The University of Texas Publication No. 4621 June 1, 1946 THE ELLENBURGER GROUP OF CENTRAL TEXAS By PRESTON E. CLOUD, JR. and VIRGIL E. BARNES Bureau of Economic Geology John T. Lonsdale, Director Prepared in co-operation with the United States Geolol!'ical Survey _ Issued December, 1948 PUBLISHED BY THE UNIVERSITY OF TEXAS AUSTIN Publications of The University of Texas PUBLICATIONS COMMITTEE E. J. MATHEWS C. D. LEAKE C. F. ARROWOOD A. MOFFIT R. H. GRIFFITH A. SCHAFFER A. C. WRIGHT Administrative Publications E. J. MATHEWS B. MCLAURIN C. F. ARROWOOD C. D. SIMMONS L. L. CLICK ROGER J. WILLIAMS The University publishes bulletins four times 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. 4601 is the :first publication of the year 1946.) 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, 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 THI: UNIVERSITY OF TEXAS PRESS The University of Texas Publication No. 4621: June 1, 1946 THE ELLENBURGER GROUP OF CENTRAL TEXAS By PRESTON E. CLOUD, JR. and VIRGIL E. BARNES Bureau of Economic Geology John T. Lonsdale, Director Prepared in co-operation with the United States Geological Survey Issued December, 1948 PUBLISHED BY THE UNIVERSITY FOUR TIMES A MONTH AND ENTERED AS SECOND·CLASS MATTER AT THE POST OFFICE AT AUSTIN, TEXAS, UNDER THE ACT OF AUGUST 2.4, 1912 The benefits of education and of useful knowledge, generally diffused through a community, are essential to the preservation of a free govern­ment. Sam Houston Cultivated mind is the guardian genius of Democracy, and while guided and controlled by virtue, the noblest attribute of man. It is the only dictator that freemen acknowledge, and the only security which freemen desire. Mirabeau B. Lamar CONTENTS PART 1; GENERAL DISCUSSION PAGE 9 Abstract -----------------------------------------------------------------------------------------------------------------------------------­Introduction ------------------------------------------------------------------------------------------------------------------------------------------­ 11 Acknowledgments ---------------__ ----------------------------------··---------------------------------------------------14 Local pronunciations of place names ------------------------------------------------------------------------------15 Glossary of selected technical terms ------------------------------------------------------------------------------------------15 Methods and techniques _________ ----------------------------------------------------------------------------------. --------------------19 Basis of correlation ----------------------··---------------------------------------------------------------------------­ 22 General stratigraphy ----------------·--------------------------------------------------------------------------------------27 Introductory notes ----------··-----------------------------------------------------------------------------------· ---27 Pre-E!Ienburger beds --------------------------------------------------------------------------------------------------------------------29 Riley formation ------------------------------------------------------------------------------------------------------------------------29 Wilberns formation __________________________________ ---------------------------------------------------------------29 Ellenburger group ---------------------------------------------------------------------------------------------------------------------------30 Summary statement --------------------------------------------------------------------------------__________ -----------------30 General features of the group --------------------------------------------------------------------------------------------31 Tanyard formation ____ -----------------------·----------------------------____________ ----------------__ 35 Threadgill member --------------------------------···------------------------------------------------.________ ----------------37 Staendebach member ---------------------------------------------·-----------------------------------------------------37 Gorman formation --------------------------------------·-----------------------------------------------------------------------38 Honeycut formation ______ -----------------------------------------_________ .-----------------------· --40 Beds that overlap the Ellenburger _____________ -----------------------------------------------_--------------------------------42 Introductory remarks -----------------------------------------------------------------------------_ -----------------------42 Devonian ----------------------------------------------------------------------------------------------------------------------------42 Mississippian ______________ --------------------------------------------------------··-----------------------------42 Ives breccia ------------------------------------------------------------------------------------------------------------46 Chappel limestone ---------------· -------------------------------------------------------------------------------------------49 Barnett formation ----------------------------------------------------------------------------_____ -----------------------------52 Pennsylvanian -------------------------------__________________ -----------------------------------------------59 Regional correlation ----------------· ______________ -----------60 Introductory remarks ----------------------------------------------------------------------------__________________ _ 60 "Missouri and Arkansas ------------------------------------------------------------------------------------------------------------------62 62 Ozark region -----------------------------------------------------------------------------------------------------------------------­ 63 Oklahoma ----------------------------------------------------------------------------------------------------------------------------·--Arbuckle and Wichita Mountains --------------------------------------------------------------63 Texas --------------------------------------___ ---------------------------------__ ------------------------------------------------65 65 Marathon basin ---------------------------------------------------------------------------------------------------------­Sierra Diablo region __ ----------------------------------------------------------------------·----------------------------------------66 Beach Mountain ---------------------____ -------------------------------------------------------------------__________________ _ 66 Diablo Plateau ---------------------------------------------------------------------------------------------_________________ 71 Hueco Mountains ------------------------------------------------------------------------------------------------­ 72 Franklin Mountains -----------------------------------------------------------------------------------------------------­ 72 Colorado ---------------------------------------------------------------------------------------------------------------------·--------------__ 75 Rocky Mountain Front ----------------------------------------------------------------------------------------_____________ _ 75 Other regions ________________ ---------------------------------------------------------------------------···-----------------------------------­ --76 Upper Mississippi Valley _------------------------------------------------------------------------------------------------------_ 77 Champlain Valley in Vermont and New York-----------------------------------------------------------------------77 Vicinity of Phillipsburg, Quebec ---------------------------------------___ -------------------------____________________ _ 78 Southern Appalachian region __________________________ -----------------------------------------------------------------78 Lithogenic and paleoecologic speculations ------------------------------------------------------------------------------------------79 Introductory statement --------------------------------------------------------------------------------------------------------------------79 Recent sediments potentially comparable to Ellenburger rocks -------------------------------------------------79 Factors limiting analogy ----------------------------------------------___ _____________________ _ 79 Geographic, sedimentary, and biologic features of the Bahama Banks ____________________________ 81 _ The chemical precipitation of calcium carbonate from saline waters _______________________________ 84 _ Mineral character of chemically precipitated calcium carbonate in recent seas 87 Genesis of the lithic constituents of Ellenburger rocks _____ ___________________ -----------------------89 I,imestone ---------------------------------------------------------------------------------------------------------------------------­89 Dolomite -----------------------------------------------------------------------------------------------------------------------­89 Chert -----------------------------------------------------------------------------------------------------------------------------­ 95 Quartz-sand -----------------------------------------------------------------------------------------------------------------­ 96 PAGE Glauconite -----------------------------------------___ .·-------------~-----__________ __________ -------------------97Ecologic implications of Ellenburger life ___ .. ______________ _____ ________ ___ __ _ ____ 98 Probable oceanography and ecography of the Ellenlmrger sea___________ -------------------------------100Comparison of the Ellenburger sea with other ancient marine provinces ________________________________ 105 Older seas ___________________________________ -------·· ________________________________________ .. __ ----------------------------_____ 105 Coeval provinces of the mid-Continent sea ----------------------106 Geologic history _ ------------------__ -·--------------______ ----------------------_________ ___ ____ _____ 109The post-Ellenburgcr unconformity ________________________ _________________________ 109 Resume of inferred Paleozoic events in the Llano region _ __ _________________________ ____ 111 Paleontography ___ ______________________ --------------------------------------------------· ---------------------113Introductory remarks _____________________ _ _ _ __________________ _ _______________ ___ -------------------___ 113 Fossils of the Ellenburger group _________ _____ _________________ ___ 115 Fauna! differentiation of the Ellenburger from underlying and overlying rocks _____ _ 117Interregional index fossils of the Lower Ordovician carbonate rocks __________________ _____ ] 18 Geologic structure ------------------------------------_______ __ ________________ ---------------------------------118 Faults --------------------------------------------------------­ ____ __ _____________________________ -----------------------------------118 Detection of faulting ----------------------___________ __ ______ __ -----------------------------119 Major fault trends __________ ----------------------------_---------------------__ --··----------------------------------------119Minor fault patterns -------------------------_______________________ -------------------------------------------120 Age of faulting ---------------------------------------______ ----------------_____________ ---------------------120 Folds _____________________ -------·-------------------------________ _________________ ______________________ 121 Collapse contacts and structural sinks ________________________ ________ ________ 121 Joints -------------------------------------------------------______________________ ____________________________ ] 22 Petrography ---------------------··-------------------------------------------------_ _______________________ ____ 122 Studies of thin sections __________________________ . ________ __________ _____________________ 123 Carbonate rocks ---------------·---------------------­ _ 123 Cherts ____________ -------------------------------------------· __ . ___ _____________ __ 124. Economic resources ______ ------------------____ . ·--------------------------__ _________ ______ ______________ ________ __ 125 Subsurface resources ______ ---------------------125 Oil and gas --------------------~--------···------------______________ -----------------------------------------------125Water ____ ------------------------------------------------------------------------_____ _________________ 126 Outcrop resources _____ ------------------------_______ _ _ ____ ________ ___ _-----------------------------------------------127 Stone used chiefly because of its physical properties ------------------________________ 127 Stone used chiefly because of its chemical composition ----------------------131 Water --------------------------··--------------------------------_·----------------------------------_______________________ 133 PART 2: LocAL STRATIGRAPHY Bald Ridge area, MeCulloch County ________________ _______________ _____ _______________ ____________ ____________ 135 Introductory statement ----------------------_______________________ ----------------------135 Upper Cambrian __________ -------------------------------------------------------------------------------------------------135Wilberns formation ______________________ -------------------------------------------------------------------135 Lower Ordovician -------------------------------------_____ ______ _______________ _------------------------------136 Ellenburger group --------------------_ ___ _________________ ---------------------------------------136 Tanyard formation, Threadgill member _ ___ ________________ _______ _------------------------136Tanyard formation, Staendebach member _ ________________ _______________________ 137 Gorman formation _______________ _ _ __________________ _________ ---------------------------___ 137 Devonian _________________________ ·--------------------------------------------­ ------------------------------------138Carboniferous -------------------------------­ __________________ ________________ ----------------------138 Mississippian -··----------------------------------------------·--__________ ---------------------------------------138 Pennsylvanian _______________ -------------------------------------------_____ _ _____ __ --------------------------139 Description of the McCulloch County sections---------------------------------------·-----------------------------------139 Highway 87 section ---------------------------__ --------"----------------------------------------140Camp San Saba section ______________ _____ ----·-----------____ -----------------------------------------------148 Bald Ridge section ----------------------------_____ ___ __ 148 Bear Spring area, Mason County _ ---------------------------------------------------------------------------------154 Introductory statement ------------------------------------------------------------·-------------------------------------154 Upper Cambrian ____________ ___________ ---------------------------------_ -·------------------_ 155 Wilherns formation ----------------------________________________ ______ -------------------------------------------155 Welge sandstone member __ -------------------------------------------------------------------155Morgan Creek limestone member _ _ _____________ -------------------____________________ 155 Point Peak shale member __________________ -----------------------------------------155 San Saba limestone member __ ---------------------,----------------------------------------156 Lower Ordovician ___ -----------___________________________ ------------------------------157 Ellenburger group _________ _________________ __________________ ___________________ . -----------------------157 Tanyard formation, Threadgill member _____ ______________ 157 Tanyard formation, Staendebach memher _ _ _ _______ -------158 Gorman formation ______ --------------· ________ ··---------------·---------. ___ -----------158 The Ellenburger Group of Central Texas PAGE Devonian -------------------------------------------------------------------------------------------------------------------­ 159 Carboniferous -------------------------------------------------_____ ------------------------------------_____________________ 160 l\llississippian ----------------------------------------------------------------------------------------------------------------------160Pennsylvanian ----------------------------------________ ---------------------------_________________ ------------------------161 Desc1iption of the Llano River sections -----------------------------------------------__ -----------------------------------.162 Rattlesnake Hill section --------------------------------------------------------------------163 Pete Hollow section -------------------------------___ --------------------------------------------------------_______________ 170 Bluff Creek section _________ ------------------------__________________________ ------------------------------___________________ 179 Mason section --------------------------------·---------------------------------------------------------------------------------------185Buck Spring section (supplementary) ______________ _____________ _________________ 186 Lange's Mill area, Gillespie County ----------------__________________ -------------------------------------------------187 Introductory statement -------------------------------------______ -----------------------------------------. ___________________ 187 Upper Cambrian __ ______ ----------------------------------------------------------------_ 187 Wilberns formation -------------------------· . ______ ------------------------------------------_________________ 187 Welge sandstone member _________________ ---------------------___________ --------------------------187 Morgan Creek limestone member ___ ________ --------------------------------------------183 Point Peak shale member ----------------------------_-----------------------------188 San Saba limestone member -----------------------------_______________________ 183 Lower Ordovician ----------------------------------_____ ------------------------------------------------------188 Ellen burger group ____ ----------------------------------------------------------------183 Cretaceous _ ----_ __ ___ ----------------------------------------------------_ -----------------··-139 Cenozoic ____________________ .. _________________ -----------------------------------------­ 189 Description of the Threadgill Creek section __ -------------------189 Cherokee area, San Saba County ------------------------------------______ _________________________________ _ 192 Introductory statement _-----------------------------------------____________ __________________________________________________ _ 192 Upper Cambrian -------·· _ __ ------------------------------------------------------------------________________ _ 192 Wilberns formation ---------------------------------------------------------------------------------------------------·--192 Pre-San Saba strata _______ ---------------------------------------------_ --------------------------192 San Saba limestone member _ -----------------------------______ __ __ __ _ ____________ _ 193 Pedernales dolomite member ________________ ---------------------------------------------------____________ _ 194 Lower Ordovician _____________________________________________ ----------------------------------------------------­ 194 Ellenburger group --------------------------------------------------------------------------_---------------------------194 Tanyard formation, Threadgill member _________________ _----------------------_______________ _ 194 Tanyard formation, Staendebach member _____________ ------------------------------_______________________ _ 195 Gorman formation ----------------------------------------------------------------------------------------------· 195 Honeycut formation _____________ _________________ ____ . ---------------------------___________________ _ 197 _ Carboniferous ----------------------------------------------------------------------------------------------___________________ 198 Description of the Cherokee sections ---------------------------------------------_____________________ 198 Cherokee Creek section ____ ---------------------------199 Harris ranch section (supplementary) ----------------------------219 Salt Branch section ________ ----------------------_ ------------------------------------------219 _ Kirk ranch section ---------------------------------------------------------------------------------------____________________________ 220 The northeastern areas --------------------------------------------------------------------224 Introductory statement _______ -------------------------------------------------------------------------_________________ 224 Tanyard area, Burnet and San Saha counties --------------------__________________ 225 Upper Cambrian ---------------------------------------------------------------------------------------------------------------------225 Riley formation _ -------------------------------__________________ 225 Wilberns formation ---------------------------------------------------------------------------------------------------------225 Welge sandstone member ---------------------------------------------__ ---------------------------_______ 225 Morgan Creek limestone member __________ ----------------------_____________ 225 Point Peak shale member _------------------------------____ ____ _______________________ 225 San Saha limestone and Pedernales dolomite members ______________________________________ 226 Lower Ordovician ______ ----------------------------------------------------------------------------------__ 226 Ellenburger group ----------------------------------------------------------------------------------------------------------226 Tanyard formation, Threadgill member _____ ---------------------------------------------------226Tanyard formation, Staendebad1 member ----------------------______________________________________ 227 Gorman Falls area, San Saba, Lampasas, and Burnet counties --------------------------___________ 227 Lower Ordovician -------------------------------------------------------------------------------------------------------------227 Ellenburger group -------------------------------------------------------------------------------227Tanyard formation, Staendebacb member __________________________ 227 Gorman formation --------------------------------------------_______ __ 227 Honeycut formation -------------------------------------------------228 Carboniferous ------------·---------------------------------------------------------------229Supplementary information, Gorrnan Falls and Spicewood Creek sections ____ ------------____ 230 Description of the Gorman Falls section ----------------------------------------------------------------------------230 The University of Texas Publication No. 4621 PAGE Description of the Spicewood Creek section ------------------------------------------------------------------------______ 238 Description of the Smith ranch section (supplementary) ---------------------------------------------------_____ 243 Description of the Tanyard section ------------------------------------------------------------------------------------_______ 244 Description of the supplementary sections in the Tanyard area ---------------------------------------------250 The Moore Hollow and Warren Springs areas, Riley Mountains, Llano County ------------------------------251 Introductory statement ---------------------------------------------------------------------------------------------------------------------251 Upper Cambrian ----------------------------------------------------------------------------------------------------------------------253 Riley formation -------------------------------------------------------------------------------------------------------------------------253 Hickory sandstone member _______ -------------------------------------------------------------------------_____ ______ 253 Cap Mountain limestone member ---------------------------------------------------------------------------------· 253 Lion Mountain sandstone member _____ ___ -----------------------------------________________ ____ ____________ 253 Wilberns formation -------------------------------------------------------------------------------------------------------------------253 W elge sandstone member ____ -------------------------------------------------------··---------··-------------------254 Morgan Creek limestone member ____ -------------------------------------------------------------------------------254 Point Peak shale member ---------------------------------------------------------------------------------------------254 San Saba limestone member ---------------------------------------------------------------------------------------------255 Pedernales dolomite member ---------------------------------------------------------------------··----------------------255 Lower Ordovician --------------· ----------------------------------------------------------------------------------------------------------------255 Ellenburger group ----------------------------------------------------------------------------------------------------------------· 255 Tanyard formation, Threadgill member --------------------------------------------------------------------------256 Tanyard formation, Staendebach member ______ -----------------------------------------------------------_____ 256 Gorman formation ----------------------------------------------------------------------------------------------------------256 Honeycut formation -------------------------------------------------------------------------------------------------------------257 Carboniferous ----------------------------------------------------------------------------------------------------------------------------------257 Description of the Riley Mountain sections --------------------------------------------------------------------------------_ 260 Warren Springs section -----------------------------------------------------------·--------------------------------------------------261 Moore Hollow section -------------------------------------------------------------------------------------------------_____________ 275 East Canyon section (supplementary) --------------------------------------····----····---------------------------------__ 287 Backbone Mountain area, Burnet County _ ----------------------------------------------------------------___ ______ 287 Introductory statement ------------------------------------------------------------------------------------------------------------------------287 Upper Cambrian -----------------··-------------------------------------------------------------------------------------------------------------------290 Riley formation ------------------------------------------------------------------------------------··----------------------------290 Wilberns formation ------------------------------------------------------------------------_____________________________ 290 Lower Ordovician -----------------------------------------------------------------------------------------------------------------------------291 Ellenburger group --------------------_________ -----------------------------------------------------------------------------------------291 Tanyard formation, Threadgill member ---------------------------------------------------------------------------291 Tanyard formation, Staendebaeh member ----····------------------------------------------------------------------291 Gorman formation ----------------------------------------------------------------------------------------------------------------291 Honeycut formation --------------------------·-----------------------__________________ ----------------------------------------292 Description of the Backbone Ridge sections ----------------------------------------------------------______________ 292 Roaring Spring section ---------------------------------------------------------------------------------------------------------------293 Mill Creek section ------------------------------------------------------------------------------------------------------------------298 Backbone Mountain section ------· --------------------------------------------------------------------------------------------303 Supplementary information, Backbone Ridge section ------------------------------------------------------------308 Johnson City area, Blanco County ---------------------------------------------------------------------------------------------------------308 Introductory statement ----------------------------------------------------------------------------------------------------308 Upper Cambrian -----------------------------------------------------------------------------------------------------------------------------309 Riley formation ----------------------------------------------------------------------------------------------------------------------309 Hickory sandstone member --------------------------------------------------------------------------------------------309 Cap Mountain limestone member ------------------------------------------------------------------------------------309 Lion Mountain sandstone member --------------------------------------------------------------------------------310 Wilberns formation -------------------------------------------------------------------------------·-------------------------------310 ~:!~:ns~r~~knl~m~=~~=r~~~};-~~--:::::::=::::=::::=::=::::==:::=::::::::::::::::::::::::::::::::::=::=::=:::: ~~~ Point Peak shale member -----------------------------------------------------------------------------------··-----------310 Pedernales dolomite member -------------------------------------------------------------------------------------------311 Lower Ordovician ··---------------------------------------------------------------------------------------------------------------------312 Ellenburger group -----------------------------------------------------------------------------------------------------------------------312 Tanyard formation, Threadgill member -----------------------------------------------------------------------312 Tanyard formation, Staendebach member ____ ---------------------------------------------------------312 Gorman formation --------------------------------------------------------------------------------------------------------313 Honeycut formation ------------------------------------------------------------------------------------------------------------314 Devonian -------------------------------------------------------------------------------------------------------------------------------------___ 315 Carboniferous ------------------------------------------------------------------------------------------------------------------------------315 Mississippian --------------------·-----------------------·--------------------------------------------------------------------------------315 Pennsylvanian -------------------------------------------------------------------------------------------------------------318 The Ellenburger Group of Central Texas PAGE Lower Cretaceous ---------------------------------------------------------------------------------------------____________ ---------------------318 Description of the Johnson City sections ---------------------------------------------------------------------------------------319 Downstream Honeycut Bend section ________ _ ----------------------______________ ------------------------_______________ 320 Rough Hollow section ----------------------------------------------------------------------------------------------------------------327 Upstream Honeycut Bend section -------------------------------------------------------------------------------------------331 Downstream Pedernales River section _____________________ ---··--------------------------_______ 337 Towhead Creek section -------------------------------------------------------------------------------------------------------------339 Upstream Pedernales River section -------------------------------------------------------------------------------------------341 Scott Klett ranch section ____ --------------------------------------------------------------------------_________________________ 343 Notes on the stratigraphy of areas not mapped in detail ----------------------------------------------------------------------344 Big Saline Creek area, Kimble County ------------------------------------------------------------------------------------344 Calf Creek area, Mason County ______ __________________ Rumsey Creek area, McCulloch County _________________ Lost Creek area, McCulloch County ____ ________________ Vicinity of Ellenburger Hills, San Saba County _ Pillar Bluff area, Burnet County _____________________ ··-----------------­Goodrich ranch area, Burnet County _____________ . Vicinity of Marble Falls, Burnet and Llano counties _ Rowntree well No. 1 Kott, Gillespie County PAR'!' 3: SUPPLEMENTARY DATA ------~-----------------344 ______ 345 ____________________________________ 346 -------------------------------------34B --------------------------------349 ___________ ---------------------------------349 _---------------------------------------349 -----------------------------------351 Section at the north end of Beach Mountain, Culberson County, Texas ---------------------------352 Section at the south end of the Franklin Mountains, El Paso County, Texas __________________ 361 Section in Williams Canyon, El Paso County, Colorado ----------------------------------------------------------369 Lower Ordovician and Upper Cambrian fossils in the Wichita Mountains, Oklahoma ------------------372 Lower Ordovician and Upper Cambrian fossils in the Arbuckle Mountains, Oklahoma ________________ 373 Chemical data -------------------------------------------------------------------------------------------------------------------------------377 Analyses of dolomites in the Johnson City area _--------------------------------------_________________________________ 377 Analyses of carbonate rocks in the Cherokee area ----------------------------------------------------_______ 380 Analyses of core-hole samples, Victoria Gravel Company quarry for magnesium ore 382 Representative thin sections of Ellenburger rocks ---------------------------_____ 391 Description of thin sections from the Cherokee area ----------------------------------------------___ 391 Description of thin sections from the Johnson City area --------------------------------------------------395 Bibliography -------------------------------------------------------------------··-----------------------------­Index ________________ -------------------------------------------------------------------------------------------------------­ ILLUSTRATIONS FIGURES-­ I. Provisional correlation of the Devonian in central Texas -------------------­ 2. Provisional correlation of the Mississippian in central Texas ____ ---· ----------399 -----------461 PAGE ---------------43 44 3. Schematic representation of the Mississippian in central Texas __ --------------------------------45 4. 5. 6. 7. 8. PLATES 1. 2. 3. 4. 5. 6. 7. 8. Graphic history of the correlation of the Barnett formation, 1926 to 1946 --------------------------54 Map of the south end of the Franklin Mountains, El Paso County, Texas, showing loca­tion and approximate geologic relationships of the EI Paso section _________ ___________________ 73 Profile of the El Paso section, Franklin Mountains, El Paso County, Texas ______ Facing p. 73 Geologic map of an area in the vicinity of the Victoria Gravel Company quarries, Burnet County, Texas __________ ----------------------------------------------------------------------------------------128 Geologic map of a fault-wedge 2 miles west of Longhorn Cavern, Burnet County, Texas __ 289 (in pocket)­Index map of central Texas showing location of areas where the formations of the Ellenburger group bave been mapped. Geologic map of the Cherokee area, San Saha County, Texas. Geologic map of the Johnson City area, Blanco County, Texas. Geologic map of the Bald Ridge area, McCulloch County, Texas. Geologic map of the Bear Spring area, Mason County, Texas. Geologic map of the Lange's Mill area, Gillespie County, Texas. Geologic map of a part of the Cherokee area, San Saba County, Texas. Geologic map of the Tanyard area, Burnet and San Saba counties, Texas. PLATES (in pocket) -continued­ 9. Geologic map of the Gorman Falls area, San Saba, Lampasas, and Burnet counties, Texas. 10. Geologic map of the Moore Hollow and Warren Springs areas, Riley Mountains, Llano County, Texas. 11. Geologic map of the Backbone Mountain area, Burnet County, Texas. 12. Geologic map of a part of the Johnson City area, Blanco County, Texas. 13. Geologic map of Honeycut Bend, Blanco County Texas. 14. Correlation of the Ellenburger group and associated rocks in central Texas. 15. Provisional correlation of some Lower 01dovician rocks in the mid-Conlinent. 16. Columnar sections and provisional correlation oI the El Paso and Beach Mountain sec­tions, El Paso and Culberson counties, Texas. PLATES-continued- FACING p AGE 17. View north 25° west up Llano River from a hill south of White's Crossing, Bear Spring area, Mason County, Texas _ -----------------------------------------------------------------------------------------406 18. Stromatolitic bioherm at the top of the Point Peak shale member of the Wilberns for· mation, Mason County, Texas ------------------------------------------------------------------------------------408 19. Rocks of the Wilberns and Tanyard formations in Gillespie and Mason counties, Texas_ 410 20. Rocks of the Tanyard and Wilberns formations in San Saba and Mason counties, Texas __ 412 21. Terrain and rocks of the Tanyard fornrntion in San Saba and Burnet counties, Texas ______ 414 22. The aggregate quarry and plant oI the Victoria Gravel company, 6 miles soulh oI Burnet, Burnet County, Texas -----------------------------------------------------------------------------------------------416 23. Types of chert in the Tanyard and Honeycut formations, San Saba and Burnet counties, Texas -------------------------------------------------·---···------------------------418 24. Rocks of the Gorman formation in the Bear Spiing area, Mason County, Texas ----------------420 25. Rocks of the Gorman formation in San Saba, Lampasas, and Mason counties, Texas ________ 422 26. Rocks of the Gorman and Honeycut formations in San Saba County, Texas ---------------------424 27. Devonian rocks in Blanco and Burnet counties, Texas ______________________ ------------------------------------426 28. Mississippian rocks in McCulloch, Blanco, and Mason counties, Texas --------------------------------428 29. Profile views of the Beach Mountain section, Culberson County, Texas --------------------------------430 30. Lower Ordovician and Quaternary rocks in central Texas and central Colorado ----------------432 31. Stereograms of the Tanyard formation and the Cambrian-Ordovician contact in the Cherokee area, San Saba County, Texas ---------------------____________ ________________________ 434 32. Stereograms of the Tanyard formation and the Cambrian-Ordovician contact in the Cherokee area, San Saba County, Texas ---------------------------------------------------------------------436 33. Stereograms of the Tanyard-Gorman contact in the Che1okee area, San Saba County, and of pre-Gorman strata in the Moore Hollow area, Llano County, Texas __________________ 438 34. Stereograms of the Gorman-Honeycut contact and of en echelon faulting in the Chero­kee area, San Saba County, Texas . ______ ---------------------------------------------44.0 35. Stereograms of the Gorman and Honeycut formations and the Carboniferous overlap in San Saba and Lampasas counties, Texas --------------------------------------------------· --------------------------442 36. Stereograms of the Cambrian-Ordovician boundary and the Carboniferous overlap in the Bear Spring area, Mason County, Texas -· ______ -· ------------------__ ---------------------------------444 37. Stereotriplet of the Cherokee Creek section, Cherokee area, San Saba County, Texas ______ 446 38. Fossils from the Wilberns and Tanyard formations ________________________ __________________ 448 39. Fossils from the Tanyard formation ______ ----------------------------------------_____ . ····---------------------450 40. Gastropods from the Tanyard formation _______________ ---------------------------_ ---------------------452 41. Fossils from the Gorman formation -----------------------------__________ -----------------------454 42. Fossils from the Honeycut formation ___ ------------------------________ ________________________________ 456 43. Fossils from the Honeycut formation -----------------------------------·-------------------____________ ·----------458 44. Fossils from beds above the Ellenburger group __________ ------------------------460 45. Diagrammatic representation of the Ellenbm ger group and Wilberns formation in the Llano uplift of central Texas ---------------------------------------------------------------------------Jn pocket TABLES-­PAGE 1. Thicknesses of u~its of the Ellenburger group (Lower Ordovician) and contiguous Upper Cambrian carbonate rocks as measured in various parts of the Llano uplift____ 28 2. Grain sizes of dolomites in the Cherokee Creek section ____________ ___ ____________________ 196 3. Physical and chemical data for samples from the Moore Hollow and Warren Springs sections, by S. S. Goldich and E. B. Parmelee _ ----------------------___________________________________ 258 THE ELLENBURGER GROUP OF CENTRAL TEXAS PRESTON E. CLOUD, JR.,1 AND VIRGlL E. BARNES2 PART I GENERAL D1scussroN ABSTRACT The incomplete sequence of Lower Or­dovician carbonate rocks known as the Ellenburger group consists of the Tan­yard, Gorman, and Honeycut formations, named in ascending order. It is essenti­ally equivalent to the lower half of the Lower Ordovician of the Ozark uplift of Missouri and Arkansas. Outcropping Ellenburger rocks younger than the Jef­ferson City group of the Ozark uplift arc unknown. The Ellenburger has its maximum development at the surface in the southeastern corner of the Llano up­lift (1820 feet). It thins to only 830 feet in the northwestern corner, mainly by pre-De­vonian truncation, but in part by west­ward thinning of the Tanyard formation. The rocks of the Ellenburger group are essentially nonglauconitic, and, at most places, are sparingly fossiliferous. Its limestones are dominantly sublithographic and very light gray, the purer ones com­monly being in the upper part of the section. Its dolomites range from micro· granular to coarse grained; the coarser grained, paler ones ordinarily being in the lower part of the section, with the finer grained, more brightly colored ones above. The carbonate rocks that overlie the Ellenburger are nondolomitic, typic­ally dark in color, commonly granular, and are apt to contain organic remains in relative abundance. The limestones of the Upper Cambrian in central Texas are ordinarily granular, glauconitic, and 1Geologist, Geological Survey, U.S. Department of the Interior. 2Geologist, Bureau of Economic Geology, The University of Texas. Published in part from the Fort Worth Geological So· ciety ReYolving Publication Fund of The Univcisity of Texas. Prepared under coOpcrative nrrnngcmcnt between the Bureau of Economic Geology, The University of Texas, and the Geological Survey, U.S. Department of the In· terior. darker in color than those of the Ellen­burger; and the dolomites are apt to be darker in color and finer in grain than the immediately superjaccnt dolomites of the Ellenburger. The basal third of the complete Ellen­burger sequence constitutes the Tanyard formation, correlative with the Gascon­ade dolomite {including Van Buren for­mation) of Missouri. It averages 590 feet thick, thinning westward. The Threadgill member constitutes its lower part, with the Staendebach member above. The dolomites of the Threadgill mem­ber are medium to coarse grained, and the distribution of limestone and dolo­mite is irregular. Dolomite predom­inates, except in the western part of the region where the member is wholly lime­stone. The dolomites of the Staendehach member are dominantly fine to medium grained. The member is wholly dolo­mite at places in the western part of the Llano uplift; but, to the east, erratically occurring limestone is conspicuous and locally dominant. Chert is rare in the Threadgill member, or consists of prin­cipally drnsy and dolomoldic types. The Staendcbach member, especially toward the east, differs from the Threadgill in containing compact cherts that weather smooLh and shiny white. Much of' this chert in the Staendebach is oolitic. With rare and inconspicuous exceptions, grains of quartz-sand are absent from the Tan­yard formaLion. The Tanyard fauna is dominated by gastropods and ccphalopods. The Gorman formation, correlative with the Roubidoux formation of Missouri, comprises the middle third of the Ellen­burger group. It averages 470 feet thick. Dominantly microgranular to very fine grained dolomite constitutes a lower dol­omitic facies; except in the northwestern corner of the uplift, where the grain size of the dolomite is coarser, and where The University of Texas Publication No. 4621 erratic lateral transition to calcitic rocks occurs. The upper half of the formation is principally limestone, with a median parvafacies of dolomite coming in east· ward and with the uppermost limestones unus~ally pure and thickly bedded. The chert of the Gorman formation is apt to be chalcedonic to porcelaneous. Sand is a characteristic feature of the Gorman formation, not as well-defined beds, but as scattered to abundant grains in dolo­mite, limestone, or chert. Where sand occurs in Ellenburger rocks it constitutes presumptive evidence, but not positive proof, that the strata are younger than the Tanyard formation. Moreover, it sug­gests that they are older than the lower 50 feet of the Honeycut formation. Rela­tively large low-spired gastropods are the most conspicuous fossils in the Gorman formation. Archaeoscyphia has been found in the Gorman formation only near its middle, essentially dividing the calcitic facies above from the dolomitic facies below. The upper third of the Ellenburger sequence, essentially correlative with the Jefferson City group of the Ozark up­lift, is called the Honeycut formation. It is known to be present only in the east­ern part of the Llano region, and there attains a thickness of about 680 feet. It disappears by truncation west of 98° 55' longitude in western San Saba County. Where fully represented it is divisible into three facies, a lower one of alter­nating limestone and dolomite, a middle dolomitic facies, and an upper calcitic facies. Lithically, individual samples of limestone, dolomite, or chert are very like those of the Gorman formation. Sand grains are much less common than in the Gorman formation, and are rare at most places above the lower 50 feet. Ceratopea and Archaeoscyphia are locally common and some beds containing them are useful local datum-markers. The fauna of the Honeycut formation is dominated by gastropods of many sizes and shapes, and the sponge Archaeo­scy phia. Trilobites are commoner than in lower Ellenburger strata except in the upper 100 feet of the Tanyard formation. As the standard section of the Ellen­burger group, the authors have selected the Lower Ordovician part of the com­posite Riley Mountain section, for reasons given on page 252. This section is de· scribed on pages 260-283 and is section number 6 of Plate 14. The type section of the Ellenburger is the composite Tanyard­Gorman section (p. 224), described on pages 230-249 and diagrammed as section number 5 on Plate 14, but it is too inacces­sible and widely spread for convenient standard usage. Evidence is advanced in favor of the hypothesis that the limestones of the Ellenburger group probably originated as chemically precipitated aragonite muds, under sedimental conditions similar to those found today in the Bahama Bank region, southeast of Florida. Much if not all of the dolomite, it is suggested, re­sulted from alteration at or near the sea floor of the original calcium carbon­ate muds, through interaction with mag­nesium salts in the basal layer of sea waler or in percolating magnesium solu­tions. The finer the grain, the more nearly penecontemporancous with original sedimentation are the dolomites thought to be. Coarser grain sizes suggest slower alteration by percolating waters below the sea floor at some deferred stage in dia­genesis. Although there is no direct evi­dence that the more finely granular, lat­erally extensive dolomites are not primary, published information on the relative solu­bilities of the various salts of calcium and magnesium in sea waters at present ap­pears to favor the deduction that pene­contemporaneous alteration is a more im­portant dolomitization process than pri­mary precipitation. Lateral transition be­tween limestones and diagenetic dolomites (including penecontemporaneous dolo­mites) may be related in part to the dis­tribution of the minerals calcite and aragonite in the sediments being altered, and in part to variations in original per­meability. Metasomatism is thought to take place according to the law of equal volumes. It is held that porosity of the diagenetic dolomites probably results from the slow leaching by meteoric waters of incompletely dolomitized rocks, resulting in removal of calcite particles and inclu­sions as well as some corrosion of the marginal dolomite, Surface leaching of The Ellenburger Group of Central Texas the dolomites of the Staendebach member of the Tanyard formation, as suggested by the concentration of springs on its out­crop, probably o:ITcrs maximum opportu­nity for the development of continuous porosity in Ellenburger rocks. OpLimum condiLions for the accumula­tion of petroleum in Ellenburger rocks below ground, therefore, may be expected where dolomites of the Tanyard forma­tion, and especially the Staendebach mem­ber, are unconformably overlain by im­permeable beds; assuming favorable struc­Lural conditions, a source of oil, and a development of Lhe rocks similar to that known at the surface. The ultimate source of petroleum might be extraneous or autochthonous. Traces of petroleum have been detected in some of the finer grained Ellenburger and Cambrian dolomites at the surface. The typically microgranular to very fine grained dolomites of the Gorman for­mation could form a seal to upward migrating petroleum in the dolomites of the Staendebach member of the Tanyard formation, if subsurface leaching, like sur­face leaching, were able to develop perme­ability in the Staendebach without making the Gorman permeable. Supplementary accumulations of petroleum could also be related to caverns, channels, or breccia­tion in limestones at the unconformable zone; or to lateral irregularities in granu­larity and mineral composition. Plate 14 serves as a graphic summary of Ellenburger stratigraphy and correla­tion in the Llano region and Plate 15 summarizes its regional correlation. The other maps, charts, and figures graphi­cally present much of the factual data on which the report is based, as well as some · of the interpretations made. Twenty-seven fulltone plates illustrate some of the fos­sils, characteristics of the terrane and vegetation, and gross lithic features. The body of the report is divided into three parts, in order to separate general descriptions and conclusions from purely documentary detail. In Part 1 is presented information related to the Ellenburger group of central Texas as a whole. The writers here discuss the stratigraphy of the Ellenburger group and pertinent de­tails of the beds that overlie and underlie these rocks; consider some Ellenburger correlatives from other regions; investi· gate the probable lithogeny of the Ellen­burger rocks and ecography of the Ellen­b~rger sea; and summarize the geologic history, paleontography, geologic struc­ture, petrography, and economic resources of the Ellenburger group. Here also the basis of correlation is summarized, selected technical terms are defined, and other features of general interest are briefly noted. Part 2 comprises the local strati­graphic detail and descriptions of sec­tions, and Part 3 includes supplementary data of various sorts. INTRODUCTION Critical work on the Lower Paleozoic rocks of central Texas began with that of Dake and Bridge (1932), and subse­quent studies by Bridge and later by Barnes served to delineate the larger features of the Ellenburger problem. A preliminary report by the present authors and Josiah Bridge (1945) summarized what was then known of the outcropping Ellenburger rocks, and Barnes and Cloud (1945) described a representative sequence of Ellenburger strata. The present report gives the final results of continuous co­operative work by geologists of the Texas Bureau of Economic Geology and the U.S. Geological Survey between Decem­ber, 1943, and June, 1946. It is intended to be a basic study of Ellenburger rocks, with emphasis on features of possible sig­nificance to the search for new sources of petroleum. In order that it may be complete, many of the data given in the preliminary reports are repeated here, and some portions are taken bodily from those papers, with appropriate revision. The present report summarizes the stratigraphy of the Ellenburger group and correlates it with other strata on the basis of detailed local observations in various parts of Lhe Llano uplift of central Texas. The areas worked in detail were selected on the basis of reconnaissance studies at many localities, and are thought to be representative of the part of the Llano uplift in which they occur. It was con­sidered betlcr for the purposes of this report to coordinate by reconnaissance a The University of Texas Publication No. 4621 large amount of detail from strategically spaced local areas than to cover a wide area diffusely. Primarily it is desired to tell what subdivisions may be made of the rocks studied, how to recognize these subdivisions and correlate them from place to place, and how the information obtained may be applied. To accomplish this the report first gives definitions of some of Lhe descriptive terms used, then states the methods followed and the sorts of evidence on which subdivision and correlation were based, and finally pro­vides descriptions of the units themselves and a discussion of the cultural and eco­nomic applications of information de­rived from their study. This is accom· plished in Part 1, whereas Part 2 pre­sents the descriptive details of local areas, and Part 3 comprises supplementary data of various sorts. The index map (Pl. 1) shows the gen­eral region involved, and the various other illustrations serve as a graphic summary of the report. A generalized diagram (Pl. 4,5) to aid the reader in visualizing the various units and facies being discussed is reprinted from a Nalional Research Council publica­tion (Cloud and Barnes, 1946, p. 88a). This plate, as well as Plates 1 through 16, is in the pocket acr.:ompanying this publication. The immediately ensuing remarks out­line the more general features of the Llano uplift. A fuller account is given by Sidney Paige (1912) in the Llano-Burnet folio, and the bibliography lists many papers that deal with various aspects of the region. The geologic map of Texas by Darton, Stephenson, and Gardner (1937) shows the general geologic setting, but a recent map of the Llano region by F. B. Plummer (1945) presents considerably more detail for the Carboniferous rocks and the local geography than does the State map. Rocks of the Ellenburger group are known to crop out only in the Llano region, a basinlike structural uplift at the geographic center of Texas, between latitudes of 30° 10' to 31° 20' north and longitudes of 98° 05' to 99° 40'. It is rimmed by the Edwards Plateau on the west, south, and east, and adjoins the Carboniferous rocks of the Osage Plains at the norlh. Primarily it is a region of pre-Cambrian and early Pale­ozoic rocks that have been exposed by gentle doming and erosion of the over­lying Cretaceous and late Paleozoic slrata. In this sense it is essentially oval; about 80 miles long in a northwest-southeast direction, 55 miles wide, and about 4400 square miles in area. This is the region referred to in the present report as the Llano uplift, the Llano region, or sim­ply as central Texas. It is the same as the Central Mineral region of some re­ports. The general area affected by dom­ing and faulting, however, is probably more than 100 miles in diameter and ex­Lends inlo the adjacent subsurface. The Llano region is historically a posi­tive one, being similar to such other iso­lated positive regions as the Black Hills, the Ozark uplift, and the Adirondack Mountains. From it extend such subsur­face structures as the Bend arch to the north and the Concho arch to the north­west. The pre-Cambrian rocks were in­tensively folded, faulted, and injected with igneous intrusions before Upper Cambrian time, and late Paleozoic fault­ing affected all rocks of pre-Canyon age. Presumably other significant movements that aIIected the Llano uplift during Pale­ozoic time were epeirogenic, and the epeirogcnic history of Lhe region is re­flected in the succession and character of the sedimentary rocks preserved upon it. Post-Paleozoic faulting within the Llano region is not known, although the Edwards Plateau within which it lies was raised by the arcuate line of Balcones and associated faulting that borders it to the east and south. The important late Paleozoic faults strike mostly in the north­east and southwest quadrants, and the block-faulted ridges which they make are elongated in the same direction. These faults are steep to vertical, commonly braided and branching, locally en echelon, and subject to abrupt changes in strike. Dips of beds are mostly gentle in the Paleozoic rocks where they are not col­lapsed or adjacent to faults, and the best places to measure sections are in high hills or adjacent to major faults that have The Ellenburger Group of Central Texas steepened dips without sending branch faults into the section. By far the grealer part of the Llano region is underlain by pre-Cambrian rocks, the Paleozoic slrata being preserved either as marginal deposits, or as out· lying block-faulted peninsulas or monad· nocks in the pre-Cambrian terrane. Schistose and granitic rocks that are poorly resistant to weathering comprise the larger part of the pre-Cambrian, ex­pressing themselves topographically as shallow basins and low domes. The gneissic and finer grained igneous rocks, however, are commonly resistant to ero­sion, forming rugged hills and peaks, and the granite locally forms imposinP­e~foliation domes. These and the stce; sided, block-faulted Paleozoic ridO'es b . h give t e region a rugged and mountain­ous appearance when viewed from some places, but relief seldom exceeds 400 to 600 feet and the basinlike Llano "uplift" is entirely below the level of the adjacent Edwards Plateau. Drainage of the Llano uplift is accom­plished almost entirely by Colorado River and its tributaries. The most important tributaries are Llano River in the south­ern ha!