percent of the rock.) . n Quotation mark1 enclose one mineral phase with a range 1n chemical composition. Thus the maximum number of minend constit· uents in any Mica Mine rock is five, with three and four the general rule. If, following Turner (1948, p, 78), these, rocks are considered a5 a six-component system, it is reasonably certain that they are equilibrium assemblages. In the following discussions of Mica Mine as­semblages and construction of AKF and ACF diagrams, calculations are made following Eskola (1915) . In analyses of Mica Mine rocks it is assumed for the purposes of calculation that the ignition loss is all water. The q uartz-microcline-albite-muscovite­biotite assemblage.-Analyses of represent­ative rocks of this assemblage are given in Table 3. In general these rocks are charac· terized by an excess of alumina (Al20 3 : (K20 +Na20 +CaO) > 1) and an ex· cess of potash. Eskola (1915, p. 125) states that rocks containing an excess of potash contain biotite or muscovite and possibly microcline but no anthophyllite, alman· dine, cordierite, or andalusite. Rocks de­ficient in potash invariably contain at least one of the minerals of the last-mentioned group and no microcline. Three analyzed samples from the quartz. ite-muscovite schist sequence and a repre­sentative analysis of pegmatite of the Mica Mine area are shown in an AKF diagram, figure 16. In the analyses of these three rocks all iron is reported as Fe20 3, and consequently in order to show the effect of possible combinations of amounts of ferrous and ferric iron in the rock, the analyses are plotted on the AKF diagram with the iron calculated respectively as fer­rous and as ferric (fig. 16). Figure 16 shows that three of the rocks are characterized by an excess of potasli (shown by the presence of microcline), the remaining analysis, number 3, falling into the almandine-muscovite-biotite field. No almandine, however, was observed in this rock or in any of the sections studied from this sequence. It is suggested that the ex· cess amount of Al20a formed muscovite at the expense of microcline while the rela­tively high Mg content (with respect to Fe) is responsible for the presence of a high­magnesium biotite and the absence of al­mandine. The FeO and MgO in this in­stance act as a single component in the bio­tite. Eskola ( 1915, p. 123) says that it is the imperfect miscibility of the ferrous and magnesium compounds that causes the ap­pearance of almandine. According to Es­kola, MgO and FeO, in a rock too rich in the latter to form cordierite, behave as two microcline Fie. 16. AKF diagram for rocks with excess SiO. and AJ,O, (after Turner, 1948, fig. 20), on which are superimposed chemical analyses of rocks from the Mica Mine area ( l through 4). a = total iron calculated as Fe20a; b =total iron calculated as FeO. Rocks whose compositions fall within the field microcline-muscovite-biotite have excess K.O. (1) Representative pegmatite (micro· cline perthite-albite (An1)-quartz-muscovite). (2) Feldspathic quartzite (microcline-quartz-albite­muscovite). (3) Biotitic muscovite schist (quartz-muscovite-biotite). (4) Muscovite schist (quartz· muscovite-microcline). See Table 3 for chemical analyses. independent components and a new phase, almandine, appears without causing the disappearance of any other phase. Some question may be raised on the idea of Jim. ited miscibility of FeO and MgO since complete mutual substitution of FeO and MgO is known to take place in many min­erals. The question here is the extent to which MgO can substitute for FeO in al­mandine. Since the work of Ford (1915) the series pyrope-almandine-spessartite has generally been considered a continuous solid solution series. Nevertheless (a) cor­dierite and almandine and (b) biotite and almandine occur together in equilibrium assemblages, and where this occurs MgO and FeO are acting as separate components. Bowen ( 1925, p. 283) says: The grouping of, say, ferrous iron and magnesia as a single component may lead to no difficulties in the great majority of rocks, but in those few that are formed under conditions that lie within what may be termed inversion intervals, a number of phases too great for agreement with such grouping may be found even with perfect equi­librium. Eskola (1915, p. 123), Barth (1936, p. 819), and Turner (1948, p. 49) all remark on the limited extent to which MgO can substitute for FeO in almandine within the range of physical conditions corresponding to a medium to high grade of metamor­ phism. It seems certain, therefore, that under some conditions MgO and FeO do act independently. Further work is needed on the physical and chemical conditions that cause such behavior. The quartz-biotite· "albite-oligoclase"-a/.. mandine assemblage.