£ and San Saba River in the northern half of the region, hut many other pei ennial slreams of considerable volume hdong to the same drainage sys­tem. Rll.infall averages alrnut 20 inches a year and is largely seasonal, so that streams are subject to wide fluctuations in volume and those that are perennial owe their tenure to springs. Streams in the pre-Cambrian areas ordinarily have broad valleys, are of low gradient, and commonly arc choked with waste but the more actively degrading stream~ that transect the Paleozoic rocks commonly occupy deeper, narrower valleys or sleep· walled gorges. The Llano region and adjacent parts of the Edwards Plateau are for the most part more thickly wooded than the plains. country to the north, west, or south, and central Texas seems truly verdant when approached from these directions. Juniper, locally called cedar, the usage fo !lowed in this paper, abounds in areas 0£ limestone, especially in the eastern part of the region. Scrubby deciduous oaks thrive on sandstone and granite. Mesquite and various spiny bushes choke the ground underlain by argillaceous and schistose rocks and will grow in any area of low relief that develops even a moderate soil cover. Dolomitic rocks are apt to be the most sparsely wooded, ordinarily ex­pressing themselves as rolling hills or uplands that support clumpy growths of live oak and occasional cedar or mes­quite. In rugged and well watered sec­tions, however, even the dolomites may be thickly overgrown with cedar, and in the western part of the uplift the vege­tation is apt to be relatively sparse ancf to contain a preponderance of semiarid types of plants. Many kinds of cacti abound in the region, as do several sorts of yucca and various spiny bushes of semiarid types. The better stream val­leys, however, support a good growth of deciduous trees such as pecan and varie· ties of oak, elm, gum, and poplar. The Llano region embraces parts of ten counties. Its principal industry is ranch­ing and the ranches range in size from under 1000 to about 80,000 acres, with the average ranch comprising perhaps 2000 acres. It is reasonably well watered and, although there is not enough soil at most places for farming, it makes good pastureland where properly cared for. In general it is thinly populated. The principal towns of the region, with average populations of 2000 to 6000, are Llano, Mason, Brady, San Saba, Lam· pasas, Burnet, Marble Falls, Johnson City, and Fredericksburg, and all of these except Marble Falls arc county seats. Good roads serve much of the region, and the more important of these are paved. Those that are not paved may he difficult after heavy rains hut are gen­erally passable. Where public roads are not available pasture roads make most parts of the country accessible by auto­mobile lo those who have the good will of the ranchers. Rail service is available to the northeastern half of the region. Close collaboration wa~ maintained be­tween lhe authors al all stages of the field work and in the preparation of the final report. Responsibility for the doc­ The University of Texas Publication No. 4621 umentary data is indicated at appropri­ate places, but general conclusions sug­gested were jointly considered and are a matter of joint responsibility. In the mat­ter of the actual writing, Barnes is the author of sections on "Geologic struc­ture," "Petrography," and "Economic re­sources" in Part 1; the descriptions of the Lange's Mill, Tanyard, Gorman Falls, Backbone Mountain, and Johnson City areas, and the notes on the Sudduth section and the well called Rowntree No. 1 Kott in Part 2; and the sections on "Chemi­cal data" and "Representative thin sec­tions of Ellenburger rocks" in Part 3. Cloud is responsible for the actual writ­ing of the remainder of the report and for most of the paleontologic work. The aerial photographs on Plates 31 to 37 are reproduced through the courtesy of the U.S. Department of Agriculture; other photographs are by Cloud unless otherwise credited. ' As one may see by reference to the en­suing section, this report is, in a very real sense, the product of collaboration between members of the cooperating organizations, the petroleum geologists of the southern mid-Continent, colleagues from other in­stitutions and regions, and the people of central Texas. If, upon critical examina­tion, it he found to have merit, there will be many to share it with. For the faults that are sure to be found, the writers wish no indulgence. They have themselves no­where hesitated to criticize where they judge facts inaccurate, procedure mislead­ing, nr inference unwarranted, and they expect their own work to receive the same objective treatment. ACKNOWLEDGMENTS Sincere thanks are here expressed to the many who have aided, advised, and befriended the writers in the ways noted below. Dr. Josiah Bridge most generously made available his <"ntire collection of unpub­lished notes on the Llano region, consulted with the authors in the field on several occasions, identified some of the fossils, and gave general scientific supervision and enthusiastic encouragement to the studies made. Mr. L. E. Warren helped with the field work from the beginning and drafted all geologic maps that are not in color (Pis. 4-13). He also compiled or helped with the compilation of most of these maps from field data. Without his meticu­lous and cheerful cooperation, the comple­tion of this report would have been in­definitely deferred. The work done by the geologi;ots of the U.S. Geological Survey was for the Fuels Section and was under the direct admin­istrative supervision of Messrs. H. D. Miser and A. A. Baker. Each gave the project his full support from the begin­ning, and Mr. Miser consulted with the authors in the field on two occasions and critically reviewed the report in typescript. His continued understanding interest in the work and the objectives of the writers will be largely responsible for any merit their paper may be found to have. Director John T. Lonsdale, of the Texas Bureau of Economic Geology, and his predecessor Dr. E. H. Sellards both took an active and appreciative interest in the work and each has inspected it in the field. Through their courtesy office facilities were made available to Cloud during his work at Austin. All of the sections described were sam­pled by Leo Hendricks with the help of A. J. Crowley and L. E. Warren, and simulated cuttings and insoluble residues were prepared from these samples under the direction of Dr. Hendricks. These, as well as cuttings and residues from several wells north and west of the Llano uplift, were made available to the writers through the courtesy of Dr. Hendricks. Mr. R. L. Heller assisted with the map­ping in parts of the Cherokee area and independently mapped a large area near its center. His call to service with the armed forces in August 1944 deprived the Ellenburger surface project of a valued associate. Mrs. Mildred Cloud helped faith­fully with many of the mechanical details of field work and did most of the typing. Drs. R. S. Bassler and G. A. Cooper made the facilities of the U.S. National Museum available to Cloud during the spring of 194,6. Dr. S. S. Goldich and Mr. E. B. Parmelee, of the Argicultural and Mechanical College of Texas, made The Ellenburger Group of Central Texas available the results of their chemical and petrographic studies of the compo­site Riley Mountain section. Mr. H. C. Counts of the Victoria Gravel Company furnished drill-hole analyses from their Sudduth property. Dr. E. B. Knopf loaned a copy of a translation of parts of Sander's "Beitrage zur Kenntnis der Anlagerungsgefuge,'' prepared by her with the collaboration of E. D. McKee. Mrs. Frances Brock Turley drafted the geologic maps of the Cherokee and Johnson City areas (Pls. 2 and 3). Mr. S. H. Purcell drafted Plate 14, the locality overlay for Plate 1, and figures 3, 5, 7, and 8. Miss Dorothy Elizabeth Hill drafted Plate 16 and figure 6. Mrs. Florence Smith Hf'ath drafted the original base of Plate I. Miss Jessie C. Kirby drafted Plate 15 and fig­ures 1, 2, and 4. Mr. F. B. Plummer furnished ozalid prints of his unpublished incomplete county maps of Burnet, Mason, and McCulloch counties for use in recon­naissance studies. Dr. H. B. Stenzel offered critical advice on the arrange­ment and content of maps and charts. Miss Josephine Casey prepared the final typescript for the printer, and made many useful editorial suggestions. Many of the petroleum geologists who attended the field conference on the Ellen­burger group from June 15 to 23, 1945, entered discussions of Ellenburger stra­tigraphy and made suggestions about its presentation that were of great help to the authors in preparing the final report. Members of the Committee on Geologic Names of the U.S. Geological Survey efficiently performed the chore of clear­ing the report for publication, and Dr. J. B. Reeside, Jr., critically reviewed the final typescript. Particular sections of the report were read and criticized at the authors' request by Josiah Bridge, G. A. Cooper, C. E. Decker, W. H. Hass, P. B. King, J. B. Knight, Mrs. E. B. Knopf, H. S. Ladd, A. K. Miller, John Rodgers, H. C. Stetson, and James Steele Williams. The authors are deeply grateful to all of these for the time they so generously gave to the im­provement of this report. Dr. G. A. Cooper identified or checked the identification of the Mississippian macrofossils cited in this report, and Dr. W. H. Hass reported on the conodonts obtained. Dr. J. B. Knight advised on the generic identification of some of the gastro­pods, Dr. Edwin Kirk identified some Mississippian Pelmatozoa, and Dr. A. K. Miller contributed suggestions about the goniatites. Some additional acknowledgments are made at appropriate places in the report, but a very great debt must be satisfied by a general expression of thanks to the many ranchers and other local residents of central Texas who befriended the authors with unfailing courtesy in ways too numerous to mention in a report of this nature. LOCAL PRONUNCIATION OF PLACE-NAMES The following list gives the accepted local pronunciations of geologically im­portant place-names in the Llano region where pronunciation is not evident from spelling Barnett (barn' et) Burnet (burn' et) Chappel (chap' el) Lampasas (lam -pas' as) Llano Clan' o) Pedernales ( pei'd' en -al' es) San Saba ( san sa' ha) SLaendebach ( stln' de biik) Sudduth (sud' i:ith) Tanyard (tan' yard) Welge (wel' gel GLOSSARY OF SELECTED TECHNICAL TERMS It has been the aim of the writers to reduce their observations to approximate systemalic categories, recognizable in the field and expressed consistently by un­ambiguous terms in such a manner as to be most usable in comparative work. Within the limits of existing terminology they have chosen descriptive rather than genetic terms, and they have preferred obvious combinations of familiar words to the coining of new terminology. Published terms and symbols are adapted to present purposes without ref­erence to etymology, history, or original aulhorship except in special instances. This is not an oversight, but an implica­tion that the terms or symbols are so The University of Texas Publication No. 462.l well established or so obvious that they may he considered a part of the English language, or of geologic symbolism, and thus common property. In the instance of words of classical origin the authors accept miscegenous words whose mean­ ing is clear in preference to coining new words. The prefixes sub and semi, al­though of Latin derivation and classically to be used only in combination with Latin roots, are here used indiscriminately as accepted prefixes in the English lan· guage-sub to mean almost or nearly; semi to mean half, denoting a median position in a gradational series of any sort. For definitions of terms other than those here noted, the reader is referred lo Grabau (1924), lo Twenhofel et al. (1932), to Hatch, Rastall, and Black (1933), or to Rice (1945). It is hoped that clarity of presentation will be im­proved by definition of the meanings attributed to the following terms: Aphanitic-see sublithographic. Bedding-in the Johnson City and Lange's Mill sections the bedding is mostly described in generalized terms. The expression "thin!) bedded" indicates beds roughly less than 3 inches thick, "medium bedded" 3 to 10 inches thick, and "thickly bedded" more than 10 inches thick. The word "massive" has been used chiefly to describe the indistinrtly bedded, microgranular to very fine grained, sphenoid­ally jointed dolomites. Benthos or henthon-bottom dwellers; henthonic --bottom dwelling. Bioherm--"moundlike, lenslike, or otherwise circumscribed structures of strictly organic 01 igin, embedded in rocks of different lith· ology" (Cummings, 1932, p. 333). See Plate 18; Plate 19, figure A_ Term proposed by Cummings and Shrock (1928, p. 599). Whether bioherms were also reefs must he judged on the basis of information commonly not avail· able to the geologist. Biohermal structures may coalesce to form biostromes (Pl. 33, fig. B). Biostrome-lat?rally exlensive betided structures ronsisting of or built mainly by or through the life activities of sedentary organisms, and not swelling into moundlike or lenslike forms (Cummings, 1932, p. 33tJ.). Not properly ap­ plied to shell breccias, limesand hanks, or other deposits of detiital origin. (Pl. 25, fig. B.) Calcarenite-a rock resulting from the litbifica­tion of detrital calcium carbonate wherein the calcitic partides exceed 0.05 mm. in diameter_ Not used in the present report. See lime-sand· stone and limesand. Calcilutite-a rock resulting from the lithifica­tion of calcium carbonate muds, of detrital origin (Grabau, 1913, p. 290). The sublitho­graphic limestones of the present report are not calcilutites, being of dominantly chemical origin. See lime-mudstone and sub-lithographic limestone. Calcilic dolomite-a carbonate rock that effer­vesces slightly under dilute hydrochloric acid. Caliche-a variously defined term here used to refer either to precipitated calcareous crusts of water-soluble minerals in the zone of ~oil formation or to the precipitated calcareous bonding material in certain detrital accumu­lations. In central Texas the mineral compo­sition of caliche is essentially calcium car­bonate, with minor impurities. See travertine. Cannonball-a term here used to indicate sub­spheroidal to ellipsoidal halls of chert and partially chertified limestone; varying in tex­ture from solid granular to porous and chalk­like chert, through chert with inclusions of limestone, to siliceous limestone. Generally with a fragmental structure, commonly con­taining fossils or Iragmenls of them, and not uncommonly with scattered to abundant ovoid bodies or cloudy indistinct markings such as characterize pellet limestones, they are strik­ingly different from other types of chert and from thP rock containing them. They are sug­gestive of lime-mud balls (pp. 95-96) in various stages of chertification and are commonly associated with chert of similar character that is spread out in irregularly lobate forms as though acrumulated in minor local depressions or squashed out to that form while yet plastic. This type of chert has not been unequivocally recorded below the Archaenscyplda zone of the Gorman formation and is generally uncom­mon below the base of the Honeycut forma­tion. The term, cannonball, is not here used to desiirnate concretionary chert that is con­centrically handed or chalcedonic to porce­laneous in texture. Chalcedonic-see texture (chert). Chert-a collective term used to include all varieties of cryptocrystalline to suhcrystalline quartz aggregates ordinarily occurring in sedi­mentary rocks. Chert-matrix sand-chert containing varying but relatively large amounts of ordinary quartz­sand grains, Clastic-produced by mechanical processes. See detrital. Coarse grained-see texture (hulk rock). Color-pending the preparation and adoption of usable standard rock-color charts, rock-colors are described subjectively, using ordinary color-terms in general usage. In their original studies of samples from the Cherokee and Johnson City areas, Barnes and Cloud fol­lowed Cullison and Ellison (1944) in deriv­ing specific color-terms hy actual comparison of rock-samples with the charts in the Maerz and Paul color dictionary (1930) and found The Ellenburger Group of Central Texas the comprehensiveness and general plan of this "dictionary" an aid to systematization. Due to the fact that its cost ($12.00) is pro­hibitive to general usage by geologists and to the unusual nature of many of the names applied to common rock-colors, its use was finally abandoned, but the conceptions of many of the color-names given are traceable to this book. Compact-constituent particles closely united or packed. It is to be assumed that all rocks here described are compact unless stated to be other­ wise. See dense. Crypto-oolitic-applied to oolitic chert where the character of the ooids and of the matrix is so similar that the oolitic structure is not readily detectable with the naked eye previous to examination with a hand lens. Crypto­oolitic chert is commonly pseudospicular, and some may represent the results of chert re­placeme11t of pellet limestones. Dense-same as compact; or refers to ratio of mass to hulk or volume. Inaccurately used by some authors in a restricted sense to de­note smooth-fracturing rocks which are not visibly granular. See calcilutite, compact, chalcedonic, porcelaneous, sublithographic, lithographic, vaughanitic. Detrital or elastic limestone-refers to limestone that is composed of fragments worn from cal­careous ;,keletons or pre-existing rocks by me­chanical processes. Dolocastic2~applied to previously dolomitic or dolomoldic chert wherein the dolomite rhombs have been replaced or their molds refilled with some mineral other than dolomite. Such chert, with pseudomorphs of quartz, was col­lected near the middle of the Staendehach member of the Tanyard formation on the east side of State highway 16 and about 9.65 miles by speedometer south of San Saba and has been seen but not collected at a few other localities, See dolomoldic. Dolomite-a calcium-magnesium carbonate rock that effervesces very slightly or not at all under dilute hydrochloric acid. Van Tuyl (1914, p. 257) shows that the name dolomite was orig­inally and is properly a rock term. Dolomitic limestone-a carbonate rock that effer· vcsces moderately under dilute hydrochloric acid. Dolomoldic~applied to previously dolomitic or interstitial chert from which the dolomite has been removed, leaving rhombic holes or mold6 where the dolomite was. Dolomitic and in­terstitial chert become artificially dolomoldic in the preparation of insoluble residues in the 2'1Tl1e writers accept the recommendation of Ireland and others (Bull. Amer. Assoc. Petr. Geol., vol, 31, pp. 1482, 1'183, 19·17) that dolomorphic is preferable to dolocastic as an adjective descriptive of the type chert hcic noted. No change is made in the text because of the rarity of this type of chert and a wis11 to emphasize the incon~uity of tl1e use of the word dolocastic for a chert containing rhomboidal molds. laboratory. Dolomoldic chert is commonly but erroneously referred to as dolocastic. See dolocastic. Eustatic-refers to changes in the volume of the oceans, or to movements of the oceanic basins, affecting the total volume of water on the continents and raising or lowering the entire surfaces of epicontinental seas. Fine grained-see texture (bulk rock). Glauconite-a complex hydrous silicate of potas­sium and ferric iron typically occurring in sedimentary rocks of marine origin as small ovoid pellets or globules of a shiny dark green to yew green color. According to Hatch and Rastall (1938, pp. 141-142), glauconite may behave as a mobile mineral and may be taken into solution or redeposited. Inasmuch as the determinations of glauconite here noted are based almost entirely on visual inspec­tion, the term glauconite, with or without the modifying word "globular," is used only to denote the ordinary type composed of small ovoid pellets. The words "interstitial glau­conite" are here used to denote other occur­rences resembling or thought to he glauconite. Most of the flaky green minerals seen in the carbonate rocks of central Texas are argil­laceous, rather than glauconitic. Granular--composed of grains or closely packed crystals. Granularity-see texture (bulk rock and chert). Interstitial-used in this paper to refer to chert or glauconite that occurs between the grains of a crystalline rock and has its form deter­mined or strongly modified by the crystallinity of the host rock. Lime-mud and lime-mudstone-calcium carbon­ate mud and the rock resulting from its lithi­fication. See sublithographic limestone. Limesand and lime-sandstone-here used in rather loose interchange to refer to conspicu­ously detrital and commonly friable limestones. The limesands and lime-sandstones of this re· port could properly be called calrarenites, and the names are not here extended in ordi­nary description to very finely detrital and well indurated lime-sandstones, or to the shell­hreccias and granular limestones of the Cam­brian. Limesilt and lime-siltstone----detrital calcium car­bonate wherein the calcitic particles are less than 0.05 mm. in diameter, and the rock resulting from its lithification. Limestone-a carbonate rock that effervesces strongly under dilute hydrochloric acid. Lithographic-see texture (bulk form). Littoral-between tides. May be taken as the upper part of the neritic zone. Sometimes confused with neritic, which see. Medium grained-see texture (bulk rock). Microgranular-see texture (bulk rock). Nektos or nekton-free swimmers; nek1onic­free swimming. The University of Texas Publication No. 4621 Neritic-between low tide and depths exceeding 100 fathoms, that is, less than 100 fathoms deep but not ordinarily exposed to the atmos­phere. See littoral. Oiicastic-if used should be applied TO previ­ously oolitic substances wherein the individual ooids have been replaced or their molds re­filled with some substance other than that originally composing the oiiids. The use of this term for chert involves the question of whether most if not all oolitic chert is not really oocastic; and, if not, how an oocastic chert is to he recognized. Ooid-the individual tiny spheroid in an oolitic rock. Here used in preference to the term oolith of some authors to avoid confusion with oiilile and oolitic. The guffixes lith and lite are both derived from the Greek root lithos, meaning a stone, and therefore prop­erly denote the (iolite rock ilse!I. The com­bining form, oii, from the Greek for egg, be­comes ooid, meaning like an egg, and logi­cally refers to the original roe-like particles. See also oolite. Oiilith--see ooid and oolite. Oolite-a rock with a considerable proportion of tiny spheroidal bodies varying as to micro­structure but commonly comprised of concen­tric shells. or with a radial structnre. and known as ~oids from their resemblance to fish roe. The common usage of this term is fol­lowed for cherl as well as bed rock, even though it may be reasonable to argue that most ooids were primarily calcitic and that the chert is therefore oucastic. Likewise it is probable that some of the cherts the writer5 call oolitic really represent chert replacement of pellet limestones wherein the included pellets were largely spheroidal. Such pellets commonly form the nuclei of true ooids. SeP also ooid. Oomoldic-applied to previously oolitic or ooca~­tic chert from which the original ooids or their replacements have been removed, leav­ing subspherical holes or molds where the ooids were. Pellet limestone-descriherl by Hatch, Rastall, and Black (in Hatch and Rastall, 1938) as a limestone consisting of "small ovoid bodies, aggregated to form a rock superficially re­sembling an oolitic limestone or an oolite sand. The constituent grains show no orig­inal concentric structure, although in some cases they have acted as nuclei for ooliths." H. B. Moore (1933) shows that modern' muds are pelleted by various mud-feeding inverle­hrates, which pass the sediment through their bodies and leave the rejected material in the form of smooth or longitudinally groved ellip­soidal grains and rod-shaped pellets. Tn re­cent marine sediments these may form "np to 30 per cent and in rarer cases as much as 100 per cent of the deposit" (Moore, 1933, p. 19). Although more than one type is found, the commonest is a simple ovoid type whose nature suggests that it is that of either a polychaete worm or a mollusc (Moore, 1933, p. 25). Porcclancous-see texture (chert) . Pseudospicular-applied to crypto-oolitic cherts wherein the outlines of the ooicls are discon­tinuous; resulting in a pattern resembling Lhe spicular meshwork of certain lithistitl sponges. Quartzosc-containing inclusions or comprised of visibly crystalline quartz. Used especially in describing chert to contrast with typP-s composed entirely of cryptocrystalline to suh­granular varieties of quartz. Reticulate weathering-applied to limestones having an internal meshwork of dolomite that weathers to dark, rough, interlacing welts on the lighter, smoother, relatively depressed sur­ face of the limestone. Sand-aggregated detrital particles larger than 0.05 mm. in diameter. Where chemically bonded or otherwise lithified the resulLing rock is called sandstone. Min era logic composit10n is varied, but nnless otherwise specified is understood to be quartz. Semichalcedonic-see texture (chert). Semiporcelaneous-see texture (chert). Silt-aggregated detrital particles smaller than 0.05 mm. in diameter. Smoolh-fracturing-considercd to he an inherent chara('ter of certain Lextural types (see Lex­turc, chert, and bed rock), just as a rough fracture is of others, and ordinarily not spe­cifically mentioned. Sphenoidally jointed-irregularly "jointed" in blocks of roughly spltenoidal shape with slightly curved smooth ourfaces. The surfaces follow no detectable pattern and are charac­teristically a weathering feature of the micro­granular to very fine grained dolomites. Spiculite--a rock in which sponge spicules are volumetrically important. Stromatolite-a laminated but otherwise struc­tureless object; commonly of some particular shape, ordinarily with a surface of attach­ment. in the larger forms, and probably of algal origin, hut not biologically classifiable (see Cloud, 1942). Stromatolitic-of the nature of or pertaining to stromatolites. Structural sink-a collap~e or subsidence feature having the structure but not the topographk expression of a sink. Snbchalcedonic~sce texture (chert). Suhgranular-sce texture (chert). Sublithographic-see texture (bulk rock). Suhporcclaneous-see texture (chert). Texture (hulk rock)-the previously published grain size scales differ widely and are not satisfactory for the sedimentary cai bonat~ rocks here considered. The following scale is used by the authors: lithographic; smooth-fracturing, with no granularity detectable except at very high enlargements, and of sufficiently uniform The Ellenburger Group of Central Texas character to be potential lithographic stone. sublithographic; smooth-fracturing, with no granularity detectable except at very high enlargements, but not of sufficiently uniform character "to be used commercially as litho­graphic stone. A compromise term mean· ing the same thing as aphanitic does for igneous rocks. Many of the limestones de­scribed as sublithographic in the present report are in part pellet limestones, which locally results in a granular appearance; but the pellets themselves are mostly of sublitho­graphic texture. The sublithographic lime­stones of this report are lime-mudstones of probable chemical origin (that is, calcipul­verties or hydrocalcilutites; Grabau, 1913, pp. 283, 297). rnicrogranular; visibly granular, but indi­vidual grains mostly not resolvable with the aid of an ordinary 10-power hand lens and commonly requiring better than 25­power magnification for resolution. Dimen­sions of grains mostly less than 0.05 mm. very fine grained; individual grains mostly not resolvable to the naked eye but dis­tinct under an ordinary 10-power hand lens. Dimensions of grains averaging 0.05 to 0.20 mm. fine grained; individual grains mostly re­ solvable with the naked eye, and with di­ mensions of most grains from 0.2 to 0.6 mm. medium grained; dimensions of individual grains, averaging between 0.6 am! 1.2 mm. coarse grained; dimensions of individual grains mostly exceeding 1.2 mm. The last five terms are used principally to classify dolomites, most limestones being re­ferred to simply as either sublithographic or granular. Weather conditions may affect grain size determinations of dolomites in the field. Dright sunlight reflected from cleavage sur­faces not uncommonly results in grain size determinations a grade coarser than might he made on a cloudy day. Although not practicable in the field, exact measurements with a microscope and ocular grid shollld be made in laboratory studies of samples and we1I cuttings. Texture (chert)--the various cryptocrystalline to subcryst.alline varieties of quarlr. collectively known as chert show a variation in their lust.er and manner of fracture that probably reflects their state of granularity. Although a loose usage of the three terms chalcedonic, porcelaneous, and granular suffices for general discussion, in detailed descriptions and more specific discussion the following terminology i' used: chalcedonic; translucent, smooth-fracturing chert with a shiny luster. The cherts here called chalcedonic are commonly variegated like agate and have a luster more like that variety of chalcedony than the waxy luster commonly associated with the term chal­cedony when used in a restricted sense. subchalcedonic; not quite chalcedonic, grad­ing toward semichalcedonic. sernichalcedonic; subtranslncent to opaque, smooth-fracturing chert with a rather dull luster and ordinarily of some dull color. porcelaneous; opaque to subtranslucent, smooth-fracturing chert with a dull luster; typically china white and with the faintest suggestion of granularity on the fractured surface. The texture is that of the finest porcelain or chinaware, su,h porcclaneons; not quite porcelaneous, grading toward semiporcelaneous. semiporcelaneous; opaque, dull-lustered chert that feels rough on a fractured surface but is not visibly granular. sub granular; opaqlle, uneven-fracturing, dull­ lustered chert that feels and looks rough but is not visibly granular. granular; visibly granular chert but ordi­ narily without the individual grains resolv­ able to the naked eye. Travertine--"a concretionary calcium carbonate formed by deposit" (Webster). Here applied to handed or laminated deposits of Ire~h-water limestone precipitated from moving water. See caliche. Vaughnitic-see sublithographic; see Kindle, 1923, p. 370. \'ery fine grained-see texture (bulk rock). METHODS AND TECHNIQUES The whole effort of the authors and their assistants in the field was geared to obtaining measurements of representative sections, to describing and marking them in such a manner that they could be fol­lowed by others, and to rer:ording laLcral variations away from the lines of sec­tion. Tn spite of !he large number of variables dealt with in anv one area, their Iluctuations from area t~ area, and the necessity of recording them in the field without bulky equipment, the authors strove for the greatest feasible accuracy in all significant details. To what de­gree the various details are significant from place to place is, of course, a mat­ter of judgment, but a judgment that had to be exercised if the work was to be brought to a conclusion. The degree of reliability that may be attributed to the data presented is di­rectly proportional to the accuracy of the techniques used and the methods followed in obtaining and recording them. These are briefly outlined in order that the reader may better judge the limitations of the work. 20 The University of Texas Publication No. 4621 The work done went through three phases: (1) detailed study of particular areas to establish key faunal and lithic sequences, accompanied by office studies of aerial photographs in an effort to rec­ognize photographic similarities and select promising areas for further study; (2) reconnaissance study on the ground of areas judged to be promising, and selection of certain of these areas for detailed study; ( 3) detailed studies and measurement of sections in selected an~as. The second phase of the work was the shortest of the three. It was followed by a progress report (Cloud, Barnes, and Bridge, 1945), whereas the present re­port comprises the results of all the work and cites the detailed documentary evi­dence for these results. Measurement of each of the sections of Ellenburger and related rocks described in Part 2 was preceded by detailed map· ping of a sufficiently large area to estab­lish measurable and structurally uncom­plicated areas of section, as well as to work out lateral variations in the map­able characters of the rocks. Both geol­ogy and base data were plotted in the field on aerial photographs. All contacts and faults were walked out except where so evident on the photographs that there was no chance of misinterpretation, and the patterns so obtained were checked by clos~ly spaced cross-section traverses. All cultural data and all places where the drainage pattern was in question were checked on the ground, and a car has been driven over all roads shown on the maps. It is probable that the authors or their assistants have been within 500 feet of every point in the areas mapped, and, in most instances, closer. Most mapping was done on vertical aerial photographs of the U.S. Depart· ment of Agriculture enlarged to the ap­proximate scale of l :7920, except in the Cherokee and Bald Ridge areas where contact prints at the approximate scale of 1 :20,000 were used. In the Bald Ridge area, and locally in other areas, data were mapped on the contact prints and later transferred to the enlargements. All maps were plotted planimetrically from the enlarged aerial photographs except that of the Cherokee area (Pl. 2) which was plotted from the contact prints. The geologic map of a part of the Cherokee area (Pl. 7) was prepared from photo­static enlargements of Plate 2. The IO maps of small areas in the vicinity of measured sections (Pls. 4~13) are here reproduced at the approximate scale of 1 :10,000. The colored maps of the two larger demonstration areas (Pls. 2 and 3) are given at the approximate scale of 1 :31 ,680. That of the Johnson City area was pantographically reduced to the drafted scale of 1 :20,000 from the en­larged aerial photographs. Many of the maps deviate slightly from the intended scales, due to irregularities in reproduc­tion, but the graphic scale should be ac­curate within the limits of the system and data used in compilation. All maps were compiled by the admit­tedly crude method of center-point con­trol, and errors were distributed as evenly as possible. The amount and kind of ground control varies from place to place and is given under the individual areas. Because of the relatively slight topo· graphic relief at most places it is believed that the center-point compilations make about as satisfactory planimetric maps as could be compiled short of resorting to the multiplex machine or similar com· plicated and expensive processes. They are better than the average plane-table map available to geologists, because linear features can be traced out for their full length in relatively true perspective to the geologic boundaries. The average user o:f these maps, while warned of gen­eral cartographic inaccuracy, will prob­ably not notice inaccuracies in detail un­less he runs fairly close instrumental sur­veys. There is, of course, no doubt that good topographic base maps would have enhanced the usefulness of the geologic maps. In descriptions of particular localities all distances given are airline unless stated to be by speedometer or by road. Airline distances and directions were ob­tained by measurements from aerial pho­tographs, or by pace and compass tra­verses. The approximate scales of aerial photographs in the several areas were checked or determined by random meas­ The Ellenburger Group of Central Texas urements between recognizable points on several photographs; the measurements being made by alidade, by taping, or by comparison with instrumental traverses of the Texas Stale Highway Department. Sections were measured directly in the field and adjusted to computations based on vertical and horizontal measurements along the lines of traverse. The described sections follow routes that give the best combinations of continuity and exposures in minimum horizontal distances. Lateral shifts are made to avoid faults, to ob­tain better exposures, or to secure a topo­graphic advantage. The horizontal length of traverse may he reduced by increased topographic relief or by steeper dips of bedding. Where topography is the major factor, the accuracy in direct measure­ment or graphic computation is inversely proportional to the length of traverse for any given thickness of rocks, and iL is the experience of Lhc authors that essen­tially the same is true where length of traverse is related to dips up to about 15 degrees. The personal error in direct measurements, however, tends to increase rapidly as the dip steepens from about 15 degrees and probably just about offsets the advantage gained by shortening of traverse at 20 degrees. Low dips result in other disadvantages, and the optimum dip is thought to be between 6 and 15 degrees. Effort was made to find the most continuous sections, but only two are not composite (Cherokee and Highway 87) and one of these (Highway 87) is unsat­isfactory because of low relief, low dips, and poor exposures. The composite sec­tions are joined on the basis of detailed mapping, and their joinings probably in­volve no greater error than is involved in many of the lateral shiits. Neither direct measurements nor com­putations are any better than the data on which they are based. Avoidance of strucLural complications in measured sec­tions of Ellenburger rocks in the Llano region depends on detailed mapping. In the non-dolomitic Cambrian and post· Ellenburger rocks, bedding alignments are generally much better defined than in the Ellenburger, permitting more ready de­termination of faults from study of aerial photographs. In these rocks favorable areas for measuring sections may be sug­gested from study of the aerial photo­graphs alone and checked by means of less detailed mapping than is called for in selecting measurable sections of the Ellen­burger strata at most places. Once assured of a structurally uncom­plicated line of section, attitude of the beds is the most important item. Dips lower than 6 degrees are commonly diffi. cult to read accurately, and unless ex· posures are very good, minor undulations may escape notice. Most dips were meas· ured in whichever combination of the following three ways seemed the more feasible: (1) contact measurement with a Brunton compass on a fiat object of the maximum convenient length laid on a good bedding surface, ( 2) sighting nor­ mal to the dip with a Brunton compass, (3) sighting parallel to the dip with a Brunton compass. Ordinarily the dip finally accepted was the average of a number of readings, and dips used in measurement were averaged continuously from data along and adjacent to the line of section. At a few places dips were in­&trumentally determined by the three­point method with an alidade and plane table. It is considered likely that the inac­ curacies in reading dips and tracing out lateral shifts are largely compensatory, and that the limit of error in measure­ rnen ls is probably within 10 per cent. The unavoidable inaccuracy, however, is such that extreme care in ~btaining com­ putative data or applying small local cor­ rections did not seem wananted. Instru­ mental traverses were made of some sec· tions, buL computations of others were based on horizontal distances scaled from aerial photographs and elevations deter­ mined with a Paulin precision altimeter. \Vhcre computations checked direct meas­ urements within reasonably close limits no correction was applied. The general system of measurement and description of detailed sections found preferable is as follows: (l) the gen· cral line of section having been worked out by mapping, the best actual route for it to follow, including necessary lat­teral shifts, it scouted out and marked; (2) atLitudes of beds are determined as The University of Texas Publication No. 4621 frequently as practicable along and near the line of section and averaged at ap· propriate intervals; ( 3) traverses are made of the line of section to determine elevations and, in some instances, dis­tances between points, and thicknesses between recognizable data are computed; (4) the section is directly measured and marked on the ground and checked at appropriate intervals with the computed thicknesses; (5) the section is finally de· scribed in detail. The general system sug· gested resulted from trial and error in the measurement and description of early sections. It was found impracticable to perform all the steps suggested at once. Even though the work was done by teams its coordination was difficult, things were forgotten, and confusion and inaccuracy resulted. It is better that the whole job be done by two people as a set of sepa­rate operations, except that the actual composition of the description should be a single continuous operation by one per­son. However, it is advantageous to dic­tate the description to an assistant, be­cause of the many operations involved in composing a detailed description. Direct measurements of strata were made with a Brunton compass set at the proper angle of inclination and used on a measured and graduated staff. In or­der to facilitate sampling, the sections are marked at approximately 5-foot in· tervals with paint-spots of some conspic­uous color. Yell ow or orange were found to be more conspicuous lhan other colors, and the Sherwin-Williams trimbrite paint No. 19604, called "fiesta yellow" proved to be the brightest and most durable of the paints tried. In addition to the usual equipment and techniques employed in study and descrip· tion of outcrops and sections, it was found helpful to carry a dropper-bottle of di­lute hydrochloric acid, and it wa~ found very useful to develop the techmque of seeing properly paired aerial photographs as a stereoscopic unit without optical aids. As in other areas, familiarity with the local rocks permitted ready visual differ­entiation between limestone and dolomite in most instances, and the dolomite was so habitually crystalline and dull weath­ering that chemical testing was mostly a confirmatory matter. The searches for sand grains in the sparsely arenaceous rocks of the Ellenburger group were greatly expedited and facilitated by the simple procedure of dragging a hammer blade across the outcrops. Disseminated silica of any sort draws instant protest from the hammer, and many sparsely arenaceous intervals were located in this manner. Without vertical aerial photographs the work would have been vastly more difli­cult, but it must not be supposed that the aerial photographs are a panacea for all the difficulties of areal geology. They are perhaps at their maximum usefulness in regions such as the Llano uplift where topographic distinctions arc well ex­pressed but not extreme, where vegeta· tion is mostly neither dense nor very sparse, and where soil cover is not great. They furnish many useful clues in the way of topographic and vegetal patterns, and in stratigraphic and structural align­ments of various sorts. They show, in most instances clearly, the cultural fea­tures in existence at the time they were taken. They guide one to the outcrops and show him where outcrops are un­likely to be found. But all these things must be checked on the ground, and it is not uncommon for such checking to yield negative results. Their use can best be learned by practice, but even one skilled in their interpretation will not always find it easy to keep himself lo­cated in very thick woods, in formerly wooded areas that have been cut over, or in very sparsely vegetated areas of gentle topography. Unaided stereoscopic vision should be cultivated by anyone who contemplates using aerial photo· graphs for geologic work, and he should have access to a good book on their in­terpretation, such as that of H. T. U. Smith (1943). BASIS OF CORRELATION Correlation of the various rock units mapped within the Llano region is based on fossils, lithic character of the domi­nant rock, macroscopic.ally visible acces­sory lithic constituents, topographic ex­pression, and vegetation. Regional corre­ The Ellenburger Group of Central Texas lation, on the other hand, is based wholly on fossils. For correlation on physical evidence in and near the Llano uplift the distribution of quartz-sand and glauco· nite and the grain sizes of the dolomites are particularly helpful. Topographic ex­pression and vegetal paltcrns are most helpful in mapping on aerial photographs. Approximate ranges in the Ellenburger group and Wilberns formation of 36 genera other than Ceratopea, of 6 "species" of that gastropod operculum, and of the trilobite subfamily Saukiinae are graphically shown on Plate 14. Jn. asmuch as most of the genera here listed are short ranged, specific determinations are given for few of them. Some of the index fossils from the Ellenburger group and associated rocks arc figured on Plates 38 to 44, and £annal differentiation is discussed under "Paleontography."2 b Physical criteria applied to the subdi­vision of the Lower Ordovician and Upper Cambrian carbonate rocks of cen­tral Texas and the correlation of their various units are as follows: l. Distribution of glauconite.-Within the experience of the authors in the mid­Continent region, the presence of glau­conite as a common accessory mineral is presumptive evidence of a Cambrian age for the rock in which it occurs. Such glauconite is likely to be globular if from limestone and interstitial if from dolomite. The occasional occurrence of glauconite m rocks of known Lower Ordovician age is not regarded as a refu­tation of this general principle, nor docs the mere absence of glauconite neces· sarily suggest a Lower Ordovician age. Glauconite is locally common near the base of the Tanyard formation in the western part of the Llano uplift, has been seen at olher levels in the Ellenburger group, and occurs in parts of the Lower Ordovician of west Texas. It is abundant in the Bliss sandstone of the Franklin Mountains, which is provisionally consid­ered Lo be Lower Ordovician but may 2bNcw name<; fo1 some of the fossils he1e hsted and illustiated will be found m papc1s by BuJgr. nnd Clout! (gast1opods and tnlobltes, 1917; species of leratopea, m press) and by Cloud (lnachiopodi:i, in p1css). It rs suggested that tho mtele'3ted ieudet consult these papers fu1 appropn~ ate systemaL1c changes. be Upper Cambrian in age. The sedi­mentational implications of glauconite are discussed on pages 97-98. 2. Distribution of quartz-sand.-The relative local abundance of quartz-sand other than detrital chert, the frequency in vertical distribution of arenaceous zones, and the character of the sand are of critical importance in the stratigraphy of the Ellenburger and associated Upper Cambrian rocks in the Llano uplift. In their field examinations of surface ex­posures, as well as their examinations in the office with a binocular microscope of chip-samples and uncontaminated hydrochloric acid residues from surface sections, the writers nowhere observed sand from the Tanyard formation and at only a few places above the basal 50 feet of the Honeycut formation. However, de­tailed petrographic examination by Dr. S. S. Goldich and Mr. E. B. Parmelee of the hydrochloric acid residues from samples from the composite Riley Moun­tain section revealed sand grains positively at 180 feet above the base of the Tanyard formation and possibly at 7, ] 13, 513, and 548 feet above its base. Thus sand does occur rarely in the Tanyard formation, at least toward the south side of the Llano uplift. The simple presence of sand grains in Ellenburger rocks, therefore, suggests, although it docs not prove, a post-Tan­yard age; and sand in relative abundance indicates the Gorman formation or the lower 50 feet of the Honeycut. As a rule, sand grains from the Ellenburger are scattered, clean (unstained), very smoothly frosted, well rounded, and small but poorly sorted within their relatively nar­row size-limits. Where sand occurs in the Cambrian rocks of the Llano region, it is ordinarily more abundant, more coarsely frosted or pitted, less well rounded, and generally composed of larger and relatively better sorted grains than are normally found in the Ellcn­burgcr. The Cambrian sand grains are locally subangular to angular, locally have recomposed crystal faces, and com­monly are stained with iron oxides. Such sand grains with ferruginous stains as occur in the Ellenburger suggest deriva­tion from wnes of exceptional permea­bility, or relatively thick arenaceous in­tervals such as occur locally at the Tanyard­Gorman boundary or within the Gorman formation. In other areas of outcropping Lower Ordovician rocks that rim the Permian basin region the distribution of sand does not follow the rules outlined above, and at some intermediate parts of the subsurface they must cease to be ap­plicable. 3. Grain size of dolomites.-ExcepL in the northwestern corner of the Llano uplift (Bald Ridge area), grain size of the dolomites is an important clue to correlation of the Ellenburger and asso­ciated carbonate rocks of the Upper Cam· brian. The applications of this clue will be evident from Plate 14, table 2, the "Explanations" on the various geologic maps, and the descriptions in the text. Microgranular dolomite is excessively rare in the Tanyard formation, and, ex­ cept in the lower part of the Staendebach member, very fine grained dolomite is uncommon in the Tanyard. Conspicuous zones of microgranular to very fine grained dolomite feature the lower part of the Gorman formation, the middle of the Honeycut formation, and loca\ly the upper or lower parts of the Pedernales dolomiLe member of the Wilberns for· mation. The dolomites of the Threadgill member of the Tanyard formation are characteristically medium to coarse grained, and those of the Staendebach member are mostly fine to medium o-rained. Except in the northwestern ~orner of the Llano region there is a conspicuous break in grain size of the dolomites at or near the Tanyard-Gor· man boundary, from coarser below to finer above. Except in the southeastern corner of the region the Cambrian-Ordo· vician boundary displays a similar though reversed break where it is in dolomite. A less conspicuous break from coarser grained dolomites below to finer grained dolomites above is a common feature of the contact between the Threadgill and Staendebach members of the Tanyard for­mation where it is in dolomite. 4. Granularity of the limestones.­Whereas essentially nondolomitic, granu­lar limestones characterize the Upper Cambrian; sublithographic limestones, if not from Cambrian bioherms, suggest the Ellenburger. 5. Types of chert.--The several signifi­cant varieties of chert are here briefly discussed and listed in the approximate order that they affect the selection of boundaries from bottom to top of the section, note being made of relative abun­ dance or absence. Except in the Pedernales dolomite member of the Wilberns formation chert is very rare in the Upper Cambrian of the Llano uplift. Finely vesicular, dolomoldic or oomol­ dic, platy-looking chert in tones of yel­ lowish to olive or greenish brown, tan, and beige is characteristic of the upper part of the Pedemales dolomite member of the Wilberns formation at some places. Oolitic and oomoldic chert is rare in the lower 90 to 300 feet of the Ellen­ burger group (Threadgill member of the Tanyard formation), but such chert is locally common at higher and lower levels. The upper one-fourth to one-half of the Tanyard formation in the north­ ern and western parts of the Llano uplift is featured by dull, russet-weathering, laminated, oolitic to oomoldic chert (oolitic laminar chert), and at places by free siliceous ooids. Semichalcedonic to subchalcedonic cherts are commonly oolitic and chalcedonic cherts are occa· sionally so. Conspicuously dolomoldic chert and in· terstitial chert such as would appear in hydrochloric residues as a highly cellu­ lar or "lacy" dolomoldic chert is much more common in the lower 520 to 660 feet of the Ellenburger strata (Tanyard formation) than above. Quartz druse and conspicuously quartz­ ose chert are abundant in the Tanyard formation and locally abundant in the underlying Pedernales dolomite member of the Wilberns formation. They are · much less common in the Gorman and Honeycut formations, although they may be found throughouL the Ellenburger group. The insoluble elements of this type are best displayed on weathered sur­faces and may be represented only by highly disseminated silica in samples from below ground. The Ellenburger Group of Central Texas Sparingly dolomoldic porcelaneous and chalcedonic chert is rare in the lower 90 to 300 feet of the Ellenburger group (Threadgill member of the Tanyard formation) at most places, and may be uncommon in the lower 500 feet. The several varieties and grades of chalce· donic chert are normally common in the Gorman and Honeycut formations, and, except in the western part of the region, in the Staendebach member of the Tan· yard formation. Porcelaneous chert is normally common in the lower part of the Gorman formation, and, again ex· cepting the western part of the region, in the Staendebach member of the Tan· yard formation. In the eastern areas the rather abrupt appearance, in the Ellen· burger group, of porcelaneous and chal· cedonic chert marks the base of the Staendebach member of the Tanyard for· mation, but in the Bear Spring and Bald Ridge areas the lowest occurrence of such chert in abundance is in the vicinity of the Tanyard-Gorman contact. In general the occurrence of truly chalcedonic cherts in abundance suggests a post-Tanyard age and of truly porcelaneous cherts a lower Gorman age. The duller varieties of both, such as would be modified by the pre· fixes sub and semi, preponderate in the Staendebach member of the Tanyard for· mation where it is cherty, and are locally abundant in the Pedernales dolomite member of the Wilberns formation. There seems to be a general tendency for the porcelaneous cherts of the Gorman for· mation to be less quartzose, less abun­dantly dolomoldic, and ordinarily to have finer dolomolds than those from lower strata. Semichalcedonic and subchalce­donic cherts are very likely to be at least partly oolitic to crypto-oolitic and pseudo· spicular. Truly chalcedonic chert is sel· trata, conspicuously dolomoldic chert and quarlz druse are noticeably less com­ mon than in the Tanyard formation. Porcelaneous to subporcelaneous, white, shiny-weathering chert is perhaps more common, less quartzose, and with tinier, more scattered dolomolds in the lower Gorman formation than in the upper Tanyard formation, but this is only a general impression and one difficult lo check statistically. As a rule nodular and concretionary, subchalccdonic lo chalce­donic chert is more common in the Gor­man and Honeycut formations than in the Tanyard. From the Honeycut formalion the chert of the Gorman differs princi­pally in being more commonly porcela­neous, less commonly nodular or concre­ti onary; and especially in lhc general rarity of cannonball 'chert ( p. 16) The University of Texas Publication No. 4621 except in the vicinity of the Archae­oscyphia zone of the Corman formation. Scattered to moderately abundant sand grains occur intermittent! y in the lime­stones and dolomites of the Gorman for­mation, in contrast to the essentially sand-free strata of the Tanyard forma­tion and the more sparingly arenaceous beds of the Honeycut formation. One mu,;t commonly search for sand grains even in the Gorman formation; but it is, gen­erally speaking, the arenaceous part of the Ellcnburger group. Fossils and correlation.