-An analysis of this rock is given in Table 6 and plotted on the ACF diagrams shown in figure 17. The rock is characterized by an excess of alumina and a deficiency of potash. Appar­ ently the magnesia and ferrous iron are act­ ing as separate components, and therefore almandine (in the presence of an excess of alumina) appears as a separate phase. Con­ sequently this assemblage contains a high magnesium biotite and an essentially fer. rous almandine. This rock falls within the biotite-almandine field of Turner's ACF diagram (fig. 17). The quartz-biotite· "albite-oligoclase" -an­thophyllite assemblage.-No analysis of this particular assemblage was made. It con· stitutes a more potassic and sodic phase of the following assemblage and contains rela­tively less alumina than the previous assemblages. A A "ti ~ ~ O"' ~· ::ti epidote ~ ~ ~ vocant field ~ .... ;:I ~ c"--~~~~~~~~~--..,..,-..,.,....z~~~~_;_,.. c diopside w~ Frc. 17. ACF diagrams of amphibolite facies, on which are superimposed chemical analyses of rocks from the Mica Mine area (1 through 5). (a) Diagram for rocks with excess of SiO, and f deficiency of K.o (after TU.mer, 1948, fig. 19). (b) Diagram showing rocks of the gabbro series, G, and rocks of the sedimentary series, S (after Vogt, 1927, fig. 104). (Note that in racks low in Na,O the place of anorthite may largely be taken by epidote.) (1) Quartz-biotite-albite-almandine schist. (2) Andesine-homblende-anthophyllite amphibolite. (3) Andesine-homblende-almandine am­ phibolite. ( 4) Quartz-epidote-oligoclase·hornblende amphibolite. (5) Epidote-albite-(sphene) epido­ tite. See Table 6 for chemical analyses. MgSio3 F ...... CJj '° The "oligoclase-andesine" -biotite-horn­blende-anthophyllite assemblage.-This as­semblage shows a wide range in the miner­als present. An andesine-hornblende-antho­phyllite member of this assemblage was analyzed (Table 6) and plotted on ACF diagrams (fig. 17). The rock is deficient in alumina and contains a small amount of femic lime but no excess lime. (CaO in excess of ratio CaO : Al20 3 -(K20 + Na20) = 1 is called femic lime; rocks showing ratio of femic lime to magnesia more than 1 :3 may be called rocks with ex­cess lime.) As plotted on the ACF diagrams the rock falls into the almandine-anorthite­hornblende field and, on Vogt's diagram, is also within the sedimentary series. In the rock itself, however, anthophyllite and not almandine is present. Apparently in the ab­sence of enough potash and alumina to form biotite, the magnesia and the ferrous iron act as a single component in the anthophyl­lite. Almandine (3Fe0 · Al2 '. 3Si02 ) 03 could not form in the presence of the high concentration of MgO and the deficiency of Al20 8, and its place was taken by antho­phyllite (7(Fe, Mg)O ·8Si02 ·H20). The almandine amphibolite in direct contact with this anthophyllite has an excess rather than a deficiency of alumina. Apparently, if sufficient water is available the alumina ratio is critical to the appearance of antho­phyllite rather than almandine. The high "A" rating which locates this rock on the ACF diagram is caused bv the high ferric iron content which is calculated with the Al20 3 to give the "A" value. The andesine-hornblende-almandine as­ semblage.-Analysis of a sample from this assemblage is given in Table 6 and plotted on the ACF diagram in figure 17. The rock is characterized by an excess of alumina and an absence of femic lime. On the ACF diagram the rock falls into the anorthite· hornblende-almandine field and, on Vogt's diagram, into the sedimentary field. The oligoclase-hornblende-epidote-quartz assemblage.-An analysis of this rock is given in Table 6 and plotted on the ACF diagram in figure 17. The assemblage has a deficiency of alumina and an excess of lime. On the ACF diagram this rock falls within the anorthite-hornblende-almandine field and, on Yo······.. ~:.·:::>:=· -....::: \..... Biati\ ·.nthaphyllito -.. K.f!m~:¢1ite lnterbeddtd ..<:::::··· ·······. Biat;=,:~."~~~·;;;:;'~····)\ .........!...O Almondine Amph1bolit1, ·· ·······..":·... .... ······ .. ;.-~: Epidote ''.::··.•... Anthaphyll1te Amphibolito and ··.. ·. ( ··.. ·. Amphiboliti:>'··.::·:. Anthophyll ite-Hornbllndt Amphibolill )<... '.... ····..~. 4 5 "'4...._35 ··.. ··.. ... ··:";'.·.. "4.._25 ....· ~ :... ~.::··..\ "% ··~ \ ...~~A~hibolitt ""'· "35 ·.. : \35 ·. ····. i ~ Scale 0 200 400 600 FEET ,_. ~ ~ C'.':'.! ., ~·