~Although for the most part sparingly fossiliferous, both the chert and the limestone of the Gor­man formation yield fossils on search; and representative though sparse faunas have been obtained from the base to the top of the formation. Archaeoscyphia, Diaphelasma, Syntrophinella, Lecano­spira, Rhoinbella (formerly called "Roubi­douxia"), "Euconia," mostly undetermined cephalopods, and Hystricuru,s are the mosL common, and the first five genera named are especially useful in correlation. Rhombella and Lecanospira are re­stricted to the Gorman formation, occur throughout it, and are relatively common in comparison with other Gorman fossils. A possible source of confusion with Lecanospira and apparent stratigraphic inconsistency is furnished by the occur­rence in the Honeycut formation of the gastropod Bamesella lecanospiroides, in­termediate in resemblance between Lecano­spira and Maclurites, and congeneric with "Maclurea" affinis Billings. More than one whorl is required to distinguish Rhom­bella surely from Euconia (strict sense). Ilystricnrus, although suggestive of the Gorman formation, also occurs in Tanyard strata. The sponge Archaeoscyphia has been found in the Gorman formation only near the middle of the formation, at a position ranging from 210 to 263 feet above the base and 201 to 235 feet be­low the top, according to data from meas­ured sections. It occurs throughout the uplift and is an important datum marker. Ordinarily it ranges through only 3 to 5 feet of the Gorman strata; but its range is slightly extended at some places, and at one locality in Mason County speci­ mens of Archaeoscyphia were found about 25 feel above its principal occurrence. The fossils of the Gorman formation indicate its homotaxial equivalence with the Roubidoux formation of Missouri, as most generally defined, and the Longview limestone of the Appalachian region. The presence of Diaphelasma about 300 feet above the base of the Cool Creek formation in the Arbuckle Mountains of Oklahoma and, accompanied by Lecanospira, extend­ ing into strata assigned by Decker to the base of the Kindblade formation of the Wichita Mountains suggcsls general equiv­ alence helween Gorman and Cool Creek strata. However, Tanyard fossils occur about 100 foct above the base of the Cool Creek formation as drawn by Decker in the Wichila Mountains. The ranges of the more useful Gorman fossils are shown on Plate 14, their signifi­ cance in local correlation being deter­ minable therefrom. It should be noted Lliat the lowest recorded occurrence of the brachiopod Xenelasma is from the top of the uppermost bed of the Gor­ man formation, but it is locally very abundant there. Some approximate re­ gional correlates of the Gorman forma­ tion are shown on Plate 15. HONEYCUT FORMATION Type section.-AII known Ellenhurger strata above the Gorman formation at the surface in central Texas are termed the Honeycut formalion (Cloud, Barnes, and Bridge, 1945). The type section and thickest known development of this for­mation at the surface is in the reach of Pedcrnales River known as Honeycut Bend, about 5 miles east of Johnson City in Blanco County. The line of this section is shown on Plates 3 and 13, and it is described in detail on pages 321-331. Thickness.-Owing to post-Ellenburger truncation, the Honeycut formation ranges in measured thickness from a maximum of 678 feet at the type section in the southeast corner of the Llano uplift to a feather edge. It appears to be entirely absent west of 98° 55' longitude in west­ern San Saba County. The Ellenburger Group of Central Texas Lithic character.-The rock types of the Honeycut formation resemble those of the Gorman formation, differing prin­cipally in distribution, bedding, and color. For the most part the limestones of the Honeycut are more thinly bedded than those of the Gorman and more in· Limately inlerbedded with dolomite, a dif­ference most apparent at the Gorman­ Honeycut contact. The microgranular to very fine grained dolomites of the Honey­cut formation are given to relatively pale colors such as light gray to yellow­ish gray, pale beige, flax, and old ivory, as contrasted to the more vivid shades of rose and beige characterizing the dolo­mites of the Gorman formation in some parts of the Llano region. Where fully developed, the Honeycut formation may ordinarily be divided into a lower facies of interhedded limestones and dolomites. a median facies in which rnicrogranular to very fine grained dolomite predom­ inates, and an upper facies in which lime­stone predominates (Pl. 14). Chert and sand.-Comparison of thc chert of the Honeycut formation with that of the Gorman is given in a preced­ing part of this paper. Cannonballs (see pp. 16, 95-96) of chert and siliceous limestone arc typical of the Honeycut formation but uncommon below it except near !he Archaeoscyphia zone of the Cor­man formation. Sand is locally common iu the basal 50 feet of the Honeycut for­maLion hut rare above. The basal bed of the Honeycut is typically arenaceous. Fossi'.ls and correlation.-Many of the fossils described from the Jefferson Citv strata of Missouri by J. S. Cullison i~ 1944, are specifically identical with fos­sils common in Lhe Honeycut formation of Texas. They show that the base of the Honeycut formation coincides faunally with the base of the Jefferson City group 0£ Missouri, and that Lhe highest Honey· cut straLa known are not far below the top of the Jefferson City group as con­ceived by Cullison. One piece of contra· dicLory evidence is the occurrence of Tarphyceras resembling T. chadwickense U.F.M.F. at the base of the Ceratopea keithi'. zone, abont 400 feet above the base of the Honeycut formation in the type section and elsewhere along the eastern margin of the Llano region. Ulrich, Foerste, Miller, and Furnish (191,2, p. 1,) do not record Tarphyceras below the Cotter dolomite; hut inasmuch as the writers have found, in west Texas, a tarphy­ceratid cephalopod even farther below its normally accepted range than the one in central Texas (p. 367), they are not disturbed by this occurrence. Outside Missouri the Honeycut forma· Lion linds partial equivalents in approxi· mately the lower three-fourths of the Kindhlade formation of Oklahoma and in an unknown part of the Newala lime· stone of the Appalachian region. Some other correlates are indicated on Plate 15. Most importantly the Honeycut forma­tion is the zone of Ceratopea in Lhe Llano uplift. Cemtopea occurs intermittently from top to bottom of the formation, and live subzones, characterized by six :;pecies or subspecies of Ceratopea, are indicated on Plate 14. A single speci· men of a seventh species of Ceratopea, very close to if not identical with C. tennessensis Oder, was found at the Lop of the thickc:;t section of the Honeycut formation, at Honeycut Bend. These sev· eral species of Ceratopea are useful both in local and regional correlation. The gastropod Orospira also ranges from bot­tom to top o{ the Honeycut formation, and species of the trilobites f ef}ersonia, Bathyurellus, and Rananasus are impor· tant index fossils. The hrachiopod Xenelasma is common in cherl of the lower Honeycut forma· tion and typical of this part ~f the Ellen· burger group. Silicified specimens of X enelasma are also locally abundant in the top of tbe uppermost ledge of the Corman formation, where they occur with unsiJicified Rhombella and less com­monly Lecanospira. This is the lowest recorded occurrence of X enelasma from the Ellenhurger, and its highest is about 140 feet above the base of the Honeycut formation. Jn general, however, Xene· lasma appears to mark the lower 60 to 70 feet of the Honeycut formation, and its lowest occurrence essentially coincides with the Gorman-Honeycut boundary. The sponge Archaeoscyphia is common in the Honeycut strata hut rare below them, being recorded elsewhere only from The University of Texas Publication No. 4621 near the middle of the Gorman forma­tion. Ordinarily Archaeoscyphia from the Gorman is not well preserved externally, being detected principally by the spicular meshwork displayed on freshly broken surfaces of certain chert nodules. In the Honeycut strata, however, an Archaeo· scyphia assignable to A. annulata Culli­son commonly preserves its external struc­ture and general form almost as faith­fully as it does the spicular structure of the sponge wall. Although it seldom occurs in dolomite, Archaeoscyphia is ''ery abundant in some limestone beds of the Honeycut. Such beds have locally been mapped to determine structure. Although very abundant in some beds, the speci­mens of Archaeoscyphia so far seen are haphazardly scattered about as broken fragments, and no indication has yet been found that any of them are in the posi­tion of growth. With Archaeoscyphia is commonly found a Rat-ended, slightly tapering siphuncle having a flattened spiculum and therefore assigned to Mc­ queenoceras. Ranges of some of the more impor­tant Honeycut fossils are graphically in­dicated on Plate 14. BEDS THAT OVERLAP THE ELLENBURGER INTRODUCTORY REMARKS Rocks of Devonian, Carboniferous, and Cretaceous age rest with profound un­conformity on strata of various parts of the Ellenburger group. The nature and history of this unconformity and the character and correlation of the over­lapping beds are a part of the Ellen­hurger problem and hear directly on the production of petroleum from Ellen­burger strata. The ensuing discussion considers the character and correlation of the overlapping beds, whereas the nature and history of the unconformity is treated under "Geologic history." DEVONIAN The stratigraphy of the Devonian in central Texas has been considered by Barnes, Cloud, and Warren (1945, 194·7). Figure 1 indicates the sequence and cor­relation of Devonian rocks in central Texas brought out by these papers. Dis­cussion in the present paper is limited to brief notice under the areas in which rocks of Devonian age outcrop (pp. 138, 159, 315, 320, 349), the reader beimr referred for further detail to the paper~ cited. M1ss1ss1rr1AN The named Mississippian formations of the Llano region and their suggested cor­relations and relationships are shown in figures 2, 3, and 4.. In considering fig­ure 3 it should be noted that the use of the base of the Barnett formation as a datum fails to differentiate between the variation in thickness of the Barnett due to overlap and nondeposition at its base and that due to truncation at the top. Slight revision of the names by which the formations of Mississippian age were orjginally known is thought advisable. It is, accordingly, here proposed that the rocks originally designated as the Barnett shale shall be known as the Barnett for­mation (following Sellards, 1932, p. 91), the Chappel formation as the Chappel limestone, and the Ives conglomerate member of the Chappel formation as the Ives breccia, a unit of formational rank. The Barnett formation and the Chappel limestone have long been known to be of Mississippian age, but new evidence as to their limits and composition has resulted from detailed mapping of the upper surface of the Ellenburger. The Ives breccia correlates with beds consid­ered to be basal Mississippian by some and uppermost Devonian by others. Further work will probably lead to the naming of additional Mississippian units (p. 27), but the proposal of new names for Carboniferous rocks is not a func­tion of the present report. Some unnamed rocks of pre-Chappel age that occur in the Jolmson City area are described on pages 316-317, and evidence cited shows that they may correlate with the Glen Park formation and Hannibal shale. Other rocks of possible Glen Park and Grassy Creek age occur in the Lost Creek area (p. 347). Rocks of Mississippian age, though locally discontinuous, are widespread in G. A. COOPER AND OTHERS (1942) I CENTRAL TEXAS I I I EUR· UNITED STAm I Group and formation Llano RegionOPE Series Stooe ' 10 ·~~ w (!) ~at: zz I I er~& 8~ I m, ....... : <( z z - z <( <( I 5 0 z (!) : w <( I ::E ::> z <( <( <( en many z <( <( unknown0 :c (.) named > (.) w 0 - ? units (!) z er 'I :::> w ::E 'I 0.. w 0.. z z ::c ::> <( <( (.)-z (.) en w Enfield shale -? ------­ I <( z ---­ er en er w I w w IL. en Ithaca shale ~ I (!)::.:: Zesch ~<( Cornel I shale I IL. ....I formation Sherburne sand stone ..._ ' (.) z Geneseo shale ' r-.,? <( ::c Tully ~ formation "r- I <( (!) Moscow I' z 0 Ir'­ <( formation ::! z i= ::c z 0.. w (!) z ::> unknown > ::> > Ci z 0 er 1: 0 0 Lud lowville w <( i= (!) formation 0 ii: -I' w w z 1: ....I 12 Skane'ateles 0 ....I 0 ::E formation ::! ::E <( > - 0 .::c zz I w <( N Marcellus Bear Spring I ....I <( I w (.) IL. formation formation jjj I I I I '? ciz w I Onondaga -<( unknown ;:e_ ::> lit: limestone ._z o z <( ZI-Schoharie Stribling ..><: -0 u formation formation ~ N e z lJJ z ::.:: "!­ ....I <( er Oriskany g.?: z co ­ sandstone er if l I I I -~g ::! 0 w (.) I-er unknown ·-­ z 1110 0 en w w absent ~ 0 Q)~ w ::> E > --' 0 - z Pn.I !"wan l"·-A•fnnA l.J_Ll-~ ~~ 0 I DIC.An ,nA ca:: ::! a:: (!) HAl":rnT 'A c w z -::> w a:: Pillar Bluff New Scotland lit: z Ow 0 I imestone limestone 0 l!1m ....I w ::c Coer5~ans T1-r-?­ (!) lime ·one . ro--r--? JCK Fig. 1. Provisionai' correi'afion of fne Devonian m central Texas. The University of Texas Publication No. 4621 SECTION IN MIDDLE MISSISSIPPIAN OF THE SECTION AFTER MISSISSIPPI VALLEY LLANO REGION E. B. BRANSON Group Formation TEXAS (1938, pp. 5, 8, 158) . Q. => 0 ' er C) 111any named w er units .... (f) w :c 0 unknown not discussed ' Ste. Genevieve limestone ? St. Lou is limestone Spergen 1----'__ ,.. -i_m_e_s_to_n_e ~ Warsaw limestone Barnell formation Keokuk I Q. => limestone 0 er Burlington C) I I I I I w Burlington limestone C) limestone <( unknown __ll (f) 0 Fern Glen limestone z <( Chouteau a: g: forma lion Chouteau Chappel limestone w_ 3: (f) (•Fern Glen) Q. limestone 0 (f) => 0 J ~f-------~----~ er I I I I I i 5 C) Hannibal Bushberg Hannibal unknown ? >.: sandstone / formation sha le 0 I I I I I 0 :c er Glen Pork w formot ion 0 z >.: Ives brecclo Louisiana ond limestone :;over ton sno le associated unnamed rocks >~~ Grossy Creek shale Wo ­ 0 ::i: shale JCK Fig. 2. Provisional correlation of the Mississippian in central Texas. (See also pp. 27, 316--317.) S.E. N. CENTER SW. I I I I I I I Johnson City Riley Mounlalns SE. port of Type Secllon of Mouth of Joe Oovls Said Ridge Sear Spri ng area Cherokee areo Choppel llmeslone Hollow orea area I Cl ~0011. 42011. loOha to Oho Cb Cc Cl Os D EXPLANATION Mlululppicn, Bar nell rormalion Mlnissippian, Chappel ll me"St One Mluluippion, Ives br ece!o Oevon1on,Slttblinq formotJon 01von1ari, Bear Spring and Zuch formation~ Oho Ordov1c1on,Honeycur lormollon, Zone o Archaeoscyi)l'll O and Cera topeo copuhf orm •'- Oqo Ordovic lori,Gormon formc11on , Zone cl Archoeo1cyph10 .J I I I 0 40• >-3 '5 "' I ;Q73 eo ~ ~ ~ Cb ;:i 0­ :;: ~ .... "' ~ .... 0 ~ 0 Ogo Ogo L200 ­ CJ [ ;:i "' Chappel limu :one ordinarily Chappel limestone commonly >-3 qroy to oll"t·9roy 1 foud1 rare dull 1011,contoina mccrofauna ~ and small . · of Chout1ou 09•· ~ Underlain by Honeycul formation Underlain by Gorman formotio11 -+­ ttorlzontol Scale I 18 milu I. VerHcol Sc.a le Ootum I$ biCJH of Barnett formiCJ tl on. I eor... I Fig. 3. Schematic representation of the Mississippian in central Texas. ~ (Triangles along line of contact indicate solution structures and collapse.) the Llano region and are much more persistent laterally than beds of known Devonian age. Occasional glauconite globules occur in various parts of the Mississippian strata, but glauconite of this age has been seen to be abundant only locally at the base of the Barnett for­mation. The partial analysis here presented of the named Mississippian rocks was a wholly incidental by-product of a study primarily concerned with Lower Ordo­vician rocks and is not to be considered definitive; but it should serve to empha­size some of the problems and add to the groundwork from which future studies may proceed. Ives breccia.-F. B. Plummer (in Bul­lard and Plummer, 1939, p. 15) describes this unit as "the basal conglomerate of the Chappel formation, named the Ives conglomerate." The locality cited by Plummer is in southeastern San Saba County on the south side of the county road from Chappel to Cherokee, 3.2 miles by speedometer south-southwest of the rock crossing of Cherokee Creek. The character and stratigraphic position of this thin but widely distributed coarsely elastic unit is clearly displayed at this place, and there is no question about the proper application of the name. At the locality cited, the Ives breccia overlaps beds of the Honeycut formation in the zone of Ceratopea 4 and 5. A type local­ity has not yet been designated in the published record. The Ives is here recognized as a unit of formational rank, distinct from and older than the Chappel limestone, to be designated the Ives breccia because of the commonly angular character of the chert pebbles and granules which it contains. Although more sporadic in occurrence than the Chappel limestone and the Bar­nett formation which overlie it, the Ives breccia is to a large extent coextensive with them in the northern part of the Llano region. It occurs widely though discontinuously throughout the eastern and northern parts of the Llano uplift, and outcrops are common as far west as western San Saba County. Few outcrops are known from the areas mapped in the southwestern part of the uplift. Ranging in thickness from 18 inches to a feather-edge, the Ives breccia is a persistent, though intermittent, detrital zone typically comprised of relatively fresh and unabraded pebble-to granule­sized phenoclasls of chert, far exceeding in volume the siliceous, calcareous, or phosphatic matrix. Most of the phenoclasts are either freshly broken and highly angular, or they consist of unabraded chert concre­tions or nodules which are naturally ovoid in form. Although the angular pheno­clasts within the hreccia are fresh clear to their margins, the concretionary peb­bl~s commonly preserve an anciently de­ veloped patina. The roundness of the Ives phenoclasts at some localities appears to be wholly a matter of the faithful preservation of chert nodules or concre­tions in their primary form, for evidence of rounding due to abrasion has not been seen, and erosive activity appears to have been limited to the chemical weathering that freed the chert from the carbonate matrix of the underlying rocks and suffi­cient movement to shatter quantities of it. The accumulation of chert in the Ives brcccia resembles that seen on some of the more deeply weathered gentle slopes in the areas of Ellenburger outcrop at the present day, except that it is probably thicker than any of the modern accumu­lations resulting from normal weathering processes unaided by transportation. Sta­tistical studies of sizes of phenoclasts have not been made; but as a rough generaliza­tion, it may be said that they seldom exceed a diameter of 12 cm., that a diam­eter of 8 cm. is large, and that the average range in size is probably from a diameter of about 8 cm. to one of only a few mi.llimeters. Although ordinarily inconspicuous, the matrix of the Ives breccia varies consideT­ably in relative volume as well as in lithic character. At many places the phenoclasts are in contact, and the matrix is interstitial; but at some places the matrix increases in volume to form pock­ets within the breccia or even to exceed the volume of the phenoclasts. Locally, The Ellenburger Group of Central Texas the breccia seems to grade laterally into or to overlie (pp. 315-316) or underlie a reddish brown, olive gray, brownish, or yellowish impure limestone or phosphatic rock containing conodonls, fragments of bones, phosphatic pellets, and sand grains. Apparently more than one limestone or "bone bed" is involved, but relationships are obscure. The matrix of the Ives bre­cia has been found, at widely spaced lo­calities and in varying lithic character, to contain conodonts similar to those stated by Branson and Mehl (l933a) to be characteristic of the Grassy Creek shale of Missouri. Where it is ~alcareous or calcareo-phosphatic the conodonts may be freed by acetic acid, but where it is highly phosphatic, or where it is siliceous, as it commonly is where it forms a volumet­rically very small portion of the breccia, the conodonls must be mechanically oh· tained by crushing the rock. That the Ives brcccia was locally de­rived from immediately suhjacent rocks is indicated by the fact that the chert phcnoclasts in it are similar to the chert of the underlying rocks, as well as by their fresh and unabraf!ed natnre. More­over, at some places (for example about 200 feet east of the Chappel to Cherokee road from a point 1.3 miles by speedom­eter south of the rock-crossing of Cherokee Creek) specimens of Ceratopea and frag­ments of chert containing other Ellen­burger fossils similar to those of the underlying chert occur as pebbles in the Ives breccia. Although a geologically short period of aggregation of the pheno­elasts which comprise this breccia is sug­gested by their poorly sorted and una­braded character and their local deriva­tion, a relatively long period of time would be suggested by the abundance of conodonts in the matrix of the hreccia at some places, if it could be assumed that they were not reworked. The characters displayed by the Ives breccia suggest a fairly rapid marine in­vasion of a surface of little relief in an area underlain by chert-bearing rocks sub­ject to chemical weathering, following a long period of exposure to the atmosphere. The invasion was marine because cono­donts are widespread in the Ives. It must have been fairly rapid, because the pheno· clasts of the Ives breccia show little or no evidence of wear. The surface must have been one of little relief, because the hreccia is uniformly thin and does not display thick lenses such as would have accumulated in valleys or other depres­sions; moreover, it is inconceivable that so rapid a marine invasion as indicated could have spread the breccia so widely anrl so uniformly over any but a nearly flat surface already fairly well blanketed with chert. That the carbonate rocks on which the Ives breccia rests were chert­bearing and subject to chemical weath­ering is self-evident, and a long period of subaerial exposure would be required for chemical weathering to free such an accu­mulation of chert as is represented by the Ives breccia and reduce the land surface to one of such Iiule relief that the chert formed a fairly uniform blanket. Either the numerous conodonts that occur in the interstices of the Ives breccia at some places were derived from an older off­shore accumulation and elastically intro­duced during the postulated marine inva­sion, or else the ancient bottom of the Ives sea was quickly put below effective wave-base and failerl to receive significant quantities of sediment other than cono­donts for a long time. If the former alternative is the case, it may explain the uneven distribution of the conodonts, as well as their seemingly conflicting evidence at such localities as l6T--2~27A, in Blanco County (pp. 315-316). The apparent scarcity of other than locally derived elastic materials in or associated with the Ives breccia suggests either that invasion by the Ives sea was so sudden and so extensive that contaminant elastic sediments from adjacent invaded areas were not effectively transported, or that they were totally by-passed, or that a near-by source of other elastic materials was lacking. If central Texas was not a low-lying island immediately prior to Ives time, it was probably part of a larger hinterland of uniformly gentle topography. At locality 16T-2-27A1 , in Blanco County, a lower Missis~ippian macro­fauna, possibly correlative with and cer­tainly no older than that of the Glen Park formation, was found in reddish brown, bone-bearing limestone that ap­pears to underlie the Ives breccia (pp. 315­316). The same rock yielded a mixed conodont assemblage containing both Gragsy Creek and post-Grassy Creek types, according to stratigraphic ranges given by Branson aml Mehl (in Shimer and Shrock, 1944); and, on Lhe basis of criteria for the interpretation of mixf'd conodonL as­sembfages (Branson and Mehl, ] 933, pp. 265-267; ] 94.l), it would be interpreted as younger than Grassy Crcrk. Although the Ives hreccia apparently overlies this rock at locality l 6T-2 27A, at other places the breccia seems to grade laterally to a similar rock, and it is nor yet clear how many chronologic units are involved. Moreover, correlation is complicated by the fact that the conodonts of the Ives brcccia at most places would he considered a Grassy Creek assemblage (W. H. Hass, oral and written communications). Jn this connection it is of interest that Branson and Mehl state (1933a, p. 174) that "at the type locality of the Glen Park it forms limestone lenses in the Grassv Creek," and as recently as 19.38, E. If Branson shows this same interpretation diagrammatically (1938, p. 8, fig. 2) and states further that "the Glen Park is referred tQ the Devonian hv the writer (Branson) on the basis of -relationships and fauna." But the Glen Park formation (= Ham­burg oolite, preoccupied) is generally con­sidered lo be Mississippian, even by stra­tigraphers who question the age of the Grassv Creek shale and the Louisiana limestone; and James S. Williams (194,3) contends that the Louisiana limestone is of Mississippian age. The occurrence of Paryphorhynchus at the base, of Proto· canites in it, and of "Spirifer" marionen­sis throughout the Louisiana limestone (Williams, 194,3) supports Williams' con­tention, and E. B. Branson himself states (l 938, p. 127) that Paryphorhynchus "has been found only in the Lower Mississip­pian." In addition to these forms, Wil­liams (1943, p. 36) has listed 14 genera which he considers to indicate a post. Devonian age. Attention should be called to the fact that C. L. Cooper (1935), in contradic­tion to Branson and Mehl, regards the conodont assemblage of rocks generally correlated with the Grassy Creek shale (Woodford chert, etc.) as Mississippian in age. Inasmuch as the Grassy Creek conodont assemblage of Branson and Mehl, which generally characterizes the Ives breccia, is said by them to represent not only the Grassy Creek shale of the standard col­umn, but also Lhe Louisiana limestone (Branson and Mehl, 1938, p. 175) and presumably the Glen Park formation, a closer correlation of the Tves breccia than is indicated in figure 2 cannot at present be made on the basis of conodonts. A tie with lower Mississippian strata of Glen Park or possibly younger age is indicated by stratigraphic evidence at locality 16T­2-27A1 , in Blanco County (pp. 315-316), as well as at localiLy TF-395 in the Lost Creek area (p. 347). The Ives breccia in the Llano re~ion is here rcfern~d to the ba~e of the Mis­sissippian, although at a few localities (pp. 316 and 348) still older Mississippian strata may be present in the formation. AlLhough the evidence of the conodonts on this problem is equivocal, the correla­tion here proposed is supported by strati­graphic evidence indicating superjacenl relationships lQ and possibly partial equivalence with rocks containing lower Mississippian rm1crofossils at locality 16T­2-27A,, and locality TF-273, as noted above. It ic> further suggested by physical evidence as follows: (l) Thr Ives breccia is much more widespread than the known Devonian rocks of the area and nearly coextensive with the known Mississippian rocks in the northern part of the Llano region. In the southeastern part of the Llano region it widely overlaps the Devo­nian rocks, hut is in contact with rocks of Mi~sissippian or Pennsylvanian age above, and in the more wPsterly parts of the region it is largely absent. (2) Although it is itself disconlinnous, the Ives hreccia intervenes between Devonian strata and posl-Ivcs rocks at all localities where they are known to be in normal sequence, whereas both hes and younger Missis­sippian berls overlap directly and exten­sively onto Ellenlmrger rocks. A long period of suhacrial erosion and weather­ing, therefore, necessarily intervened be­ The Ellenburger Group of Central Texas tween the deposition of known Devonian rocks and the Ives breccia, as contrasted to a much shorter period of erosion, or possibly merely an interruption in or change in conditions of sedimentation, between the thin Ives breccia and the overlying Chappel limestone. (3) Re­ worked elastic fragments from the Ives, which should be very resistant and per­sistent, are absent or rare in the Chap· pel limestone, further indicating a rela­tively brief interruption in sedimentation between these two units; whereas a com­paratively long pedod of weathering was probably required to free such an accumu­lation of chert as is represented by the Ives breccia from the rocks that underlie it. (4) Faunal evidence shows that the Ives breccia is widely separated in time from the youngest beds of known Devo· nian age in the Llano region (figs. 1 and 2); and, if it is Devonian rather than basal Mississippian, it is neither a basal nor a regressive breccia, occupying a posi­tion of stratigraphic isolation unique among coarsely elastic deposits. On the other hand, it forms an ideal basal breccia for the Mississippian sequence, only very locally displaying anomalous subjacent Mississippian limestones. Chappel limestone.-OriginaIIy named the "Chappel formation" by E. H. Sel­lards (1932, p. 91) this widespread unit has been found to be persistently calcitic and can therefore appropriately bear the more descriptive appellation of Chappel limestone. Sellards applied the name to the so-called "Boone-age" limestone de­scribed by Roundy, Girty, and Goldman (1926) and stated that the type locality is "three miles southeast of San Saba" (Sellards, 1932, p. 92), a fairly evident reference to the earlier report by Roundy, Girty, and Goldman wherein Roundy de­scribes the "Boone-age" limestone as "a gray, medium-hard, somewhat crinoidal limestone exposed, so far as I am aware, only along the road to Chappel about 3 miles southeast of the courthouse at San Saba." Of the 41 species and varieties of fossils noted by Girty and Roundy as occurring in "limestone of Boone age" 38 are recorded only from their station 2623 which is described by Roundy (in Roundy, Girty, and Goldman, 1926, p. 17, ref. 2613D-2613H) as being "On the south­west side of road to Chappel, about 3 miles southeast of San Saha courthouse, at first sharp turn well up hill." Fortunately the geology and topography of the area is such that only one locality fits the description given. This locality is about 2.4 miles by speedometer south­east along the San Saba-Chappel road from the south side of the courthouse at San Saba, immediately east of and down­hill from the present road at a point just south of the V-bend made by the road about midway in its ascent of the fault­line scarp of the Simpson Creek fault zone. It is at the west side of the old road, which is downhill from and about 80 feet east of the present road at this point; between two branches of the Simpson Creek fault zone; and about 600 feet north-northwest of locality TF-22 on the geologic map of the Cherokee area (Pl. 2). The approximately 24 inches of Chap­pel limestone exposed at the type locality is a crinoidal limestone wherein abundant to scattered larger pelmatozoan fragments are tightly bonded or cemented by a matrix of fine grained limestone. The upper 10 to 11 inches is a tough, medium to rather dark gray rock with a brownish, olive, or bluish cast, fairly typical of much of the Chappel limestone on the east side of the Llano uplift. The lower 13 lo 14 inches, however, have been leached to a marly appearance and a lighter yellowish gray color, presumably by movement of ground water along the Chappel-Honeycut contact. Residues from its dissolution in acetic acid yield a few conodonts, occasionally globules of glau­conite, and rare grains of sand. At the type locality the Chappel is directly over­lain by about 50 feet of Ii.ales of the Barnett formation, and a few hundred feet to the south a lens of the Ives breccia up to 1 foot thick intervenes between the Chappel limestone and the Honeycut for· mation of the Ellenburger group. The top of the Honeycut formation at this place is in the zone of Ceratopea capuliformis, and the principal zone of Archaeoscyphia in the Cherokee area. In the eastern half of the Llano uplift the Chappel limestone is generally similar 50 The University of Texas Publication No. 4621 to that exposed in the type section, being gray to olive gray or brownish gray, tough, crinoidal limestone with fossils rare and mostly very small. It ranges in thick­ness from a feather-edge to a maximum observed thickness of possibly 14 feet in the southeastern part of the Cherokee area (TF-427), but it seldom exceeds 3 to 5 feet, and the suggested maximum of 14 feet is based on a quite unsatisfactory measurement. The macrofauna described by Girty (in Roundy, Girty, and Gold­man) was, at the time of its description "wholly unknown and ... in many re­spects peculiar," and "the species ... small, almost minute," in fact suggesting dwarfed or infantile specimens. It was provisionally considered to be of "Boone" (roughly Osage) age. In the western half of the Llano region limestones occupying the stratigraphic po­sition of the Chappel are texturally simi­lar to the Chappel limestone of the type section; but their color, in addition to and locally to the near exclusion of the gray and olive gray tones, is commonly rose or dull pink to rose brown and pink­ish orange. Although gray to olive gray beds may predominate locally in the west, few sections fail to display some he 0 a:: "' a: ... .... "' "' "'<.) Foyttttvilla sholt Batesvlll e sond1tone Moortfitld "Shale t--·---­ t I I I I I I I ---1-­Lower Caney sh.ale I __.l_ -t I I I I . I I--1­Barnell shale i IL :::> 0 a: "' a: ... .... "'... "' <.) Born•ll formalian - ...:; 0 N z ;.. a E 0: a " ~ -.. 0.. ~ ..-a -: ~ ~ -.. ,, "' <11'-­-;,; ~ .. ~ ~a: 0 H "' .. IL a IL ~ ­:> "' e!: 0 0 M ~ ! z c Borne.If 1hate "' :> 0 a:.., ...-z 0 "'0:: ""<.) 0:: "'IL IL :> "' :> 0 0:: "'... -z 0 "" 0:: ... <> 0: "' 3' 0 _, z.. -a: ::> :t.. z a: ... 1t 0 _, z.. ... "' ; a: ... IL IL ::> z... ... "';; ... -' Q 0 i . c a 0 . "a ~ a. a e " "' . c 0 N -.. 0 g ~ " .. " " ~ " .. m -a:.. .... "'~---­-.. :c <.) Uppu Barnett z.. ... Lower "' ; Borne ti . a: 0 .. .. ....,. " : L-.----­.. ,. CHESTER GROUP Foyettevilla shale Botesvllle.. 1ond11on1 ~ ! ~ ~ ci~~ Ste. Ruddell ~~ Genevieve shale-a. limestone ! '---:> 0 .. 0 ~ :E ~ a: SI. Moorefield ~ ~~.: "' Louis formalion (!) ~ ~ ~ li mestoM u "': Sper9en Speroen ond ~~ upper Worsaw Wars a• (not di1cu11ec limestone 0. 8 Keokuk Boo"'~limH.,M .... ~ chert no reference-Barnett 1holt no rehn nce CHESTER GROUP unknown . Ste. I ' '\Genevieve \ o. limestone I \ ,,,_ I \ 0 I \a: St• "' Louis l ?, ~ limes1one I \ ,. .....__.. I \ ~ Speroen / \ \ ~ limestone ii Wonow Ruddtll limestone Barnt tr shale 0. ood :::> Keokuk formation M0«0fl1ld0 ~ lim11tone formation "' '"-3 ~ ~ e· 'Cb ;:i.,... ~ ~ '"-3 ~ H fl "ti;:: O"' """'[. ;s· ~ ~ ~ ...... ~ Burfinqtori: ~ limestone ~ B4Jrlin9ton ~ lirnKlone unknown Fig. 4. Graphic history of the correlation of the Barnett formation, 1926 to 1946. The Ellenburger Group of Central Texas tites choctawensis, G. kentuckiensis, G. cumminsi, G. crenistria, and G. striatus, and what, if any, is the stratigraphic significance of possible specific distinctions in this closely similar group of shells. Plummer and Scott, as well as Miller and Furnish, state that the types of Goniatites choctawensis Shumard have been lost, neither has designated neotypes, and Shu­mard figured no specimens. The original figures of G. kentuckiensis S. A. Miller (1889, pp. 439-440, fig. 740, 2 views) are of an internal mold which does not show the important features of external ornamentation; and S. A. Miller does not discuss the external ornamentation. Of the other species discussed G. crenistria and G. striatus are European shells; but G. cumminsi is a Texas species whose types are extant. Through the courtesy of Dr. F. L. Whit­ney, Hyatt's types of goniatites at The University of Texas were made available to Cloud for study (11/24/45). He found, as had been suggested by Miller and Fur­nish (1940, pp. 362-363), that the best specimen of Glyphioceras cumminsi Hyatt in the original type lot (Hyatt, 1893, Pl. 47, figs. 36, 37) had been refigured by Plummer and Scott (1937, Pl. 7, fig. 10) as a cotype of Glyphioceras incisum Hyatt. It is suggested that this specimen be desig­nated the holotype of Goniatites cumminsi (Hyatt), and that that name be retained un­less and until the type specimens or ade­quate illustrations of the types of any sup­posed senior synonym can be produced and demonstrated by comparison to be the same species, whether it be G. choctawen­sis or G. kentnckiensis or both. In the present report the name Goniatites cum­minsi (Hyatt) refers to specimens that are slightly compressed laterally and re­semble the holotype in having Goniatites­type sutures in which the adult bifid middle lobe is narrow and exceeds half the height of the bounding lateral lobes (Pl. 44, figs. 39-40). The ornamentation consists of prominent revolving lirae and fainter transverse threads (growth lines) resulting in a cancellate pattern. This can­cellation is ordinarily clear enough to the hand lens, but, where the growth lines are subdued, it is commonly not readily evident to the unaided eye. Although Gordon (1944) recognizes Goniatites of the type of G. crenistria (his G. choctawensis = G. cumminsi of the present report) as indicative of a zone distinct from and stratigraphically lower than Goniatites of the type of G. stria'tus (his C. kentuckiensis), Miller and Fur­nish (1940, p. 362) state that "in North­ern England, G. striatus occurs with simi­lar forms ... generally regarded as specifically distinct and ... grouped with G. crenistria." Goniatites, in the strict sense, is said by Miller and Furnish (1940, p. 358) to be "represented in only the Visean," and they further restrict (p. 357) this Goriia­tites zone to the upper Visean, which they correlate with the upper Meramec. The same authors have written (Miller and Furnish, 1940, p. 358) that an upper ammonoid fauna "of Chester aae, which is characterized by Enmorphoce;""as, corre­sponds to the lower N amurian of western Europe." However, Plummer and Scott ( 1937, p. 15) record Goniatites in their Eumorphoceras bisculatum zone, and this association is confirmed by the work of the present authors (TF-422, TF-429, 205T-24). Shimer and Shrock (1944, p. 571) also have recorded "Goniatites cren· istria Phillips" [presumably the same as G. choctawensis Girty of Gordon and Goniatites cnmminsi (Hyatt) of the present report] from the Fayetteville shale of Ar­kansas, a formation generally considered to be Chester in age. Girty, himself, reported Enmorphoceras bisnlcatum from about 30 feet below the top of his Moorefield shale at Howard's Wells in Independence County, Arkansas (Girty,. 1911, pp. 141, 19-20). This would be about 30 feet below the top of the Ruddell shale of Gordon (1944) and there is no reasonable possibility of contamination at the locality mentioned. Gordon (1944) correlates the Ruddell shale with the Ste. Genevieve limestone, and to the best knowledge of the authors no one has yet correlated either the Moore­field formation or the Ruddell shale with beds of Chester age. Additional complication is suggested by Dr. G. A. Cooper's discovery of an in­complete specimen strongly resembling the typically Pennsylvanian genus Goniolobo­ The University of Texas Publication No. 4621 ceras3a in association with Eumorphoceras and with Goniatites and other fossils indi­cating equivalence to the Moorefield forma­tion and Ruddell shale at locality TF-422 in the Bear Spring area. To confine dis­cussion to unequivocally identified genera, it is seen that Eumorphoceras, supposed by some to indicate a Namurian age, and Goniatites, supposed to indicate a Visean age, have been found in the same bed and even in the same concretion by the writers. The dilemma is obvious. It appears that convincing correlation be­ -tween central Texas and European gonia­tite zones must await further study in both places, and that any tenable correla­tion of the Barnett formation with the Missis;ippian of the middle Mississippi Valley must take other evidence into consideration. It is, therefore, fortunate for the corre­lation of the Barnett formation and equiva­lent strata that the fauna of the dark shale and petroliferous limestone facies has been found in direct association with limesands containing typical Keokuk hrachiopods, thus establishing at least the lower limit of strata equivalent to the Barnett formation. This happens in the Bear Spring area of Mason County, in the southwestern part of the Llano region. In this area the Bar­nett formation is in large part represented by white to light gray, coarse to medium grained, crinoidal limesands that in parl go laterally and vertically to fine grained and sublithographic limestones and are separated by covered intervals or inter­vals of caliche that probably represent shale zones. In the Bear Spring area the relative proportion of the outcropping de­trital limestones varies from 16 to 90 per cent of the total Barnett strata by actual measurement. Where the covered intervals or those of caliche, and therefore prob­ ably the shale, account for a large propor­tion of the section, the intcrbedded lime­stones are commonly petroliferous and darker than usual, and some are brown and conspicuously pelleted, like those that occur as concretions in the shales of the east and north. Moreover, like the con­cretionary eastern limestones, they yield ••Dr. A. K. Millet and Mr. Walte1 Youngquist have in press a paper on the Barnett gonia lites in which ihcr assign this specimen to Girtyoceras mcslerianum (Gii ty). a normal Barnett fauna with obvious re­semblance to that of the Moorefield strata of Arkansas and Oklahoma. These are important considerations, for it has been known at least since C. L. Dake's work in the area in 1931 that a fauna of Osage age occurs in white and pink-stained limesands at White's Crossing of Llano River. Although it has been supposed by some that these limesands belong to the Chappel limestone, the fact is that they rest unconformably on and overlap the dark rose to gray or olive gray, much less crinoidal, compact lime­stones of Chouteau age that comprise the Chappel formation; and at one locality in the west end of the panhandle of the Pat Rogers ranch a conglomerate bed was observed between the two. From the base of the limesands at White's Crossing, Dr. Edwin Kirk identified, in addition to the uhiquitous Platycrinites, species of Or­bitremites and Metablastus, which he states (oral communication, March 18, 1946) are intermediate in aspect between upper Burlington and Iow er Keokuk forms. In addition to these, fossils such as Spirifer logani Hall, S. cf. S. washingtonensis Weller, Brachythyris cf. B. suborbicularis (Hall), Orthotetes keokuk (Hall), Dictyo­clostus cf. D. craw Jordsvillensis (Well er) , Echinoconchus cf. E. altematus (Norwood and Pratten), and a large species of Lep­taena, found in the lower 20 feet of the white limesands at White's Crossing and at TF-1'18 indicate a Keokuk age for the beds in which they occur. Many other fossils occur at these localities in addition to those named; and some, such as Ortho­teles keokuk (Hall), Brachythyris cf. B. suborbicularis (Hall), and Echino­conchus cf. E. biseriatus (Hall), range upward into higher beds at locality TF­ 418. At White's Crossing the common Bar­nett and Moorefield hrachiopod Leior­/iynchus carboniferum Girty was found in association with the Keokuk fossils noted above. At locality TF--418 the genus Goniatites occurs with a normal Keokuk assemblage at 20 feet above the base of the limesand facies of the Barnett forma­tion, and a well-preserved Goniatites oc­ curs 92 feet above the base and 48 feet below the top of the same section. In The Ellenburger Group of Central Texas addition to the evidence of Goniatites, the equivalence of the limesand section at TF-4,18 with that containing a normal Barnett fauna at locality TF-122 is shown by the occurrence in both of identical species of Pleurodictyum, Avicztlo pecten, a small "Spirifer" resembling S. bifur­calus Hall, and a brachiopod with strong resemblance to Diaphragmus fasciculatzts (McChesney). Of 10 specimens of the sup· posed Diaphragmus that were split longi· tudinally, only one showed a' dorsal valve and its characters were not clear. How­ever, one exfoliated dorsal valve is carinate at the point of geniculation, and suggests the presence of a diaphragm. This prob­able Diaphragnms occurs with the upper Goniatites at TF--418, and again 24 feet below it and 42 feet below the highest occurrence of Orthotetes lceokuk (Hall) at the same locality. At TF-422 it occurs in petrolifcrous limestone interbedded with limesands and covered intervals suggest­ing shale. With it at the latter locality, in addition to normal Barnett fossils such as Leiorhynchus carboniferum Girty, small fanoproductus, Aviculopecten :;.;p., and many Goniatites, are Archimedes sp., Echinoconchus cf. E. genevievensis Weller, "Spirifer" alf. S. bifurcalus Hall, Dictyo­clostus cf. D. inflatus (McChesney), D. cf. D. burlingtonensis (Hall), Eiimorphoceras cf. E. bisculatum Girty, and Goniolobo­ceras sp. Anyone familiar with the nor­mally cited ranges of the fossils named will appreciate the conflicting testimony given by them, if interpreted in terms of these ranges. The fact remains, however, that it has not been possible consistently to subdivide either the shale or the lirnesand facies of the Barnett formation on the basis of any evidence applied. In fact, at TF--420, in a zone of dark petroliferous limestone and shale at the middle of a 130-foot sequence of mostly very similar crinoidal lirnesands, the authors and G. A. Cooper found the following fossils in conflicting association: Pleurodictyum sp., Orthotetes aff. 0. keokuk (Hall), Brachythyris cf. B. suborbicularis, small Lino productus, Productella hirsutif onnis Walcott, Moore· fieldella cf. M. eurekensis (Walcott), Diaphragmus (?), and other fossils. Spirifer logani was not found at this lo­cality. although it and other Keokuk types of fossils were found in association with Leiorhynchus carboniferum Girty at White's Crossing (TF-184,). The conclusion that the while limesands of the Bear Spring area, including those that contain a Keokuk fauna at White's Crossing and at TF-418, are simply a facies of the Barnett formation, was reached from three lines of evidence. It was first suspected from mapping. It was later independently confirmed by W. H. Hass (letter to Cloud, October 31, 1945), who found that samples from the original Keokuk locality at White's Crossing con· tained typical Barnett conodonts, and that Barnett conodonts occurred from within at least 20 feet of the bottom to 10 feet of the top of the section at TF-418. And finally, in February 1945, G. A. qooper and the authors cracked the white lime­stones and limesands until enough macro­fossils were obtained, in addition to those previously collected, to establish that the normal Barnett fauna and that of the coarse white limesand facies are associated at TF-418 and TF-4.20. Relationships very similar to those de­scribed for the Barnett formation in the present report were found by I-I. D. Miser between the Ridgetop shale and beds of crinoidal limestone in the Waynesboro quadrangle in western Tennessee over 30 years ago and were described by G. H. Girty in 1915. Girty pointed out that "the fauna of the Ridgetop shale ... strongly and obviously resembles the fauna of the Moorefield shale," whereas the crinoidal limestones that were considered to be laterally equivalent to the Ridgetop shale by Miser contain a fauna that "in many respects presents a typical -Burlington facies." The fauna of the limcsand facies of the Barnett formation at White's Cross­ing certainly has some Burlington aspects, although they appear to be outweighed by Keokuk affinities, and the facies changes displayed by the Barnett formation sug­gest those shown by beds of Ridgetop age.* The salient faunal elements of the shale facies of the Barnett formation-Leiorhyn­chns carhoniferum Girty, rare specimens of Moorefieldella, a small Linoproductus, *See also Wihon, C. W., Jr., and Spain, W. L., Jr., Age of Mississippian "Ridgetop shale" of central Tennessee: Bull. Amer. Assoc. Petr. Geol., vol. 20, pp. 805-809, 1936. species of Caneyella, Goniatites,4 and Eumorphocetas-strongly indicate corre­lation with the Moorefield formation and Ruddell shale of Arkansas and strata of Moorefield age in Cherokee and Muskogee counties, Oklahoma. Wherever the Bar­nett formation fits into the standard sec· tion a large proportion of the Moorefield and Ruddell strata, the lower Caney shale, the Ridgetop shale, and other correlative rocks must also fit. The reverse, of course, is also true; but it may be noted that correlation of these other rocks with the formations of the middle Mississippi Val­ley has been based large]y on elements (goniatites) which, to judge from the lack of published references, are extremely rare in those formations, and the cited ranges of which have been here shown to be open to question. In fact, although the most recent study provisionally reiterates the view that the Moorefield fauna is approxi­mately St. Louis in age, its author states (Gordon, 1944, p. 1631) that "direct evi­dence to support this correlation is still lacking." Although Gordon (1944, p. 1629) states that the uppermost beds of the Boone for­mation, on which the Moorefield formation rests, are lower Warsaw in age, Girty ( 1915, p. 15) wrote of the same beds that he found "nothing in the Boone indicating an age younger than the Keokuk." Indeed Girty states (1915, p. 17) that "the pecu­liar Spring Creek fauna (equals Moore­field fauna of Gordon) need not neces­sarily be placed above the Keokuk in the time scale." The present authors submit that the evidence obtained at and in the vicinity of White's Crossing in the Bear Spring area of central Texas is cogent reason for believing that the base of the Barnett formation correlates with strata at least as old as and probably not older than the Keokuk limestone. The upper age limit of the beds in question is an open subject, and one that cannot satisfactorily be settled without a more intensive study of the faunas of the Barnett formation and its possible correlatives than has yet been made. 40stracods and conodonts occur in both the limestones and the shales of the Barnett formation and should be helpful in subsurface studies. A spedes of a compound coial resembling Pleurodictyurn is common in the Barnett in Mason County. Without doubt the possible Goniolobo­ceras can be discounted as a dating ele­ment, but there are other fossils that favor partial correlation of the Barnett forma­tion with post-Keokuk strata. Echino­conchus cf. E. biseriatus (Hall) and "Spir­ifer" aff. S. bifurcatus Hall suggest War­saw and Spergen equivalence; Archimedes is uncommon below the Warsaw limestone; Echinoconchus cf. E. genevievensis Weller suggests Ste. Genevieve equivalence; and Diaphragmus (?), suggests a Ste. Gene­vieve or Chester age. Eumorphoceras is also considered by most students4~ of goniatites to indicate a Chester (lower Namurian) age and the Dictyoclostus here compared with D. infiatus (Mc­Chesney) might be considered by some to indicate a Chester age. However, of the elements noted, Echinoconchus biseri­atus occurs in strata of Moorefield age in Cherokee and Muskogee counties, Okla­homa, and is recorded from the Keokuk by Moore (1928, p. 215); the "Spirifer" compared with S. bifurcatus is clearly a new species and cannot he considered an important dating element; it also occurs in shale of Moorefield age in northeastern Oklahoma; Archimedes has been recorded from the Keokuk limestone (Moore, 1928, pp. 208, 209) ; Echinoconchus genevieven­sis may be but a variety of the Keokuk fossil E. altcrnatus (Norwood and Prat­ten) and is too similar to he critical; Eumorphoceras is of doubtful correlative value; and a variety of Dictyoclostus in­fiatus similar to the one here noted occurs in the Moorefield (Girty, 1911, p. 42, Pl. 4). Diaphragmus (?) cf. D.? fasci­culatus (McChesney) alone remains, but a fossil that was enough like Diaphrag­mus to be identified by L. R. Laudon as Diaphragmus elegans is reported by him (Laudon, 1939, p. 329) to be common in the Keokuk equivalent of northeastern Oklahoma in association with Orthotetes keokuk and Spirifer logani. The specimen of Diaphragmus, figured by Girty (1911, Pl. 4, fig. 1{,) as coming from tbe Moore­field strata at Spring Creek, in the Bates­ville District of Arkansas, and said by hA. K. Miller (Bull. Geo}. Soc. Amer., vol. 159, pp. 117, 113, 1948) now indicates Eumurpltoceras to he restricted to the lower Meramccian. The p1esent authors would extend iLs range-line downward into the Keokuk and leave it as probable (Bisat, 1936) that it also ranges upward into rocks of .Chester age (Namurian, in part). The Ellenburger Group of Central Texas Gordon (1944) to be. a contaminant ele· ment, is very like the Diaphragmus (?) that occurs in the Barnett formation. This specimen is from a collection made by H. S. Williams in 1890 (Girty, 1911, pp. 12-14.) at a locality over 2 miles north­east of and across White River from the nearest Fayetteville outcrops (see map by Gordon and Kinney, 1944). Despite its lateral gradations the Bar­nett formation, in the full normal develop­ment of either the shale facies or the lime­sand facies, is compact and seemingly in­divisible. The lower limit of its age is virtually fixed as Keokuk, but the .upper limit might be as high as Ste. Genevieve. The present authors consider it extremely unlikely that any part of the Barnett for­mation is as young as Chester in age, and it is their opinion that it is actually entirely pre-St. Louis if not pre-Sper­gen. The greater number of species from the middle Mississippi Valley region that compare closely with Barnett species· are either restricted to the Keokuk limestone or are known to occur in it. It is possible, therefore, that the Barnett formation and its correlatives, although provisionally re· £erred to Keokuk plus Warsaw, are wholly of Keokuk age. PENNSYLVANIAN In the Llano uplift, rocks of Pennsyl­ vanian age do not ordinarily rest directly on strata of the Ellenburger group. For this reason it is not necessary in connection with the Ellenburger problem to consider the Pennsylvanian rocks in the detail de­ manded by the more commonly overlap­ ping Mississippian strata. Various strati· graphic units of Pennsylvanian age have been mapped under a single symbol on some of the geologic maps accompanying the present report, and only in the south· eastern part of the uplift (Johnson City area) have units been differentiated in the "Marble Falls limestone." Brief discussion of the Marble Falls limestone and of the structural informa­ tion furnished by the overlapping strata of Canyon age is pertinent to tbe Ellen· burger study. Moore and others (1944, p. 697) state that "studies by Plummer indicate that the so-called Marble Falls limestone really consists of two distinct formations, the lower of which is Morrowan m age and the upper post-Morrowan," and they re­ strict the name Marble Falls to the strata of Morrow age. Now whether or not both "formations" are present in the vicinity 0£ the town of Marble Falls (Thompson, 1944; pp. 417, 423; Plummer, 194.5, p. 66; Spivey and Roberts, 1946, p. 183; Thomp­son, 1947, p. 149), the waterfall there is over the highest post-Morrow beds in the type section, and post-Morrow beds are much more widely distributed in central Texas than are beds of Morrow age, occu­pying the larger part of the areas shown as "Marble Falls" on present maps. It is to be regretted, therefore, that the Pennsyl­vanian subcommittee chose as it did. A final decision on this important matter should consider the type section, general usage, and areal distribution; preferably restricting the name Marble Falls to the rocks most generally known by that name, that is, the beds of post-Morrow age, or retaining it in unrestricted usage as a group name. 'The present authors favor restricting the term Marble Falls lime· stone as a formation name to apply to post-Morrow beds below the Smithwick shale; but that is a problem demanding more extended study than they have made, and the Marble Falls limestone (unre­stricted) of the present report locally in­cludes strata that contain a Morrow fauna (pp. 229-230, 318), as well as younger Pennsylvanian beds. The beds of Morrow age, at least locally and perhaps commonly, contain limestone lenses suggesting bio­herms possibly of organic origin, and they are unconformably overlain by the younger Pennsylvanian strata. However, the two are generally similar gray to black, fine grained limestones containing black to speckled gray, nodular chert. Fusiform fusulines have not been seen in the strata of Morrow age but occur locally in both limestone and chert of the post-Morrow beds. In the Bald Ridge and Bear Spring areas, near the western edge of the Llano region, beds of Morrow age are absent, and the lowest beds of the Marble Falls limestone contain Fusiella. Globular glau· conite and phosphate pellets are commonly abundant in the basal foot of the Marble Falls limestone and have been observed locally as scattered pellets in higher strata. In the Bear Spring area, at the western edge of' the Llano region, scattered to abundant, highly colored, sub-rounded to angular, small chert pebbles occur at the base of the l\1arble Falls limestone. The Smithwick shale and beds of the Strawn formation are not known directly to overlap Ellenburger strata in the Llano uplift. However, undisturbed beds of Can­yon age have been observed to overlap faulted strata of the Tanyard formation in western McCulloch County, south of the village of Calf Creek, on the Block­house ranch. Inasmuch as the faults which cut the Ellenburger rocks also transect the Marble Falls limestone and Smith­wick shale, and probably locally the Strawn formation, but not the strata of the Canyon group or younger formations, it is evident that the major faulting in the Llano region after pre-Cambrian time was a post-Smithwick and pre-Canyon event. Evidence is not at present clear as to whether faulting occurred during Strawn time, or whether it was entirely a post­Strawn and pre-Canyon event. REGIONAL CORRELATION INTRODUCTORY REMARKS The Ellenburger group is but a part of a Lower Ordovician series of mostly ca!rbonate rocks that occur widely at and below the surface of the mid-Continent region. To understand its place in this series requires consideration of its re­gional stratigraphic and facies relation· ships. The critical sites of outcropping Lower Ordovician rocks are those which may be said to rim a broadly conceived Per· mian basin region-the Llano and Mara­thon uplifts, both of which are topo· graphic basins; the Sierra Diablo; the Hueco Mountains; the Franklin Moun­ tains; the Rocky Mountain Front; and the Wichita and Arbuckle Mountains of Okla­homa. Petroleum production from the Lower Ordovician of the Permian basin region and the structural highs that limit it is doubtless not restricted to strata of the facies revealed in the Llano uplift, and a fuller comprehension of the vari· ations displayed by these rocks in their various areas of outcrop is desirable. Reconnaissance studies of the lower Paleozoic rocks in the areas named were made, therefore; and the results of those sludies, as well as a discussion of the important Ozark sequence and brief con­sideration of other areas, is here given. The conclusions suggested are provi­sional. The time spent in the field in the various areas discussed was necessarily lim­ited. The physical aspects of correlative strata vary considerably from one region to another and cannot ordinarily be relied upon for correlation. The fauna! evidence, which is necessarily the basis of regional correlation, is not uncommonly conflicting. In some instances it is a case of one so­called index fossil against another and adjudication between such contendents involves phildiophical considerations with which some may disagree. The published faunal evidence itself is so scattered and commonly so difficult to interpret that some has probably been missed com­pletely or misinterpreted. Even where fauna! evidence is good, the limits within which unit boundaries may fall are not ordinarily as closely restricted as could be desired. It is evident that painstak­ing biostratigraphic studies in many regions will be required to solve all of our problems satisfactorily; but as a temporary measure the authors propose to state the evidence at hand, indicate contradictions, suggest reasonable infer­ences, and reserve the right to change their minds where new evidence so advises. The correlations suggested are individ­ually discussed in the ensuing pages and summarized on Plate 15. The discussions that follow are scaled to the importance, of the region consid­ered in relation to the Ellenburger prob­lem, the amount of data available, and the adequacy of the previously published record. Descriptions of three sections in Trans-Pecos Texas and central Colorado and lists of fossils from two sections in Oklahoma are given in Part 3 of this re­port, and the reader is referred to pub­lished descriptions of important sections. Because the authors understand the sequence of faunas in the Ellenburger group more clearly than in other Lower Ordovician strata, it is thought advisable to refer to fauna! equivalents in terms of the various units of the Ellenburger in The Ellenbtrger Group of Central Texas so far as practicable. Above rocks equiv­alent to the Honeycut formation it is necessary to go to the Ozark sequence of Missouri and northern Arkansas for names. Naturally, strata younger than the Honeycut formation are also younger than the Ellenburger group of the Llano uplift, and it is of special interest that the upper half of the El Paso formation at the south end of the Franklin Moun­tains and its upper two-thirds at the north end of Beach Mountain appear to be of post-Honeycut age. Two minor nomenclatural revisions are considered advisable as a result of the regional field studies made. The El Paso "limestone" of previous authors contains limestone, dolomite, minor sandstone, and local shale beds and is therefore referred to as the El Paso formation in the present report. Likewise the so-called Manitou "limestone" is largely dolomite at many localities, and is here called the Manitou formalion. It is recognized that further nomenclatural revision may prove desirable, hut provisional fauna! and physical subdivisions of well-known strati­graphic divisions of a composite nature are here indicated by letter only, pend­ing detailed studies of the rocks in question. Attention is invited to the fact that one of the most persistent fauna! and phys­ical breaks in the strata considered is between probable correlatives of the Tan­yard and Gorman formations. This is essentially the position at which Ulrich last placed his "Ozarkian-Canadian" boundary (see Ulrich and Cooper, 1938, PI. 58). It should he kept in mind in the ensuing discussion that Ulrich's Ozarkian system straddles the Cambrian­Ordovician boundary and that his Cana­dian system extends essentially from the boundary between the fauna! equivalents of the Tanyard and Gorman formations through the Lower Ordovician of most geologists. It is also worth noling that, whereas fossils in the Gorman and Roubidoux formations and their correla­ tives are generally scarce and largely undescribed, the more commonly occur­ ring faunas of the normally underlying and superjacent rocks are fairly diversi­ fied and reasonably well known. The ex­tension into rocks of Gorman-Roubidoux age of the stratigraphic ranges of cer­tain fossils generally supposed to be re­stricted to higher or lower beds should, therefore, not cause surprise. The studies on which were based the regional correlations here offered were greatly facilitated by the availability of the splendid reports on "Ozarkian and Canadian"· fossils by E. 0. Ulrich and co-authors G. A. Cooper, A. F. Foerste, A. K. Miller, W. M. Furnish, and A. G. Unklesbay (1938, 1942, 1943, 1944). These reports contain some stratigraphic incon­sistenCies, but they have at the same time furnisl1ed the tools for solution of these inconsistencies, and their publication should greatly stimulate needed biostrati­graphic studies of the Lower Ordovician rocks. Such studies in the vicinity of Phillipsburg, Quebec, for instance, might serve lo answer some of the most vex­ing problems of fauna] distribution (see Ulrich, in Ulrich and Cooper, 1938, Pl. 58, and pp. 69-70 of the present report). Inasmuch as the correlations suggested are based upon the opinions of the authors of the present report about the ranges of fossils in particular regions, some of these opinions, most of them not original, are here summarized: I. Billingsella, Eoorthis, H uenella, Owenella, and saukiinid trilobites are restricted to Upper Cambrian rocks equivalent to the Wilberns formation. 2. Any strata containing cephalopods of other than the mosl primitive types are sus· pected to be post-Cambrian. 3. Ophileta and Lytospira (Ecculiomphalus of some authors) first appear at the base of the Lower Ordovician, although Ophileta has a long range within the Lower Ordo­vician and Lytospira within the Ordo· vician. 4. Helicotoma of the type of H. uniangu­lata, Ozm kina, Gasconadia, Clm koceras, and other fossils whose ranges in cen­tral Texas are shown on Plate 14 are restricted to strala of Tanyard age. 5. The presence of either piloceratid siphun­cles or coiled cephalopods indicates post­Tanyard strata, but several types of coiled cephalopocls range much lower than has previously heen supposed. 6. Clarkella and Tetralobula suggest a Tan· yard age but range into strata of Gor­man age. Syntro phina likewise occurs in strata of both Gorman and Tanyard age and is recorded from the Upper Cambrian by Ulrich and Cooper (1938, p. 219). The University of Texas Publication No. 4621 7. Di'.aphelasma (see item 8, below), Syntro­phinella, and Lecanospira are considered to mark equivalents of the Gorman for­mation. Rhombella (formerly called "Roubidouxia") generally denotes a Gor­man age, but ranges into younger strata. 8. Barnesella (of which "Maclurea" af]inis is a species) ordinarily occurs only in strata younger than the Gorman forma­tion. This fossil and Diaphelasma occur together at the top of the Manitou forma­tion, and it is unr-ertain which should there he considered the dating element. In general, small macluritids suggest high Lower Ordovician. , 9. Xenelasma marks the lower part of the Honeycut fauna! equivalents. 10. Orospira is probably restricted to rocks of Honeycut age. 11. Other fossils, whose ranges in central Texas are shown on Plate 15, serve to identify rocks of Honeycut age. 12. Ceratopea is invariably of post-Gorman age, and various forms are useful as in­dex fossils to particular zones. 13. Tritoechia occurs in strata of Honeycut age and ranges through strata equivalent to the Black Rock formation, but most species are probably post-Honeycut. Arch­aeorthis begins in rocks of Honeycut age and ranges through strata equivalent to the Black Rock formation. 14-. H esperonomia, Diparelasma, Polytoechia, Syntrophia, and most species of Syntro· phopsts mark strata younger than the Honeycut formation. 15. Syntrophopsis magna Ulrich and Cooper, Buttsoceras, and trilobites and snails of "Middle Ordovician" aspect occur in strata equivalent to the Black Rock for­mation, the highest known Lower Ordovi­cian. Diparelasma typicum Ulrich and Cooper, suggests, hut does not prove, the same correlation. MISSOURI AND ARKANSAS OzARK REGION Perhaps the most intensively studied and best known sequence of lower Paleo­zoic rocks of the carbonate facies in North America is that of the Ozark region in Missouri and northern Arkansas, and this sequence serves as a standard to which other sections in the carbonate facies may be referred. Its. stratigraphy has been made known through the efforts of many workers, and salient fauna! elements of its rocks have been described. Although the following brief remarks necessarily lean heavily on the published record, they owe much to the informal teachings of Drs. Josiah Bridge and J. S. Cullison and are supplemented by a some­what casual acquaintance with the Mis­souri section on the part of Cloud. For further discussion of the early Paleozoic rocks of the Ozark uplift, their faunas, and their siliceous residues, the reader is referred to Josiah Bridge (1930), C. L. Dake (1930), H. S. McQueen (1931), and J. S. Cullison (1944.). The fossils leave little question as to where the correlations are between the Ellenburger rocks and those of the Ozark region, although actual placement of the boundaries of the Roubidoux formation needs clarification. Comparison of the few illustrations of Ellenburger fossils in the present report (Pls. 38--43) with pub­lished illustrations of the Lower Ordo­vician faunas of Missouri will show the reader some of the reasons for these corre­lations. Briefly, the fossils indicate that the equivalent of the Tanyard formation in the Ozark region is the Gasconade dolo­mite (including the Van Buren "forma­tion"), the correlative of the Gorman for­mation is the Roubidoux formation, and the Honeycut formation includes most of Cullison's Jefferson City group. Of the 1600 to 1700 feet of Lower Ordovician strata in the composite Ozark section (about 2200 if cumulative maximum thick­nesses are taken), the upper 700 to 800 feet apparently have no representatives in the Llano region. Of all the correlatives ' of the Ellen­burger group with which the authors are acquainted, those in the Ozark and south­ern Appalachian regions are most similar to it in faunal facies and types of chert. In Missouri the rocks equivalent to the Ellenhurger group consist of cherty dolo­mite and thin sandstone beds. Apparently limestone is absent in the rocks equivalent to Ellenburger strata, although present in the younger strata of the Smithville and Black Rock formations in the Ozark region. The Gasconade dolomite, averaging per­haps 300 feet thick, has a sandstone mem­ber at its base locally (the Gunter sand­stone), apparently does not show impor­tant grain size distinctions from the rocks immediately above and below, and con­tains compact white cherts near its base (Bridge, 1930, p. 104) as well as in the The Ellenburger Group of Central Texas upper part (Bridge, 1930, p. 113). Other­wise it has a general resemblance to the Tanyard formation where the latter is in the dolomitic facies, and the faunas of the two are strikingly similar. The Roubidoux formation, averaging about 120 feet thick, is characterized by interbedded sandstones and dolomites. Commonly three of the sandstone inter­vals are more prominent than others and express themselves topographically. Curi­ously, although sand is scarce in the Lower Ordovician of central Texas, the Gorman formation, which is the faunal equivalent of the Roubidoux here, is to some extent characterized by the presence of sand. It seems to be a general truism in Missouri, the Appalachians, and central Texas that the Roubidoux correlatives are apt to be poor in both number and variety of fos­sils. Of the described fauna! elements, Lecanospira and Rhombella are the most generally known. Their general order of appearance in strata equivalent to the Roubidoux formation seems to be Rhorn­bella at the base and Lecanospira a little higher, with both ranging to the top. At a few places Lecanospira has been seen in the basal strata. The experience of the writers in central Texas leads them to agree with Bridge (1930, p. 123) that the zone of Syntrophina campbelli (Wal­cott) is at the base of the Roubidoux for­mation and not the top of the Gasconade dolomite as stated by Ulrich and Cooper (1938, p. 218). The present report does not support Kirk's implication (1934, p. 4,57) that the "Roubidoux equivalent (in central Texas) of Dake and Bridge (1932) . . . does not range lower than the Jefferson City." The Jefferson City group of Cullison (194'4), about 530 feet thick and includ­ing his Rich Fountain and Theodosia formations, is a zone of dolomites with beds and lenses of sandstone up to 12 feet thick. The "cotton rock" which char­acterizes much of this formation is evi­dently the result of partial leaching of very fine grained silty dolomites (Culli­son, 1945, p. 49). Although fossilifer­ous cherts of the cannonball type occur in both units, there is little actual sim­ilarity between the rocks of the Jefferson City group and those of the Honeycut formation. Faunal correspondence, how­ever, is striking. This is especially true of the Rich Fountain fauna, representa­tives of which may be found in the lower 250 to 350 feet of the Honeycut forma­tion in Texas. It was not possible, on the basis of present studies, to recognize within the Honeycut formation clear-cut units equivalent to the Rich Fountain and Theodosia formations, but it is evident that both are represented. No rocks younger than the Jefferson City group were recognized in central Texas, although the El Paso formation of Trans-Pecos Texas includes strata as young as the Black Rock formation of Missouri. Except that they are · in the carbonate facies in both regions there is no apparent physical similarity between the post-Jefferson City rocks of the Ozark uplift and equivalents in west Texas. OKLAHOMA ARBUCKLE AND WrcHITA MouNTAINs The Lower Ordovician and Upper Cam­brian rocks of the Arbuckle and Wichita Mountains have been described by C. E. Decker (1939), Decker and Merritt (1928), and E. 0. Ulrich (1932) ; and the position of the Cambrian-Ordovician boundary has been discussed by Josiah Bridge (1936) and E. A. Frederickson, .Tr. (1941) . Under the guidance of C. E. Decker, W. E. Ham, H. A. Ireland, and R. H. Dott the authors spent four days in the spring of 1944 studying lower Paleozoic sections in the Arbuckle and Wichita Mountains. Previously Cloud spent a day at the well-known section along U.S. highway 77 between Davis and Ardmore (in December, 1943), guided by H. A. Ireland, and in com­pany with A. P. Wishart and R. L. Heller. Upon data obtained during these recon­naissance field studies, as well as on the published record, the following discus­sion is based. Although the Lower Ordovician rocks of the Arbuckle and Wichita Mountains are in the carbonate facies, they differ from those of the Ellenburger group in many important features. Rocks equiva­lent to the Ellenburger strata are almost twice as thick, contain dissimilar chert The University of Texas Publication No. 4621 and little of it, contain much larger amounts of elastic material in the form of disseminated sand and shale and in­terbedded shale, and were obviously bet­ter supplied with elastic materials at the time of their deposition. The Arbuckle sequence is, in fact, quite the thickest section of Lower Ordovician and late Cambrian carbonate rocks in the mid­Continent region, totaling about 7000 feet of rocks correlative with tbe Wil­berns formation, the Ellenburger group, and Lower Ordovician rocks younger than the Ellenburger. Computations and provisional location of fauna] boundaries in the Arbuckle Mountains by the present authors indicate that about 3200 feet of Arbuckle rocks are equivalent to a gen­eralized section of 1730 feet of Lower Ordovician in central Texas. The fauna] facies of the formations of Oklahoma is like that of the El Paso formation of the Franklin and Hueco Mountains in west Texas, being charac­terized by brachiopods. It contrasts with the predominantly molluscan faunas of the Lower Ordovician in central Texas, the Ozark region, and the southern Appalach ians. Physical distinctions between the Lower Ordovician formations in the Arbuckle and Wichita Mountains are not conspic­uous, and the boundaries between them were not readily apparent to the authors. However, the presence of faunal units essentially corresponding to the named formations is evident, and failure to see the boundaries can be explained by any comhinaLion of the following possible factors: (1) unfamiliarity with the area or lack of discernment on the part of the authors, (2) absence of well-defined boundaries, or (3) evidence for subdivi­ sion not yet adequately 'worked out. Evi­dently the vicinity of the Arbuckle and Wichita Mountains was the site of much more active sedimentary accumulation than central Texas or the Ozark region, and it is possible that continuous sedi­mentation may have there obscured boundaries that are elsewhere well de­fined. It is fundamental to stratigraphic thinking to keep in mind that the more complete the sedimentary record is, the fewer and the less conspicuous are strati­graphic breaks apt to be. That refine­ment in the criteria for subdivision of the Arbuckle group is possible is indi­cated by the probability that some of the formation boundaries indicated by Dr. Decker (1939) fall at different strati­graphic levels in the Wichita Mountains from their placement in the Arbuckles, and by the observed fact that the litbic boundaries between calcitic and dolomitic units in the Cambrian part of the sec­ tion are widely transitional. Provisional correlation and unit thick­nesses within the Arbuckle group are summarized on Plate 15 under sections 5 and 6. The basis for this correlation is wholly faunal. Some of the support­ing faunas are listed in Part 3 of the present report and reference to them is indicated on Plate 15 by locality num­bers. The others are listed by Decker ( 1939), in his described sections, from identifications said to be in large part made by Drs. G. A. Cooper and Josiah Bridge. Reference to Decker's report of 1939 is indispensable to critical consid­eration of the correlations suggested on Plate 15. The Lower Ordovician of the Arbuckle and Wichita Mountains, where not over­lapped by younger beds, includes beds of all ages represented in the Ellenburger group, as well as strata as young as the Odenville limestone of Alabama, the Black Rock formation of the Ozark uplift, and Unit C of the EI Paso formation of Trans­Pecos Texas. In general the McKenzie Hill formation seems to be about of Tan­yard age, the Cool Creek formation about of Gorman age, the lower two-thirds of the Kindbl ade formation of Honeycut-] effer­son City age, the upper third of the Kindblade formation and probably the lower part of the West Spring Creek formation of Cotter age, and the bulk of the West Spring Creek formation of Powell, Smithville, and at least locally of Black Rock age. Ulrich and Cooper (1938, p. 235) report the occurrence of Syntrophopsis magna Ulrich and Cooper near the top of the Arbuckle group in the Ardmore quadrangle not far west of the Chapman ranch section, and the same species occurs in the Odenville limestone, The Ellenburger Group of Central Texas the Black Rock formation, and Unit C of the EI Paso formation at EI Paso. There is no reason to doubt that de­termined search would reveal enouah faunal evidence to permit fairly clo~e placement of faunal houndaries through much of the Arbuckle group. Physical distinctions are apt to become more evi­ dent once fauna! divisions are closely worked out and coordination of the two lines of evidence may eventually permit the recognition of clearer unit boundaries than at present. TEXAS MARATHON BASIN Outcrops of Lower Ordovician rocks in Texas occur in isolated basins, plateaus, or mountains, where erosion or uplift has brought them to view. Of these, the near­est to the Llano uplift is the Marathon basin, a structural uplift of basinlike topography. Here is exposed the Mara­thon limestone and the thin Alsate shale described by P. B. King (1931, pp. 1066­1070; 1937, pp. 26-32, Pl. 2). The writ­ers and Mr. L. E. Warren, under the guidance of Dr. Josiah Bridge, spent two days studying and collecting from the Marathon limestone in November, 1944 (especially section number 4 of King, 1937, Pl. 2), and Dr. J. B. Knight and Cloud made additional collections from the Monument Spring dolomite member of the Marathon limestone on Alsate Creek in July, 191,S (see King, 1937, fig. 12). Edwin Kirk (1934, pp. 451-452) has discussed the correlation of the Mara­thon limestone and AlsaLe shale, and the present writers have little to add to his suggestions. The following discussion presents the salient described features of these rocks and adds a few previously unpublished data. No sequence of Lower Ordovician rocks discussed in the present report is more persistently different from rocks of the Ellenburger group than the Marathon limestone and Alsate shale, unless it be the elastically contaminated carbonate rocks of the Ordovician part of the Arbuckle group in Oklahoma. In fact the Marathon and Arbuckle rocks are nearly intermediate in character between the purer carbonate facies of the Ellen· burger group and the elastic facies that characterizes the Lower Ordovician of the Ouachita Mountains of Oklahoma and Arkansas. Averaging, according to King, between 500 and 1000 feet thick, the Marathon limestone consists principally of dark gray to black, sublithographic, platy, argillaceous limestone, with interbedded shale and "scattered layers of conglom· erate and sandstone" (King, 1937. p. 27), indicating its deposition near shore or within the range of offshore currents competent to carry such detritus. Only near the middle of the formation does it resemble any part of the Ellenburger group. Herc heavy-bedded, medium to light gray, sublithographic, siliceous and very slightly dolomitic limestones weather light gray with yellowish to huff siliceous films and yield minor quantities of chert. These beds, named the Monument Spring dolomite member of the Marathon lime­ stone, contain a mollusk-brachiopod fauna such as more normally characterizes Lower Ordovician rocks in the carbonate facies. In the intPrhedded limestones and shales above and helow, however, only graptolites, inarticulate brachiopods, and sponges were found. ' The Alsate shale is said by King to range from less than 20 to more than 100 feet thick and to consist of green shale with minor chert, limestone, and sandstone. Ruedcmann (see King, 1931, p. 1070; 1937. p. 32) and Kirk (1934, p. 452) consider the Alsate shale to be of high Lower Ordovician age on the basis of graptolites, and Kirk (1934, pp. 4,52, 451) provisionally correlates it with the "uppermost horizon" of Lhe El Paso formation (about Unit C), "although," he says, "it may fall somewhat lower." Overlying the Alsate shale is the Fort Pefia formation of King, the Middle Or· dovician age of which is established, and beneath the Marathon limestone is the Upper Cambrian Dagger Flat sandstone, from limestone beds at the top of which Bridge, Warren, and the writers obtained the gastropod Owenella (TF-305), an upper Wilberns fossil in central Texas. Evidence from the beds above and be­low thus seems to bracket the Marnthon limestone as younger than the Wilberns formation (uppermost Cambrian) and older than Unit C of the EI Paso for­mation (equivalent to the high Lower Ordovician Black Rock formation of Arkansas). Fossils obtained from the lower 100 feet of the Marathon lime­stone below the Monument Spring dolo­mite member by Bridge, Warren, and the writers were only an unidentified sponge, Didymograptus sp. and "Lingula" sp. (TF-3M, 306, and 307) ; and correlation within the Lower Ordovician on the basis of these fossils would be difficult to de­fend. From the Monument Spring dolo­mite member itself were obtained Cala­thiwn, an unidentified sponge, cystoid plates and columnals, Diaphelasma sp.. "Orthis" sp., unidentified gastropods, Allopiloceras cf. A. coarctum (Ulrich and Foerste), fragments of other piloceratid cephalopods, and a trocholitifl cepha1o­pod (TF-303, 308, 309, 310). This fauna is definitely post-Tanyard in age, and Diaphelasma indicates correlation with Unit A (the Gorman equivalent) of the EI Paso fonnation. Unfortunately this fossil was found in float only and its derivation is not certain. "Orthis" and the trocholitid cephalopod suggest a post­Gorman age and there is no question as to their source. It seems best, in view of the conflicting evidence, to consider the assemblage listed as simply post-Tanyard. Above the Monument Spring dolomite member were procured Phyllograptus, Didymograptus protobifidus Elles, Logan­ograptus, Dichograptus, "Orthis" sp., and fragments of a piloceratid (?) siphuncle (TF-3I2, 30I, 302). Didymograptus pro­tobi(zdus Elles is said by Decker (1944, p. 383) to occur in the Smithville for­mation of Arkansas and 80 feet below the top of the West Spring Creek for­ mation of the Arbuckle Mountains, in that portion of the formation which is of post-Cotter and pre-Black Rock age; so it is likely that about the upper 100 feet of the Marathon limestone is at least as young as Smithville in age. What is known about the Marathon limestone, then, is that it is post-Cam· brian and probably pre-Black Rock in age. Fossils of post-Tanyard age are known in the Monument Spring dolomite mem­ber of the Marathon limestone and fos­sils of Smithville or younger age above it. One may infer from this that the Marathon limestone and Alsate shale to­gether are about equivalent lo the El Paso formation and perhaps the Bliss sandstone of the Franklin Mountains. The reader who is inclined to further speculation is warned that no fauna of recognizable Tanyard age has been found in either the Marathon basin or the Franklin Mountains. SIERRA DIABLO REGION Beach Mountain.-At the north end of Beach Mountain, in the southwestern part of the Van Horn quadrangle, southwest­ern Culberson County, IIIS feet of prin­cipally dolomitic rocks are assignable to the EI Paso formation of Lower Ordovi­cian age (sec P. B. King and J. B. Knight, I944). Above the El Paso formation is the Montoya limestone of Upper Ordo­vician age and below it is I25 feet of the so-called Bliss sandstone, of which the upper 8 feet contain Lower Ordovician fossils believed to be of upper Tanyard age. The Bliss (?) sandstone rests with angular unconformity on the Van Horn sandstone of Cambrian (?) or pre-Cam­ brian age and on known pre-Cambrian rocks. The Beach Mountain section (Pls. IS, I6, and 29) displays a complete and well-exposed sequence of the El Paso for­mation, is readily accl:'ssible, and has been visited by many geologists. For these reasons, and because it offers the near­est good section of Lower Ordovician rocks west of the Llano region that can be closely correlated with those of the Ellenburger group, it is o{ foremost im­portance to regional conceptions of Lower Ordovician stratigraphy and is, therefore, described in detail in Part 3 of this report. The following observations are based on information obtained during visits to the section by Josiah Bridge, L. E. War­ren, and the authors on November II, 1944; the authors on October 9 and 10 and November 10, 1945; and Josiah Bridge, John C. Dunlap, Ralph King, The Ellenburger Group of Central Texas Eugene Callaghan, and the writers on November 9, 1945. Particular thanks are due to Dr. Bridge, who made a special trip to the region to guide the authors to the locality at which Ordovician fos­sils were obtained from the Bliss sand­stone, and to Mr. Dunlap who found the first and best specimens of the cephalo­pods obtained at this locality. Although the Lower Ordovician of the Beach Mountain section can, within lim­its, be correlated faunallv with that of the Llano region and the Franklin Moun­tains, there is little physical similarity between the rocks of the various faunal units in the regions noted. Sand occurs intermittently from bottom to top of the Beach Mountain section and forms prom­inent sandstone beds in the faunal equiv­alents of the Tanyard and Gorman for­mations. Very fine grained to micro­granular dolomite characterizes the strata of the post-Gorman fauna! equivalent. Glauconite occurs locally but is uncom­mon, and the section is essentially non­glauconitic. The Lower Ordovicia~ rocks of the Beach Mountain section resemble those of the Llano region more closely than either resembles the intervening Marathon limestone; but specific simi­larities between correlative units, other than faunal, are few if existent. The 1240 feet of pre-Montoya (Upper Ordovician) and post-Van Horn (pre­Cambrian or Cambrian [?]) rocks of the Beach Mountain section are readily divisible into four units, as follows: · Stratigraphic unit El Paso formation (lll5 feet thick) Unit C (zone of earthy dolomites) ________________ The so-called Bliss sandstone of the vicinity of Beach Mountain is of partic­ular interest to the present study. It con­sists of well-indurated beds of sandstone up to 2 feet thick, alternating with poorly indurated zones that contain con· siderable argillaceous material. Tests with dilute HCl show that most beds are slightly calcareous, and one 10-inch bed of dolomite was seen near the middle of the unit at the north end of Beach Moun­tain. The well-indurated sandstones com­monly display numerous, closely spaced, slightly undulating, vertical tubes gener­ally referred to as '1Scolithns," although the name Sabellarifex R. Richter, 1921, has been proposed for the undulating vertical tubes as distinct from the straight ones. As was pointed out by P. B. King (1940, p. 154,), the Bliss (?) sandstone of Beach Mountain is abruptly and dis­conformably succeeded by an interval of sandy and silty dolomite, dolomitic silt­stone and sandstone, and much argilla· ceous material belonging at the base of the El Paso formation. Commonly up to 10 inches of the basal El Paso is a calcareous reworked sand, and locally it contains pebbles and cobbles of quarlz and sandstone, one incompletely expo'led cobble of sandstone in a section at the north end of Beach Mountain measur­ing 10 by 5 by 2 inches. Lower Ordo­vician fossils were collected from beds 4 to 8 feet below the top of the Bliss (?) sandstone on the west side of Beach Mountain, opposite the top of Tumble- Thickness Provisional in feet correlation Black Rock fm. and --------------------------------50 Odenville ls. Unit B (813 feet thick; granular dolomite) zone of very fine grained to micro· Subunit B2 (a faunal division that is provisionally sub­divided at the base of a 50-foot zone of repeated thin arenaceous intervals into an upper portion 331 feet thick called Subunit B2b and a lower portion 334 feet thick called B2a) ____ ----------------------------------------------------·----·-----------------­ 665 Strata of post-Honey­cut and pre-Black Rock age. Subunit Bl (a fauna! division) ------------------------------------------------­ 148 Honeycut fm. Unit A (zone of sandstone, dolomite, and limestone)_________________ 252 Gorman fm. Bliss ( ? ) sandstone ______ ----------------------------------------------­ 125 Tanyard fm. Total ------------------------------------------------·-------------------------------------------------1240 Lower Ordovician. The University of Texas Publication No. 4621 down Mountain, on a spur about 700 or 800 yards south of the abandoned shafts of the Dallas Mine (TF-438). They con­sist of fragmentary specimens of a cepha­lopod referred to the genus ClarkoCPras," Lytospira sp., Ophileta sp., cf. Helico· toma of the type of If. uniarip,ulata (Hall), numerous cross sections and impressions of gastropods of debatable generic affini­ties, and many unidentified archaeostra­can crustaceans. Dr. Bridge states that this is the locality from which C. L. Dake and he obtained gastropods and the trilobites Hystricurus ci1ed by P. B. King 0940, p. 154) as evidence for the Ordovician age of the Bliss sandstone. The fossiliferous zone is below the high­est well-indurated, slightly calcareous sandstone bed containing irregular tan­gential borings and a few "Scolithus" tubes and 19 feel above a bed contain­ing the brachiopod Lingulepis (TF-438), generally regarded as an Upper Cambrian index fossil (see Cooper in Shimer and Shrock, 1944, p. 285). Although numer­ous casts and impressions and a few cross sections of gastropods were seen in and below the Lingulepis bed, none was found in place with or below Lingulepis that could unequivocally be said to he an' Ordovician form, no Lingulepis was found in the upper zone with the Lower Ordovician gastropods and cephalopods, GLarge cyrtendocerntid cephalopo\'CI vcnltad th.rn dorsad, and faiily rapidly expanded orad. The septa me concave OlcH1 am.1 the s11t111es m'lke -;Jw;hL lateral lobes, a faiily p1ominc11t dorsal saddle, and probably an inconsp11,• uous vcntrnl saddle. The s1phunclc is laige, slightly com· prc'>scd late1ully to almost c11cu1a1 111 c1os<; section, and ventral but not quite rna1ginal in posit10n. Measmementlll of an incomplete shell show that the cross sectional b1cadth was greater than .'W mm. do1so~ventrally and mo1e than 20 mm. laterally. The specimen that showi'i a. plan vif'w of the conch is applO'-lllUtcly 17 by 22 mm ac1oss and has a siphuncular diameter of 6 mm. at thar point. Pa1 tial impressions of s1phuncles up to 55 mm. long ·we1c obtained and the average di,tmeter of these at the 01al end i.~ 10 mm., with diametrts up to 15 mm. obtamcd. If the shell b1oadenr1l prnpoltionately it' adult mal section may have averaged about 28 by 37 mm. and probably attained breadths ag great as 42 by 55 mm. and no evidence of a break in sedimen­tation could be found between collec­tions TF-437 (Ordovician) and TF-433 ( Lingulepis}. However, archaeostracan crustaceans and a gastropod probably referrable to Lytospira were found im­mediately above but apparently in the same bed with Lingulepis at the north end of Beach Mountain (TF-438). Al­though the faunal evidence does not per­mit positive dating of the beds below the upper 8 feet, the abundance of gas· tropods in the Bliss (?) sandstone of the Beach Mountain sequence, helow and with as well as above Lingulepis, is a feature unusual in Cambrian strata; and, in view of the striking similarity of the various portions of this sandstone the authors are inclined to regard it as a single unit and to agree with King and Bridge Un King, 1940, pp. 154-J 55) that it is probably of Lower Ordovician age throughout. This conclusion is supported by the presence of Clarkoceras, normally an upper Tanyard fossil, and Hystricurus, normally Tanyard or higher, at the top of the Bliss (?) sandstone. If Lingulepi8 be discounted and the disconformity at the top of the Bliss (?) he regarded as the Tanyard-Gorman boundary (discussed below), the Bliss (?) sandstone of the Beach Mountain section may be consid­ered a correlative of the Tanyard for­mation of the Llano region. Comparison of the Bliss sandstone of the type section in the Franklin Moun­tains with the so-called Bliss sandstone of the Beach Mountain section shows some similarities and many differences. Each is a thick basal sand, resting un­conformably on the pre-Cambrian and overlain disconformably by the El Paso formation. Jn the Beach Mountain sec­tion this sand is 125 feet thick, in the Franklin Mountains it is about 300 feet thick. That in the Franklin Mountains is commonly quartzilic, mostly noncalca­reous, and contains some prominently glauconitic beds in the upper half; that in the Beach Mountain section is well to poorly induratcd but not quartzitic, for the most part slightly to moderately calcareous, and contains little if any glauconite but has considerable argilla­ceous material. The type Bliss contains The Ellenburger Group of Central Texas a few irregular tangential borings, few if any of the vertical "Scolithns" tubes, and no observed gastropods; whereas the so-called Bliss sandstone of Beach Moun­tain contains numerous conspicuous "Scolithus" beds and abundant impres­sions of gastropods. Some of these dif­ferences are probably altributable to the fact that the sActions compared are llO miles apart and overlap entirely difier­ent types of pre-Cambrian rocks, the Bliss sandstone at the south end of the Frank­lin Mountains resting on a rhyolite porphyry and the sandslone at Beach Mountain overlapping miscellaneous pre­Cambrian rocks in which arkosic sand­stones predominate. Correlation of the two sandstones is suggested by strati­graphic pos1t10n, paleogeography (see Schuchert and Dunbar, 1941, pp. 126, 152), and the occurrence in both of Lingulepis. It is negated by the absence of gastropods in the Bliss sandstone at its type locality, in contrast to their abun­dance in the so-called Bliss of the Beach Mountain section, and by specific phys­ical dissimilarities. Unfortunately Lingu· lepis can no more logically he nsed to correlate these two sandstones than it could be used lo prove a Cambrian age for the Bliss sandstone in its type sec­tion. The presence of abundantly glau­conitic zones in the Bliss sandstone of the type section in contrast to its absence in the Bliss (?) of the Beach Mountain section would be presumptive evidence of a Cambrian age for the former if the stratigraphic dislribution of glauconite found in the Llano region could be ex­tended to ·west Texas; hut, on the other hand, the contrasling character of the adjacent source rocks might explain this difference, as has been iepeatedly stated in papers by E. W. Galliher. The rela­tionships and correlation of the basal Paleozoic sandstones of Trans-Pecos Texas are, therefore, moot; and additional evidence must he sought before a finally tenable conclusion may be reached. The burden of proof perhaps rests with those who would dispute the cm-relation of these two stratigraphically commensu­rate basal sands. As in the case of the age of the basal sands, the evidence on the placement of the boundary between the faunal equiv­ alents of the Tanyard and Gorman for­ mations in the Beach Mountain section is contradictory. The lowest El Paso fossils collected by the authors occur 68 to 70 feet above the base of the El Paso formation (TF-433) and ~on­sist of Clarkella, Syntrophina, Ophileta aff. 0. rotuliformis (Meek), and Hormo­toma. Four to 6 feet above these (TF--434) were collected an imhricate Tetralobula, Ophileta (Ozarkispira) aff. 0. rotulifonnis (Meek), Piloceras, Allopi­loceras, and Kirkoceras'?. Ulrich, Foerste, and Miller state ( 1943, p. 25) that P. B. King and J. B. Knight found Piloceras "in the lower part of the El Paso lime­stone just above the Bliss sandstone." According to Ulrich, Foerste, and Miller (194.3, pp. 16, 19, 20, 26, and 45) the Piloceratidae, which include the cepha­lopods listed, are "exclusively Canadian (post-Tanyard) in age." On the other hand, Ulrich and Cooper report (1938, pp. 33, 42, 211, 218) that Clarkella and Syntro phina have been found only in rocks of "Ozarkian" (pre-Gorman) age, and that the same is true of T etralobula (op. cit., pp. 43, 203) ; except for its occurrence in the Tribes Hill limestone which they consider to he of "Lower Canadian" (post-Tanyard) age, but which the present authors correlate with the Tan­yard formation. Field studies in central Texas have failed to reveal Horrnotoma below the Gorman formation and have shown that Ophileta of the type of 0. ro­tuliformis (Meek) (= Ozarkispira Wal­cott, 1924) is ordinarily a Gorman or a high Tanyard fossil. They sh.ow that Syn­trophina is characteristic of the lower part of the Gorman formation and uncommon in the Tanyard; but they also suggest that Tetralobula is an index to the Tan­yard formation. From the authors' expe­rience in collecting the sparingly fossil­iferous Lower Ordovician carbonate rocks it seems more likely that the known range of Clarkella and Tetralobula may he extended by future collecting than that the range of such conspicuous and com­monly numerous objects as the siphuncles of Piloceras and Allopiloceras would be. extended. The law of probability as well as the weight of the available faunal and physical evidence is against the brachiopods, and the present report holds that the fossils listed are of Gor­man age. Further evidence bearing on the correlation of these beds is cited in Part 3 of this report, being omitted here because its validity is in question. An analogous situation is found in the vicinity of Phillipsburg, Quebec, in the Hastings Creek and Luke Hill formations, where Tetralobula, Clarkella, and Syn­trophina occur in association with Archae­orthis, Tritoechia, Diaphelasma, and Syn­tro phia (Ulrich and Cooper, 1938). The present authors have found Archaeorthis, Tritoechia, and Diaphelasma only in post­Tanyard strata; and Ulrich and Cooper ( 1938, pp. 4°2-44, 160, 245) note that, except for occurrences in the Luke Hill and Hastings Creek formations, all known occurrences of Tritoechia "are from strata high in the Canadian period," and that, adding an occurrence in erratics (?) of the Levis formation, the same is true of Syntrophia. The first appearance of Tri­toechia represents the first appearance of the brachiopod suborder Clitambonoidea, whereas Tetralobula and Clarke/la belong to the suborder Syntrophioidea which had its beginnings in Cambrian time. It is a widely accepted biostratigraphic pre­cept that first appearances of new organic groups are likely to be more pertinent to correlation than holdovers of previously established groups, and it would seem more reasonable to the authors to date the beds in question on the basis of Tri­toechia than of Tetralobula and Clark­ella, even were no other evidence avail­able. So far as the Luke Hill formaLion is concerned, however, there is other evi­dence in the form of the fauna of the Naylor Ledge formation which intervenes between the Hastings Creek and Luke Hill formations and contains a cephalopod fauna so markedly "Canadian" in aspect (Ulrich, Foerste, Miller, and Unklesbay, 1944,, p. 19) that Ulrich (in Ulrich and Cooper, 1938, p. 25, Pl. 58) was forced to postulate a submarine cave deposit in order to explain it. The present authors consider the Naylor Ledge and Luke Hill formations at least, and probably part or all of the Hastings Creek formation, to be of post-Tanyard age; and they call attention to the fact at this place as sub­ stantiation of the extended range of Clark­ ella and Tetralobula and the placement of the Tanyard-Gorman boundary of the Beach Mountain section at the Bliss (?) ­ El Paso contact. If the disconformable Bliss (?)-El Paso contact does not coincide with the Tan­ yard-Gorman contact as is here suggested, there is then a disconformity within the faunal equivalent of the upper Tanyard formation. More disconcerting, the only logical alternative position for the "con­ tact," the "faunal boundary" between the Clarkella-Tetralobula fauna and the Dia­ phelasma fauna, occurs within a series of intergrading limestone and dolomite beds. The contact between the fauna! equiva­lents of the Gorman and Honeycut forma­tions in the Beach Mountain section is most appropriately located at the top of a 4-foot bed of dolomitic sandstone and arenaceous dolomite that occurs 19 feet above the second or upper prominent bluff­forming sandstone in the El Paso forma­tion. The lowest diagnostic Honeycut fos­sils (Xenelasma) occur 4 feet above this point (TF-319) and the highest undoubted Gorman fossils (Diaphelasma) were found 133 feet below it. However, poorly pre­served syntrophioid brachiopods suggest­ing Diaphelasma were found 8 feel below the contact designated and its location 'ls probably fairly accurate. The contact chosen is a marked lithic break and places the major sand zones of the El Paso for­mation within the fauna! zone that is the more characteristically arenaceous in the Llano and Ozark regions (Gorman and Roubidoux formations) . The Gorman faunal equivalent so conceived (Unit A) has a prominent bluff-making dolomitic sandstone in the lower part, another near the top, and a third but much less prom­ inent sand at the top, with intergrading limestone and dolomite between the two principal sandstone zones and with sandy, silty, and shaly dolomite between the lower sand and the Bliss (?). Above the Gorman-Honeycut faunal boundary, and below the 50 feet of earthy dolomites at the top, the Beach Mountain section presents a difficultly divisible se­quence of predominantly very fine grained to microgranular dolomites about 813 The Ellenburger Group of Central Texas feet thick and here known as Unit B. At least the lower 14,8 feet of Unit B (Sub­unit Bl) is faunally equivalent to the Honeycut formation, and is arbitrarily separated from the 665 feet of post-Honey­r.ut and pre-Black Rock dolomites above (Subunit B2). Like any group of rocks along an undeviating line of section, the dolomites of Subunit B2 are divisible into intervals in the Beach Mountain section, but whether any of these intervals or groups of inte1vals displays persistent .Features is a problem which can be settled only by detailed areal or subsurface studies. A feature of possible zonal value in Subunit B2 is the 60 to 70 feet of recurrently arenaceous dolomite near its middle which may correspond to a 50-foot inlerval of arenaceous dolomite at the middle of Subunit B2 in the El Paso sec­tion. The observed data are recorded in detail in Part 3 of the present report and further suggestions on subdivision are made 'at that place. The upper 50 feel of the· El Paso forma­tion, called Unit C, is a zone of earthy dolomites probably correlative with the Odenville limestone of Alabama and the Black Rock formation of Arkansas. As potential petroleum reservoirs, both the Tanyard and Gorman fauna! equiva­lents of the Ileach Mountain section are significant. Each is conspicuously arena­cous in the vicinity of Beach Mountain, whereas in the Llano uplift the Tanyard formation contains no sand and the Gor­man formation very little. Stratigraphic­type traps are to be looked for in these units, particularly where the change in faciQs takes place, at some intervening point. It is also of interest that a petro­liferous odor was noticed locally in the post-Honeycut dolomites of the Beach Mountain section, especially from 500 to 515 feet below the base of the Montoya limestone. Diablo Plateau.-A difficultly measur­ able but quite fossiliferous section of the El Paso formation in a small horst on the Diablo Plateau between Black Knobs and Cox Mountain, about 17 miles airline northwest of the Beach Mountain section, was recommended by Dr. J. B. Knight as a good locality for study. This section was visited by Josiah Bridge, L. E. War­ ren, and the authors on November 12, 19f[ik It was estimated that between 34,0 and 400 feet of beds are exposed between the top of the lower prominent calcareous sandstone zone and the overlapping Hueco limestone at the top of the section. Faunal evidence proves that if this sandstone cor­relates with one of those in the Beach Mountain section it is the lower one, and the upper prominent sand of the Beach Mountain section is absent or inconspicu­ous in the Diablo Plateau section. Field studies were hurried and this point was not clearly ascertained. The exposed rocks arc largely impure, brownish gray, gran­ular, locally cherty limestones. Except for the sandstone at its base the rocks in this section are more like those of the Hueco and Franklin Mountains than those of Beach Mountain. Grains of a green mineral suggesting glauconite were noted in several beds and are locally abundant. A sample of this mineral from a ledge about 115 feet above the sandstone, how­ever, shows a flaky crystalline structure and is probably a hydrous mica or one of the group of minerals known as chamo­siles. Fossils were seen at a number of places in the section and collected at seven (TF-329 to 335) . They denote faunal equivalents of the Gorman and Honeycut formations, but no boundary was selected. If the prominent sand at the base of this sequence were the exact correlative of the basal sand of the El Paso formation at Beach Mountain and the overlying strata did not vary in thick­ness, the boundary between the faunal equivalents of the Gorman and Honeycut formations should fall 193 feet above its top and the top of the Honeycut fauna! equivalent at 341 feet. The brachiopod Xenelasma, which should occur at or just above the base of the Honeycut faunal equivalent, was found 145 feet above this prominent sand zone (TF-331), suggest­ing that the top of the lower prominent sand zone may rise in the section west­ward from Beach Mountain. The highest fossils seen in the El Paso formation at this place were Honeycut types, but they were probably near the top of the Honey­cut faunal equivalent. The University of Texas Publication No. 4621 HUECO MOUNTAINS In November 1944, in company with Josiah Bridge and L. E. Warren, the writers spent two days in the .Hueco Moun­tains, the west edge of the Diablo Plateau, studying and collecting fossils from two sections of the El Paso formation. One of these sections is in west-central Hudspeth County, northeast of the Old Padre Mine; and the other is in east-central El Paso County, north of Helms West Well. These sections were known to Barnes from <:! field trip with Leo Hendricks an~ Bruc.e Harlton in November 1941. The1r posi­tion and geologic relationships may be found on a Geologic Map of the Hueco Mountains by P. B. King, R. E. King, and J. B. Knight ( 194,5) . The following observations are principally from the sec­tion north of Helms West Well; the sec­tion at Lhe Old Padre Mine was not com­pleted but seemed to be essentially like that at Helms West Well. Cumulative estimates and measurements between fossil collections in the section north of Helms West Well amounted to 960 feet of strata of the El Paso forma­tion between the overlying Upper Ordo­vician Montoya limestone and the bolson deposits that conceal the base of the El Paso formation in the vicinity of Helms West Well. If the roughly approximate position of Lhe contact between the faunal equivalents of the Gorman and Honeycut formations in this section is compared with the contact between Units A and B of the El Paso section, about 330 feet of lower El Paso strata and all of the Bliss sand­stone would be indicated as covered by bolson deposits north of Helms West Well. The derived figure of approximately 1300 feet may be taken as the approximate thickness of the El Paso formation in the Hueco Mountains until better data are available. The lower El Paso beds and the Bliss sandstone may be seen to the south of Helms West Well toward the Old Padre Mine. The El Paso formation in the section north of Helms West Well resembles that of the El Paso section in being predom­inantly dolomite with minor sandy inter­vals below the approximate contact of the Gorman and Honeycut faunal equivalents, and predominantly granular, impure lime­stone above. It is not sufficiently dif­ferent from the El Paso section nor ac­curately enough measured to warrant description at this time, hut it is quite fossiliferous and deserves detailed study. Only 11 collections of fossils (TF-34,0 through TF-350) were made from the section north of Helms West Well, but many more could have been obtained and fossils were recorded by field identifica· tion al 22 other levels in the El Paso for­mation. The little brachiopod Archae­orthis is abundanl at many stratigraphic levels above Unit A, and exquisite speci­mens of Polytoechia were obtained from the upper beds of Unit Il. The fossils col­lected demonstrate the presence of all the units and subunits recognized in the El Paso formation of the Franklin Mountains and the reader is r~ferrcd to the descrip­tion of the El Paso section for further detail on the character of the rocks in­volved. FRANKLIN MOUNTAINS The El Paso section (figs. 5, G; Pls. 15, 16), at the south end of the Franklin Mountains, and just outside the corporate limits of El Paso, El Paso County, Texas, would make a good type section for the El Paso formation. It is lhe thickest section of the El Paso formation known to the aulliors, comprising about 1590 feet of limestone, dolomite, a little shale, and less sand; and it conlains enough fos­sils for biostratigraphic studies. Edwin Kirk (1934) discussed the El Paso sec­tion and the broader aspects of its corre­lation, and the reader is referred to his paper as directly complementary to the present one. The reader should bear in mind that Kirk's "Taffia near iones" has since become Hes peronomia (see Ulrich and Cooper, 1938, p. 119) and that the ob­served range of Calathium has ~ecn ex­tended. The present report takes exception to Kirk's view (1934, p. 457) that "the El Paso does not range lower than the Jefferson City." Like the Beach Mountain section, from which it differs lithically, the El Paso section is of primary importance to re· gional stratigraphic studies of rocks re­lated to the Ellenburger group and is Fig. 5. Map of the south end of the• hanklin Mountains, ~l Paso County, 'l exas, showing loca­tion and app1oximate geo.Jogic relationships of the El Paso section. (Topography and culture enlarged from 71;2 minute El Paso Quadrangle, U. S. Geol. Survey, ed. 1943. ) sooo' z z 0 j:: .. > .... 4400' ~ 4200' "' 4000' 200· •, 5000' 4600' EXPLANATION I 2 ~ lnte-rvols ln El Peso formation • Tf-450 Locollly from which fossils were collec!ed f F Locallty 01 which fossils were recorded • Tf· 3Sais Approx1mole slrollgrophic posilion of o conechon ot losslls nol to~en lrom lhe line or profile Intervals 1-6 ' Un1I C Intervals 7-10 ' Subunit B 2 b Inter vols 11-14 ' Subunit B 2 o Infer vols 15-17' Subunit BI lnlervols 18-27• Un11 A See Plole 16 for columnar secfloo qooo HORIZONTAL SCALE IN FEET 400' 600' aoo· 1000' 1200· 1400' 1600' 1600' '--~--'-~~-'-~~"'--~--'-~~-'-~~-'--~---L~~...b.~~..._~---1~~--1...~~...l-~--.JL...-~--1...~~....L,_~~I 1~-I Fig. ~· Profile of the El Pa•o sf'rtion, Frankltn M01mtai11f', El Paoo County, Texas. The University of Texas Publication No. 4621 described in detail in Part 3 of this report. The remarks that follow are based on observations made by the writers and L. E. Warren on November 26 and 27, 19'1,11., and by the writers on October 7 and 8, 1945. In 1944 a section at the head of McKelligon Canyon was studied, as well as the one here designated the El Paso section. The Bliss sandstone is well dis­played in the McKelligon Canyon section, and the sequence in the El Paso forma­tion, except that it shows more dolomite and is much faulted, is about the same as that in the El Paso section. The El Paso formation of the type area contains much more limestone and much less sand and dolomite than it does in the section at the north end of Beach Mountain, and although sand occurs at places from top to bottom of the El Paso section it is sufficiently concentrated to Stratigraphic zmit El Paso formation (l590 feet thick) authors limestone predominated over dolomite. The age of the Bliss sandstone in the Franklin Mountains, its type area, is not known, but analogy with the so-called Bliss sandstone oJ Beach Mountain sug­gests that it be considered lowest Ordo­vician until diagnostic evidence is found. There is, however, room enough for a Tanyard equivalent fully as thick as that of the Beach Mountain section between the lowest recognized Gorman fossils and the top of the Bliss sandstone of the El Paso section. The correlation of the Bliss sandstone is considered further under the discussion of the Sierra Diablo region and the description of the El Paso section. The 184,Q feet of pre-Montoya (Upper Ordovician) Paleozoic rocks in the El Paso and McKelligon Canyon sections are divisible into four units, as follows: Thickness Prouisi anal in feet correlation Unit C (zone of inequigranular shell-limestones, shale, and minor dolomite) -----------------------------------------------­_________________________ 36 Unit B (1014 feet thick; zone of predominaling limestone) Subunit B2 (a faunal division that is provisionally sub­divided at. the base of a SO-foot zone of arenaceous dolomito into an upper portion 335 feet thick, called Subunit B2b and a lower portion 364 feet thick called B2a) --------------------------------------------------------------------------------------------­ 699 Subunit Bl (a faunal division) 315 Unit A (zone of predominating dolomite, with arenaceous intervals) -----------------------------------------------------------------------------­ 54.0 Bliss sandstone ----------------------------------------------------------------------------------------­ 250 Total form a sandstone only at the top of the Gorman faunal equivalent (Unit A). Rocks equivalent to the Gorman forma· tion are predominantly dolomite, and granular limestone characterizes the strata younger than the Gorman. That there is some lateral gradation between limestone and dolomite both in the El Paso and the Beach Mountain sections is evident, but how much is not known. In all sections of the El Paso formation west of Beach Mountain that were examined by the 1840 Black Rock fm. and Odenville ls. Strata of post-Honey­cut and pre-Black Rock age. Honeycut fm. Gorman fm. Tanyard Im. (?) Lower Ordovician. In terms of characteristic fossils these divisions might be referred to as follows: A, zone of Diaphelasma; B, zone of Cera­topea and Archaeorthis; Bl, subzone of Orospira and Xenelasma; B2a, subzone of Polytoechia; B2b, subzone of Polytoechia and Hesperonomia; C, zone of Syntro­phopsis magna. Correlation with the divi· sions of similar designation in the Beach Mountain section is indicated by fauna! evidence, stratigraphic position, and the The Ellenburger Group of Central Texas probable continuity of arenaceous inter­vals at the top of Unit A and the base of Subunit B2b. CorrclaLion of Subunit B2 between the two sections is not directly supported by the known fauna! sequence within these units and more faunal infor­mation is needed in this part of the Beach Mountain section especially. Except that they both consist predominantly of car­bonate rocks and seem to have a certain correspondence of arenaceous zones, there is little lithic similarity between these two sections, and neither is at all similar to equivalent parts of the Ellenburger group. For detail on the El Paso formation in the El Paso section, its description, in Part 3 of this report, should be consulted. Although the general equivalence of the El Paso and Beach Mountain sections and their correlation with standard rock units in the Llano and Ozark uplifts seems to be established within indicated limits by the faunal evidence, it should be kept in mind that the exact placement of the faunal boundaries is still open to question. COLORADO RocKY MouNTAIN FRONT Rocks equivalent to the Ellenburger group in the Rocky Mountain Front are included in the Manitou formation, a stratigraphic interval of brick red to gray dolomite and limestone referred to in re­ports as the Manitou limestone and the Manitou dolomite. The Manitou formation and the Sawatch quartzite (?) upon which it rests have been described by Brainerd, Baldwin, and Keyte ( 1933) ; G. I. Finlay (1916); G. B. Richardson (1915); and others. The following remarks are based on the published record, supplemented by four days of field observation by the authors under the guidance of Harry W. Oborne in October 1945. Time in the field was limited and results inconclu­sive, but discussion may serve to indicate some of the questions which detailed work in the lower Paleozoic of the Front Range should seek to answer. The brick red arkosic sandstones and light yellow sandstones that underlie the Manitou formation in the Front Range north of Deadman's Canyon have been correlated with the Sawatch quartzite "be­cause of lithologic character and strati­graphic position" (Finlay, 1916). The cited fauna! evidence for this correlation is weak, and a published list of identifi­cations by E. 0. Ulrich (in Richardson, 1915) of fossils from the "basal red arenaceous limestone" of the Manitou for­mation contains names of fossils that would now be considered diagnostic of the Upper Cambrian along with names of equally diagnostic Lower Ordovician fos­sils. Judged from the names cited, either this collection is mixed and represents a considerable time interval, or the ranges of ordinary Cambrian and Ordovician fos­sils are hopelessly extended. The degree of apparent mixture (Billingsella and Cameroceras) is so great that the first possibility is considered the more likely, and the listing of such fossils as Billings­ella plicatella and Owenella seems to leave little room for doubt that Upper Cambrian rocks immediately underlie the Manitou formation in the Front Range. On the basis of the published information, there­fore, and not as a result of their own ob­servations in the field, the authors pro­visionally accept the designation of the sandstones below the Manitou formation as Upper Cambrian and a correlative of the Sawatch quartzite. In the considerable metamorphosis which the concept implied by the term Manitou has undergone since its proposal by Cross in 1894 the recent contribution of Brainerd, Baldwin, and Keyte (1933) scored a significant advance by the re­moval of younger rocks from the top of the Manitou limestone of Richardson (1915) and Finlay (1916). The six sec­tions examined by the present authors were at or near sections l, 3, 5, 7, 8, and 11 shown in the stratigraphic cross sec­tions of Brainerd, Baldwin, and Keyte (their figures 9 and 10) and the reader is referred to their report for visualiza­tion of local stratigraphic relationships. The present authors saw no limestone in the Manitou formation south of Wil­liams Canyon; a section west of Beulah (No. l of Brainerd, Baldwin, and Keyte) contains no representative of the Mani­tou; a section west of Wellsville (No. 11) is largely a very fine grained to The University of Texas Publication No. 4621 microgranular, gray to olive gray dolo­mite; and sections near Priest's Canyon (No. 3) and in Phantom Canyon (No. 5) are entirely of very fine grained rose to brick red dolomite with irregularly dis­tributed semichalcedonic to subchalce· donic, spiculiferous chert. On the other hand, the sections at Williams Canyon (No. 7) and Missomi Gulch (No. 8) are largely limestone. The dolomitic sections seen all rest directly on pre-Cambrian rocks, whereas Lhe calcitic sections rest on strata correlated with the Sawatch quartzite. This is in line with the concept of dolomitization as a near-shore or shal­low water phenomenon (p. 94). The dolo­mitic sections were also very sparingly fossiliferous, only one collection being ohlained from each (CF-1, CF-2, CF-3). This meager representation permits only the suggeslion that, as would be expected, the lower Manitou strata are missing where that formation directly overlaps the pre-Cambrian. The thickest, best exposed, and most fossiliferous section visited was that above the "Narrows" in Williams Canyon (Pl. 30, fig. D; Pl. LS) at the town of Manitou, and the suggestion by Brainerd, Baldwin, and Keyte that it be considered representa· tive of the type section is endorsed by the present writers. The Manilou formation in this section is approximately 200 feet thick and is comprised largely of fine grained, light gray limestone. Fossils were obtained at H levels within it ( CF-1. through CF-17). Tanyard, Gorman, and possibly Honeycut affinities are indicated by the various collections, hut the authors are unable at this time to suggest logical boundaries between faunal units. No clearly defined boundary was found be­tween the Manitou formation and the rocks correlated with the Sawatch quartzite. It was, therefore, arbitrarily placed above the highest bed containing a conspicuous quantity of glauconile. Thinly interbed­ded dolomite, earthy dolomite, dolomitic limestone, calcareous sandstone, and are· naceous dolomite and lime:3tone above the Manitou formation in the Williams Can­yon section was named Lhe Williams Canyon limestone by Brainerd, Baldwin, and Keyte ( 1933) and considered by them to be possibly of Devonian age. These beds occupy a stratigraphic pos1t1on held by the Middle Ordovician Harding sand­ stone farther south and, until faunal evi· deuce of a different age is submitted, their correlaLion with the Harding sand­ stone remains a probability. The Williams Canyon section is described more fully in Part 3 of the present report. Manitou Park is an alternative type area for the Manitou formation and the one which strict grammatical construction of Cross's original description (1894, p. 2) might consider to he the designated type area. A section in Missouri Gulch (No. 8 of Brainerd, Baldwin, and Keyte), on the mountain slope east oI Manilou Park, near the headwaters of Trout Creek, displays in the vicinity of 100 feet of Manitou strata. This section is not quite a mile by speedometer cast of Colorado highway 67 between Lhe gulch-side quarry of the Manitou Forest Experiment Sta­tion (Pl. 30, fig. C) and a Forest Service weir. The rocks included in the Manitou at this place are mostly fine grained, slabby-weathering, in part slightly dolo­mitic, rose to gray limestone with a few intervals of nodular and marly limestone near the middle. Apheoorthis is abun· dantly represented by two or more species that occur silicified in rose limestone from about 20 to 40 feet below the top (CF­18). It suggests that rocks equivalent to Lhe lower part of the Manitou formation in the Williams Canyon section are here more thickly represented, whereas most of the higher strata of the Manitou of the Williams Canyon section are absent at Missouri Gulch. OTHER REGIONS The areas of oulcropping Lower Ordo­vician rocks most directly related to the Ellenburger group have been discussed above and it is beyond both the scope of this report, and, in most instances, of the authors' experience to consider other areas in detail. Edwin Kirk (1934) has dis­cussed the correlation of Lower Ordo­vician rocks over a wide area, and the correlation chart in preparation by the National Research Council's subcommittee on the Ordovician will show the correla­tion of Lower Ordovician rocks in many parts of North America not considered in The Ellenburger Group of Central Texas the present report. Four regions contain­ing carbonate rocks of particular interest to the student of Lower Ordovician stratig­raphy are, however, briefly noled below. Except for the southern Appalachian re­gion, the descriptive and fauna! data are taken wholly from the writings of others, but the present authors are responsible for all conclusions not consislent with those generally accepted. UPPER MISSISSIPPIAN VALLEY The characters, history, and correla­tion of the Lower Ordovician Prairie du Chien group of the upper Mississippi Valley have been excellently summarized by G. M. Kay (1935), E. H. Powers (1935), and A. C. Trowbridge and others (1935), and the faunas of the Oneota and Shakopee dolomites have been de­scribed by L H. Powell (1935) and C. R. Stauffer (1937). The Prairie du Chien group, according to Trowbridge and others (1935), aver­ages 190 to 245 feet thick, and it consists principally of dolomite with minor sand­stone and siltstone. The Oneota dolomite with the Kasota sandstone and Blue Earll~ siltstone at its base locally, comprises the lower two-thirds to three-fourths of this sequence and is, judged from fossils fig­ured by Powell, of Tanyard age. At many places, a thin sandstone with locally interbedded dolomite intervenes between the Oneota dolomite and the 11ext dolomitic unit above. This sandstone has generally gone by the name of the New Richmond sandstone. Except for the stro­matolite cryptozoon, no fossils have been recorded from the New Richmond; hut iLs stratigraphic position and arenaceous nature suggest correlation with the Roubi­doux formation of the Ozark uplift and indirectly with the Gorman formation of tbe Llano uplift. A unit predominantly of (lolomite, but with minor thin sandstone beds, consti­tutes the upper one-third to one-fifth of the Prairie du Chien group and is called the Shakopee dolomite by some and the Willow River limestone or dolomite by others. Powers (1935, p. 394) states that information from well cutlings from the city well at Shakopee "shows that the Shakopee type section helongs within the lower 65 feet of the 126 feet of Oneota dolomite, which is overlain by 2--4 feet of New Richmond sandstone, and that in turn by 61 feet of Willow River dolo­mite." He says, "It is preferable to return to the term Willow River" (of L. C. Wooster, 1882). The U.S. Geological Sur­vey recognizes the name Shakopee dolo­mite, as does Dr. Stauffer, who has de­scribed its fossils. Josiah Bridge (1930, p. 129) recognized partial equivalence between Jefferson City and Shakopee strata. J. S. Cullison (194'4, pp. 23, 32) holds that "the fauna described by Stauffer (1937) from the Shakopee dolomite at Cannon Falls, Minnesota," has its closest affinilies with the middle part of his J ef­ferson City group and thus probably about the middle of the Honeycut formation; but a small capuliform Ceratopea figured by Stauffer (1937, Pl. 9, figs. 3, 9, and l 0) suggests lo the present authors the lower third of the Honeycut formation. The Shakopee fauna described by Stauffer (1937) from Stillwater, Minnesota, is recognized by both Staufier and Cullison (J 9!J 4, p. 39) as of Cotter age. Of the Prairie du Chien group in gen­eral Powers (1935, p. 390) says, "lntra­formational conglomerates, oolites, glau­conite, chert, Cryptozoon reefs, porosity, kind oI dolomite, shale and siltstone serve as means of identifying many beds." CHAMPLAIN VALLEY IN VERMONT AND NEw YonK An historic sequence of Lower Ordo­vician rocks in the Champlain Valley region of Vermont and New York State, known as the Beekmantown group, con­sists of dolomite, limestone, and sand­stone and has a total thickness of about 1300 ieet (Brainerd and Seely, 1890; see Wilmarth, 1938, pp. 145-148). This group of rocks includes the upper part of Division A and all of Divisions B, C, D, and E of Brainerd and Seely (1890). Judged from lists of fossils compiled from the reports of E. 0. Ulrich and coauthors on the "Ozarkian and Canadian" faunas (1938, 1942, 1943, 1944), it includes cor­relatives of rocks from the age of the The University of Texas Publication No. 4621 Tanyard formation through the Powell dolomite and perhaps higher. A three­ fold stratigraphic division is generally recognized. The Tribes Hill limestone is equiva­lent to the upper part of Division A and all of Division B of Brainerd and Seely. It comprises the basal portion of the Ordovician in this region, and was Jast correlated with strata of post-Tanyard age by Ulrich (in Ulrich and Cooper, 1938, Pl. 58) hut the fossils said to occur in it by Ulrich and coauthors indicate a Tanyard age to the prese:r:it writers. The boundary between the fauna! equivalents of the Tanyard and Gorman formations (Ulrich's Ozarkian-Canadian boundary) in the Champlain Valley south of Canada probably should fall at or near the top .of the Tribes Hill limestone. The middle unit of the Lower Ordo­vician in the Champlain Valley includes Division C and the lower part of Division D of Brainerd and Seeley, consisting of dolomite with interheds of sandstone fol­lowing a basal calcareous sandstone about 60 feet thick and grading to limestone in the upper part. It is known to many geologists as the Ogdensburg formation, contains Lecanospira in the lower part (lower part of Division C, fide Bridge, 1930, p. 124), and is therefore at least in part of Gorman age. Like the equiva­lent Gorman and Roubidoux formations, the Ogdensburg is characteristically more arenaceous than the immediately overly­ing and underlying rocks. It is apparent from the contained faunas, and it has long been known, that the upper unit of the Champlain Valley sequence, comprising the upper part of Division D and all of Division E of Brain­erd and Seeley, and known to many geologists as the Fort Cassin or Cassin formation or limestone, is of late Lower Ordovician age, younger than rocks equivalent to the Gorman formation. The fossils listed by Ulrich and coauthors in­dicate that its upper portion is at least as young as the Powell formation of the Ozark region and possibly younger, and Cullison (1944, p. 46) states that, "The general aspect of the Powell fauna is like that of the Ft. Cassin beds of Vermont." VICINITY OF PHILLIPSBURG, QUEBEC The Lower Ordovician carbonate rocks in the vicinity of Phillipsburg, Quebec, are of special biostratigraphic interest be­cause of the peculiar mechanism postu­lated by Dr. Ulrich to explain the apparent mixture of "Ozarkian" and "Canadian" faunas in the "Hastings Creek and Luke Hill formations" (see Ulrich and Cooper, 1938, Pl. 58). A similar problem faced the present authors in the lower part of the Beach Mountain section (pp. 69-70) hut they were prevented by unimpeachable physical evidence from postulating either a submarine cave, such as the "Naylor Ledge formation," or a collapse of younger beds into older, such as they have evi­dence for at many other places. An interpretation of the Phillipsburg sequence that would he consistent with the evidence in the Beach Mountain sec­tion would move Ulrich's "Ozarkian­Canadian" boundary (coincident with the boundary between the fauna! equivalents of the Tanyard and Gorman formations) downward to a point within or at the base of the "Hastings Creek formation." SOUTHERN APPALACHIAN REGION The Lower Ordovician rocks of this re­gion and of the Ozark region of Missouri and northern Arkansas, as was mentioned thereunder, arc more like those of the Ellenburger group in their faunas and the types of residual chert they yield on weathering than any other strata of like age known to the authors. Like the Ellen­burger they include both limestone and dolomite, but unlike the Ellenhurger they contain definite beds and zones of sand at several stratigraphic levels. Weathering is so intense and outcrops so few in the part of the region familiar to the authors (northeastern Alabama) that the propor­tions of limestone and dolomite and their lateral behavior are not known, hut it is safe to say that lateral transition occurs there as it does in Texas. The Lower Ordovician of Alabama has been described and some of its fossils figured by Charles Butts (1926, pp. 87­ 101, Pls. 15-19). From the base upward, the named stratigraphic units are the The Ellenburger Group of Central Texas Chepultepec dolomite, ihe Longview lime­stone, the Newala limestone, and the Oden­ville limestone. According to Butts the Chepultepec dolomite is "at least 1000 feet thick" at Chepultepec, while the Long­view and Newala Hmestones are said by him to be about 500 and l 000 feet thick, respectively, in the Cahaba Valley region. The Odenville limestone is reported to outcrop at only one locality, where it is ahout SO feet thick, but its fossils have been found in thB residuum above the Newala limestone at other places. In Cherokee County, in northeastern Ala­bama, the Chepultepec dolomite and the Longview limestone appear to be a little thinner, and the Newala limestone much thinner, than the same units in the Cahaba Valley, and no trace of the Odenville limestone was found. Exposures of the Longview in Cherokee County were mostly dolomite and sandstone, but limestone does not outcrop well in that area. Equivalence of the Lower Ordovician rocks of the southern Appalachian region to those of the Ellenburger group and El Paso formation is as follows: the Chepultepec dolomite correlates with the Tanyard formation; the Longview lime­stone is approximately equivalent to the Gorman formation; the Newala lime­stone is in part equivalent to the Honeycut formation, but includes younger rocks as well; and the Odenville limestone is about equivalent to Unit C of the El Paso formation. LITHOGENIC AND PALEOECOLOGIC SPECULATIONS INTRODUCTORY STATEMENT The genesis of sedimentary rocks in­volves three closely related problems: (] ) under what conditions were the orig­inal sediments formed and brought to rest; (2) what was the character of the original sediments; and (3) through what diageµetic processes, acting at what time, did the rocks in question attain their present lithic character. The character of the original sediments is indicative of the environment under which they formed and came to rest, and the study of the rela­tions of this environment to the life of the times is paleoecology. The sorts of recent sediments that might, upon diagenesis, form rocks comparable with those of the Ellenburger group has important bearing on the prohable genesis of the lithic constituents of the Ellen­burger rocks. The discussion that follows attempts to correlate observed lithic and biologic features of the Ellenhurger rocks with known sedimentary and biologic con­ditions in recent seas. It is of value to do this for the reason that the origin and accumulation of pe­troleum is ultimately related to biologic and sedimentary processes, and to the en­vironment in which they had their being. In the ensuing discussion the descrip­tive and theoretical background, on the basis of which geologic conclusions are reached, is presented first. This is followed by sections on "Prohable oceanography and ecography of the Ellenburger sea" anrl "Comparison of the Ellenburger sea with other ancient marine provinces." The reader interested in conclusions alone may turn to these. RECENT SEDIMENTS POTENTIALLY COMPARABLE TO ELLENBURGER ROCKS FACTORS LIMITING ANALOGY Inasmuch as carbonate sediments capa­ble of forming dolomites without the addition of magnesium are not known to he forming anywhere in the world today, the probable origin of the limestones of the Ellenburger group is here primarily considered. The do 1 omites are reserved for discussion under "Genesis of the lithic constituents." The typical limestone of the Ellen­burger group is a calcite rock so extremely fine grained that it is described as sub­lithographic. It is apt to contain minor irregu1ar argillaceous films, patches of limestone pellets, and local small patches or veinlets of calcite. Above the Tan­yard formation it locally contains scat­tered sand grains, and chert is intermit­tently abundant above the lower part of the Tanyard formation. Glauconite is ex­Lremely rare and, for practical purposes, the rock is essentially non-glauconitic. Where it does not break along the minor The University of Texas Publication No. 4621 argillaceous films its fracture is smooth and almost conchoidal. Terrigenous ma­terials are scarce in limestones of the Ellenburger group. Slight stratigraphic and regional varia­tions are noticeable. The occasional oc­currem~es of sand grains in the known out­crops are, with rai·e and very inconspicu­ous exceptions, restricted to beds younger than the Tanyard formation; and the lime­stones of the Threadr;ill member of the Tanyard formation contain a greater pro­ portion of silty material in the western part of the region than they do in the cast. Now, although calcium carbonate oc­curs very widely in recent marine sedi­ments, even accounting for about 32 per cent of deep sea sediments (Vaughan, 1924a, pp. 312-313). places where -it is being formed in sufficient purity to com­prise a potential limestone are areally restricted. Biogenic and elastic accumula­tions, such as coral and algal assemblages and their associated elastic debris, colonies of other sessile bcnthos, coquinas or shell-breccias, limesands, and Globigerina­or pteropod-ooze, account for most mod­ern accumulations of relatively pure cal­cium carbonate. Obviously, the sublitho­graphic limestones of the Ellenburger group do not belong with any of these, although algae may have played a part in their deposition. In fact, the field of the chemically precipitated calcium car­bonate sediments must be searched to find similarities with limestones of the Ellen­burger group. Inasmuch as the Ellen· burger fossils demonstrate an essentially marine origin, only sediments formed in or in close association with marine waters are here considered. It is generally recognized that chemical precipitation of calcium carbonate may occur through the agency of either physi­ cal (physicochemical) or biologic (bio­chemical) processes that disturb the equilibrium of sea-waters that are sat­urated or supersaturated with calcium car­bonate. The organic factors have been stressed by some and the physical factors by others, but experimenlal work shows that both physicochemical and biochemi­cal precipitation can occur. Johnston and Williamson (1916, p. 733) point out that "in order to decide definitely if a natural water is saturated with respect to calcite one must know: (a) the con­centration of free C02 in the water, (b) the temperature, ( c) the concentrations of the other constituents present"; and that "of these the third is the only one which has in general been satisfactorily ascertained." Howf)ver, most investigators now "agree that the surface !avers of the ocean water in tropical and s~btropical regions arc saturated or even svpersaturated with reference to CaC03" (Vaughan, 1924,a, p. 325. See also Johnston and William­son, 1916, p. 735; and Black, 1933a, p. 457). Moberg and others (J 934, p. 272) indicate the possibility of relatively heavy supersaturation for the warm surface waters, although they questioned as ex­cessively high, supersaturation ratios such as were derived from data similar to theirs by Wattenberg and other German workers. C. L. Smith (1940, p. 171) says, "It is well known that the surface water of the Atlantic Ocean is heavily supersaturated with calcium carhonate." Revelle (1934, p. l) states that "Although the experimental results obtained by var· ious authors for the solubility product are not in agreement, it is at least certain that surface sea water at a temperature of 30° C. is saturated with calcium car­bonate." Ca(HC0 Calcium carbonate in solution is com­monly said to occur as the bicarbonate, 3) 2, although, of course, what is actually present in other than negligible amounts is Ca++ ions and HC03-ions. The status of calcium carbonate as precipi­tate or solute is thought to be related pri­marily to the concentration of co2, the pH, and interconnected factors. CaC03 precipitated from the saturated surface waters of the oceans is subject to re­solution where it sinks into cooler waters capable of holding more C02 , and therefore likely to be undersaturated with respect to CaC08 • A natural environment of chemically precipitated calcium car­bonate sediments in the sea is seen to be one of shallow waters in a tropical or semitropical region, where the circula­tion of the waters is restricted, yet suffi­cient to allow replenishment or replace­ment from time to time by waters rich The Ellenbnrger Gronp of Central Texas in Ca++ and HCOa-ions. The precipita· tion of CaC03 through interactions of other soluble compounds with CaSO 1 is noted on page 85. The present known sites of the un· doubted occurrence in the sea of chemi· cally precipitated CaC03 , identified by the occurrence of aragonite needles, are the Bahama Banks (many references), "some of the flats behind the Florida Keys" (Vaughan, 1924a, p. 321), and "within an inclosed lagoon at Maiao, in the South Pacific" (Thorp, 1936, µ. 92). The environment in all instances is one of shallow, warm, relatively sluggish waters; but from what is known of the sediments being formed at these places, those of the Bahama Banks most nearly recall the limestones of the Ellenburger group. Temporarily reserving comment on the mineralogic composition of the calcium carbonate muds on the Bahama Banks, we may note remarks, inspired by one (Maurice Black) who was intimately ac· quainted with them, on their probable lithification features: "Aragonite mud behaves, in many respects like a clay sediment. ... On lithification, the ara­gonite recrystallises as a dense and tightly welded mass of calcite granules ... and . . . shrinkage cracks are formed. These spaces later become filled with (relatively) coarsely crystalline calcite deposited slowly from solution." (Hatch, Rastall, and Black, 1938, p. 174.) Black also be­lieved that "Amongst the older limestones, the calcite mudstones are the lithified rep· resentatives of the modern aragonite muds. . . . The purer calcite mudstones are pale grey in colour, and have a smooth, almost conchoidal fracture.... Thin sec­tions show that these rocks consist of extremely minute crystals of calcite, the individuals being so small that they render the slice semi-opaque. Running through this fine-grained rock are irregularly branching little veins of clear calcite, in comparatively coarse crystals." (Hatch, Rastall, and Black, 1938, pp. 173-174.. ) The similarities to the limestones of the Ellenburger group are apparent. An environment of origin for the lime­stones of the Ellenburger group similar to that on the present Bahama Banks is indicated. Needless to say, an isolated "hank" is not required; although the Ellcnburger sediments could have been formed in an area rdatively elevated with respect to the surrounding sea floor. It has been pointed out that the reduced cir­culation and other conditions on wide shoal ·water shelves and hanks "resemble in some respects the conditions in iso­lated lagoons" (Hatch, Rastall, and Black, 1938, p. 171); and the protected bank areas are, in a sense, evaporating pans. Similar conditions may well have been, and probably at times were, wide­spread in epicontinental seas in shallow areas not receiving quantities of ter­rigenous sediments; such as might he found far from shore, adjacent to land approaching base level, or separated from the shore by relatively deep, current· swept channels. Grwf:RAPHIC, SEDIMENTARY, AND BroLoc1c FEATUims OF THE BAHAMA BANKS It may he profitable to examine more closely the geography and sedimentary features of the Bahama Banks to see what sorts of conditions are there displayed. A map of the entire arfm involVf~d is given in a report by Drew (1914, Chart A) and some additional maps of interest are given by Drew (1914, Chart B) ; Field (1928, p. 122); Field nnd others (1931, fig. 1) ; Bavendamm (19,)2, figs. 1-2); Black (1933a, fig. l; 1933b, fig. I); Thorp (1936, fig. 3); and C. L. Smith (J 91.0, figs. M-52). From these maps it may be seen that the complex of shoaly banks, islands, and intersecting channels of the Bahama Banks comprises a general shoaly area southeast of Florida and north of the east end of Cuba. The banks themselves extend to within 60 miles of Florida and 50 miles of Cuba, but are separated from both by deep channels. The general area involved is kidney shaped, convex at the southwest, and elongated in a north-northwest to south-southeast direction. Tt is about 450 miles long and 170 miles wide at the middle, tapering bluntly to either end. Within the general area outlined about 30,000 square miles consist of "banks" The University of Texas Publication No. 4621 I which are today receiving or have re­cently received precipitated calcium car­bonate muds. Those parts of the hank areas that are swept hy currents tend to be surfaced by limesands (Black, 1933a, p. 462), but below the seasonably stag­nant waters in the lee of (west of) Andros Island an area of about 4500 square miles (up to 70 miles wide and 90 miles long) is known to be floored with deposits that contain a large proportion of aragonite needles (Black, 1933a, p. 461, fig. 1; Field et al., 1931, fig. 1). This area of known aragonite muds is itself about as large as the surface expression of the Llano uplift (approximately 4400 square miles) , and the total area of potential calcium carbonate precipitation is about the size of Maine or one-ninth as large as Texas. Great Bahama Bank, to the south, is separated from Little Bahama Bank, to the north, by a deep channel. It is divided into a smaller eastern and a larger west· ern lobe by a tongue of the ocean. This tongue of the ocean is 120 miles long, averages 25 miles wide, and ranges in depth from 1200 fathoms at its north end to 795 fathoms at its south end. It is a deep czd-de-sac in a shallow bank. Great Bahama Bank itself is mostly less than 3 fathoms deep, and apparently does not exceed 5 fathoms in depth except on its shelving margins. On the west side of the tongue of the ocean lies Andros Island, 140 miles long in a north-south direction, protecting a large part of the western lobe of Great Bahama Bank from cur­rent sweeping and encouraging conditions favorable to the precipitation of calcium carbonate muds. Little Bahama Bank, on the north side of Providence Channel from Great Bahama Bank, is bounded at the south by Bahama Island and at the east by Great Abaco Island. Theoretically the sheltered bank area west of Great Abaco Island should constitute an evaporating pan similar to that west of Andros Island and likewise favoring the accumulation of calcium carbonate muds. The entire shQal area is sharply delimited, dropping abruptly, "at a gradient of one in three for the upper part of the slope, to depths varying between 2,500 and 6,000 feet" (Black, 1933a, p. 455). "According to gravity data the Bahama Block is not underlain by igneous rocks, ... is approximately in isostatic equili­brium, and has undergone recent oscilla­tion in relation to sealevel. Structural phenomena on the margins of the block indicate long and persistent fault-zones; the block has risen or is rising on the east and is sinking or has sunk on the west" (Field and others, 1931, p. 783). Of the sediments Black says (1933b, p. 167) : "The rocks which form the islands are very pure limestones of Pleisto­cene Age, and the modern sediments on the banks are also entirely of calcite and aragonite, without any admixture of silice­ous or argillaceous material (Thorp, 1936, p. 82 says-no appreciable terrigenous material) . . . . Andros Island . . . is the largest island in the Bahamas. Along the east coast there is a well developed bar­rier reef backed by a narrow ridge of limestone hills.... Low-lving marshy plains, interspersed with shallow lakes and outcrops of limestone are found in the interior, whilst in the westernmost part of the island . . . the country con­sists of white, unconsolidated limestone­mud . . . with bare white drewite6 flats, (and) an intricate system of tidal creeks and mangrove swamps, which ren­der it liable to heavy flooding under favorable circumstances." Drew (1914, p. 33) continues this description as fol­lows: "Towards the west of the island (Andros) the land is remarkably flat, and near the coast consists of white chalky mud, which has partially dried, and in places has formed a harder crust on the surface. These half-dried mud-flats slope almost imperceptibly into the sea, and are continuous with the submarine flats which extend some sixty miles off the coast with an average depth of two to three fathoms. Tli,e mud forming these submerged flats is very soft, and near the coast it was easily possible to push a ovnughan (1924. p. 316) pointed out that the teun "d1ewite," as originci.lly described by Field (1920, Car­ negie Inst. Washington, Yea:i.book 18, p, 197) was Hpplicd to fine giained, mostly detrital muds in TortngM! lagoon, and that it is not similar t[) the mainly chemically pre­ cipitated calcium carbonate muds to which Kindle (1923) applied it {Tho1p, 1936, pp. SO. 81) and for which Black here uses it. The Ellenburger Group of Central Texas twelve-foot sponge pole down t.o its full length into it without touching any harder material. The surface layer of the mud for a depth of about 6 inches is of a creamy white color, but helow that it is of a grayish tinge and has a slight odor of sulphureted hydrogen." According to Field and others (1931, p. 774) "the bulk of the fine-grain mud . . . is composed of crystalline grains of calcite and aragonite, 0.05 to 0.01 milli­meters, and fine needles of aragonite 0.003 to 0.005 millimeters in diameter." Black states (1933a, p. 461) that "In the centre of the shoals west of North Andros Island, Aragonite needles make up about 20 per cent of the sediment, the other constituents heing shells of foraminifera, chips of mollusc shells, and some lime­stone grains." References to mud cracks and ripple marks in the submerged muds were not noted, but dessication cracking of the muds on Anfeet :'.:t:::2~ Fig. 7. Geologic map of an area in the vicinity 9£ the Victoria Gravel Company quarries, Burnet County,' Texas. The Ellenburger Group of Central Texas stone only because it is farther from the crushing plant. It is in the top of the Threadgill member of Lhe Tanyard for­mation. Adjacent ground both north and south from the present quarries is favorable so far as railroad transportation is con­cerned; but much of the surface in these areas is of low relief, making it difficult to start a quarry. The Backbone Moun­tain area of the present report (Pl. 11) is favorably situated for establishing quar­ries, and only short railroad spurs would be needed. In fact Backbone Ridge (Bur­net County) in general is the most acces­sible portion of the Ellenburger outcrop area for location of quarries. Long Mountain in Llano County is topped by Ellenburger rocks, beneath which is much Pedernales dolomite. The southwestern end of Long Mountain is along a railroad, offering a favorable site for quarry location even though the initial work would be difficult because of the steepness of this end of the mountain. A quarry from which crushed stone was pro­duced for construction of Buchanan Dam is located at the northeastern end of Long Mountain. No effort has been made to determine whether this quarry is in the Pedernales dolomite member of the Wil­berns formation or in the Tanyard forma­tion. It has not operated since the com­pletion of the dam. Small inliers of the dolomitic facies of the Gorman formation are situated along Mesquite Creek a short distance both east and west of a railroad, north of Lake Vic­tor, Burnet County. A small quarry in the western inlier opened for road mate­rial has long been abandoned. These in­liers are in valley bottoms and, in general, are not favorably located for the establish­ ment of large quarries, since stripping of the overlying Cretaceous rocks would soon become necessary. A small abandoned quarry, described under locality Bl-2 in The University of Texas Publication 4.246, is located 7.7 miles west of Johnson City and about 100 yards downstream from the highway crossing over Rocky Creek. The quarry is in the Pedernales dolomite member of the Wilberns formation and the stone was probably used for local road construction. The localities where Ellen­burger and immediately subjacent Cam­brian carbonate rocks are most accessible to railroad transportation have been enu­merated. Another prominent group of Ellenburgcr outcrops runs roughly paral­lel to and about 4, miles south of the rail­road between San Saba and Brady in the northern part of the Llano uplift. The Cherokee area includes the more easterly of these outcrops, and quarry sites could be located anywhere along the Simpson Creek fault zone. The crushed stone now produced from the Ellenburger by the Victoria Gravel Company is used mostly for railroad bal­last, road metal, and for concrete aggre­gat~-Since the quarry opened in 1937 about 4 million tons of crushed stone has been produced. The quarry in the SLaende­bach member of the Tanyard formation contains an abundance of chert, which prohibits the use of this stone for any chemical use. The quarry in the Thread­gill member of the Tanyard formation is low in impurities, and Lhe stone may be used for practically any chemical use for which a high grade dolomite is needed. This rock is also of value for crushed stone and as stone chips to be used for stucco, roofing, and terrazzo. Explanation of figure 7, illustration on opposite pagc.-Formations and members of the Ellen­burger group of Ordovician age present are as follows: Og(mg), dolomitic facies of the Gorman formation (in part collapsed into the Tanyard formation); Ots(ca), calcitic facies and Ots(mg), dolomitic facies of the Staendehach member of the Tanyard formation; Ott (mg), dolomitic facies and Ott (ca), calcitic focies of the Threadgill member of the Tanyard formation. Cambrian formations and members present arc as follows: Wilberns formation--ewp, Pedernales dolomite; cwpp, Point Peak shale; ewm, Morgan Creek limestone and eww, Welge sandstone members; Riley formation--erL Lion Mountain sandstone member. In the vicinity of the quarry for magne~ium ore, areas outlined by dots and labeled A indicate rock containing in excess of 30 per cent MgC03, and areas labeled B indicate rock containing less than 30 per cent MgCO,. Symbols such as 9-5E indicate the locality of a fossil collection. Base from U. S. Department of AgriculLure, Soil Conservation Service, aerial photographs flown by Park Aerial Surveys, Inc., 1939-1940. Geology by Virgil E. Barnes and Lincoln E. Warren, 1944-45. The University of Texas Publication No. 4621 In much of the eastern part of the Llano region, the microgranular dolomites of the Gorman formation are varicolored in yellowish. pinkish, and brownish tones such as beige, rose beige, rose pink, cin­namon pink, tan, old ivory, and nutria. Many of these colors are desirable for terrazzo chips, and the latter have been produced from several small pits (see Univ. Texas Pub. 424,6, locality Bu-4.4), mostly about 3.5 miles south of Sudduth along the eastern side of U.S. highway 281. The most highly colored microgranu­lar dolomite in this area is near the base of the Gorman formation where it is faulted against the Hickory sandstone member of the Riley formation. The stone is of a dark red color. Another pit from which some chips for terrazzo, as well as crushed stone for high­way surfacing and concrete aggregate, were taken is located near Sudduth at the intersection of the railroad and the highway. This pit is in coarse grained dolomite, probably of the Threadgill member of the Tanyard formation, but it could be in the coarse grained portion of the Pedernales dolomite member of the Wilberns formation. The stone is mottled pinkish gray to brownish gray and is de­scribed under locality number Bu-36 in The University of Texas Publication 4246. Other localities from which terrazzo chips have been obtained are described under locality numbers Bu-20, Bu-33, and Bu-39. Locality Bu-20 is located al@g Hamilton Creek at the south city limits of Burnet where the dolomite is colored pink with breccia-like areas of ivory. This deposit is an inlier surrounded by Cretaceous rocks and probably formed as a weathering breccia or caliche on the pre-Cretaceous surface. An ivory colored limestone occurs at locality Bu-33, about 1.5 miles by road northwest of Burnet, along Hamilton Creek. Dolomite locality Bu-39 is east of the highway about 1 mile south of Burnet. The pits from which ter­razzo chips are produced are mostly small, and, as their operation is mostly periodic, some may have been owrlooked. Also other pits have probably been opene'~ since the field work for Publication 4246 was completed. The Ellenburger rock is good for filter stone, and the fines from crushing when properly screened can be used for stone sand. The University of Texas Publication 4246 covers the use of the Ellenburger as a building stone and discusses 28 locali­ties where deposits have been or might be of value for building and monumental Slone. Five quarry sites of the Vermont Marble Company in San Saba County, all in limestone, are discussed under locality S-1. One of these, in the Threadgill mem­ber of the Tanyard formation (Pl. 20, fig. C), has stone of good quality. A small quarry in the Staendebach member of the Tanyard formation, in the southeast corner of the W. R. Payne ranch, contains much chert and the stylolites are rather open. Limestone in a pair of small quar­ries high in the Staendehach member, along State highway No. 16 on the H. C. Perry ranch, is also highly cherty. In a quarry in the calcitic facies of the Gorman formation, above McAnelly Canyon on the J. C. Creamer ranch, the stone is in­ferior in grade, the stylolites being open and the weathering of pyrite nodules causing unsightly stains. The two small quarries that comprise the fifth site of the group are also in the calcitic facies of the Gorman formation, along the San Saba-Chappel road one-fourth of a mile south of the Simpson Creek fault zone. In addition to those noted, two other small quarries in limestones of the Threadgill and Staendebach members of the Tanyard formation are shown on Plate 2. The only quarry containing promising stone is located in the Threadgill member of the Tanyard formation (Pl. 20, fig. C). Even here only a portion of the beds quarried is desirable; and were all beds in the sequence to be used, the quality of the product would be lowered. The field work for the building stone publication (Univ. Texas Pub. 4246) was done befor any of the present detailed information on the Ellenburgcr was known; conse­quently the sampling of the Ellenhurger was done in a hit-or-miss fashion. If the present information had been available, sampling would have been largely con­fined to the limestones of the Threadgill member of the Tanyard formation, the The Ellenburger Group of Central Texas uppermost portion of the calcitic facies of the Gorman formation, and the upper cal­ citic portion of the Honeycut formation. The upper calcitic portion of the Honey­ cut formation, being present only in a limited area of Blanco County and pos· sibly in a small area in eastern Burnet County, was not sampled at all during the building stone work. About 17 dolomite deposits are de­ scribed in the building stone publication. Few of these contain stone which is of building or monumental grade, and most of the localities are recommended only as being suitable for crushed stone. STONE UsED CHIEFLY BECAUSE OF !Ts CHEMICAL COMPOSITION Producing areas.-The only Ellenburger rock so far quarried because of its chemi­cal composition was dolomite that served as an ore of magnesium in the pl ant op­erated by the International Minerals & Chemical Corporation near Austin, Texas. A small amount of the same "ore" was shipped to the Mathiesen Alkali Company, Lake Charles, Louisiana. The quarry from which this rock was obtained is about 6 miles south of the town of Burnet in Bur­net County. It was operated by the Vic­toria Gravel Company and produced about one-quarter of a million tons of magne­sium ore. The geology in the vicinity of Victoria Gravel Company property is shown in figure 7. The quarry for mag­nesium ore is located west of the railroad and is in the top of the Threadgill mem­ber of the Tanyard formation. A fault divides the area shown in figure 7 into two parts. The western part con­tains Cambrian rocks that include from south to north the Lion Mountain sand­stone member of tbe Riley formation and the Welge sandstone, Morgan Creek lime­stone, Point Peak shale, and Pedernalcs dolomite members of the Wilberns forma­tion. The San Saba limestone member of the Wilberns is absent. East of the fault all of the rocks are Ordovician in age; the top portion of the Threadgill member of tl_ie Tanyard formation, the Staendebach member of the Tanyard formation, and the bottom portion of the Gorman formation are represented. The Threadgill member of the Tan· yard formation outcrops in two separate patches, the more northerly of which con­tains the quarry for magnesium ore. This outcrop is limited by normal overlap of the Staendebach member to the east and by faulting at the west. Rocks of the Threadgill member are brought to the surface at this place in a northeastward­plunging anticline. They belong mostly to the dolomitic facies of the Threadgill with only one small outcrop of limestone noted between drill holes 27 and 28. The out­crops southwest of drill hole number 25 are very poor, and the boundary between the Threadgill and Staendehach members in this area has been drawn on the basis of analytical data. The high silica content of holes 28, 29, and 31 (see "Chemical data," pp. 389-390), suggests that solu­tion of the underlying Threadgill member has allowed cherty dolomite of the Staen­debach member to collapse into the Threadgill member. Drill hole number 28 is in a dense clump of trees and under­brush between drill hole 6 and the one outcrop of limestone, and the relative density of vegetation suggests soil forma­tion and water circulation such as is favored by collapsed areas. If this is a collapsed area it would not have to he much larger to include drill holes 29 and 31. On the basis of chemical data the Threadgill member has been divided into zones that are marked on figure 7 by dotted lines. The two zones labeled A have a content of MgC08 above 30 per cent, ranging up to 4.7.90 per cent in drill hole 31; and the two zones labeled B have a content of MgC03 below 30 per cent, the lowest recorded being 16.35 per cent for drill hole 27. The chemical data from drill holes on this property substan­tiate the lateral gradation so often mapped during the work on the Ellenburger prob­lem. 'It also makes it evident that the only safe way to predict the chemical properties of rock in the Threadgill member is from analysed samples from holes drilled on a closely spaced grid. The only rock of chemical grade within the area covered by figure 7 is in the Threadgill member. From the patch of the dolomitic facies of the Threadgill member of the Tan­ yard mapped in the southern part of figure 7 similar coarse grained dolomite extends almost 2 miles to the south and includes an unknown amount of coarse grained dolomite of the Pedernales dolo· mite member of the Wilberns formation. The coarse grained Pedernales dolomite is probably of about the same grade as the dolomite in the Threadgill if analyses of the two obtained in Blanco County are indicative. The length of outcrop in this fault block is about 1.2 miles, and the surface, except in the southeastern portion, is flat and not well suited for the location of a quarry. The next fault to the east of the area shown in figure 7 drops the top of the Threadgill member to slightly more than one-half mile upstream from the falls on Hamilton Creek at Mormon Mill. Good quarry siLcs exist from 0.7 to 1.3 miles southeast of Sudduth station along the bluIT of Hamilton Creek and along a side drain of Hamilton Creek. Drilling, of course, would be necessary to determine the grade of the dolomite. The entire thickness of the Staendebach member of the Tanyard formation is ex­posed within the area mapped, and the quarry from which crushed stone is pro­duced is located near the middle of the dolomitic facies of the member. The cal­citic facies of the Staendebach member occurs at its top. It is laterally gradational to dolomite, with some sections devoid of limestone. The Gorman formation, com· posed mostly of microgranular dolomite and a few limestone beds, outcrops within the map area only in the vicinity of Honey Creek. The Tanyard-Gorman contact, especially in the area just east of figure 7, is in large part a collapse contact, with the Gorman at one place being dropped down many feet into the dolomite of the Stacndebach. Two small areas of Gor· man are collapsed into the Staendebach within the map area. Areas of potential prodnction.-Analy­ ses were made by Mr. R. M. Wheeler of selected portions of the sections of lime­stone and dolomite in t.Q.e Johnson City area and in the Cherokee area. Other sec· lions measured contain stone of chemical grade for which it would be desirable to have analyses, but furLhcr analytical serv­ices were not available. The analyses are listed under "Chemical Data" on pages 377-381, and the position of each sample in the section is given so that it can be compared with the graphic and written descriptions. In the Johnson City area analyses num­ber 1 and 2 are of rocks within the Point Peak shale member of the Wilberns forma­tion. Analyses 3 to 10, inclusive, are of the lower fine grained portion of the Pedernales dolomite member and are high in insoluble material and rather urii­formly high in magnesia. Analyses 11 to 20 are of the upper coarse grained portion of the Pedernales dolomite and, except for samples 11 and 17, are low in insoluble material and uniformly high in magnesia. Analysis 21 is of high grade rock strad­dling the Cambrian-Ordovician boundary. Analyses 22 to 28 are of the bottom por­tion of the Threadgill member of the Tan­yard formation and are similar to the coarse grained dolomites of the Peder­nales except that they contain about half as much Fe20,,. Analyses 29 to 35 are of the lower portion of the Staendebach member of the Tanyard formation and are all high in insoluble material and uni­formly high in magnesia. Analyses 36 to 4,9 are of the dolomitic facies of the Gor­man formation and are rather uniformly high in insoluble material and are slightly low in magnesia. Analyses 50 to 66 are of the middle dolomitic facies of the Honeycut formation and are similar to the dolomites in the Gorman formation. In the Cherokee area analyses 67 to 73, inclusive, are of the lower part of thP Threadgill member of the Tanyard for­mation. They are uniformly low in in­soluble material and, as compared with similar rocks in the Johnson City area, somewhat low in magnesia. Analyses 7!J to 83, of the upper part of the Threadgill member, have a wide but not uniform variation in insoluble content and an erratic and slightly low magnesia content. Analyses 84 to 92, of limestones from the calcitic portion of the Staendcbach member, are mostly very high in insoluble material and show an excessive amount of magnesia. Analyses 93 to 102, of the dolo­mitic facies of the Gorman formation, arc The Ellenburger Group of Central Texas considerably higher in insoluble material than equivalent rocks in the Johnson City area and lower in magnesia content than the same rocks. Analyses 103 to 109 are of limestone from the calcitic facies of the Gorman formation. The bottom three samples are rather high in magnesia, and two of the samples are high in insoluble material. The top four samples are from the prominent massive limestone zone at the top of the Gorman formation and are exceptionally pure limestone so far as magnesia is concerned and only slightly high in insoluble material. Analyses 110 to 113 are of limestones in the calcitic facies of the Threadgill member of the Tanyard formation from the Kirk ranch section and are laterally equivalent to dolomites analysed in the Cherokee sec­tion. Sample 113 is a dolomitic limestone, and the other three are almost free of magnesia. The insoluble material is ex­ceedingly high in these samples as com­pared with the equivalent samples from the Cherokee section. As the analyses were made from samples taken primarily for studies of insoluble residues, the chances are that the insoluble portion is abnor­mally high in most of them. From the above analyses it appears that the purest dolomites are from the Thread­gill member of the Tanyard formation and the coarse grained portion of the Pcdernales dolomite member of the Wil­berns formation. The purest limestone is probably in the topmost portion of the Gorman formation, and, where low in net­work dolomite, limestones in the Thread­gill member of the Tanyard might also be pure. It is possible that the insoluble ,material in the upper part of the Gorman at other places may be lower than the perhaps "loaded" analyses indicate it to be in the Cherokee area. Additional analyti­cal work should be done on the limestones both of the Threadgill and the top of the Gorman. Moreover, the upper limestone facies of the Honeycut formation in Honey­cut Bend should be analysed. Except for those mentioned under the section on "Producing Areas," above, few favorable occurrences of either the Thread­gill member of the Tanyard formation or the Pedernales dolomite member of the Wilberns formation are near a railroad. The most accessible area is probably south of Post Mountain near Burnet. No area of limestone in the Gorman formation is known to be really close to a railroad, and the Mill Creek portion of the com­posite Backbone Ridge section is probably the nearest. The upper massive limestone of the Gorman formation is well exposed and appears to he exceptionally pure in the vicinity of Longhorn Cavern. Outcrops of this upper limestone of the Gorman formation are within a few miles of a railroad in the Cherokee area (Pl. 2), and other areas about equally distant from the same railroad are probably present be­tween San Saba and Brady. All other known outcrops of this portion of the Gorman formation are far from railroad transportation. The upper limestone zone of the Gor· man formation and the dolomites of the Threadgill member of the Tanyard forma­tion and the Pedernales dolomite member of the Wilberns formation are suitable for agricultural "limestone." These "lime­stones" are relatively hard and expensive to crush, and for this reason may not be able to compete with the softer Cretaceous limestones. For the production of sodium carbonate by the Solvay process the upper limestone of the Gorman in places might be low enough in silica, and if limestones of the Threadgill member of the Tanyard were investigated in detail some would undoubtedly be found to contain less than one per cent silica and be low in magnesia. About the same specifications as for the Solvay process are required for limestone to be used in the Bayer process for the production of aluminum oxide and in the manufacture of sugar. At many localities limestones and dolomites of the Ellen­burger group are suitable for use in the production of calcium carbide, lime and carbon dioxide, dolomite refractories, filler for fertilizers, flux, glass, mineral feeds for stock, paper, poultry grit, technical carbonate, and probably for the production of many other commodities. WATER The number of springs issuing from each unit of the Ellenburger group in the The University of Texas Publication No. 4621 areas mapped has been enumerated on page 127. Almost half of these issue from the dolomitic facies of the Staendebach member of the Tanyard formation, and almost a quarter of them issue from the Honeycut formation. The flatter areas are deficient in springs, most of them being located in areas of considerable relief or along faults. Surface water also is lim­ ited in most of the flatter areas and must be supplemented for stock raising by im­ pounding water in earth dammed storage ponds locally known as tanks. Care must be taken in locating tanks on portions of the Ellenburger. Most of the microgranu­ lar dolomites are essentially impervious, and tanks situated on them, if the dam is constructed out of impervious mate­ rials, will ordinarily hold water. Expe­ rience shows that tanks are also likely to hold water if built on the Pedernales dolo­ mite member of the Wilberns formation regardless of its grain size, on either dolo­mites or limestones of the Threadgill mem­ber, or on limestones of the Staendebach member of the Tanyard. Many tanks built on the dolomitic facies of the Staendebach member wil1 not hold water and others will hold only a limited amount. Tank building in the calcitic portions of the Gorman and Honeycut formations can be successful if cavernous areas are avoided and the tanks are built on beds of micro­granular dolomite which are locally com­mon even in the caleitic portions of these formations. The major springs for the areas mapped are shown on the geologic maps and some are mentioned in the text description of the areas. In general, springs shown on the geologic maps may be expected to flow or seep for a good part of the year, but few are truly perennial or of large volume. Many other springs exist in areas of Ellen­ burger outcrop not mapped for the pres­ent publication. The Texas State Board of Water Engineers in cooperation with the United States Geological Survey has issued mimeographed reports on the "Rec­ords of wells and springs, drillers' logs, water analyses, and maps showing loca­tions of wells and springs" for Blanco County, 1932; Gillespie County, 1937; Mason County, ]940; and San Saba County, 1939. In these reports formations from which springs issue are not given, but in areas mapped the formation is known. The following information on the How of springs at some specific date is taken from the above noted publications on San Saba and on Blanco counties. Name of spring Gallons per minute San Saba County Gorman Falls area Gorman Spring 900 measured Sulphur Spring 600-700 estimated Clark Spring 50 estimated Seven Springs 250 measured Between Gorman and Tanyard areas Post Oak Spring 350 measured Jennings Creek Spring 650 measured South of Tanyard area along Fall Creek Boiling Spring 1900 measured Cherokee area Rough Creek Spring 220 measured Barnett Spring 405 measured McAnelly Spring 300 estimated Mud Spring 5 estimated Two unnamed springs 2-3 estimated Blanco County Johnson City area Unnamed (Crofts ranch) 60 measured Unnamed (Hyatt ranch) 5-10 estimated Salter Spring (Wier ranch) 10 estimated A group of springs known by the name of Three Springs, along Pedernales River, not included in the issued report, probably flow several hundred gallons per minute. The only spring mentioned in the Ellen­burger of the Bear Spring area in Mason County is Bear Spring and no estimate of its flow is given. By comparison with the springs noted, it is estimated that a num­ber of the other springs mapped in the various areas probably flow several hun­dred gallons a minute. PART 2 LOCAL STRATIGRAPHY BALD RIDGE AREA, McCULLOCH COUNTY INTRODUCTORY STATEMENT In a logical succession of Ellenburger sections from northwest to southeast in the Llano uplift, the thinnest and most abnor­mal is that displayed in the Bald Ridge area, in southeastern McCulloch County. The irregular area mapped lies between the towns of Brady and Camp San Saba and is approximately bounded on the southwest by U.S. highway 37, on the south by San Saba River, on the east by a north-south power line, on the northeast by the Brady-Vaca road, and on the north by an arbitrary line. About 11 square miles was mapped geologically between the latitudes of 31° 00' to 31° 05' north and longitudes of 99° 15' to 99° 19'. Plate 4 shows the geology of the area at a scale of 1:10,000. Control for the base map of the Bald Ridge area was taken from a Texas State Highway Department traverse of U.S. high· way 37. The base was plotted and adjusted by L. E. Warren from data recorded by the authors on aerial photographs of the U.S. Department of Agriculture. Ro:ks mapped within the Bald Ridge area mclude the Point Peak shale and San Saba limestone members of the Wilbcrns formation (Upper Cambrian), the Tan­ yard and Gorman formations of the Ellen­ burger group (Lower Ordovician), a small patch Qf Devonian rocks, and strata of Mississippian and Pennsylvanian age. All of the Honeycut formation and the upper 130 to 230 feet of the Gorman formation arc missing as a result of pre-Devonian truncation. The structure of the area is simple, a few very minor faults and occasional col­lapse structures being the principal com­plications. Dips are mostly quite low, and a few gentle rolls repeat beds in the north­ern part of the area. Because of the low dips, gentle topography, poor exposures, collapse structures, and structural rolls, stratigraphic measurements were difficult to make and the accuracy of those made is not guaranteed. The Bald Ridge area is of special in­terest in the Ellenburger problem because it lies at the northwestern corner of the Llano region and displays facies changes in the lower part of the Gorman forma­ tion that are of significance in predicting what the Ellenburger rocks of the adjacent subsurface might be 1ikc. Vegetal and topographic patterns as ex­pressed on aerial photographs are helpful in mapping. The limestones of the Thread­gill member of the Tanyard formation and the San Saba limestone member of the ~ilberns formation merge in a topograph­1cally rough area of fairly abundant cedaI in which bedding patterns are very evident. This pattern is distinct on the photographs from that of the Staendebach member of the Tanyard formation and the Gorman formation, which merge in an area of s~ooth and gently rolling topography with scattered clumps of live oak. Both l~ve oak and cedar grow on the higher limestones of the Gorman formation and tree growth is mostly thicker than on the underlying dolomites. Depressed areas, fuzzy with mesquite, betray the occurrence of shales of the Barnett formation; whereas the interbcdded limestones and marly zones of the Penn_sylvanian show conspicuous vegetal bandmg of cedar and mesquite. Several sections measured in and near the Bald Ridge area are described on pages 140-153 and diagrammatically represented on Plate 111,. UPPER CAMBRIAN WILBERNS FORMATION . The Welge sandstone and Morgan Creek limestone members of the Wilberns forma­tion are not present in the Bald Ridrre map area but are well displayed hetw~en its southeast corner and the mouth of Katemcy Creek, along San Saba River. The Welge sandstone is brown to brownish yellow, The University of Texas Publication No. 4621 sparingly glauconitic, and about 21. feet thick. The Morgan Creek limestone is granular, glauconitic, red to pink in the lower part and gray to greenish gray above, and about 125 feet thick. Eoorthis is abundant 68 feet above the base of the Wilberns formation at Flatrock Crossing of San Saba River, northeast of Camp San Saba. Elvinia, Pterocephalia, and other trilobites occur just below it and thumb­nail-shaped Billingsella is common from just below to several feet above it. The lower llO feet of the Point Peak shale member of the Wilberns formation consists of interbedded green shales; platy, crinkled, micaceous, green limestones; and granular, gray to grayish brown, glau­conitic limestones. The upper 80 feet is a zone of sublithographic to microgranu­lar, greenish to brownish gray to woodash gray, stromatolitic limestones locally inter­bedded with or laterally displaced by shale like that below. Local zones of fine grained dolomite occur near the top of this biohermal zone. On Plate 4, this upper 30-foot zone of almost continuous stro­matolitic bioherms is erroneously included with the San Saba limestone member. Such an inflexible application of member lithol­ogies, although defensible on some grounds, was shown by later study to obscure the actual stratigraphic relation­ships of the bioherms, but the map was already in the hands of the engraver. Pleclotrophia bridgei Ulrich and Cooper and a new species of Paleostrophia occur together about 20 feet below the principal zone of stromatolitic bioherms at the top of the Point Peak shale mem­ber. Stromatolitic limestone is also lo­cally interbedded with shales at and below the zone of Plectotrophia. The San Saba limestone member of the Wilberns formation in the Bald Ridge area consists of about 200 feet of mostly granular, intermittently glauco­nitic, brownish to greenish gray to gray­ish brown limestone that weathers gray to bluish and brownish gray to grayish brown. A conspicuous zone of yellowish limestone crowded with saukiinid trilobites marks the base of the member, and trilo­bites are locally abundant throughout it. LOWER ORDOVICIAN ELLENBDRGER Guoup Only about 830 feet of Ellenburger rocks are present in the Bald Ridge area, the Honeycut formation and the top beds of the Gorman formation being absent. Tanyard formation, Threadgill mem­ber.-The boundary between the Thread­gill and Staendebacl). members of the Tan­yard formation in the Bald Ridge area is drawn at the line of chemical change from limestone below to dolomite above. Thus the Threadgill member corresponds to the original "Threadgill limestone" of Bridge and Barnes (in Barnes, 191.1.). The low­est semiporcelaneous to subchalcedonic white-weathering cherts occur in the gen­eral vicinity of the transition from lime­stone to dolomite, and the stratigraphic thickness involved in the transition is so slight that it serves very well in place of the more difficultly mappable chert boundary. Limestone, with rare thin intcrbeds of dolomite, comprises the approximately 255 feet of the Threadgill member of the Bald Ridge area. It is predominantly sub­lithographic, essentially nonglauconitic, pearl gray and old ivory to light brown­ish gray, and weathers to medium to light bluish gray or woodash gray ledges sep­arated by intervals of poor exposure. Sections of Lytos pira and 0 phileta as well as dolomitized trails are locally abundant on bedding surfaces, but in gen­eral they arc less abundant and the lime­stone beds mostly thicker than in the Threadgill member of the Bear Spring and Lange's Mill areas. Gasconadia, oc­casional breviconic cephalopods, and abundant silicified Finkelnburgia occur locally. Quartz druse occurs on some beds as scattered rosettes and occasional incrus­tations. Near the top of the member at most places are thin plates and extensive lenslike inclusions of tan-tq russet-weath­ering chert composed of silicified frag­ments of fossils and pelletlike debris. Locally semiporcelaneo~s to subchalce­donic, nodular chert occurs near the top of the member, and more rarely at lower positions. The Ellenburger Group of Central Texas Separation of the Threadgill member of the Tanyard formation from the San Saba limestone member of the Wilberns forma· tion below is based on the change from the granular and glauconitic limestones of the latter to the sublithographic and essen· tially nonglauconitic limestones of the former, as well as on faunal distinctions. Tanyard formation, Starndebach mem· ber.-Except for the local occurrence of meshwork dolomitic limestones near the base and at the top, the approximately 280 feet of beds included in the Staende­ bach member of the Bald Ridge area is wholly dolomite. The dolomite of the lower third is mostly very fine grained to fine grained and light to medium gray to yellowish and brownish gray. At places it contains plates and lenses of subchalce· donic to porcelaneous, locally oolitic, chert rhat weathers light tan to shiny white. The dolomite of the upper two­thirds of the member is fine to medium grained, in large part vuggy, and light gray to light brownish and yellowish gray. It yields much more chert than the lower third, and its chert is for the most part conspicuously dolomoldic and drusy, with local blocks and layers that are subchalce­donic to chalcedonic and weather shiny white. Much oolitic to crypto-oolitic chert was seen and oolitic laminar chert occurs locally in the upper beds. Subchalcedonic to chalcedonic, commonly oolitic, white­wcathering cherts are locally conspicuous in the vicinity of the Tanyard-Gorman boundary. The Tanyard-Gorman boundary is diffi­cult to map in this area because of the relatively coarse grain size of the lower Gorman strata. It is placed below the lowest sand, the Tanyard formation being wholly nonarenaceous, and the local con· centr~tioJ:J. of while-weathering smooth· fracturing cherts near the boundary is a considerable aid to its approximate loca­tion. Typical Tanyard fossils such as Ozarkina, H elicotorna, and the dasyclada­ccan alga Rhabdoporella range to the top of the Staendebach member, and faunal distinction from the overlying Gorman for­mation is possible at most places where determined search is made. Gorman formation.-Pre-Devonian trun­cation in the Bald Ridge area removed all Honeycut strata, as well as the upper beds of the Gorman formation; and Devonian and Carboniferous rocks rest directly on beds possibly as much as 230 feet and as little as 130 feet bel~w the top 0f the Gorman formation. The remaining strata of the Gorman formation differ from those in other parts of the Llano region in that the lower beds are principally fine to medium grained dolomites that grade laterally to limestone, whereas microgranular dolomite is a rar­ity. Because they are so unusual in this part of the section, the intertonguing limestones within the mainly dolomitic lower part of the formation are shown on Plate 4, but dolomites within the mainly calcitic zone are not separately mapped. Tbe failure of the criterion of grain size of dolomites to be applicable in selecting the Tanyard-Gorman boundary in the northwest part of the Llano region was disappointing and bodes difficulty in subsurface work northwest from the Llano country. Fortunately sand ranges to the base of the Gorman formation in this area, while sand in the Tanyard forma· tion at the surface of the Llano uplift has so far been found only as extremely rare grains in the Moore Hollow area (p. 255). The concentration of porce­laneous to chalcedonic and locally oolitic white-weathering cherts in the vicinity of the Tanyard-Gorman contact is an aid to its location, and fauna! evidence serves to limit it closely. As in other areas Rhombella ranges to the base of the Gor­man formation, whereas Lecanospira first appears somewhat above the base. The fine lo medium grained dolomites of the Bald Ridge area are light gray, grading to brownish gray and grayish brown, and the limestones into which they grade commonly contain an internal dolo­mitic meshwork that weathers into reticu­late relief. The uncommonly occurring very fine grained lo microgranular dolo­mites display darker colors, and the upper limestones are purer and dominantly sub­lithographic. Chert is intermittently abundant in the Gorman formation of the Hald Ridge area, The University of Texas Publication No. 4621 occurring as nodules, lenses, plates, and thin bands that are mostly chalcedonic to porcelaneous and locally oolitic. Centers of the ooids are not uncommonly sand grains. Of more local occurrence is quartz druse, dolomoldic chert, and at places oolitic laminar chert such as normally characterizes the upper beds of the Tan­ yard formation. Of local prominence in the float are stromatolitic structures of the archaeosolon type (parallel pipes) outlined or replaced by drusy and dolo­ moldic chert. The principal zone of A~chaeoscyphia in the Gorman formation occurs about 210 feet above its base and a second oc­ currence was noted locally about 15 feet higher. Archaeoscyphia was also noted in beds apparently above its principal zone east of the Devonian joint-filling at TF-233 (marked F). Structural sinks are common in the vicinity of the Archaeo­ scyphia zone and its inferred trace on Plate 4 at many places crosses such col­ lapsed areas on a projected line, even though higher beds are actually known to be at the surface at such places. Its trace outlines well the gently synclinal struc­ ture of the area in which are the more prominent collapsed extensions of the Mississippian strata. DEVONIAN Medium yellowish brown, to olive gray, fine grained, slightly arenaceous, fossil­iferous limestone of Devonian age (TF­233) is plaslered into crannies and irregu­lar surfaces of the Gorman formation and occurs as float for about 250 feet along an east-west line about midway between High Lonesome Mill and the May Brook Kothman ranch headquarters. This rock apparently represents a joint-filling in rocks that are probably little above the principal zone of Archaeoscyphia in the Gorman formation. It is further described, referred with query to the Zesch formation, and its few fossils listed in a report by Barnes, Cloud, and Warren (1947, p. 139). CARBONIFEROUS MISSISSIPPIAN Only the Chappel limestone and the Barnett formation are known to represent the Mississippian of the Bald Ridge area. The Chappel limestone is well displayed at the northeast end of a large collapsed area on the May Brook Kothman' ranch, between the ranch headquarLers and the windmill known as High Lonesome. Ex­posures that extend north from fossil lo­cality TF-406 display about 25 feet of granular, crinoidal, rose pink to olive gray and plain gray limestones that are spottily fossiliferous. Some of the fos­siliferous pockets contain a rich, varied, and well preserved fauna of Chouteau age, including forms identified by Dr. G. A. Cooper as Stenoscisma sp., Sedenticellula sp., Shumardella obsolens (Hall), Rhyn­chopora cooperensis (Shumard), Produc­tella sp., Avonia sp., Plicatifera boonensis !Branson), Brachythyris chouteauensis Weller, B. girtyi Branson, Spirifer striati· formis Meek, S. aff. S. gregeri Weller, Pseudosyrinx, n.sp., Tylothyris sp., Cyrtina burlingtonensis Rowley, Plectospira sex­plicata (White and Whitfield), Reticularia cooperensis (Swallow), Athyris lamellosa (Leveille), and Cranaena aff. C. occiden­talis (Miller). From i! maximum observed thickness of about 25 feet, the Chappel limestone of the Bald Ridge area thins to a feather edge, occurring erratically along the collapsed Ellenburger-Mississippian contact. It var­ies liulc in appearance from that displayed in the vicinity of locality TF--406, but is rarely so fossiliferous. Excellent collec­tions of the fauna of Chouteau age may be obtained, however, at the margins of a caliche pit in a structural sink at the seventh culvert north of San Saba River along U.S. highway 87 about 6.7 miles south-southeast from the courthouse at Brady (TF-385 tQ 389c), as well as northwest from the locality. The Barnett formation of the Bald Ridge area consists of thin limeslonc beds interspersed in covered and relatively de· pressed areas that support a thick growth of mesquite and are judged to be under­lain by shale. Caliche is characteristic of such ~hale areas, and gray to olive gray The Ellenburger Group of Central Texas calcareous shale may be seen in the caliche pit mentioned above, in the structural sink on U.S. highway 87. The areas of mesquite concentration make fuzzy patches on the aerial photographs, offering a ready clue to the occurrence of outlying patches of the Mississippian strata. The limestone that occurs in the Bar­nett formation of this area includes both medium gray Lo yellowish or brownish gray granular limestone and medium to rather light olive gray or mouse gray sublitho­graphic limestone. They resemble some of the limestones found in the lower part of the Marble Falls limestone, and the poor exposures at many pl aces cause diffi­culty in drawing the Barnett-Marble Falls boundary. This boundary was placed largely on a fauna! basis, fusiform fusu­lines resembling Fusiella and locally a large bellerophont occurring to the base of the Marble Falls limestone, whereas the limestones of the Barnett formation contain species of Aviculopecten, Bem· bexia, rarely Goniatites, a small Linopro­ductus, a small Pustula, and a large Or­biculoidea such as were seen in known Barnett strata of the Bear Spring and Cherokee areas. On Plate 4 the pattern­less patches immediately northwest of TF-406 and just north of culvert 10 that are surrounded by the symbol for the Barnett formation are now known on fauna! evidence to represent limestones of the Barnett formation. Local revision of the Barnett-Marble Falls boundary in other parts of the area may prove advis­able when fossils are collected more extensiveI y. The thickness of the Barnett formation varies from a feather edge to possibly as much as 50 or 60 feet, bul it is diffi­cult to estimate and the upper figure is little better than a guess. PENNSYLVANIAN Pennsylvanian strata mapped in the Bald Ridge area are limited to the Marble Falls limestone. That formation consists of granular to locally sublithographic, gray to brownish gray and black lime­stone interbedded with marly limestones and calcareous shales. The limestones lo­cally contain coal black chert or black and light gray speckled chert. Only its lower beds were mapped and no estimate of its thickness was made. It is over­lapped by strata of Canyon age north and northwest of the area mapped, and it rests on the Barnett formation, the Chap­pel limestone, or the Gorman formation. Fusiform fusulines resembling Fusiella extend to the base of the formation, and it is unlikely that beds of Morrow age are included in the Marble Falls lime­stone of the Bald Ridge area. Other fos­sils seen were large bellerophonts recalling Bellerophon crassus Meek and Worthen, small silicified high-spired gastropods, "Spirifer" cf. S. rockymontanus Marcou, Marginifera sp., and large Linoproductus. DESCRIPTION OF THE McCULLOCH COUNTY SECTIONS The HiglJ,way 87 section, along the Brady-Mason road (U.S. 87) between San Saba River and a point 4.6 miles by speedometer northwest, includes the San Saba limestone member of the Wilberns formation and all units of the Ellenburger group present in the Bald Ridge area. Poor exposures, gentle dips, and gentle topography contrive to make measurement of this section difficult and unreliable; but it is of such importance to show the char­acter of the Ellenburger group in the northwestern part of the Llano uplift that an approximation was attempted. The Camp San Saba section, outside the Bald Ridge map area, completes a com­posite section of the Ellenburger group and the Wilberns formation for the north­western part of the Llano region. Because of the unusual development of the lower part of the Gorman formation and the relationships at the Tanyard-Gor­man boundary, a third section was located on the south slope of Bald Ridge, below the windmill known as High Lonesome, where the best exposures of this part of the Ellenburger sequence were found. Reconnaissance studies in other parts of McCulloch County indicate that the se­ quence of units involved in the Ellen­ burger problem is as well displayed in the Bald Ridge area as anywhere else in the northwestern part of the Llano region, and the area is accessible and structurally simple. The sequence, in the Bald Ridge area, of stratigraphic units between the base of the Wilberns formation and the Carboniferous strata i!'l essentially as follows: Thickness Stratigraphic unit in feet Mississippian (variable thicknesses of the Barnell formation and Chappel limestone) Lower Ordovician-Ellenburger group (830 feet) Gorman formation (295 feet) Calci tic facies __________ 295-90 Dolomitic facies _________________ Oc--205 Tanyard formation (535 feet) Staendebach member (280 feet) Calcitic facies _____________ ---------------_ 0-23 Dolomitic fade~ 280'--257 Threadgill member (225 feet) Calcitic facies ___ _________________________ __ 255 Upper Cambrian (539 feet measured) Wilberns formation (5.39 fef't) San Saba limestone member___________ 200 Point Peak shale member _____ _ 190 Morgan Creek limestone member____ 125 Welge sandstone member____ 24 Approximate total thickness of Ellen· burger group and Wilberns forma­tion ----------------------------------------------------1370 For measurements of the Camp San Saba section above the W elge sandstone the authors are indebted to Dr. Josiah Bridge. In the same section the Welge sandstone member was measured and the Morgan Creek limestone member was re­measured by Barnes and Cloud in Janu­ary 1946. The Highway 87 section was marked and described, and the Bald Ridge section was measured, painted at 5-foot intervals, and described by Cloud in April 1945. The lower part of the Bald Ridge sec­tion is on the T. Brook ranch, the middle part on the Fred Otte ranch, and the upper part on the May Brook Kothman ranch. The Highway 87 section traverses a num­ber of properties as shown on the geologic map of the area. The Camp San Saba section is mostly on the T. Brook ranch. The location of the Bald Ridge section and its offsets is shown on the geologic map of the Bald Ridge area (Pl. 4) by lines of inverted V's and rows of dots. The Highway 87 section follows the Brady­Mason road and does not require a special symbol. The location of the Camp San Saba section is described thereunder. HIGHWAY 87 SECTION The Highway 87 section, as described below, includes approximately 830 feet of the Gorman and Tanyard formations and 230 feet of the Wilherns formation. The Wilberns strata described constitute the total thickness (200 feet) of the San Saha limestone member in its type sec­tion, as well as 30 feel of stromatolitic limestone here included in the Point Peak shale member. It is roughly estimated that an additional 50 feet of the biohermal limestone intervenes between the base of the described section and the characteristic shales of the Point Peak shale member. ThiGknesses given for this section were not measured directly because it would be impossible to do so with any semblance of accuracy. After the area was mapped and the route of section traversed to ob­tain and balance all available data on the attitudes of the beds, the section was described by intervals whose limits were located on the aerial photographs and paced to recognizable points such as high­way culverts. A profile was then plotted at vertical and horizontal scales of l inch lo 200 feet from data derived from an instrumental traverse furnished by the Texas State Highway Department. On th,is profile were superimposed the contacts of the various intervals described, their boundaries being projected at the angle of dip determined from a balance of all apparent attitudes obtained. Thicknesses of the intervals were scaled directly from Lhe constructed profile and applied to the described section. Because the Highway 87 section is not marked at regular intervals it is difficult to sample. In order to minimize this diffi­ culty the stratigraphic position is given for the floor of each of the 10 culverts between San Saba River and the top of the section, as well as for a few other recog­nizable points. Yellow 1.1umbers were painted on these culverts, beginning with the first one north of the river. and their positions on the geologic map of the Bald Ridge area are shown by the symbols Cl, C2, etc. The contacts of the various strati­ The Ellenburger Group of Central Texas graphic units, as well as the principal Gor· way bridge over San Saba River. It is man zone of Archaeoscyphia, were marked about 30 feet stratigraphically below the with yellow paint spots on fence posts base of the San Saba limestone member or rocks. of the Wilberns formation. The top of this section is at an altitude of 1778 feet on U.S. highway 87, approxi­mately 4,,6 miles by speedometer northwest of San Saba River and just north of cul­vert 10. Straligraphically it is at the boundary between the Gorman formation and the overlying Mississippian strata. The base of the section is at an altitude of 1570 feet at the north end of the high­ Desc1 i ption Ellenburger group: 830 feet thick Go1man fmmation: 295 feet thick Calcitic facies: 90 feet thick Description Qf the Highway 87 section is based largely on observations made on the west side of the road and in the adja­cent pastures. Neither the description nor the measurements approximate the degree of detail or accuracy to be expected in the descriptions of the other sections in this report. Thickness infeet Inter-Cumu-Feet above val lative base J. Inlerbedcdd limestone and dolomile; the sequence from bottom to top being essentially 15 feet of dolomite, 5 feet of limestone, 3 feet of dolomite, and 17 feet of limestone but ohscurcd by poor exposures, aherrant dips, and an incipient structural sink centf~red on culvert 8 (altitude 1773 feet at the top, of sect.ion)­ t.he dolomite is predominantly very fine grained to microgran­ ular, in part grading to fine grained; mostly light gray or brown­ ish gray, grading in part to brown or nutria. The limestone is snhlithographic; pearl gray towarrl ash gray, in part with irregular greenish argillaceous films. Beds poorly exposed and being generally indeterminate, hut apparently ranging from less than 1 inch to about 24 inches thick. Weathers brownish gray to gray where dolomite anrl medium light bluish gray where Jim.,,stone. The inlerval is in large part covcrerl, except for some very conspicu­ ous ledges, and the character of the underlying beds is revealed principally by abundant rubble at the surface. Minor chert Of' curs intermittently throughout interval ] , and may be seen in place as compicuous rings around stromatolite heads at the base of the calcitic upper portion of the interval. It is semichalccdonic to chalcedonic or suhporcelaneons, in part hiGhly oolitic to rrypto-oolitic, bluish to brownish gray to while. Locally float blocks of fine grained quartz druse arranged in con­spicu011sly 11nd11latinp: bands suggest archaeosolon-like stromatolites. Small, well-ro11ndec!, frosted sand grains, poorly sorted within their small size-limits, are scattered to abundant in some of the finer grained dolomite beds. "Fuconia" was noted hut not collected in limestone near, the middle of interval 1 and Rhombella was noted but not collected in limestone on the trace of the top of the interval about 1300 feet northeast of the line of section on the old Brady-Mason road. Culvert 3 (altitude 1802 feel) is approximately 5 feet, cul­vert 9 (a!Litude 1783 feet) about 10 feet, and culvert. 10 (altitude 1770 feel) about 30 feet above the base ~f interval 1. Mi,sissippian strata occupy a structural sink between intervals 1 and 2 on the highway. Culvert 7 is about on the contact between the Barnett formation and the Chappel limestone at the south side of 1his collapse feature. 2. Mostly limestone, with occasional thin interbeds of dolomite above 50 the principal A1 chaeoscyphia berl-4:he limestone is sublitho­grapliic, in part a fine pellet limestone; woodash gray to pearl gray, in minor part grading to light brownish gray and light grayish yellow, and in parl with irregular greenish to yellow­ish argillaceous films. The dolomite, where it occurs, is fine 40 40 1020-1060 90 970-1020 The University of Texas Publication No. 4621 Thickness in feet Inter-Cumu· Feet above Description val lative base grained to very fine grained; yellowish brown to nutria. Beds from a fraction of an inch to 24 inches thick; discontinuous, but in part fairly well exposed. Weathers solution-pitted; medium to light bluish gray to woodash gray, with dolomite beds weather· ing yellowish to brownish gray. Minor chert occurs, being mostly subchalcedonic, but in part grading to porcelaneous or chalk textured in the weathered perim· cters, and locally oolitic. The principal zone of Archaeosc) phia in the Gorman formation occurs about 5 feet above the base of interval 2, and a second Archaeoscyphia bed was seen about 20 feet above the base of the interval. Base of calcitic facies of Gorman formation at base of interval 2 (altitude approximately 1797 feet). Total thickness of facies 90 feet. Dolomitic facies: 20.S feet thick 3. Dolomite, in part grading laterally to limestone from about 20 90 180 880-970 to 45 feet above the base-the dolomite is predominantly fine grained, locally grading to medium grained, in part vuggy; but from about ]5 to W feet below the top of the interval it is microgranular to very fine grained. It is light gray, in part grading to brownish gray and grayish brown. The limestone is sublithographic, in part a fine pellet limestone; grading to fine or medium grained where dolomitic. It is pearl gray to woodash gray, with irregular greenish and yellowish argillaceous films, and in part with pink blotches and specks where dolomitic. For the most part very poorly exposed, weathering to a covered interval displaying rough to smooth, in part pitted cobbles of dolomite in the float and occasional ledges. The minor occurrences of limestone weather medium to light bluish gray to woodash gray, in la1·ge part reticulate. Chert occurs occasionally in the interval, being abundant only near the base. It is porcelancous to chalcedonic, locally with scat· tered small dolomolds, in minor part with fine drnsy coatings, and china white lo bluish gray to dull gray. Interstitial chert is ahun· dant in some of the dolomite. "Euconia" was noted in the chert about 10 feet above the base of the interval and Lecanospira and "Euconia" were collected from the chert at about the middle of the interval (TF-277). Culvert 6 (altitude 1790 feet) is approximately 65 feel above the base and 2S feet below the top of interval 3. 4. Dolomite-microgranular, woodash gray to white, bedding indeter­5 185 875-880 minate. Weathers to a covered interval displaying smooth, woodash gray to white cobbles. Minor chert is present; being subchalcedonic, in part micro· oolitic, in part quartzosc, bluish gray to white, and weathering shiny white. 5. Dolomite-· -very fine grained at the base, grading to fine or medium 30 21 .5 84..5-87.5 grained ahove; light gray with local yellowish tinges; bedding inde· terminate. Weathers lo a mostly covered interval strewn with smooth to rough, light gray to light yellowish gray cobbles. Oolitic, bluish gray to white, white-weathering chert occurs locally. Interstitial chert is very abundant in some of the coarser grained blocks. 6. Dolomite; in part grading to limestone at the top and base of 80 29.5 765-845 the interval on the west side of the road, and from about 20 to .1S foet above the base on the east side of the road-the dolomite is fine to medium grained, in part vuggy; light gray. The lime· stone is sublithographic; in part a fine pellet limestone; pearl gray to woodash gray, in part with greenish lo yellowish irregular argillaceous films and locally with yellow to pink splotches of dolomite. Bedding indeterminate, exposed ledges uncommon. Weathers to a largely covered surface strewn with rough, largely The Ellenburger Group of Central Texas Thickness in feet Inter-Cumu· Feet above Thickness in feet Inter-Cumu-Feet above Description val lative base pitted, medium to light gray to yellowish gray blocks of dolo­ mite, and with occasional inconspicuous ledges. The limeslone weathers medium bluish gray and is largely reticulated with dolomite. Chert is common and locally very abundant, hoth as loose pieces and in the dolomite blocks; and one large mass was seen in place in a ledge near the base of the interval. It is chalcedonic to subgranular, int large part crypto-oolitic to micro-oolitic and oolitic, locally with scattered dolomolds or included dolomite rhombs, bluish gray to white with local brownish bands and tinges, and weathers shiny white. Interstitial chert is also locally abundant. Sand grains were found by Barnes in dolomite about 11 feet helow the top of the interval on the east side of the road. A few grains of sand were also seen in float of oolitic chert near the base of the interval. An east-west fence abuls both sides of the highway about 8 feet stratigraphically above the base of interval 6. It is about 773 feet above the base of the ~ection, at an elevation of 1729 feet. Base of dolomitic facies of Gorman formation at base of interval 6 (altitude 1726 feet). Total thickness of facies 205 feet, and of forma­ tion 295 feet. Tanyard formation: 535 feet thick Staendebach member: 280 feet thick Dolomitic facies: 280 feet thick 7. Dolomite-predominantly fine grained, locally grading to medium 190 485 575-765 grained, and in part very fine grained in the lower beds; in large part vuggy in the upper half and less commonly below. Light gray to light brownish and yellowish gray, grading to grayish brown and woodash gray; bedding quite indeterminate. Weathers to rubble-strewn slopes with very few recognizable ledges, the character of the interval being determinable from abundant, blocky rubhle that weathers rough, pitted, and medium gray to light yellowish gray. The base of interval 7 is marked by a prominent zone of con­ spicuously drusy chert that weathers to closely spaced large blocks. These blocks represent a layer several feet thick that falls about 6 feet vertically below the top of a 1718-foot hill on its south side, and are well exposed on the east side of the road. This chert is mostly conspicuously and finely dolomoldic and drusy, yellowish white to bluish gray and light yellowish brown. Locally, however, large blocks, patches, and irregular layers of it are subchalcedonic to chalcedonic, streaked and blotched in tones of bluish gray to white, and weather to shiny white or hluish white patches in the larger chert masses. Chert is generally abundant as float throughout this interval; being largely $imilar to that described except that it is generally more dolomoldic and less drusy, with the different types varying in predominance and also containing much oolitic to crypto-oolitic and locally oolitic laminar chert. Tnterstitial chert occurs in the dolomite but is seemingly not common. A few fossils were seen in chert along the trace of the lower part of interval 7 near the highway, Ilelicotoma cf. H. uniangu­ lata (Hall) and Ozarkina being the only ones identified and re­ corded. 0 phileta and chertified Rhabdoporella were found by Barnes about 10 feet below the top of the interval on the west side of the highway. Although no fossils were collected along the line of section, several collections of fossils and notations on their occurrence indicate that Ozrrrlcina, Sinuopea, Chepulta­ pecia, Helicotoma cf. H. uniangulata (Hall), Paraplethopeltis, and cephalopods occur at the top of interval 7 along its trace. Description val lative base The floor of culvert 4 (altitude 1737 feet) is approximately 50 feet and that of culvert 5 (altitude 1684 feet) is approxim~tely 90 feet above the base of interval 7. 8. Dolomite, with very minor inclusions of limestone near the base­ 90 575 485-575 very fine grained to fine grained, in part vug;gy; light to medium gray, in part grading to yellowish, brownish and pinkish g;ray. Beds 1 to 14. inches thick. Weathers to largely covered slopes strewn with blocks of dolomite that weather fairly smooth to ' pitted and medium g;ray lo iron gray, grading to light yellowish gray near the base. Occasional to fairly abundant platps and thinly extensive lenses of chert ?ccur at 40 feet above and again at 65 feet above the base of rnterval 8. It is semichalcedonic in part micro-oolitic to crypto-oolitic, locally porcelaneous to chalk textured, light tan to shiny white. Similar chert, ;eemingly more chalcedonic and more commonly weathering white, was noted along the trace of the base of interval 8 and even into the limestone of the upper part of interval 9 abont 0.5 mile northeast and again 0.5 mile southwest of the line of section. Minor quartz druse occurs on some beds. The floor of culvert 3 (altitude 1708 feet) is roughly 5 feet above the base of interval 8. Base of Staendebach member of Tanyard formation at the base of interval 8 (altitude 1708 feet). Total thickness of member, which is entirely in the dolomitic facies in the line of section, 280 feet thick. Threadgill member: 255 feet thick Calcitic facies: 255 feet thick 9. Limestone; with rare thin interbeds of dolomite probably not 100 675 385-485 totaling over 3 feet thick, and in very minor part with an irreg­ ular internal meshwork of dolomite ­ predominantly sublitho­ graphic, in conspicuous part a fine pellet limestone and locally with coarser pellet layers that have a granular appearance; light brownish gray to pearl gray and old ivory, grading to pink or grayish yellow, in part with irregular yellowish patches where dolomitic, and locally with pale dusty green to yellowish irregu­ lar argillaceous films. The minor dolomite beds are very fine grained, yellowish g;ray to grayish and pinkish yellow. Interval 9 begins at the base of a conspicuous 27-inch bed, with the overly­ ing beds ranging from 1 to 27 inches thick and with beds over a foot thick forming a conspicuous part of the total thirkness. Wcathers to fairly conspicuous, smooth to solution pitted, medium to light bluish gray ledges that locally grade to woodash gray and are separated by intervals of poor exposure. Very fine grained quartz drnse occurs as rosettes and occasional incrustations that ai·e scattered to abundant on a number of beds. Thin plates and extensive lenslike inclusions of tan- to russet­ weathcring "pebble" chert composed of silicified fragments of fos. sils and pelletlike debris occur about 9 feet below the top of interval 9, with a second occurrence about 16 feet lower. 0 phileta, a trilobite with affinities to Leiostegium, and other fos­ sils were collected from this "pebble" chert 9 feet below the top of the interval (TF-411). Besides these, scattered to abundant calcitic and dolomitic sections and impressions of Lytospim cf. r. gyrocera (Roemer), Ophileta cf. 0. polygyrata (Roemer), Gas­ conadia, and less commonly cephalopods were seen on occasional bedding surfaces. The center of an intersection of the present. highway with the former highway (altitude 1689 feet) is about 10 feet above the base of interval 9. JO. Limestone-predominantly sublithographic, in part a coarse pellet 155 830 230-385 limestone, grading to a granule conglomerate with a granular appearance similar to some of the beds in interval 11. Pearl gray The Ellenburger Group of Central Texas Thickness in feet Description and old ivory to light brownish gray; less commonly grading to light greenish gray, grayish brown, and brownish yellow, and in part with pale dusty green to yellowish irregular argillaceous films. Some of the beds arc mottled with an irregular, yellowish, dolo­ mitic meshwork, especially near the base; but the dolomitized trails and fossils so common in equivalent beds in the Thread­gill Creek and Llano River sections arc much less conspicuous in the Highway 87 section_ Beds from a fraction of an inch to 12 inches thick, in general averaging less than 8 inches thick. Weathers to generally discontinuous, poorly exposed, medium to light bluish gray to woodash p;ray ledges that are locally mottled in tones of yellow to buff where dolomitic. The lower 25 feet especially shows mottled beds, with woodash gray to white patches of sublithographic limestone in darker gray to buff-weathering limestone or slightly dolomitic limestone which is in part gran­ular. The occurrence in the limestone ledges of the sublitho­graphic patches that weather woodash gray to white and similarly weathering plates of limestone in the float, characterizes the basal limestones of the Threadgill formation in this area, as in other areas where they rest upon the San Saba limestone member of the Wilberns formation. No glauconite was seen in any of the beds in interval 10 in the line of section, but corroded globular glauconite was seen at a few places in the area in heels probably equivalent to the lower part of the interval_ Occasional rosettes of fine-grained quartz druse were seen on some of the heels, and chert was noted in one bed abont 10 feet below the top of the interval. The chert occurs a5 small, irreg­ular, nodulelike inclusions; being semiporcelaneous, grading to chalk textured in the weathered perimeter, light. brownish gray to grayish brown or dirty white, and weathering tan. In general, elements that would he insoluble in dilute hydrochloric acid are relatively rare in interval 10. Dolomite rhombs would doubtless be common in acetic acid residues. Silicified Finlcelnburgia cf. F. bellatnla* Ulrich and Cooper was coIIected from the basal part of interval 10 about 4.90 feet south­west from the highway (TF-4.09). Inasmuch as this locality is in a structural sink its actual position in the section is diffi­cult to judge. Silicified specimens of the same species of Finlc­elnburgi,a were obtained in the line of section about 9.5 feet above the base of interval 10, on the west side of the highway and about 300 feet north-northwest of culvert 2 (TF-410). Silicified Finlc­elnbnrgia cf. F. buttsi* Ulrich and Cooper was obtained from lime­stone 30 to .50 feet abov<" the Tanyard-Wilberns boundary about 0 . .5 mile west-southwest of U.S. Highway 87 and beyond the limits of the area mapped (TF-229). Gasronadia and Ophileta were noted in place as calcitic impressions in the limestone about .5 feet above the base of the interval but were not coIIected. Occa­sional bedding surfaces throughout the interval display calcitic or dolomitized sections of Lytospira cf_ L. gyrocera (Roemer), Gasconadia, Ophileta cf. 0. polygyrata (Roemer), Sinnopea and occasionally cephalopods, but such occurrences seem to b~ less common than they were in the Llano River section. The floor of culvert 2 (altitude 1638 feet) is about 60 feet above the base of interval 10. Ease of Threadgill member (entirely calcitic) o{ Tanyard formation, and Cambrian-Ordovician boundary, at ba5e of interval 10 (altitude 1647 feet). Thickness of member and calcitic fades 2.55 feet, of furmation .535 feet. Upper Cambrian: 230 feet described Wilberns formation: 230 feet described San Saba limestone member (type section): 200 feet thick :f'Cloud (in press) refers these specimens to his new species Finkelnburgia helleri. Inter-Cnmn-Feet above val lative base Thickness in feet Inter-Cumu-Feet above Description val lative base 11. Limestone, in part with yellow-to huff-weathering dolomitic 140 970 90-230 patches-except for the upper and lower few feet, which contain sublithographic, medium bluish and yellowish gray to brownish gray patches, the limestone in this interval is conspicuously gran­ ular. It varies from microgranular to coarse grained and in general is crowded with fine to coarse pelletlike bodies. Its color is mostly brownish to greenish gray or grayish brown, with local patches and specks of yellow bLifI or cinnamon where dolomitic, and locally with scattered to abundant green specks of globular glauconite. Beds from a fraction of an inch to 6 inches thick. Weathers to thin, in part solution-pitted, in general poorly ex­ posed ledges that are brownish gray to greenish and bluish gray or grayish brown; in part with yellow to buff tinges, mottles, and hlotches; and locally with very light bluish gray to wood- ash gray mottles and patches. The upper aud lower few feet esnecially are marked by thin beds that weather mottled in yellow to buff and light bluish gray to woudash gray. Globular glauconite ocon s ocrasionally throughout interval 11, being quite abundant in some beds. Acetic acid residues would probably yield abundant dolomite rhomhs as well as glauconite. Several of the beds are crowded with fragments of trilobites and gastropods, with Kingstonia-1ike trilobites occun-ing in medium to coar~ely granular limestone near the top of thP interval. No fossils were collected, however. The attitudes and sequence of the heda in the upper part of the interval ll are obscured by collapse structure, and in gen­ ernl the Cambrian-Ordovician contact in the vicinity of the high­ way is not well marked. - The floor of culvert l (altitude 1605 feet) j, about ] 5 feet above the base of interval 11. l 2. Limestone--conspicuously granular in appearance, varying from 60 10.30 30­ 90 very fine to coarse grained; in general crowded with fine to fairly coarse pelletlike bodies, and with several heds rrowded with fragments of trilobites and other fossils, especially in the lower few foet. H.a1e interbPd' of limestone in inlerval 12 are similar to that in interval ].':\. Grayish brown to brownish gray. grading to yellowish brown and yellowish gray: in part. with scat­ tered to abundant green specks of glauconite, and with occasional beds that are quite green. Beds 1 to l 6 inches thick, mostly less than 8 inches thick. Weathers gray to hrownigh gray, in part with yellowish tinges. Glauconite was the only ''insoluble" noted, except that acf'Lic acid residues of many beds would probably yield abundant dolo­ mite grains. The glauconite is generally not abundant, but it is common enough to be found without difficulty, and some thin beds are quite crowded with glauconite globules. Saukiinid trilobites, determined by Dr. Josiah Bridge as Sau­ lciella sp., were collected in an outlying pocket of the basal beds of interval 12 in December. 1940, during a field trip led by Bridge and Barnes. The lower few feet of the interval is highly fossiliferous, being crowded with trilobites. Base of San Saba limestone member of Williern~ fnnnntion at base of interval 12. Thickness of member 200 feet. Point Peak shale member: 80 feet described l 3. Limestone, with occasional zones of dolomite; compnsrng the up­ 30 1060 0­ .30 per beds of a zone of stromatolit.ic bioherms---the limestone is sublithographic to microgranular, in part a fine pellet limestone; greenish to brownish gray to woodash gray or light yellow, in part with yellowish argillaceons films. It weatherb mcdimn hlnish gray to woodash gray. The dolomite is very fine-grained to microgran­ ular and· brownish to yellowish gray. It weathers medium gray to brownish gray. Beds generally fairly thick; those measured The Ellenburger Group of Central Texas Thickness in feet Description being from less than 1 inch to 14· inches thick, but with some beds probably a good deal thicker than this. Most beds in interval 13 weather to large, subcircular, hum­ mocky, reticulated reef masses. Stenopilus was collected near the base of the interval in December, 1940, during a field trip led hy Bridge and Barnes. Interval 13 and the approximately 50 fret of section below it to the base of the lowest prominent slromatolitic limestones are included in the Point Peak shale member because the normal association of the bioherms at this position, judged from the Llano region as a whole, appears to be with the Point Peak shale mem­ ber, and because shale appears lo occur hetween the hioherms in the Bald Ridge area. Beds below the base of interval 13 are not formally described be­cause they are almost wholly covered by alluvium in the vicinity of the highway. The stromatolitic nature of some of these beds jg well displayed in a small park on the south side of San Saba River. Pro­jection of beds both eastward and westward along the north hank of the river leads to the supposition that additional hiohermal lime­stones there intervene between those of the park ~nd the base of interval 13 in the part of the section above noted as being covered. and the shale that locally intervenes between the bioherms may be seen at the mouth of Hudson Creek or in a road material pit 3 miles sonth-southwest from the highway bridge over San Saba River. ThP massive hiohermal limestones at the hase of the zone of hioherm5 out­crop prominently in the river hed for a distance of about 370 feet downstream from the highway bridge. Because the bottom of the river channel below the highway hridge (altitnde 1531 feet) is 39 fret bPlow the base of interval 13 (altitude 1570 feet), the strata in interval 13 are almost flat, and tllf' basal biohermal bed is about 10 feet thick, 50 feet is considered a fair estimate for the thickness of the stromatolitic Point Peak strata below interval 13. The contact of the San Saha limestone and the Point Peak shale memhers of the Wilberns formation is wrongly shown at the base of the zone of stromatolitic bioherms on the map of the Hald Ridge area (Pl. 4). Although it was ~tated by Dake and Bridge (1932, p. 727) that "the reef material is embedded in sbaly layers," and the local presence of shale hetwecn the hiohermal limestone was observed by the authors, it was not considered appropriate, at the time the map was made, to include these limestones in a memhcr characterized by the dominance of shale. After the map was in the bands of the engraver additional work by the authors in other areas showed that the normal association of the bioherms at this position in the Llano region as a whole is with the Point Peak shale member, and that consistency is best served by a similar assignment of the biohermal limestones in the Bald Ridge area. The hiohermal lime­stone extends higher in the section along U.S. highway 87 than it does a shmt distance to the west, where a prominent shale band at the top of the biohermal zone can be traced for several miles southwest from the western limit of the Bald Ridge map area. Base of Highway 87 section at base of interval 13; altitude 1570 feet. At extreme north end of concrete span over San Saba River, on west side of road. Total approximate thickness of section 1060 feet, of which 230 feet belongs tu the Wilberns formation of Upper Cambrian age and 830 feet to the Tanyard and Gorman formations of Lower Ordovician age. Inter-Cumu-Feet above val lativc base CAMP SAN SABA SECTION The Camp San Saba section supple­ments the Highway 87 section by furnish­ing information on the Wilberns forma­tion below the upper biohermal limestones of the Point Peak shale member. In this section the beds above the Welge sand­stone member of the Wilberns formation were measured along the left (northeast) bank of San Saba River in the vicinity of Flatrock Crossing, at the village of Camp San Saba and a little over 1 mile down San Saba River from the bridge at U.S. highway 87. The Welge sandstone was measured in a bluff immediately east of the juncture of Katemcy Creek and San Saba River, not quite one-half mile down San Saba River from Flatrock Crossing. The Morgan Creek limestone member of the Wilberns is very well ex­posed in the vicinity of Flatrock Cross­ ing and a bed-by-bed measurement of .it with a steel tape by Barnes checked closelv with an earlier measurement by Dr. Bridge. The Camp San Saba section is struc­turally continuous with the Highway 87 section and the units studied are uncom­plicated and well exposed. They are, from the base of the Wilberns formation up­ward, as follows: 24 feet of the Welge sandstone member, ] 25 feet of the Mor­gan Creek limestone member, and llO feet of the Point Peak shale member to the zone of stromatolitic bioherms. Eoorthis texana Walcott and many other fossils are abun­dant about 70 feet above the base and Plectotrophia bridgei Ulrich and Cooper is locally abundant near the top of the section. The general characters of the units measured are shown graphically on Plate 14.. BALD RIDGE SECTION The Bald Ridge section, as described below, includes 187 feet of the Gorman formation and 23 feet of the Tanyard formation. It is described for the pur­pose of showing how different the devel­opment of this part of the Ellenburger sequence is from that displayed in the Highway 87 section, 1.3 to 1.5 miles southwest of here. In contrast to the prin­cipally dolomitic Gorman strata below Archaeoscyphia in the Highway 87 sec­tion, the Bald Ridge section is largely cal­citic. Moreover, limestones occur on boLh sides of the Tanyard-Gorman boundary in the Bald Ridge section, whereas the same boundary is within a sequence of dolo­mites in the Highway 87 section. In both sections the criterion of grain size of the dolomites, widely applied in the place­ment of the Tanyard-Gorman boundary in other parts of the Llano region, was useless. However, as in other parts of the region, sand does not occur in the Tanyard formation. In the Bald Ridge area the lowest occurrence of sand in the Ellenburger approximately coincides with the Tanyard-Gorman faunal boundary and a zone Qf blocky white-weathering chert, the three together constituting the basis on which the boundary was here mapped. The top of the Bald Ridge section is about 300 feet north of and a little be­low the base of High Lonesome, a wind­mill at' an elevaLion of approximately 1840 feet at the northeast end and high­est part of Bald Ridge (Pl. 4). Strati­graphically it is about 187 feet above the base of Lhe Gorman formation and pre­sumably not far below the principal zone of Archaeoscyphia in the Gorman which occurs at approximately 210 feet above the base of the Gorman formation in the Highway 87 section. The secLion termi­nates at the top of Bald Ridge, with the nearest exposures of Carboniferous rocks in a collapsed syncline 0.5 mile north­west, but at a considerably lower alti­tude and probably not over 50 feet above the highest beds described. The base of the Bald Ridge section is aL an altitude of about 1745 feet in a draw Lhat is the principal wel-wealher tributary to Hudson Creek, at the south foot of Bald Ridge, and ahouL 0.55 mile airline south and slightly east from the top of the section. The section follows the draw for about 2000 to 2100 feet and then ascends an open hillslope to Higl1 Lonesome. The Ellenburger Group of Central Texas Thickness in feet Inter- Cumu- Feet above Description val lative base Ellenburger group: 210 feet described Gorman formation: 187 feet described 1. Limestone; with occasional interbeds of dolomite, especially 15 15 19.5 -210 in the lower 5 feet and from 206 to 208 feet (altitude approximately 1840 feet at top of section)-thc limestone is mostly sublitho·graphic, in part a fine pellet limestone, and in part grading to fine grained where dolomitic; pearl gray to light brownish and yellowish grny. The dolomite is fine grained and light gray to light pinkish gray. Beds from a fraction of an inch lo possibly as much as 12 inches thick. Weathers to irregular, solution-pitted ledges or largely covered intervals strewn with thin slabs; medium to light bluish gray to woodash gray. The dolomite weathers medium to light gray. Chert occurs at 196.5 feet as chalcedonic to subchalce­ donic, dark gray to bluish gray to white, shiny, white-weath­ ering blocks in the float. At 197 feet is a layer of semi­ chalcedonic, oolitic chert tending to be laminar and rang­ ing up to several inches thick. From 198 . .5 to 200 feet are lenses and irregular inclusions of semichalccdonic to chal­ cedonic chert that is oolitic to crypto-oolitic, bluish gray to white, and weathers shiny white. At 20.5 to 206 . .5 feet is fairly abundant float of chert that is semichalcedonic to chalcedonic, in part crypto-oolitic, in part wilh chalk-tex­ tured patches, datk gray to brownish or bluish gray to white, with the colors irregularly streaked. At 207 feet are many small nodules and irregular inclusions of chert that is subchalcedonic to chalcedonic, in part grading to chalk textured, and largely crypto-oolitic and pseudospicular. B bed at 19.5 feet, a 6-inch bed of dolomite. Offset 100 feet east-northeast if going up in the section and west-southwest if going down. 2. Limestone, capped by 6-inch bed of dolomite-sublitho­ 17 . .5 32..5 177 . .5-195 graphic, in part a fine pellet lim estonc; pearl gray to woodash gray, in minor part grading to a light greenish gray, and in part with irregular dusty green and yellowish argillaceous films; beds from a fraction of an inch Lo 10 inches thick. Weathers to in"egular, solution-pitted, light to medium hluish gray to woodash gray ledges. The 6-inch bed of dolomite at the top is fine grained; woodash gray to white, and wealhers rough and medium gray. Chert is abundant as float at 178 feet, apparently being derived from this part of the section. It is chalcedonic to subchalcedonic, wilh occasional qua1tzose patches, bluish to brownish gray to white, and weathers shiny white. Promi­ nent, large, extensive lenses of white-weathering chert are abundant in place from 182 to J8.5 feet; in part bowing up through the beds as large concentric stromatolitic masses up to 6 feet or more across. This chert is chalcedonic to semichalcedonic, in part highly oolitic, locally with quartz­ ose patches and in minor part a granule conglomerate; it is dark gray to bluish, brownish, or white and weathers shiny white to dull white. This may be the source of much of the chert seen in interval 3. Sand occurs as scattered small grains in limestone at 187 feet. A bed at 177.5 feet, a prominent ledge of limestone. Offset 250 feet east-northeast if going up in the section and west-south­ west if going down. Description 3. Limestone; locally with irregular dolomitic inclusions, but otherwise apparently quite pure-sublithographic, in part a fine pellet limestone, locally grading to fine grained where dolomitic; pearl gray to woodash gray; beds from a frac­tion of an inch to 12 inches thick. Weathers to a series of terraces, with heavier ledges forming sleps which are separated by benches strewn with thin slabs of limestone and pebbly to blocky float of white-weathering chert. The weathered color of the limestone is medium to light bluish gray to woodash gray or almost white. The pebbly to blocky chert float on• the benches was especially noted at 146 to 147, 151 to 153, 156 to 157, 161 to 163, 166 to 168, 170 to 171, and 174 to 176 feet. It is generally similar at all these places; being subchalcedonic to chalcedonic, in part micro-oolitic, locally with quartzose patches, bluish gray to white, in part streaked in varying tones of the named colors and ivory yellow, and weather­ing white and shiny. It was not determined whether this chert was weathering out at any or all of the places men­tioned or how much has floated down from above, but it seems likely that some of it was locally derived. Conspic­uously oolitic, gray to brownish gray chert, in part tending to be laminar, occurs in place as thin layers that in part run together to form thicker layers at 141 to 142 feet. Minor inclusions of chalcedonic, white-weathering chert occurs at 148 feet. Just below 174.5 feet the limestone con­tains scattered angular grain to granule size inclusions of quartz and an occasional small sand grain. Sand was noted only as occasional grains in the lime­stone at 174.5 feet. Lecanos pira was seen on the trace of the limestone at about 140 feet, about 60 feet northeast of the line of sec­tion in the axis of the draw. 4·. Dolomite and limestone; inter bedded, laterally intergrading, and in part probably mixed by collapse-the dolomite is fine to medium grained and light gray; the limestone is sublithographic, in part a pellet limestone, and pearl gray with yellowish to pale salmon tinges where dolomitic. Bed­ding indeterminate. Weathers medium gray to yellowish gray where dolomite, medium to light bluish gray where limestone. Chert occurs at about 132.5 feet as shattered nodules and blocks of float that are partly massive and partly cav­ernous. It is semichalcedonic to chalcedonic, in part con­&picnously oiilitk to crypto-oolitic and micro-oolitic, bluish gray to white, and weathers mostly shiny white. Locally it contains abundantly scattered, small, subround to round sand grains and included flat pebbles of limestone that weather out to give a cavernous appearance. Sand grains occur in the chert at 132.5 feet, as noted above. 5.. Mostly covered except for the basal foot; dolomite was seen in place and limestone in the float-the dolomite is fine grained and light gray. The limestone float is sublith­ographic, in large part pelletted, pearl gray. A nodule of chert seen in the float at 115 feet was sub­chalcedonic to chalcedonic, bluish gray, brittle, and weath­ered yellowish white. At 115.5 to 116 feet is float of porce­laneous to chalk-textured, white, conspicuously and finely dolomoldic chert and coatings of fine grained quartz druse. Chert projects from the hillside at 121 feet indicating a Thickness in feet Inter-Cumu,. Feet above val lative base 40.5 73 137 -177.5 87 123 -137 9 96 114 -123 The Ellenburger Group of Central Texas Thickness in feet Inter-Cumu-Feet above Description val lative base bed or an extensive lens ahont 5 inches thick near this point. It is subchalcedonic to chalcedonic, in minor part grading to porcelaneous, in part ~onspicuously oolitic to crypto-oolitic, bluish to brownish gray to white, weathers shiny white, and has scattered small, well rounded sand grains throughout. Interstitial chF1i is abundant in the dolomite. Sand occurs as small, well-rounded grain, abundantly scattered in chert at 120 and J21 feet. 6. Covered interval; with float 0£ chert, limcs1one, and dol­4 100 ]10 -114 omite---the limestone and dolom]1e arc similar lo that de­scribed in interval 9. The chert is subchalccdonie, in minor parl crypto-oolitic, bluish gray to white, and weathers shiny while lo bluish white or dull tan. It may be from above. 7. Dolomite--fine grained to vc1y fine grained, light g1ay to 5 105 105 -110 light brownish gray, bedding indeLerminate. Weathers to a largely covered interval strewn with smooth, medium gray cobbles. Interstitial chert is ahundanL. Fine grained quartz drnse occurs as films and rosettes on the dolomite. 8. Dolomite-very fine grained to microgranular; light gray, 5 110 100 -105 in minor part grading to beige; bedding indeterminate, ex­posures poor_ Weathers to smooth plates and cobbles on the hillslope and in the draw. 9. Mostly limestone; in large part with an irregular internal 25 135 75 -100 meshwork of dolomite, in part. grading laterally to dolo­mite, and with occasional interbeds of dolomite--the lime­stone is sublithographic, in large part a fine pellet lime­stone; pearl gray to woodash gray, in minor part grading to light yellowish gray. The dolomite b medium to fine grained, in part vuggy; light gray to light brownish and yellowish gray, in minor part with salmon pink tinges. Beds from a fraction o( an inch to 14 inches thick, ex­posed only in the draw. Weathers medium to light bluish gray, reticulate where dolomitic limestone and medium gray where dolomite. Interstitial chen oc< urs in some of the dolomitic layers. Sand occurs as small, well rounded, frosted grains that are scattered to fairly abundant in a LS-inch limestone bed at 97.5 feet. Unrecognized gastropods were seen in the limestone at 92 feel. The limestone from 83 to 34 feet is stromato­ litic, with dolomitization following a concentric pattern in circles 5 to 16 inches in diameter. l0. Dolomite-fine to medium grained, in large part vuggy; 32 167 43 -75 light gray, in part with pinkish tinges; beds from 1 to 16 inches thick. Weathers to medium to light gray ledges that are in large part pitted and rough. Interstitial chert occurs intermittently throughout and is abundant in some beds. 11. Limestone and dolomite, interbedded and laterally intergrarl­14.5 181.5 28.5-43 ing-the limestone is snblithographic to microgranular, in part a fine pellet limestone; pearl gray to woodash gray, in part with pinkish tinges. The dolomite is medium grained, in part vuggy; light gray, in part with brownish to yellow­ish tinges. Beds from a fraction of an inch to 3 inches thick, slabhy and poorly exposed away from the draw. Weathers medium to light bluish gray, with woodash gray to while patches, in the calcitic parts and medium gray where dolomitic; rough and in part reticulate. The University of Texas Publication No. 4621 Description Chert occurs from 29 to 30 feet as minor angular inclu· sions that are subchalcedonic, hluioh gray to white, and white weathering. At 34 to 35 feet it occurs as an irregu­lar meshwork in the limestone and dolomite; being sub­chalcedonic to chalcedonic, in part with chalk-textured patches, and bluish gray to bluish white. From 35.5 to 36.5 chert is abundant as rough, ropy excrescences that are semichalcedonic, crowded with indistinct pinpoint marking~, bluish to brownish gray or bluish white, and tan weather· ing. At 41 feet chert occnrs as irregular plates that are semichalcedonic to suhchalcedonic, chalk textured, and locally porcelaneous; bluish white to chalk white, grading to bluish gray; and tan weathering. At 43 feet are promi­nent lenses of chert that are chalcedonic to subchalcedonic, in part quartzose, brittle, bluish gray to white and pale carnelian, and weather shiny white to tan. Sand grains are abundantly sc;attcred in a 5-inch bed of conglomeratic pellet limestone at 30 feet. The individual grains are small, frosted, and well rounded to poorly rounded; and the conglomeratic fragments range from the size of granules to flat pebbles 2 inches across and 0.5 inch thick. "Euconia" was collected from 34 to 35 feet (TF-4.00). The limestone at 29.5 feet is stromatolitic, with dolomitiza­tion following a concentric pattern. 12. Limestone, dolomite, and chert; the limestone and dolomite apparently intergrading-the limt'stonc is sublithographic, in part a fine pellet limestone, in part with scattered to abundant euhedral rhombs and irregular inclusions of dolomite, pearl gray to woodash gray. The dolomite is medium grained and light yellowish gray. Bedding thin but indeterminate. This basal zone of the Gorman formation characteristically weathers to a chert-strewn Lench or ter­race, the chert weathering to shiny, white or bluish white blocks apparently derived from fairly thick lenses varying in position within the interval. Chert in the line of section is abundant from 23 to 26 feet and from 28 to 28.5 feet. That at 23 to 26 feet is mostly highly oolitic or pelletted, in part quartzose, in minor part with scattered dolomolds, locally subchalcedonic to porcelaneous, and bluish to brownish gray to white. Close examination shows that this chert is in part truly oolitic and in part a chertified pellet limestone. The lower 18 inches is in part a granule conglomerate that contains subangular to subrounded inclusions of limestone and occa­sional sand grains. The chert from 28 to 28.5 feet is sub­chalcedonic to chalcedonic, in part with porcelaneous to chalk-textured patches, in part slightly quartzose, locally crypto-oiilitic to oolitic, and bluish gray to white. Intersti­ tial chert occurs in the dolomite. The chert described is similar to that which occurs widely at the base of the Gorman formation in the Bear Spring area, and which there locally contains Rhombella. However, no Rhombella was found at this locality or from this chert anywhere in the Bald Ridge area. Sand occurs in the chert of the lower 18 inches of in­ terval 12 as scattered, small, well rounded grains that in part form nuclei for ooids. Such grains were especially com­ mon about 1 foot above the base of the interval. Thickness in feet Inter- Cumu- Feet above val lative base 5.5 187 23 -28.5 The Ellenburger Group of Central Texas Description "Euconia" and Ophileta were collected from the chert at 25 feet (TF-399). The limestone at 27 feet is stromat­olitic, with dolomitization following a concentric pattern in circles from 6 to 13 inches in diameter. Base of Gorman formation at base of interval 12 (altitude about 1750 feet). Total thickness of formation described 187 feet. Tanyard formation: 23 feet described Staendebach member: 23 feet described Calcitic f acies: 23 feet thick 13. Limestone; for the most part with an irregular internal meshwork of dolomite and in part grading to dolomite-­the limestone is sublithogrnphic, in part a fine pellet lime­stone, in part grading to fine and medium grained where dolomitic; pearl gray to woodash gray, grading to light brownish gray. The dolomite is medium to fine grained and light yellowish to pinkish or brownish gray. Beds less than 1 inch to 18 inches thick. Weathers rough, re­ticula!e, hummocky, medium bluish gray with darker brown­ish gray reticulations representing the internal dolomitic mesh work. Interval 13 represents an approximate maximum thickness for the calcitic facies of the Stacndebach member in the Bald Ridge area, and through a large part of the area this facies is entirely absent. At approximately 0.25 mile south of the line of section the limestone di0mppears, and 0.3 mile to the east-northeast it has disappeared or thinned drastically. Chert is abundant throughout the interval. A meshwork of subchalcedonic to semkhalcedonic, medium to dark blu­ish gray chert is present from 0 to 1 and 4, to 7 feet, weathering to a ropy, tan fretwork on the surface of 1he beds. Oolitic chert that is in part laminar and weathers punky, cavernous and russet is abundant from 7 to 9 and 17 to 19 feet. Abundant chert between 13 and 17 feet is subporcclaneous to chalk textured, in part finely drusy, in part dolomoldic, chalk white, and weathers punky, cavernous, rough, blocky, and tan to i usset. Thin plates and lenticles of semichalcedonic, slightly calcitic, gray to white, tan-weath­ering chert were seen at 7.5 feet. Fossils are common in chert in place at 6 feet. Those noted were Schizo pea, Helicotoma cf. H. uniangulata (Hall), and Ozarkina cf. 0. complanata Ulrich and Bridge. Ozarkina complanata Ulrich and Bridge, Helicotoma cf. H. uni­angulata (Hall), and a brevicone cephalopod were collected from chert between 13 and 17 feet (TF-397). Ozarkina, H elicotoma, Clarkoceras (?) and a trilobite were obtained from granular to chalk-textured chert along the trace of the upper foot of this interval, 300 feet east of the line of section (TF-398). The limestone at 20 feet is stromat­olitic, with dolornitization following a concentric pattern in circles up to 2 feet in diameter. Base of calcitic facies o{ Staendebach member of Tanyard for­mation and base of Bald Ridge section at base of interval 13; altitude roughly l 74.5 feet. At axis of a draw about 0.5' mile south and slightly east from the windmill known a& High Lone­some, at the south foot of Bald Ridge. Total thickness of section 210 feet, oI which 23 feet belongs to the Tanyard and 187-to the Gorman formation. Thickness infeet Inter- Cumu- Feet above val lative base 23 210 0 -23 BEAR SPRING AREA, MASON COUNTY INTRODUCTORY STATEMENT Lying in southwestern Mason County, 7 to l l miles airline southwest to west· southwest of the town of Mason, the Bear Spring area displays a' sequence of Ellenburger rocks that may be taken as representative of the southwestern part of the Llano region. About 15 square miles was mapped geologically between the latitudes of 30° 38' to 30° 4,3' north and longitudes of 99° 18' to 99° 24'. Plate 5 shows the geology of the area at a scale of 1:10,000 and Plate 36 includes , stereograms of parts of the area al the approximate scale of 1:20,000. Control for the base map of the Bear Spring area was taken from an alidadc and plane-table traverse by Barnes and L. E. Warren along the county road in the southeastern part of the area. The base was plotted and adjusted by L. E. Warren and Cloud from data recorded by the latter on aerial photographs of the U.S. Department of Agriculture. The Bear Spring area includes ground on both sides of the Llano River, from White's Crossing to the mouth of Bluff Creek. It is most aecessihle from the 'southeast side, the distance hy the county road from Mason to White's Crossing (Doell highway no. 1) being 10 miles. Ingress to the southwest side of the area may be had from the same county road by way of pasture roads on the Tom White ranch. From the north side, the area may be reached from the old Mason­Streeter road via the C. S. Vedder ranch and pasture roads to Bear Spring, Air· heart Spring, and Gunstock Springs. The northwest portion of the area may be reached by going south from U.S. high­way 187 at the west edge of Streeter on pasture roads through the Bob Hoffman ranch to the mouth of Bluff Creek. Rocks mapped in the Bear Spring area include the undifferentiated Wilberns for­mation (Upper Cambrian), the Tanyard and Gorman formations of the Ellen­burger group, two new Devonian forma­tions (the Bear Spring and Zesch forma­tions), and heds of Mississippian and Pennsylvanian age. The Honeycut forma­tion is missing due to pre-Devonian truncation. The names of the Devonian formations do not appear on Plate 5 be­cause the map had been sent to the en­graver before evidence relating to the Zesch formation was finally worked out. Structurally the Bear Spring area is bounded on the southeast by the Honey Creek fault and on the northwest by the Bluff Creek fault. The latter is of pecu­liar structural interest because of the fan-like "unraveling" or diffusion into minor zones of failure that it undergoes at either end of a short, clear-cut, pro­found rupture. Branch faulting is com­plex in the southeast corner and at the west side of the area, and several obscure faults near the middle of the area com­plicate the measurement of sections. Dips of strata are mostly very gentle and sel­dom exceed 7°, except in the vicinity of some faults, the flexed west side of the faulted Honey Creek syncline, and near collapse structures. For the most part the larger vegeta­tion is sparse in the parts of the Bear Spring area that are underlain by pre­Carboniferous rocks, and consists of scat­tered live oak, cedar, and mesquite, with local growth of deciduous oaks on the Cambrian sandstones. Low and spiny bushes such as the scrub-mimosa known as catclaw, black persimmon, agerita, blue-brush, bee-brush, and others are scat­tered over the area, along with many kinds of cactus and several of yucca. Parts of the area underlain by Carbonif­erous rocks are mostly more thickly wooded; the Mississippian strata com­monly bear a thick and locally impene­trable chaparral of blue-brush and other low and spiny hushes, whereas the Pennsylvanian Marble Falls limestone supports cedars and the Smithwick shale is marked by scrub-elm, scrub-oak, black persimmon, agerita, hlue-brush, and bee­ hrush. The composite Llano River section dis­plays the Ellenburger and related rocks of the Bear Spring area, and the sections which comprise it are described on pages 163-187 and diagrammatically represented on Plate 14. The Ellenburger Group of Central Texas UPPER CA MBRlAN WILBERNS FORMATION All units of the Wilberns formation, except the Welge sandstone member, are present in the Bear Spring area and may be differentiated. They were not sepa· rately mapped because the areal repre­sentation of members below the San Saha limestone member is slight and their de­lineation wonld have prolonged mapping in the area without materially enhanc· ino-the usefulness of the map. A 7-foot sa~dstone near the top of the formation was mapped for most of its length be­cause it furnished a structural datum parallel and close to the Cambrian-Ordo­vician boundary. Welge sandstone member.-:--This mem­ber is not actually present m the area mapped, but poor exposures may be seen northeast of the Bear Spring area at the base of a section about 3 miles southwest of the Mason courthouse. Here it is about 25 feet thick and consists of brown, spar­ingly glauconitic sandstone. Morgan Creek limestone member.---:The Eoorthis zone of the Morgan Creek lime­stone member is the lowest bed exposed on the south bank of Llano River east of the Honey Creek fault. A minor fault 80 feet southeast of the Honey Creek fault at this place raises the Eoorthis zone about 5 feet to expose the beds immedi­ately below it. A complete and well ex­posed section of the Morgan Creek lime­stone member above the Eoorthis zone at this place displays the granular, glauco­nitic, greenish gray to gray limestones characteristic of the upper Morgan Creek throughout the Llano region. The thick· ness of the Morgan Creek limestone in Lhe Mason section is 120 feet. Point Peak shale member.-The shaly portions of the Point Peak shale mem­ber are poorly exposed and not well rep­resented in the area mapped. However, the stromatolitie bioherms at the top of the member are well displayed along Bluff Creek, and Point Peak strata occur in some of the draws east of the Honey Creek fault and above the Morgan Creek limestone in the southeastern corner oJ the area. ' A prominent northeast-trending branch fault that may be seen in the east bank of Honey Creek 700 feet northeast of the Mrs. .T. Rogers ranchhouse raises beds to the southeast and is responsible for the exposures of the Morgan Creek limestone and Point Peak shale members on the south bank of Llano River east of White's Crossing. The thickness of the Point Peak shale in the Mason section is about 155 feet. Magnificent exposures of the stromato­litic bioherms at the top of the Point Peak shale member, and of the inter­hedded shales and limestones below them may be seen in a bluff extending about 0.4 mile along the south (right) bank of Llano River between the mouth of Mill Creek and a point 0.2 mile west of the mouth of Honey Creek (Pl. 18; Pl. 19, fig. A). The interbiohenn beds contain more limestone than shale, but the latter is a conspicuous lithic element and there is no doubt that the bioherms are prop­erly associated wilh the Point Peak shale at this place. The bioherms extend up­ward from a floor of Point Peak strata, commonly with compaction basins he· neath them. Apparently they grew con­siderably above the sea-floor of their time, for peripheral beds abut them with­out noticeable intertonguing; and, whereas some of them are wholly within the Point Peak shale member, others extend upward into the stratigraphic level of the San Saba limestone member, with San Saba beds deposited against them and over them. The overlying beds, moreover, display marked dips that can only in part be explained by compaction and are perhaps largely initial.' In the bluff described the bioherms are about 30 to 60 feet thick, and vary slightly in strati­graphic position. A prominent shale zone (marked x on PI. l8 and Pl. 19, fig. A) that abuts the most prominent bioherm in the bluff a little above its middle is above the next several bioherms east of it, and the prominent limestone bed (marked y on Pl. 18 and Pl. 19, fig. A) through which its base extends forms a base for the same bioherms over which the shale zone extends. The stromatolitic limestone of the .bio­herms is microgranular to sublithographic and medium to light brownish, greenish, or olive gray. The interhioherm beds consist of granular, brownish to green­ish gray, glauconitic limestone; platy green limestone wilh crinkled surfaces; and green calcareous shale. Below the biohermal zone, shale constitutes more of the total thickness and limestone less. The Wilberns graptolites described by C. E. Decker in 1945 came from the Point Peak shale member just east of the Bear Spring map area and about a mile north of the mouth of Honey Creek. Stratigraphically they occur about 120 1feet below Lhe Lop of the Point Peak shale member and 380 feet below the top of the Wilberns formation. Decker gives their position as 85 feet below Lhe top of the Wilberns, their collector evidently having taken the biohermal limcslones of the upper Point Peak shale as the base of the Ellenburger. Pleclotrophia was not found in or near the Bear Spring area, but the related huenellid brachiopod Mesonomia is as­sumed to mark its approximate position, and it was noted 6 to 16 feet below the hiohermal zone at the top of the Point Peak shale member. San Saba limestone member.-0£ the 260 feet of beds that represent the San Saba limestone member in the Bear Spring area, quartz sand is intermittently abundant through 80 to 100 feet. Four zones of calcareous sandstone or highly arenaceous limestone are recognizable locally, but at other places the two lower zones spread out and run together. The upper sandstone, known in the field as the fourth or 7-foot sandstone, is per­sistent throughout the small area mapped, averages between 7 and 8 feet thick, and maintains a position 28 to 35 feet below the Cambrian-Ordovician boundary as mapped. This variation might indicate in­consistency in the selection of Lhe Cam­brian-Ordovician boundary, variation in position of the 7-foot sand, or even minor disconformity at the systemic boundary. The 7-foot 'sandstone is a very useful reference datum for the Cambrian-Ordo· vician boundary in the Bear Spring area and is easy to trace because black per­simmon thrives on it, showing as a nar­row dark band on the aerial photographs (Pl. 36, fig. B) . In general black per· simmon and scrub-oak grow well on the Cambrian sandstones, probably because they are local aquifers, and clusters of these trees suggest the presence of sand. The individual sand grains in the San Saba limestone member of the Wilberns formation are small, moderately frosted and pitted, subround to round, and not uncommonly stained pink or brownish orange. In general they are less well rounded, less smoothly frosted, and bet­ter sorted than the very small clean sand grains that characterize the Gorman for. mation of the Ellenburger group. The limestones of the San Saba lime­stone member in the Bear Spring area are mostly granular, fresh surfaces vary­ing in color through brownish, yello~v­ish, or greenish gray to grayish brown and weathered surfaces being medium brown­ish to bluish gray. They are commonly silty and dolomitic, weathering yellow to buiI or even brick-red in such instances. Glauconite occurs inlermiltenLly through­out the member. The limestones of the San Saha in the Bear Spring area are characteristically more impure and dirtier in appearance than those of areas to the east. Recurrent sand beds are distinctive of the San Saba limestone member not only in the Bear Spring area but generally west of longitude 99° 20'. During recon­naissance studies along San Saha River on the Blockhouse ranch in northwest­ern Mason County and south and south­east of the village of Erna in west-central Mason and southeastern Menard counties sand zones of varying thickness were seen at several stratigraphic positions within the San Saba limestone member. The greatest continuous thickness of sand observed was 4,7 feet on the Blockhouse ranch, and the next greatest was about 40 feet in a faulted bluff on the west side of Leon Creek 0.9 mile southeast of Erna. Thin­ ner sandstones at other stratigraphic po­sitions were observed at both of these localiLies, but the sands observed in the three areas noted do not correspond when plotted with reference to the faunally established Cambrian-Ordovician bound­ary and are therefore apparent local de­ The Ellenburger Group of Central Texas velopments within a generally arenaceous San Saba limestone member. Mechanical and chemical analyses of some sand from the San Saba limestone member are given by F. B. Plummer (1943), who calls attention to its pos­sible industrial applications. Due to numerous high-angle faults and the prob­able lateral discontinuity of these sand tongues attempted commercial exp}oila· tion should be preceded by detailed geo­logical mapping in order to locate the thicker and more extensive of the sev­eral sand intervals. Possibly the thicker and more continuous calcareous sands in the lower part of the Cap Mountain lime­stone member of the Riley formation would be more suited to commercial ex­ploitation. LOWER ORDOVICIAN ELLENBURGER Gnoup The most complete composite sequence of Ellenburger rocks in the Bear Spring area includes only 973 feet of beds, the Honeycut formation being absent. Tanyard formation, Threadgill member. -As mapped in the Bear Spring area the Threadgill memher of the Tanyard formation is wholly limestone, corre­ sponding to the original "Threadgill lime­ stone" of Bridge and Barnes (in Barnes, 1944). The member was so mapped as a matter of convenience, the lowest por­ celaneous to chalcedonic white-weather­ ing cherts occurring in Lhe general vicin­ ity of the transition from limestone to dolomite, but not in sufficient abundance to be readily mapped. The chemic~! transition in the western part of Lhe Llano uplift normally involves so small an amount of secLion as to make a rea­ sonable member-boundary, the limestones below being assigned to the Threadgill member and the dolomites above to the Staendebach member of the Tanyard for. matfon. Measured thicknesses of the Threadgill member in the Bear Spring area range from 265 to 294 feet. The limestones of the Threadgill mem· 'her in the Bear Spring area are pre­ dominantly sublithographic, essentially nonglauconitic, and pearl gray to light brownish or bluish gray to woodash gray. For the most part they are thinly bedded, and bedding surfaces are commonly covered with sections or selectively do­lomitized casts of Lytospira gyrocera (Roemer), Ophileta polygyrata (Roemer), and trails made by them. Both fossils and trails commonly are selectively dolo­mitized, weathering in relief and to a darker color than the adjacent rock. A few of the dolomitized markings pene· trale the rock and are probably borings, but most are restricteq to bedding sur­faces and are undoubtedly the trails of creeping organisms. Most of these trails are discontinuous, relatively smooth, dol­omitized welts and were probably made by some buoyant benthonic organism such as the gastropods named. A few corru­gated vertebra-like markings may have been made by trilobites. Tetralobula texana Ulrich and Cooper marks a zone about 40 to 45 feet above the base of the Threadgill member, and a distinctive robust species of Finkelnburgia·* occurs about 260 to 285 feet above the base. The limestones of the Threadgill mem­ ber of the Tanyard formation differ from those of the San Saba limestone mem­ ber of the Wilberns formati.on in bein~ purer, predominantly suhlithographie, and essentially nonglauconitic. The Cambrian­ Ordovician boundary is not abrupt, how­ ever, and sedimentation may have been essentially continuous across it. In the Bear Spring area it is drawn at the base of a 5-to 10-foot zone of mottled rock that consists of irregular inclusions of huff dolomitic limestone i.n light gray pure limestone, or the opposite. The transition between the two is fairly sharp, yet at the same time sufficiently grada­ tional in appearance that the effect is not Lhat of a conglomerate hut rather of chemically varying portions of the same bed. In mapping, this was referred to as the buff-bed and made a good bot­ tom to Lhe Tanyard formation at most places, in that it contained the lowest very light gray suhlithographic limestone, was above most of the observed glauco­ nite, and was essentially coincident with the Cambrian-Ordovician faunal bound· ary. Abundant glauconite was seen in a ] -inch zone about 5 feet above this buff­ hed at many places and probably marks *Finkelnburgia obesa Cloud (in press) . The University of Texas Publication No. 4621 a persistent zone, hut Gasconadia was ~een between it and the top of the huff. bed. Locally a second huff-bed occurs above the first, and somewhat similar 'beds below it, so care must he taken as to which bed is mapped. In Lhis connec­tion the 7-foot sand near the top of the Wilberns formation is of great help. Occasional beds of fine grained dolo­mite occur in the upper part of the Threadgill member of the Tanyard and rare globules of glauconite were seen locally in the upper beds. Small quantities of silt grains were seen in siliceous residues from the lower 130 feet of the Thread­gill member and occasionally above. As in other parts of the Llano region, no sand was found below the Gorman for­mation hut the occurrence of silt grains in the Tanyard formation of the western areas prognosticates a possible influx of sand in the Tanyard formation of the subsurface west of the Llano region. Tanyard formation, Staendebach mem­ber.-In the Bear Spring area the beds mapped as the Staendebach member are wholly dolomite. They overlie limestones assigned to the Threadgill member of the Tanyard formation and underlie micro­granular to very fine grained, locally arenaceous dolomites belonging to the Gorman formation. The measured thick­ness of the Staendebach member in the Pete Hollow section is 229 feet, of which the lower 19 feet intergrades with lime­stone. Local variation in thickness re­sults from downward extension of the dolomitic facies and lateral gradation to limestone, but these do not ordinarily involve notable stratigraphic intervals. The change from limestone of the Thread­gill member to dolomite of the Staende­bach member is generally marked by a decrease in the abundance of cedar, in­crease in abundance of live oak, a clump­ier pattern of tree growth, and disappear·· ance of strong bedding patterns. The dolomite in the lower half of the Staendebach member is fine grained to very fine grained, grading locally and in minor part to microgranular. In color it is light to medium brownish to yellow­ish gray, grading to nutria. That of the upper half is mostly fine grained to medium grained, rarely grading to very fine grained. It is commonly vuggy and its color is pearl gray to light brownish or yellowish gray, grading to woodash gray. Chert is scarce in the Staendebach member of the Bear Spring area except locally in the upper part. Oolitic to oomoldic chert occurs sporadically and coatings of quartz druse are locally con­spicuous in the lower half of the mem­ber. In the upper half of the member dolomoldic and in part quarlzose cherts, as 'well as oolitic to oomoldic chert and free siliceous ooids, are locally abun­dant, and commonly are associated with lesser quantities of chalcedonic, porce­laneous, or subgranular white-weathering chert. As in the Bald Ridge area chal­cedonic to semichalcedonic, and porcela­neous to scmiporcelaneous, white-weath­ering cherts are locally abundant near the top o,£ the Staendebach member, oc­curring on both sides of the Tanyard­Gorman contact as local accumulations of float suggesting lenslike masses such as that illustrated by figure C of Plate 21. The chalcedonic lo semichalcedonic varieties are commonly oolitic. Ozarkina, Helicotorna, and other Tan­yard fossils occur to within a few feet of the top of the Staendebach member, furnishing faunal control of the physi­cal evidence used in mapping the Tan­yard-Gorman boundary. However, the genera that characterize the upper 90 to 100 feeL of the Staendebach member in the eastern areas (e.g., Paraplethopeltis and Ribeiria; Rhabdoporella perhaps occurs, p. 169) were not found in the Bear Spring area. Although the Tanyard­Gorman contact is locally irregular (Pl. 24, fig. B) and may he disconformable it is doubtful that Lruncation could have been sufficient tQ eliminate the upper faunal zone of the Tanyard formation, and facies changes may account for its apparent absence. Gorman forrnation.-The lower half of the Gorman formation in the Bear Spring area is wholly dolomite, and the upper half is principally limestone. The dolo­ mite (Pl. 24•, fig. B) is microgranular to' medium grqined, with microgranular The Ellenburger Group of Central Texas being a very conspicuous but not a pre· dominant grain size. In color it varies from light to medium brownish to yel­lowish gray to light gray, light brown, pinkish gray, beige, brownish beige, or rose beige. The limestone (Pl. 24, fig. C; Pl. 25, fig. B) is predominantly sub­lithographic and woodash gray to pearl gray. It is more thickly bedded and markedly purer than that of the Thread­gill member of the Tanyard formation in this area and is further distinguished by lacking the dolomitized gastropods and gastropod-tracks and by containing relatively abundant chert. As in other areas the upper beds are notably pure and thickly bedded limestones. Sand is characteristic of and recurrent throughout the Gorman formation as scat­tered to abundant grains in dolomite, limestone, and chert. These grains are typically small, well rounded, smoothly frosted, clean, and poorly: sorted within their small size limits. They occur to the base of the formation, combining with microgranular dolomite to set it apart from the Tanyard formation below. Chert is more abu.ndant in the Gor­man formation of the Bear Spring area than in any but the highest beds of the underlying Tanyard formation. It occurs as nodules, lenses, plates, irregular inclu­sions, and incrustations that are com­monly chalcedonic to semichalcedonic and oolitic, less commonly porcelaneous, locally granular, and locally dolomoldic or drusy. Incrustations or rosettes of quartz druse are locally conspicuous, and chert­matrix sand was noted at several places. Chalcedonic lo subchalcedonic and partly oolitic cherts are conspicuous on both sides of the Tanyard-Gorman boundary in parts of the area, and some of the ooids in the Gorman cherts have sand grains for centers. The concentration of such che1 t at the formational boundary is a great aid to ils quick, approximate location, but exact determination of its position depends en fossils, and on the distribution of sand and microgranular dolomite. There is a geneta! topographic and vegetative break at the Tanyard-Gorman boundary, the dolomitic facies of the Gonnan formation being expressed as a gent!y rolling upland that supports scat· tered clumps of live oak, a thin growth of mesquite, and local cedar; whereas the ground underlain by dolomites of the Staendebach member of the Tanyard sup­ports few mesquites. Rhombella occurs in the basal chert of the Gorman formation and ranges through the entire unit. Lecanos pira first appears a little higher but ranges to the top. Syntrophina is a common elemeul in porcelaneous cherts about 70 to llO feet above the base of the formation. Archaeoscyphia characterizes a zone 220 feet above the base of the formation but was found locally aL positions about 25 feet below and 6 feet above its principal occurrence. The 450 feet of Gorman strata in the Dear Spring area probably displays almost the full normal thickness of the formation, but it is directly overlain by beds of Devonian or Mississippian age (Pl. 28, fig. C; Pl. 36, fig. A) . Strata of the Honeycut formation are missing due to pre-Devonian truncation, but the former presence of Honeycut strata at least as young as the zone of Ceratopea 5 is demonstrated by the occurrence of that "species" of Ceratopea as pebbles in the Devonian Zesch formation and the Mississippian Ives breccia in the valley of Honey Creek 300 yards west-southwest of the rock cabin on the Roy Zesch ranch. Pre-Mississippian weathering apparently imparted a yellow to buff discoloration to beds at the upper surface of the Gor­man formation so lhat limestones along this zone of weathering can be mistaken for dolomite. This discolored zone served as a clue to the location of some of the smaller patches of Mississippian that are mapped, and suggests the recent removal of the Mississippian from the tops oi '3ome hills where none is mapped. DEVONIAN Rocks of lower Middle Devonian and probable Upper Devonian age occm in a structural sink 200 to 300 yards west­southwest of the rock cabin on the Roy Zesch ranch and southwest of Honey Creek in the northeast corner of the Bear Spring map area (PL 5; Pl. 36, fig. A). The lower uniL consists of granular, yellowish or greenish brown to white, in part cherty limestones containing a fauna of lower Hamilton age and is called the Bear Spring formation. The upper unit is a thin, highly siliceous limestone or leached silica rock containing a fauna suggestive of that occurring in. the lthaca and Cornell shales but with faunal affinities possibly as low as the Tully formation and as high as the Enfielrl shale. It is called the Zesch formation. A peculiar olive green to brown, finely laminated, ferruginous and phosphatic silica rock occurs at the margins of the structural sink, as float only, suggesting that it represents remnants of rock that were dragged loose during collapse and probably came from below the Bear Spring formation. This rock contains no fossils, its stratigraphic relationships are not known, and it is not deemed worthy of a name. These rocks are described by Barnes, Cloud, and Warren (1947) and their probable correlation is shown in figure 1 of the present report. The names of the Devonian formations do not appear on Plate 5 because the map had been sent to the engraver before evi­dence relating to the Zesch formation was finally worked out. CARBONIFEROUS MrssrssIPPIAN Mississippian rocks of the Bear Spring area comprise two small patches of the Ives breccia and good representations of the Chappel limestone and the Barnett formation. The character"3 of these rocks are described in another part of this re­port (pp. 42-59) and only local details need concern us at this place. Sections of the Mississippian were measured at six places in the Bear Spring area and probably all outcrops of Mis­sissippian rocks in the area were seen. The Chappel limestone consists of larger crinoid fragments in a finely granular matrix and is dark rose lo gray · and olive gray in color. Macrofossils are rare at most outcrops, but Brachythyris choitteauensis Weller, Plicatifera, and other fossils of Chouteau affinities were obtained at TF-4171 and TF-417c, just off the northeast edge of the map. Cono­donts were found in most of the samples taken and according to W. H. Hass (oral communication) are of the same types that occur at the type locality of the Chappel limestone. The Chappel lime­stone of the Bear Spring area ranges from a maximum of about 35 feet thick to a feather edge. It rests directly on the Gorman formation except at one locality where the Ives hreccia and De­vonian strata intervene, and for the most part it is collapsed i.nto the Gorman for­mation to some extent, a notable struc­tural sink being in a draw about 400 yards northeast of Rattlesnake Hill (Pl. 36, fig. A). Mixing of float from the Chappel limestone with weathered and discolored cobbles of Gorman rocks is commonly intimate and it is locally diffi­cult to say exactly where the contact between Mississippian and Ordovician should be drawn. The Barnett formation of the Bear Spring area consists of white to light gray crinoidal limesands and finer grained limestone, local tongues of dark petroliferous limestone, dun to huff pel­let limestones, and caliche-covered zones probably representing shale: There seems to be a general increase in proportion of limesand and decrease in shale and petroliferous limestone from north to south along the Barnett outcrop belt in this area.' Shale was noted locally in the dominantly covered zones and there is little doubt that they are underlain by shale at most places. The Barnett for­mation of this area varies from as much as 140 feet thick in the southeastern corner of the area (TF-4.18, TF-1.20) to a feather edge. Variations in thickness ap­parently result both from truncation of the upper beds and nondeposition at the base of the formation, as well as prob­ah le variation in the rate of accumula­tion of the original sediments. Where the shale intervals and petrolif­crous limestones are well developed they support a dense chaparral of blue­brush and other stiff and thorny bushes that gives the approximate position of the Ellenburger-Mississippian boundary. However, the nonpclroliforous while lime­sands and Chappel limestones do not differ vegetatively from the limestones of the Ellenburger group and careful map­ping is necessary to locate all lobes and The Ellenburger Group of Central Texas outliers of Mississippian limestones (see Pl. 36, fig. A). At many places the lower 20 to 40 feet of the Barnett formation is com­prised of white to very light gray lime­stones and crinoidal limesands with an apparently sharp break to interbedded shales, petroliferous limestones, and lime­sands above. A macrofauna with obvious Keokuk affinities (p. 56) was found in the lower limesand at White's Crossing (Pl. 28, figs. D, E, and F) and other places, and in the early phases of the mapping an attempt was made to sepa­rate these basal beds as a distinct unit intervening between Barnett and Chappel strata. However, continued mapping brought out the fact that lateral shifts of facies took place and that where, by nondeposition, a higher limesand came to rest upon Ellenburger rocks it could not ordinarily be distinguished from the lower limesands. It was also once sup­posed that significance might he attached to size of the detrital particles or to the occurrence of concretions of finely mot­tled, white to light gray, subporcelane­ous chert, or cavernous crinoidal chert in the lower white limestones assigned to the Barnett formation at localities TF-417, 420, 421, and a few other places in the Bear Spring area. But, again, as the mapping continued it be­came increasingly difficult to make any consistently satisfactory physical distinc­tion between the lower limesands and the upper strata of the Barnett forma­tion in the Bear Spring area. These vari­ous attempted subdivisions were actually mapped on the aerial photographs in order to see their special relations, but were finally given up as meaningless. The essential unity of the sequence is further shown by the occurrence of a tongue of shale and petroliferous lime­stone with Leiorhynchus carboniferum Girty, Productella cf. P. hirsutif or mis Walcott, Moorefieldella cf. M. eurekensis (Walcott) and other Moorefield fossils in the midst of a 140-foot limesand sequence at locality TF-420, about one-half mile west of White's Crossing, and by the occurrence of Goniatites at several places in another ] 40-foot sequence of white limes ands containing Keokuk and War­ saw types of fossils (p. 56) at TF-418 in the southeastern corner of the Bear Spring area. According to W. H. Hass, the conodonts from samples collected by him and the writers at White's Crossing, TF-416, TF-418, TF-420, and TF-422 are like those from the Barnett forma~ tion at and in the vicinity of its type locality in San Saba County (oral com· munication). The equivalence of the petroliferous shales of the Barnett forma­ tion in its type area with the limesands of Keokuk and Warsaw age of the Bear Spr~ng area is thus supported by the macro­ foss1ls, the conodonts, and by detailed mapping (see also p. 57). The lithic break from the dark com­ pact Chappel limestone to the light and commonly friable limesands and lime­ stones of the Barnett formation is marked at most places, and at one locality, near the head of the draw at the southeast side of a panhandle pasture on the P. G. Rogers ranch, limestone conglomerate occurs between them. At some places, however, dark limestone suggesting that of the Chappel beds actually occurs as lenses in the Barnett limesands, and if exposures are poor or fossils scarce such situations may be quite confusing. Such a situation may be seen at White's Cross­ing, immediately north of the more con­spicuous outcrops along a fence that trends north-northwest. A few cobbles of limesand and lime· stone occur in the limesand matrix at White's Crossing, but they contain fos­sils similar to those of the matrix and apparently indicate no more than rapid deposition and consolidation in waters that were at times turbulent. The Barnett formation rests on the Chappel limestone or the Gorman forma­tion of the Ellenburger group. The gently synclinal structure of many of the out­liers of the Barnett formation indicates solution below it, with resultant settling of the Barnett strata into the underlying• beds, and at a few places it has actu­ally collapsed into direct contact with Ellenburger rocks. It is not, however, so conspicuously collapsed as is the Chap­pel formation. PENNSYLVANIAN Pennsylvanian rocks in the Bear Spring area consist of the Marble Falls lime· stone and the Smithwick shale. Neither was measured or studied in detail. The Marble Falls limestone is mostly quite fine grained, gray to black, and locally contains quantities of black and gray speckled chert. It is several hun­dred feet thick. Macrofossils are locally abundant, but few were collected. Fora­minifera are abundant in some of the chert and limestone and fusiform fusu­lines probably referable to Fusiella were found in the basal beds at many places. Beds of Morrow age are thus apparently absent. Late in the v1rork on the Bear Spring area it was discovered that the basal foot or two of the Marble Falls limestone was conglomeratic, containing scattered sub~ angular to angular small pebbles and granules of varicolored chert in a gray­ish to yellowish brown matrix. This zone of chert pebbles was observed at widely separated localities and apparently oc­curs in the subsurface west of the Llano region, but it has not been seen east of the Bear Spring area. It is possible thal local occurrences of dark. microgranular to sublithographic limestones at the top of the Barnett formation may have been included with the Marble Falls limestone at a few places Jue to failure to recog­nize the basal conglomerate of the Mar­ble Falls early in the mapping. At most places the Marble Falls lime­stone contains scattered clumps of cedar and live oak and is sparsely vegetated in comparison to the shaly zones of the Barnett formation, the Smithwick shale, and the Marble Falls limestone of areas to the east. It is more thickly vegetated at other places, however, and generally supports a better growth of trees than the Ellenburger rocks of the Bear Spring area. Vegetational banding is common on the Marble Falls outcrops. The Smithwick shale of the Bear Spring area is a dark gray fissile shale with siltstone and limestone beds near the base. Its top is not preserved and its thickness is unknown. It weathers to a soft yellow clay, being so nonresistant to weathering that few outcrops are known in the area. UndouLtedly most of the alluvium in Lhe valley of Honey Creek between the Honey Creek fault and the outcrop belt of the Marble Falls lime­stone is underlain by Smithwick shale. Above the level of the present valley floor of Honey Creek, and best displayed on the southeast side of its valley, is an older terrace capped by alluvium and displaying a few small outcrops of Smith­wick strata. This terrace is thus mapped ab Smithwick shale on most of the south­east side of the valley, but its few rem­nants on the northwest side are mapped as alluvium. Where it is near enough to the surface lo influence the vegetation the Smithwick shale fosters dense thickets of scrub-elm, ;.cruL-oak, black persimmon, blue-brush, ngcrita, and bee-brush. Among fossils collected from the Smith­wick shale at TF-156, Dr. J. B. Knight identified a new species of Neilsonia and a new species of a new genus of high­spired gastropods, which he states to be characteristic of the Cumminsia aplata zone (Plummer. 1945, p. 73) of the lower Smithwick shale southwest of Bend and at other localities. DESCRIPTION OF THE LLANO RlVER SECTIONS The Rattlesnake Hill, Pete Hollow, and Bluff Creek sections, measured along and near the Llano River in the Bear Spring area, comprise a composite section dis­playing the full thickness of the Ellen­ burger group and the San Saba lime­stone member of the Wilberus formation, in this part of southwestern Mason County. The Mason section, a supple­mentary section, 6.5 miles by speedom­eter northeast of White's Crossing fur­nishes information on the Wilberns for­mation below the San Saba limestone member and completes the composite Llano River section, while the supplemen· Lary Buck Spring section furnishes a check on the measured thickness of the Threadgill member of tbe Tanyard for­mation. Iu the Bear Spring area the Honeycut formation is entirely absent and the Carboniferous strata rest directly on the Gorman formation. An interesting feature of the Bluff Creek section is the presence of four prominent arenaceous zones in the San Saba limestone mem­ ber of the Wilberns formation. The sequence of stratigraphic units in the The Ellenburger Group of Central Texas composite Llano River section is as follows: Thickness Stratigraphic unit in feet \fississippian (variable thicknesses of the Barnett formation and Chappel limestone) ~ower Ordovician-Ellenburger group (973 feet) Gorman formation ( 450 feet) Calcitic facies ---------------------------___ ___ 237 Dolomitic facies ----------------------------213 Tanyard formation (523 feet) Staendebach member (229 feet) Dolomitic facies ________ _________________ 229 Threadgill member (294 feet) Calcitic facies ____ _______________ ______ 294 Jpper Cambrian (560 feet measured) Wilberns formation (560 feet) San Saba limestone member___ ____ 260 Point Peak shale member________________ 155 Morgan Creek limestone member___ 120 W elge sandstone member ______________ 25 \.pproximate total thickness of Ellen­ burger group and Wilberns for­ mation -------------------------------1530 Each of the three principal sections lescribed is a unit structurally separated 'rom the others and selected to present he best exposures in the shortest hori­ontal distance. Although they are de­cribed as individual sections, they are iresented graphically, along with the sup­1lementary Mason section, as a single :omposite section on the local correla­ion chart. The Rattlesnake Hill, Pete Hollow, and Huff Creek sections were measured, 1ainted, and described by Cloud in Feb­uary, 1945. They are marked at ap­1roximately 5-foot intervals with yellow aint spots, and a number showing ac­cial feet above the base of the particular ection is painted every 50 feet or less. .ontrol for computed measurements was 1ken from large scale (approximately , inches to 1 mile) aerial photographs nd intersecting vertical traverses made ·ith a Paulin precision altimeter to de­~rmine elevations based on numbers of ~adings averaged to the nearest 5 feet )r each point of determined elevation. he computed measurements checked so loselv with field measurements made ith ~ Brunton compass that no adjust­ient was made, although reexamination £ the sections indicates that some inter­ vals include less and others more than 5 feet of section. The Mason section was measured by Barnes and Cloud in Janu­ ary 1946, and the Buck Spring section by Cloud in February 1945. The Rattlesnake Hill section is mostly on the Roy Zesch ranch, but in part also on the Mary Rogers and Melvin Capps ranches. The Pete Hollow section and the supplementary Buck Spring section are entirely on the Tom White ranch. The Bluff Creek section is on the Bob Hoffman ranch, crossing the property line to the Ben Jordan ranch in the upper few feet. Property ownership for the Mason section was not ascertained. The locations of the various sections on the geologic map of the Bear Spring area (Pl. 5) are shown by lines of inverted V's. Offsets are indicated by dotted lines except where self-evident, and the letter S indicates the supplementary Buck Spring section. RATTLESNAKE HILL SECTION The Rattlesnake Hill section (Pl. 24, figs. A and B), as described below, in­ cludes 450 feet of the Gorman and 5 feet of the Tanyard formation. This is a com­ plete and probably a maximum thickness for the Gorman formation in the Bear Spring area, and comparison with other sections suggests that a very few more feet of section would have displayed the Gorman-Honeycut contact. The top of this section is at an alti­tude of approximately 1705 feet at the northwest side and almost at the top of Rattlesnake Hill, about 0.9 mile airline north of White's Crossing of Llano River. Stratigraphically it is at the top of the Gorman formation and is overlain by a 12-foot rubble-covered interval probably representing the Barnett formation, and followed by about 23 feet of medium gray to brownish gray, cherty Marble Falls limestone (undifferentiated) con­taining macrofossils and small fusulines and extending to the top of Rattlesnake Hill. The base of the section (Pl. 24, fig. B) is at an altitude of about 1375 feet, about 0.75 mile airline northwest up Llano River from the top of Rattlesnake Hill and 5 feet stratigraphically below the Tanyard-Gorman contact. Thickness in feet Inter-Cumu-Feet above Description val lative base Ellenbmger group: 455 feet described Gorman formation: 450 feet thick Calcitic fades: 237 feet thick 1. Limestone (altitude approximately 1705 feet at top of sec­67 67 388 -455 tion)-predominantly sublithographic, in minor part a brec­cia and in part with irregular argillaceous films. It is of apparently great purity to the naked eye. The color is woodash gray to pearl gray with minor yellowish gray to yellowish olive beds. The beds range from a fraction of an inch to 24 inches thick where measurable, but are mas­sive and hummocky-weathering in the lower 6 feet and for the most part relatively massive in appearance, averag­ing perhaps 8 to 16 inches thick. Weathers to light to medium bluish gray, smooth to solution-pitted, conspicuous ledges. Chert occurs as scattered nodules, lenses, and irregular inclusions and was specifically noted at 402 and 444 feet. It is predominantly chalcedonic and subchalcedonic, in part oolitic, and locally with included sand grains; bluish to brownish gray varying to white and dull carnelian, in part color laminated; and weathers shiny white to rough and dull or white. Grape-sized rosettes of quartz drnse were noted at 430 and 434 to 435 feet: and the upper 5 feet is characterized by rough, russet-weathering, granular, fossiliferous chert externally resembling cannonball chert but only questionably approximating the textural detail of that chert type. Scattered sand grains were noted in some of the chert and limestone float (abundant at 402, 410, and 453 feet) but were seen in place only at 41.6 feet where they occur in great abundance in a dull orange to brownish gray, mottled, calcareous chert. Cross sections of Lecanospira were noted in limestone float at 395 feet and in chert float at ~07 feet. A few Lecano­ spira were collected (TF-293) from chert float 15 feet above the north end of bed D, or by projection at 403 feet in the line of section. These were very abundant at the locality where collected but search up the slope and laterally yielded no additional specimens, so it is likely that they were derived from a local occurrence very near the gathering place. Hor­ motoma and other high-spired gastropods were collected from the rough chert in the top 5 feet of the interval (TF-183), and Ophileta and poorly preserved cross sections of unidenti­ fied gastropods were ohscrved in lime,tone at 4°50 feet. D bed at 388 feet; a prominent ledge at top of interval 2, in part covered laterally. Shift about 370 feet south-southwest if going up in the section and north-northeast if going down. The offset on this bed is based on a projection along the west slope of Rattlesnake Hill, along which the rocks are generally well exposed, and was checked by a shift on bed C, 37 feet stratigraphically below. Bed C is marked below the southeast end of bed D in an undescribed alternate segment of the section. 2. Limestone-sublithographic, woodash gray; beds from a 4 71 384 -388 fraction of an inch to 22 inches thick. Weathers light to medium bluish gray. Minor chert occurs in the basal part as thin plaLes; be­ ing semichalcedonic to suhchalcedonic, bluish gray to white, and in part oolitic with scattered grains of sand. 3. Dolomite, in part slightly calcitic and with minor inter­24 95 360 -384 beds of and lateral gradation to limestone in upper part­fine grained to microgranular, yellowish beige; beds 1 to The Ellenburger Group of Central Texas Description 24 inches thick. Weathers light to medium yellowish gray, poorly exposed. 4. Limestone, with dolomite beds from 312 to 313.5 and 341 to 343 feet-predominantly sublithographic, in minor part a granule conglomerate wilh a medium to fine grained matrix and locally grading to medium grained where dolo­mitic; woo dash gray to pearl gray; beds from a fraction of an inch to 24 inches thick with the more massive beds commonly weathering to thin plates. Weathers to uneven, solution-pitted, light to medium bluish gray surfaces. The dolomite is medium to fine grained, in part vuggy, and light gray to yellowish gray from 312 to 313.5 feet and microgranular to very fine grained and yellow beige from 341 to 343 feet. Chert is fairly abundant in the interval, both in place and as float. It was noted in place at 307.5, 309, 315, 317, 334, and 344 to 355 feet. The chert in the lower 10 feet is chalcedonic to subchalcedonic, oolitic to crypto-oolitic, with included rhombs of dolomite. It is mostly of a strik­ing brownish orange to carnelian color but grades in part to brownish or bluish gray to white. That at 315 feet occurs as an anastomosing network in limestone, weathering to a dull ropy surface, and is white and slightly calcitic on the fresh surface. In the upper 45 feet the chert is mostly chalcedonic to semichalcedonic, in part oolitic to crypto-oolitic, and in part with scattered sand grains; weal hering typically to shiny while surfaces. At 353 feet is a thin film of russet-weathering quartzose chert with scat­tered ooids and sand grains. Sand grains were noted in abundance in some of the chert and limestone lloaL especially at 322 and 353 feet and in place in limestone from 309.5 to 310 feet and in chert at 317, 334, and 345 feet. These grains are small and well rounded, but irregular in size within their small limits. "Hormotoma" was noted in ropy chert at 315 feet and Lewnospira was collected from limestone ledges in place between 350 and 355 feet (TF-292). C bed, at 351 feet, was traced laterally about 400 feet southeast along the sonthwest slope of Rattlesnake Hill to check the offset on bed D, 37 feet slratigraphically above. 5. Dolomite-microgranular, in minor part grading to very fine grained; beige to rose beige, wilh purplish to purplish rose bands, streaks, and mottles. Deds 4 Lo 18 inches thick, poorly exposed. Weathers to mostly smooth, uneven, light to medium yellowish gray to brownish gray ledges. 6. Limestone, grading laterally to dolomite in upper 3 feel­predominately suhlilhographic, hut in minor part a granule breccia with sublithographic fragments of limestone in a fine to medium grained dolomite matrix and in part with irregular greenish yellow argillaceous films; woodash gray to pearl gray, in part with yellowish pink dolomite rhombs or patches; beds from a fraction of an inch to 30 inches thick with more massive beds commonly weathering to thin plates. Weathers to uneven, medium to light bluish gray surfaces which are commonly pitted or radially grooved by solution. Chert occurs as layers or extensive flat lenses, 2 to 4 inches thick, in the lower 20 feet; and as nodules, lenses, and irregular inclusions in the upper portion of interval 6. Cherty zones were noted at 229.6 to 230, 235, 236.5, 242.5, Thickness in feet Inter- Cumu- Feet above val lative base 60 155 300 -360 3 163 292 -300 63 231 224 -292 166 The University of Texas Pnbl£cation No. 4621 The Ellenburger Group of Central Texa8 Thickness in feet Inter- Cumu- Feet above Description val lative base 265 to 266, and 288 lo 290 feet. The chert is chalcedonic to semichalcedonic, conspicuously oolitic at 236.5 feet, and commonly banded with brownish to bluish gray and white and in part clear brown streaks. Sand grains occur in limestone about 30 feet above the base and 10 feet below the top of interval 6. Snail impressions were noted on some of the limestone ledges but none were collected or identified. Chertified Archaeoscyphia occurs in the basal .3 feet (TF-163), mark­ ing the principal Gorman zone of Archaeoscyphia in the Bear Spring area. An occurrence o{ Archaeoscyphia 25 feet below the principal zone was noted about 0..5 mile north of the line of section, and another occurrence about 6 feet above at a locality 0.9 mile north of the line of section. R heel at 224 feet. Shift about 380 feet south if going up in ihe section and north if going down. The hasis [or this offset is the sponge Archaeoscyphia which occurs immediately above. At the south end of the offset (altitude 1490 feet) the dolo­ mite limestone contact is somewhat farther below bed B than at the north end (altitude 1530 feet) due to lateral transition. 7. Limestone, in minor part with irregular dolomitic inclu­ 6 237 218 - 224 sions-sublithographic, grading to medium grained where dolomitic; woodash gray to pearl gray, with yellowish specks and mottles where dolomitic; bedding irnlistinct. Weathers uneven, light to medium bluish gray to almost white. Chert occurs as float in blocks which are subchalcedonic to subgranular or chalk textured, in part slightly quartzose and with scattered tiny dolomolds, white to light brownish gray, and weather whitish. Base of calcitic facies of Gorman formation at base of inter­ val 7 (altitude approximately 1530 feet) Total thickness of facies 237 feet. Dolomitic facie;: 2.13 feet thick 8. Dolomite-medium to fine grained, in part vuggy; light 8 245 210 - 218 gray; bedding indistinct. Weathers rough, in part pitted, medium to light brownish to yellowish gray. Chert occurs as nodules and irregular inclusions which are in part chalcedonic to porcelaneous, in part oolitic to micro-oolitic, and locally contain scattered sand grains. Sand grains were noticed in dolomite float near the base of the interval and in some of the chert. The 210-foot horizon in the Rattlesnake Hill section is at the west end of a west-northwest trending spur, near its truncated terminus in a 160·foot bluff on the northeast bank of Llano !liver, and the 210­ to 218-foot interval is measured over a distance of about 450 feet along the south edge of the spur­ crest in the direction of dip. Little reliance can he placed, therefore, upon the accuracy of the measured thickness of in­ terval 8. 9. Dolomite-medium grained to microgranular: being medium ll5 360 95 -210 to fine grained, and in part vuggy from 95 to 112, 122.5 to 128, 129 to 137, 14.6 to 148, 155 to 159, 162 to 192, 197 to 202.5, and 208 to 210 feet; and microgranular, in Description minor part varying to fine grained and vuggy, from 112 to 122.5, 128 to 129, J37 to 14.6, 148 to 155, 159 to 162, 192 to 197, and 202.5 to 208 feet. The medium to fine grained dolomites are light gray to light yellowish or brownish gray. The microgranular dolomites range from light yellowish or brownish gray Lo light beige, and are in part mottled and f>treakecl with darker beige to nutria. The beds range from 1 to 24 inches thick, with more mas­sive-looking beds in an adjacent cliff. Weathers medium gray or brownish gray to light yellowish gn1y, being com­monly rough and pitted in the medium to fine grained portions and tending to be sphenoidally jointPd with smooth surfaces in the microgranular pm:tions. Chert occurs intermittently as nodules, lenses, irregular inclusions, and as abundantly interstitial cherL Lhat shows, on weathering, as dolomoldic :,urfaces. It was specifically noted at Jl7 to 118, 124 lo 126, 128, 1311 lo 136, 158 to 160, 163, and 191 to 193 feet. It is cbakedonic to por­celaneous or chalk textured, in part olilitic or with tiny indistinct cloudy pellctlike markings, and in part inter­stitial or dolomoldic. The color varies Crom light bluish gray to brownish gray, grading to china white where inter· stitial or porcelaneous. Many bedding and joint surfaces are conspicuously coated with fine to coarse grained quartz druse. Sand occurs ab scattered lo fairly abundant grains in some oI the chert and dolomite. IL was noled in dolomite at 112, 119, 125, and 168 feet and in chert at 119, 139, and 192 feet; and it is sufficiently abundant at 195 feet to constitute in part a chert matrix sand. The individual grains are well rounded, frosted, and poorly sorted within their small size-limits. Poorly preserved specimens of "Euconia" and a small, moderately high-spired, sharply keeled gastropod were noted in chert at 128 feet. Chert similar to that from which Syntrophina campbelli (Walcott), and Ilormotoma were ob­tained about 0.4 mile northwest of here (TF-291) was noted at 117 to 118 feet, but although suggestions of a small syn­trophid brachiopod were seen in this chert no definitely recog­nizable fossils were obtained. l 0. Dolomite-predominantly microgranular, in minor part grad­ing to very fine grained especially at 67.5 feet and 77 feet, in minor part vuggy; color light gray to light yellow­ish gray to beige or brownish beige; beds apparently rang· ing from 2 to 24 inches thick, but more massive-looking in an adjacent cliff. Weathers to sphenoidally jointed ledges with smooth, or rarely pitted, surfaces; medium to dark gray or hrownish gray to light yellowish gray. Nodules and irregular inclusions of chert are ;;cattered through interval 10, and were specifically noted at 68, 75, and 92 feet and along the trace of the interval at its base. This chert is mostly chalcedonic to subporcelaneous and in part chalky. It is in part quartzose, locally oolitic, and locally contains scattered ooids and sand grains. The Thickness in feet Inter- Cumu- Feet above val lative base 32 392 63 -95 Description col?r of a f:esh surface is light bluish to brownish gray to white, and it commonly weathers shiny white. Some bed­ding and joint surfaces coated with quartz druse. Small, well-rounded, frosted sand grains; varying consid­erably in size within their small dimenoions; are abundant in the dolomite at 69 and at 77.5 feet and scattered grains were noted between 84 and 87 feet a~d in some other beds. Small nodules of chert matrix sand are conspicuous at 69 feet, and at 92 feet sand is abundant in chert nodules. A bed at 63 feet. Shift about 2500 feet southeast if going up in the section and northwest if going down, following the approximate trace of bed A along the northeast (left) bank of Llano River. A minor syncline causes bed A to be concealed by alluvium for about 350 feet near the midlength of this oftset; but as nearly as could be determined from tracing, geometric projec­tion, and comparison of chert and lithology, the shift is made within a relatively small limit of error. The contact between the predominantly microgranular dolomites above and the fine to medium grained dolomite below is fairly persistent, and the inter­mittent occurrence of prominently developed white-weathering chert in the stratigraphic vicinity of bed A, and commonly ju5t above it, is noticeable. The white weathering chert, however, varies somewhat in character, position, and quantity. 11. Dolomite-fine to medium grained, in part vuggy; light gray to light yellowish giay, in pa1t wilh darke1 streaks; bedding indistinct. Wealhe1s to pooily exposed. rough, in part pitted, medium to light yellowish giay ledges and scattered cobbles. Chert is ahundant in this interva I both as float and in place, being pa1 ticulaily noted at 47 to 48 feet but seeming to appear and disappear when tiaced lale1ally. It occurs as len~es, nodules, and irregular inclusions and is subpor­celancous to chalcrdonic, in pa1t oolitic, locally qna1tzose, and in pal! oligbtly dolornoldic. It weathers conspicuously while and shiny except where dulled by films of qua1tz druse. Qua1t7 drnse also occurs as rosettes and incrusta­tions on the dolomite. Sand occurs as scattered grains in some of the chert and in dolomite at the top of the interval. Synt1ophina was collected from float of subporcelaneous to subgranular white chert at 60 feet (TF-289), but prob­ably this Syntrophina bearing chert floated down from a 2-foot ledge of chert that occurs 50 feet above the top of interval 11, and toward the top of a northwest-trending spur, at a sll atigraphic position oI about 108 to llO feet above the base of the Go1man fo1mation. Additional col­lections of Syntrophina campbelli (Walcott) were made from chert float 10 to 35 feet st1 atigraphically above the top of interval 11 (TF-290) and from the ledge 50 to 52 feet above it (TF-291). 12. Dolomite, all but lower 12 inches covered in the line of section-a little to the soulheasl of the line of section it is microgranular, in part grading to very fine grained espe­cially in upper 5.5 feet; light gray to light yellowish gray or beige; beds 6 to 42 inches thick. Weathers medium to light brownish gray. Chert occurs as nodules and irregular inclusions, espe­cially at about 34 feet. It is chalcedonic to subchalcedonic, light bluish gray to white, and weathers shiny white. Quartz Thickness in feet Inter- Cumu- Feet above val lative base 20 412 43 -63 12 424 31 -43 The Ellenburger Group of Central Texas Thickness infeet Inter- Cumu- Feet above Description val lative base druse occurs as conspicuous incrustations and rosettes in the lower part of interval 12. 13. Dolomite-medium grained, in part grading to almost coarse 12.5 4·36.5 18.5­ 31 grained; light brownish to yellowish gray; beds 2 to 19 inches thick. Weathers to rough, uneven, medium to light brownish gray to yellowish gray ledges. Interstitial, chalk-textured, white chert is abundant in some beds. Chert also occurs as irregular inclusions that are porcelaneous and white. Nodules and inclusions of a conspicuously ooliLic, light brownish to bluish gray, shiny white-weathering chert occur at 28.5 feet. Sand grains appear to be present as centers for some of the oiiids in the chert at 28.5 feet and scattered grains were seen in dolomite near the middle of the interval. An essentially east-west offset of about 200 feet between inter­ vals 14 and 13 was accomplished by shifting on the top of interval 14 about 100 feet east along the north bank of Llano River and then sighting with a Brunton compass set for a 2° east dip to the east side of a draw at a point 15 feet northeast of a northwest-trending fence. 14. Dolomite-microgranular in minor part grading to very fine 13.5 450 5 - 18.5 grained from 5 to 13 feet, medium grained to fine grained from l 3 to 14 feet, fine to very fine grained from 14 to 15.5 feet, and medium to fine grained from 15.5 to 18.5 feet; light gray to light brown, beige, or faintly pinkish gray, grading to light olive gray in the upper foot; beds 4, to 25 inches thick. Weathers to haekly or ~phenoidally jointed ledges which are dark to medium gray with a brownish tinge. Chert occurs as small concretions, nodules, and irregular inclusiom at 10 to 11 feet. It is suhchalcedonic to porce­ laneous to chalk texlmcd, in pall oolitic and in part with scattered ooids, and while to bluish or brownish gray. The medium grained dolomite fiom 1.S to l'l feet is conspicu­ ously inle1laced and coated wiLh fine to coarse grain~d quartz druse, accompanied by minor oolitic to oiimoldic in­ crustations and interstitial chert. Sand grains occur sporadically throughout the interval but are mosl abundant in the lower 25 inches and quite conspicuous in the lower 8 inches. They are small, well rounded, and frosted. Tiny interstitial inclusions of a greenish substance that is probably glauconite occur near the base of the interval and again at J 1.5 feet. Samples isolated by dissolution of the matrix in hydrochloric acid were a brighter green than the interstitial glauconite of the Cambrian Pedernales dolo­ mite but resemble it in other respects. Base of dolomitic facies of Gorman formation at base of in· terval 14 (altitude 1380 feet). Total thickness of facies 213 feet, and of formation 450 feet. Tanyard formation: S feet described Staendebach membe1: 5 fed described Dolomitic facies: 5 feet described 15. Dolomite, in part slightly calcitic-medium to fine grained, 5 455 0 - 5 in part vuggy; light gray to light brownish gray; bedding indeterminable. Weathers rough, irregularly jointed, in minor part pitted, medium to dark brownish gray. Excrescences and surficial coatings of fine to coarse grained quartz druse and oolitic to oiimoldic chert occur throughout the interval. Minor chert containing "Rhabdo­ porella"-like objects occms in the basal ledge and imme­ The University of Texas Publication No. 4621 Description diately below. Irregular inclusions of white-weathering oolitic chert occur at the top. &attered sand grains occur in the upper parl of the interval and some of the ooids in the upper oolitic chert appear to have sand grains for centers. This sand is by no means conspicuous in a ham! specimen aml must be searched for with a hand lenq_ Perhaps tlw sand-bearing beds should be included in thf' Gorman formation; but the prominent grain size break occurs a little above the lowest sand, which i'3 assumed in th is instance to repre­sent a premature contamination or a secondary introduc­tion, due to disturbance by wave aclion or by burrowing organisms of the upper layerq of the Tanyard formation before diagenesis. Interval 15 mark-. tlw top of the Tanyanl formation as mapped at this place and is overlain by microgranular dolomite of the basal Gorman formation which is conspic­ously sandy in the lower 8 inches. The dt>signated forma­tional contact is clearly visible and is irregularly undulat­ing, due to slight disconfonnity if not to solution by ground water. Visible apparent truncalion at the contact is about 4 to 6 incltl:'s and geometric projection we'3tward along the low bluff at this place snggcsls that it may he greater. There is, however, no evidence to preclude the possibility that this irregular contact was developed by solution. Ozarkina, Ophileta, a high-spired gastropod suggestin?, Hormotoma, and Ectenoce1as? were collectc:t feet, and is illustrated hy figure D of Plate 20. Fossils occur intermittently throughout the interval and are extremely abundant on some bedding surfaces. Gas­conadia is especially abundant at 152 to 154 feet, where it is associated with Lytospira, Ophz'.leta, and brcvicone cepha­lopods. Ophileta is abundant at ] 59 to 160 feet, Ophileta and Lytospira at 165 to 168 feet, Lytospira at 199 to 202 feet, and Ophilcta and Gasconadia at 230 to 231.6 feet. A smooth-headed trilobite with 7 or 8 thoracic ser;ments was collected at 174 feet by Leo Hendricks (TF-4,:JO). Com­monly the fossils, like the trails they make, are preserved in relief on the bedding surfaces by dolomitic replacement. A little interstitial green material that is probably glau­conite was noted in the dolomite near the top of interval 14. Except for tbis, rosettes of limonite after pyrite and the pale argillaceous films were the only macroscopically visible "insoluble" elements noted. Thickness in feet Inter- Cumu- Feet above val lative base 85.8 399.8 147.2-233 The Ellenburger Group of Central Texas Thickness in feet Inter-Cumu-Feet above Description val lative base 15. Dolomite, slightly calcareous, grading laterally to limestone and dolomitic limestone-fine grained, light yellowish gray; beds 1 to 13 inches thick. Weathers gray to buff. 16. Limestone, in part very slightly dolomitic-mostly sublitho­graphic, grading to fine grained where dolomitic; mostly pearl gray, with light yellowish gray inclusions where dolo­mitic. Beds a small fraction of an inch to 5 inches thick; in part very thinly platy, with argillaceous material along the partings. Weathers medium to very light bluish or yellowish gray, with yellowish to buff reticulations of dolo­mite prominent in some beds. Rosettes of limonile after pyrite were the only macro­scopically visible "insoluble" elements noted, except for the argillaceous films between the platy beds. Silt grains are relatively common in the siliceous residues from the rocks below 130 feet, but less common above. Ophileta and Lytospira are not uncommon throughout in­terval 16, and broken surfaces of the rock show cross sec­tions of trilobites and cephalopods. From 124 to 355 feet the section follows excellent exposures on the south bank of Llano River; below 124 feet it is on a sparsely cedared hillside south of the river and east of the mouth of Pete Hollow. X bed at 124 feet; a conspicuous 9-inch bed; shift about 450 feet east-northeast if going up in the section and west-south­west if going down (altitude approximately 1420 feet at west and 1395 feet at east end). 17. Limestone-as in interval 18; beds 1 to 10 inches thick. W hed at 115 feet, an R-inch ledge; shift about 130 feet east­northeast if going up in the section and west-southwest if going down (altitude approximately 1440 feet at west and 1325 feet at east en_d). 18. Limestone-in all respects similar to the pure ledges in the upper part of the interval helow, except that it is slightly darker; beds 1 to R inches thick. V bed at 109 feet, a prominent 16-inch ledge; shift about 640 feet east-northeast if going up in the section and west-south­west if going down (altitude approximately 1470 feet at west and 1435 at east end). 19. Limestone-mostly pure and sublithographic, but in part with minor inclusions of fine grained dolomitic limestone, and in minor part with thin beds of limestone-granule con­glomerate. Mostly pearl gray, grading in minor part to light brownish gray, with buff to yellow streaks and blotches where dolomitic. Beds a fraction of an inch to 12 inches thick. Weathers to smooth, light to medium bluish gray ledges alternating with poorly exposed intervals covered with thin, smooth slabs that weather very light gray. Interval 19 differs from interval 20 principally in having more prominent ledges and in the greater purity of the limestone, reticulate weathering beds being present but not conspicuous. The quantitatively insignificant siliceous residues from in­terval 19 apparently consist wholly of silt particles less than 0.05 mm. in diameter and scattered to moderately abundant clear flakes of detrital muscovite. Globular glauconite was fairly abundant in a thin slab of limestone found at 67 feet in interval 19. It may have weathered out from this part of the section, but could also be a stray piece. Artifacts were found in the vicinity and it is near a pasture road. 1.7 401.5 145.5-147.2 21.5 4,23 124 -145.5 9 432 115 -124 6 438 109 -115 54 492 55 -109 The University of Texas Publication No. 4621 Thickness in feet Inter- Cumu,­ Feet above Description val lative base Fossils occur intermittently through the interval but are not generally conspicuous in the line of section. Those noted were Lytospira, Gasconadia, Ophileta, and a small brevicone cephalopod. A thin bed crowded with Gasconadia was seen at 80 feet. 20. Limestone, in minor part slightly dolomitie-snblithographic 39 531 16 -55 to fine grained, with minor medium grained portions; in part a limestone-granule conglomerate with sublithographic granules of pure limestone in a granular matrix of slightly dolomitic pellet limestone. Pearl gray to light bluish or brownish gray, with buff blotches and streaks where dolo­ m1t1c. Beds a fraction of an inch to 6 inches thick. W eath­ ers to fairly smooth, slabby, medium to light bluish gray ledges alternating with poorly exposed mtervals strewn with '>mooth weathering slabs that are of such a light gray color that they give the appearance of being almost white in the sunlight. Some ledges are reticulated with raised huff colored markings where &lightly dolomitic. Occasional small rooettes of limonite were the only mac­ roscopically visible "insolubles., seen, bnt moderate quanti­ ties of fine silt and scattered clear dctrital mica Hakes appear in the quantitatively insignificant •iliceous residues from this interval. Measurements with a graduated scale in a microscope indicate that the silt grains average leos than 0.05 mm. in diameter. Fossils occur intermittently through most of the interval hut are ordinarily not noticeably abundant. Those seen were species of Tetralobula, Finkelnburgta, Lytospira, Ophileta, Gasconadia, a short blunt hrevicone cephalopod, and a few trilobites. The lowest Lytuspira was at 28 feet. Silicified Tetralobula was collected at 41.5 feet (TF-192) and seen but not collected at 45 feet. Silicified Finkelnburgia cf. F. bellatula Ulrich and Cooper* was collected from 44.3 to 4.S feet (TF-193) and again at 52 feet (TF-194). 21. Covered, but with olabs of limestone from less than an inch 11.5 542.5 4.5-­ lC> to a few inches thick scattered over the gently sloping sur­ face of the interval-on a fresh surface these limestone slabs are sublithographic to medium grained; pearl gray to brownish gray, in part with slightly dolomitic buff mot­ tles. They weather very light gray to white. The surface at interval 21 is also fairly well salted with slabs of the upper Wilberns limestone, supplied eithe1 by the stream that intermittently Hows in Pete Hollow, or by Llano River at a higher stage than its present course. Glauconite grains are common in some oi the more gran­ ular beds represented by float between 10 and 15 feet in this interval. Fossils noted were Gasconadiu, cf. Sinuopea, and a "mall blunt brevicone cephalopod. Fossiliferous slab<; arc not un­ common on the surface of the interval, especially in the upper 5 feet, and one of the slabs containing clearly iecog· nizable Gasconadia and cephalopods contains granular glau­ conite. Silicified Finkelnburgia was seen in glauconitic lime· stone float al 12 feel, the opecimens being discarded fol­ lowing preparation and identification. 22. Limestone, in part slightly dolomitic-oublithog1aphi~ to 0 -4.5 medium grained; in part a granule·conglomerate with s11h­ rounded inclusions of sublithographic limestone in a matri" of slightly dolomitic pellet limestone. Light bluish gray -"Equahi l'wkelnlmrgfo lielln1 Cloud (in p1cgs), The Ellenburger Group of Central Texas Description where pure limestone, varying to huff or yellow where dol­omitic. Some of the rock consists simply of irregular in­clusions of huff dolomitic limestone in light gray pure lime­stone, or the opposite; the transition between the two types being fairly sharp, yet at the same time sufficiently gracla­tional in appearance that the effect is not that of a con­glomerate hut rather of chemically varying portions of the same bed. Beds I to 6 inches thick. Weathers to rough surfaces, mottled in light bluish gray and buff, with some heels weathering light woodaoh gray to almost white. No fossils were seen in this interval. The base of interval 22 is thought to he slightly above the actual base of the Ordovician. The lrnff-mottled lower 2 feet of this interval is very similar to the buff beds typi­~al of the lower 5 to 10 feet of the Ordovician throughout the Bear Spring area. The projection of the Wilberns­Tanyard boundary from the west side of Pete Hollow brings it immediately below the point where the section begins. The 7-foot sand which occms 28 to 35 feet below the base of the Tanyard formation throughout the Bear Spring area crosses the mouth of Pete Hollow, and meaY­urements indicate that there is room for about 35 feet of strata in the covered interval between it and the base of the Pete Hollow section. The base of the Pete Hollow section is, therefore, thought to be 0 to 7 feet above the base of the Tanyard founation; and, as a matter of conveni­ence, is assumed to be coincident with it. Base of Threadgill member oI T!\.llyaracl formation at base oI interval 22. Total thickness of member, which is entirely in the calcitic facies, 294· feet thick. Total measured thickness of formation 523 feet. It may he noted here that an artesian well at lht> Melvin Capps ranch headquarters is said to obtajn its water from sand at a depth of 586 feet. Assuming that this a qui fer is the upper 7 -foot band of the San Saba lime· stone member of the Wilberns formation, the measured thick­nesses show that it is about 551 feet below the Tanyarcl-Gor­man contact. The residue of 35 Ieet is approximately correct for the stratigraphic distance above the Tanyard-Gorman con· tact at the Capps ranch headquarters and the well essentially checks the measured thickness of the Tanyarcl formation. Base of Pete Hollow section, a segment of the composite Llano River section, at base of interval 22; altitude approximately 1425 feet. About 500 feet south of the water's edge on Llano River at the mouth of Pete Hollow. Total thickness of Ellenburger rocks incluclecl in this section 547 feet, 523 of which belong to the Tan­yard and 24 to the Gorman formation. Thickness infeet Inter- Cumu- Feet above val lative base HUJFI•' CREEK SECTION The Bluff Creek section includes 18 feet of the Tanyard formation and 262 feet of the Wilberns formation. Of the latter 260 feet are assignable to the San Saba limestone member of the Wilberns for­mation, but the basal 2 feet are in a stro­matolite bioherm that should be included in the Point Peak shale member of the Wilberns. Sand is an interesting feature of the San Saba limestone member of the Wil­berns formation in this section, occur­ring as four fairly well-defined zones of calcareous sandstone and as intermittently scattered grains or patches of calcareous sandstone between these four zones. The upper 7-foot sand zone is known to be persistent throughout the Bear Spring area· and the others are probably locally persistent, for sands were seen in the The University of Texas Publication No. 4621 approximate positions of the lower zones wherever this part of the section was tra­versed in the Bear Spring area. To the east, however, the sand disappears and, except for occasional grains, sand was not seen in the San Saba limestone member of the Threadgill Creek section 20 miles to the southeast or of the Brady-Mason highway section 22 miles north-northeast. Sand zones are present, however, in the San Saba limestone member of the Wil­berns in northwestern and west-central Mason County and one or more of these was named the "Erna sand" by F. B. Plummer (1943). The top of the Bluff Creek section is at an altitude of approximately 1630 feet at the peak of a west-southwest trending spur and about 0.6 mile airline north­northeast of the mouth of Bluff Creek (Pl. 5). Stratigraphically it is 18 feet above the diastem chosen as the Cam­brian-Ordvician contact and the base of the Ellenburger group. The base of the section is at an alti­tude of 1425 feet about 1200 feet airline west-southwest from the top, at the mouth of a draw on the east side of Bluff Creek. Description Ellenburger group: 18 feet described Tanyard formation: 18 feet described Threadgill member: 18 feet described Calcitic facies: 18 feet described J. Limestone, in part slightly dolomitic (altitude approximately 1630 feet at top of section)-sublithographic to medium grained, in part a granule breccia with matrix and pheno­clasts of similar lithology or with matrix slightly dolo­mitic; pearl gray to woodash gray with buff to yellow mot­tles and streaks where dolomitic. The dolomitic inclusions are especially prominent in the lower 8 feet, and the basal foot is a conspicuous buff bed that Iooh like an earthy dolomite but fizzes strongly upon application of HCl indi­catin11: that it is only slightly dolomitic. Beds poorly exposed, probably few of them exceeding a few inches in thickness. In contrast to the darker weathering limestones of the Wil­berns formation below, the limestones in this interval weather conspicuously light gray or even white, with buff to yellow blotches representing the dolomitic portions. Scattered glauconite pellets are present about 5 feet above the base of this interval and lateral tracing has shown that a 1-to 3-inch conspicuously glauconitic zone is fairly persistent at about this position. Inasmuch as globular glau­conite is very rare in beds considered to be of Ordovician age throughout most of the Llano region, it is possible that the Cambrian-Ordovician houndarv should be drawn above this occurrence. However, the sublithographic smooth­fracturing, white to very light-gray-weathering lithology so characteristic of the Threadgill limestones of the western part of the Llano uplift begins as minor inclusions in a persistent thin huff bed below thi5 glauconitic zone and becomes increasingly prominent above it. Hence the base of the huff bed is regarded as the base of the Ordovician and the glauconite is considered to represent either a be­lated manifestation of Cambrian conditions or reworked material. Fossils are fairly common in this interval, consisting of cross sections in the limestone, but none were collected. Ophileta cf. 0. polygyrata (Roemer) is most abundant but Gasconadia cf. G. putilla (Sardeson), Lytospira cf. L. gyrocera (Roemer), and a small blunt brevicone cephalopod Thickness in feet Inter-Cumu-Feet above val lative base 18 18 262 -280 The Ellenburger Group of Central Texas Thickness in feet Inter· Cumu· Feet above Description val lative base also were noted. These fossils apparently occur to the very base of the Tanyard formation, for blocks containing them have floated down over the Wilberns formation below to as low as a point 12 feet stratigraphically below the Wil· herns-Tanyard in the measured section. The lowest Lyto­ spira noted in place in the Bear Spring area was found about 6 feet above the base of the basal buff-mottled bed of the Ordovician. Base of Threadgill member (entirely calcitic) of Tanyard for­ mation, and Cambrian-Ordovician boundary, at base of interval 1 (altitude about 1615 feet). Thickness (incomplete) of mem­ ber and formation in Bluff Creek section 18 feet. Upper Cambrian: 262 feet described Wilberns formation: 262 feet described San Saba limestone member: 260 feet thick 2. Limestone-microgranular to coarse grained, includes fairly 28 46 234 - 262 abundant small limestone pellets or micro-ooids and scat· tered irregularly shaped inclusions as we!l as individual small buff-weathered dolomite rhombs. Grayish brown to brownish, yellowish, and greenish gray; beds poorly ex­ posed and apparently none over a few inches thick. Weath· crs medium brownish to bluish gray on the gentle slope where exposed ledges are few. The float in the upper 12 feet of interval 2 appears to be derived largely from the overlying Ordovician beds, thereby obscuring the Cambrian-Ordovician boundary. How­ ever, thin, granular, brownish gray limestone ledges of the Wilberns type of lithology project from several places in the obscured interval. Rare grains of sand occur in the lower part of interval 2. Scattered globular glauconite occurs throughout the inter­ val and is abundant in some layers. Plethometopus and other trilobites were collected from 237 to 240 feet (TF-191). Similar trilobites were noted in a ledge at 252 feet and another ledge near the top of the interval, hut these were not collected. 3. Calcareous sandstone and arenaceous limestone, in part 7 53 227 - 234 slightly dolomitic-fine to medium grained, light yellowish gray with buff and brown streaks, beds 1 to 7 inches thick. Weathers to rough, irregular, poorly exposed, buff to medium brownish gray ledges. The individual grains of sand, like those stratigraphically below, are small, moderately frosted, and subround. How· ever, few pink-stained grains are present. This sand has been traced through most of the Bear Spring area and has been found to maintain a uniform thickness of 7 to 8 feet, to average close to 30 feet of stratigraphic separation from the base of the Ordovician above, and to be marked by a concentration of black persimmon bushes so that it shows on the aerial photographs as a narrow dark band, Glauconite occurs as scattered pellets, which are locally fairly abundant. Trilobite fragments are fairly common in the more coarsely granular limestone lenticles. 4. Limestone; mostly a zone of flat-pebble (shingle) conglom· 28.5 81.5 198.5-227 erate-matrix, phenoclasts, and nonconglomeratic beds vary· ing from microgranular to medium grained, with most con­ spicuously conglomeratic beds being at 202, 207 to 212, and 214 to 216 feet; medium brownish, yellowish, or olive gray with huff dolomitic streaks, the phenoclasts generally being The University of Texas Publication No. 4621 Description darker in color than the matrix; beds poorly exposed, appar­ently not exceeding 8 inches thick. Weathers medium blu­ish to brownish gray. Scattered grains of glauconite occur in various parts of interval 4 and are conspicuous in a few thin beds but not generally abundant. 5. Limestone in large part coarsely silty to very finely sandy and slightly dolomitic except for thin zones of tlat-pebble a few >Cattered globules in various parts of the interval and is fairly abundant in a thin bed a little below the middle. Calvinella and other trilobiteb were collected at l 38 feet (TF-187). The gastropod Owenella was noted at 148 feet. At 150 feet the upper surface of the rock io crowded with narrowly conical fossils averaging 0.8 to 1.0 inch long and 0.2 inch in diameter at the wide end, composed of several similar cones packed one inside the other, and sugge5ting a primitive Salterella-likt> rephalopod (T.l•~-188). Owenella and fragmentary trilobites were rollertt>d at l S2 feet (TF-188a). 'l. Limestone; in considerable part arenaceow,, similar lo that of interval 11-weathers as interval II, except that the upper 7 feet weather to a lighter lrnff or even to a medium hlnish gray color. The rock is very sandy, grading to calcareous sand5tone, from 114.5 to 11 S.5, 123 to 125, and 126 to 126.5 feet. Jn the intervening portions it tends to be silty. Glauconite is generally uncommon, hnt "·atterrd grain., occur in a few thin beds. Trilohite fragments occnr in the more calcareous and coarsely grained rocks in this interval. 10. Limcstone--medium to fine grained, yellowish to brownish ·. gray, beds 1 to 9 inches thick. Weather5 to inconspicnons, medium to light bluish gray ledges. Sand and glauconilc arc uncommon except that locally the top 4 inche5 of the interval are conspicuously arenacC;· ous. Coarse silt grains are fairly common in the upper part of the interval. l J. Silty and slightly dolomitic linw,stone with occasional pock­ets and beds of sand-the limestone is mostly very fine grained, grading to medium grained. It generally appears buff to brick red on a broken surface, hut thi at 272.5 fret aud c;tatiou 17 i., ctt :!69.S !eel in section. val latiiJe base 2 2 293 -295 3 10 235 -21}3 6 16 279 -285 3 19 276 -279 l0.5 29.5 265.5-276 The Ellenburger Group of Central Texas Thickness in feet Inter· Cumu· Feet above Description val lative base ·----~· ---------------­ 6. Dolomite-fine grained, yellow beige, one bed. 1.5 31 264 -265.5 Station 16 at base of interval. 7. Limcstone-suhlithographic, mottled, old iv01 y to reddish 15 46 249 -264 buff, hazel, and buttercup yellow; beds 4 inches to a foot or more thick. CalciLic fossil;, are abundant on the bed­ ding surfaces of the top 11 feet. Station 15 at base of interval. 3. Covered-at the foot of the valley wall Lo the east, float 19 65 230 -249 suggest' that this interval is rather thinly bedded limestone. Station 14 at base of covered interval. 9. Limest.one-suhlithographic, mottled old ivory with a few ll 76 219 -230 areas hazel or darker, medium to thinly beddell. Station 11 at base of interval. JO. Limestonc--sublithographic, mottled old ivory Lu reddish 16 92 203 -219 buff, medium to thinly bedded. Calcitic fo~sils arc Lyto­ spira on top surface. Gasconadia, cephalopods, and other fossils at 216 feet. Station 10 at bottom of interval. 11. Limestone-sublithographic, mottled, old iv01 y, medium lo 30 122 173 -203 thinly bedded. Calcitic fossils on Lop surface are 0 phileta, Lytospira, and cephalopods. Station 9 at bottom of interval. U. Limcstone--sublithographic, mottled, old ivo1y in top half; 41 163 132 -173 mostly brownish gray to beige in lower part; medium to thinly bedded. Chert smoky gray, quarLzose, rare. Calcitic fossils are Lytospira and Ophileta on top surface. Station 8 is at bottom o E interval and at top of a small falls. 13. Limestone-suhlithographir, mottled, old ivo1y and reddish 31 194 101 -132 lmfI, medium Lo thinly bedded. Calcitic fossils include Gas­ conadia and other fossils. Station 7 at bottom of inte1 val. 14. Limestone--sublithographic. motLled, brownish gray, me· 15 209 86 -101 dium to thinly bedded. Calcitic fos.ih are Lytospira rn top surface. Station 6 al boLLorn of interval. 15. Limestone-snblithographic, mottled, between o!tl ivory and 7 216 79 -86 brownbh gray and some reddish bu!I, medium to thinly bedded. Calcitic fossils arc Lytospirn in the top surface. Station 5 at bottom of interval is a chiseled triangle cut in a prominent ledge at the mouth of Mormon Creek (east side). 16. Limestone~oublithographic, moLtled old ivot y with some 11 227 68 -79 reddish buff, medium to thinly bedded. Calcitic fo;sils in· elude Ophileta in top surface. Station 4 at bottom o± interval. l 7. Limestone-snblithographic, mottled old ivmy, medium to 7 234 61 -68 thinly bedded. Calcitic fossils include Sincwpea in upper surface am! trilobites at base. Station 3 at bottom of interval. 18. Limestorte-8Ublithographic, mottled old ivm y Lo medium 24 25B 37 -61 gray and brown, medium to thinly hedded. Station 2 at bo~tom of interval. 19. Limestonc-5ublithographic, mottled brownio,h giay to old 22 280 15 -37 ivory with top 5 feet reddish buff to fawn, medium to thinly bedded. Station 1 at bottom of interval i~ on Cambrian-Ordovician contact. LTpper Cambrian IPilberns formation San Saba fimestonP menibe1 (Lop 15 fN·t sampled) 20. Limestonc~-granular, light to medium gray, alternately lllt'­15 295 0 -15 dium to thinly bedded, glauconitic. The University of Texas Publication No. 4621 CHEROKEE AREA, SAN SABA Fossils were collected from 160 locali­ COUNTY ties in the Cherokee area. Of these col­ lections 16 were from the Wilberns for­ INTRODUCTORY STATEMENT The Cherokee area, as here considered, is in southeastern San Saba County, be­tween the towns of San Saba and Cher­okee. The area that was mapped geologi­cally lies between latitudes of 30° 59' to 31° 12' north, and longitude of 98° 36' to 98° 46'. It is approximately bounded by the Cherokee and Little Llano River fault zones on the east, the Simp­son Creek fault zone on the north, the Cherokee-San Saha road (State highway 16) on the west, and the Cherokee-Chap­pel road on the south. The 85 square miles mapped geologically comprise Plate 2, at the approximate scale of 1 :31,680 and on a planimetric base. The Cherokee area is divisible into two unequal major fault blocks, here called the principal block and the southeast block. The south­east block lies between the ends of the Cherokee and Little Llano River fault zones, and the geologic features of part of that block are shown at the approxi­mate scales of 1 :10,000 and 1 :20,000 in Plates 7 and 37. General geologic rela­tionships of the area mapped are shown on a planimetric base at a scale of 1 inch equals 1 mile by F. B. Plummer (194,0). Control for the base map of the Cher­okee area was taken from Texas State Highway Department traverses of State highway 16 and U.S. highway 190, in combination with the U.S. Coast and Geo­detic Survey's base line between Dee and Cherokee triangulation stations. The base was plotted and adjusted by Cloud and R. L. Heller from data recorded by them on aerial photographs of the U.S. Depart­ment of Agriculture. Rocks mapped within the Cherokee area include several members of the Wilberns formation (Upper Cambrian), all units of the Ellenburger group, and strata of Mississippian and Pennsylvanian age. No Devonian strata were noticed, but small patches or pockets of Devonian rocks could have been overlooked along the Ordovician-Mississippian boundary. Dips are mostly very gentle except in the vicin­ity of some faults and collapse contacts. mation, 50 from Lhe Tanyard formation, 50 from the Gorman formation, 38 from the Honeycut formation, and 9 from Car­boniferous rocks. In addition, fossils were seen hut not collected at many localities, including about 50 which were recorded and mapped. The Cherokee Creek section, described on pages 199-219 and diagrammatically represented on Plate 14., displays a char­acteristic development of the Ellenburger rocks in the Cherokee area. Its location is shown on Plates 2, 7, and 37, whereas Table 2 presents a statistical evaluation of the grain sizes of the dolomites to be found in the various formations of the Ellenburger group in that section. UPPER CAMBRIAN WILBERNS FORMATION Pre-San Saba strata.-In the Cherokee area the strata mapped as undifferentiated Wilberns include the Point Peak shale member, the Morgan Creek limestone member, and locally the Welge sandstone member. At the base is the Welge sandstone member, about 9 feet thick, relatively fine grained, rusty, sparingly glauconitic, and sparkling from reflections of recon­ stituted quartz. The Morgan Creek limestone member is granular, reddish in the lower part and gray above, and fairly glauconitic. The basal layers are arenaceous, grading to the Welge sandstone; and locally a 30-foot shaly zone occurs at the middle of the member. The Morgan Creek lime­ stone is locally highly fossiliferous, and the important zone fossil Eoorthis occurs about 58 feet above the base of the mem­ ber ( 67 feet above base of Wilberns for­ mation) and just below the 30-foot shaly zone where that is present. Topographi­ cally the Morgan Creek limestone mem­ ber expresses itself as a subordinate strike ridge lying below a scarp occupied by the Point Peak shale member and sep­ arated from that scarp by a gentle strike valley. Where the middle shaly zone is present the member expresses itself as a The Ellenburger Group of Central Texas pair of subordinate strike ridges, with the shaly zone between, the dip slope of the upper strike ridge extending below the vallev at the base of the Point Peak scarp, and 'the scarp slope of the lower strike 1 idge facing the flat at the top of the Riley formation. The Point Peak shale member of the Wilberns formation consists of intcr­bedded greenish shale and brownish, greenish, or grayish fragmental limestone with local beds of microgranular to sub­lithographic limestone. It crops out on a prominent scarp capped by or giving way to the relatively thickly bedded limestones of the San Saba limestone member of the Wilberns. The top of the Point Peak shale member in the Cherokee area is drawn at the highest appearance of a significant quantity of shale, commonly judged from topographic expression. A feature of the Point Peak shale is the occurrence, in close association with the more shaly portions, of impure green micaceous limestones that weather to thin plates with crinkled surfaces that are transected by calcite veinlets. In its typical expression of minor lime­stone scarps and shale benches interrupt­ing a regional strike scarp, one of the minor limestone scarps in the Point Peak shale member is the Plectotrophia bed. In the Cherokee area Plectotrophia nor­mally occurs in a single bed 2 to 2.5 feet thick and 30 to 50 feet below the base of the San Saba limestone member as mapped. At one place Plectotrophia was found in two limestone beds having a total vertical range of 'about 5 feet, but in the Cherokee area it most commonly occurs only in the top few inches of one bed, and locally it is so abundant in these few inches that the matrix is scarcely more than a cementing medium about the exquisitely preserved silicified shells. P. bridgei Ulrich and Cooper is the com· mon species, but P. alata (Walcott) seems to occur locally in the same stratum. Associated with Plectotrophia and ranging up to about 15 feet above it is an alale species of Billingsella (also silicified), serving to give the approxi­mate position of Plectotrophia where found. A small species of Billingsella also occurs just below the San Saba strata, but no Billingsella has been found at a higher stratigraphic position than this in the Cherokee area. Lithically the Plectotrophia bed is a medium to coarse grained, fragmental, glauconitic, fossiliferous limestone that is mottled or speckled in tones of brown, gray, and green (glauconite globules). It weathers medium gray to rose gray. Immediately below the Plectotrophia bed in the Cherokee area is a shaly zone con· taining beds of flat-pebble limestone con­glomerate resembling lithified beach shin­gle. Although it was nowhere measured, this conspicuously conglomeratic zone is roughly 50 to 60 feet thick at most places and may extend to the base of the Point Peak shale member at some places. San Saba limestone member.-This member of the upper Wilberns formation lies between the Point Peak shale and the Pedernales dolomite members in the Cher­okee area. It grades upward and later­ally to the Pedernales dolomite member, the two being essentially equivalent facies. Its thickness is variable and at most places uncertain; but it is probably nowhere less than 70 feet thick, and was estimated to be about 140 feet thick southeast of the Harris ranch headquar­ters in the southeastern corner of the Cherokee area. The limestones of the San Saba mem­ber are sublithographic to unevenly granular and commonly contain numer­ous small round stromatolites about as big as an ordinary concord grape and known by the name of "girvanella." The matrix limestone is ordinarily some light Lone of gray or yellowish gray and is commonly mottled with light bronze to gold and pale green streaks or blotches; whereas the more compact girvanellas maintain even gray tones. The San Saba strata weather to smooth ledges in even tones of medium gray, commonly with a brownish tinge; sections of contained girvanellas ordinarily weather to lighter colored circles. In the Cherokee area the basal lime­stone beds of the San Saba typically cap a scarp, the face of which consists of the Point Peak shale member of the Wilberns formation. The succeeding lime­stones dip gently north in undulating The University of Texas Publication No. 4621 wrinkles below an area of gentle topog­raphy and thick cedar, to merge almost imperceptibly with the Pedernales dolo­mite member somewhere in this feature­less jungle. Not only the character of the terrain, the transitional nature of the San Saba-Pedernales contact, structure, and gentle and undulating dips, but even dolomitization along joints conspire to confuse the geologist; and he is fre­quently uncertain as to the actual super­position of the beds. Under these circum­stances it is likely that the San Saba­Pedernales contact may not be accurately located throughout its trace and that some of the outlying and inlying patches shown on the map represent recurrences of lithic types. Pedemales dolomite member.-This up· permost member of the Wilberns forma­tion lies between the San Saba limestone member below and the Tanvard formation of the Ellenburger group ~hove, grading downward and laterally to the San Saba member, an essentially equivalent facies. The thickness of the Pedernales dolomite member is variable and at most places uncertain; but within the area mapped it is nowhere less than 70 feet thick nor more than 150 feet thick, and it was esti­mated to be about 90 feet thick south­east of the Harris ranch headquarters in the southeastern corner of the Cherokee area. The Pedernales dolomite member of the Wilberns formation in the Cherokee area is fine to medium grained, tending to be coarser grained in the lower part and fine to very fine grained toward the top. Its fresh color ranges through medium tones of gray or brownish gray, and green spots of interstitial glauconite, as well as pinkish and yellowish tinges, are not uncommon. The weathered surface is smooth, commonly hummocky, and col­ored in dull medium tones of gray or brownish gray. In the Cherokee area the Pedernales dolomite member is slightly cherty, with the chert limited to a belt in its middle or upper part. At places this cherty zone is immediately overlain by the truncat­ing dolomites of the Tanyard formation, and, at some places, notably southeast from the line of the Cherokee Creek sec­tion, pebbles of what appears to he Peder­nales dolomite are included in the basal bed of the Tanyard formation. The chert of the Pedcrnales member in the Cherokee area is typically minutely spongy or vesicular; abundantly and finely dolomoldic or, less commonly, oomoldic; and platy in structure. Freshly broken surfaces are colored in tones of yellowish to olive brown and beige, or, where internally weathered, russet to brick red. This chert weathers to dull tones of brown. Stenopilus, Plethometopus, Euptychaspis (Dake and Bridge, 1932, p. 740, Pl. 12, fig. 3) , and fragments of other fossils have been found in chert of the Peder­nales in the Cherokee area, indicating general equivalence with the Eminence dolomite of Missouri. LOWER ORDOVICIAN ELLENBURGER GROUP Tanyard formation, Threadgill member. -In the Cherokee area the basal, coarse to medium grained, nonglauconitic, light gray to white dolomite of the Threadgill member of the Tanyard formation rests disconformably (see above) upon the upper fine grained, interstitially glauconitic, me­dium gray or brownish gray dolomite of the Upper Cambrian Pcdernales dolomite member of the Wilberns formation (Pl. 20, figs. A and B). The approximate position of this contact is marked top· ographically as well as lithologically, be­cause a little down dip from it the flat and typically cedar-covered lowland of the Pedernales outcrop belt breaks to the rough bald or sparsely wooded hills of the Tanyard formation (Pl. 31, fig. B; Pl. 32, fig. B). The thickness of the Threadgill member in the Cherokee area probably averages about 260 feet, its measured thickness in the Cherokee Creek section. Although dolomite predominates, lime­stone (Pl. 20, fig. C) occurs erratically in the lower part of the Threadgill mem­ber, shifting abruptly to dolomite along the strike and back to limestone again for no evident reason. The maximum thickness of the limestone in the Cher­okee area is about 65 to 75 feet, on the Porch ranch. None was seen in the Cher­ The Ellenburger Group of Central Texas okee Creek section, but slightly over 40 feet was measured in the Kirk ranch sec­tion a mile south-southeast of the Cher­okee Creek section. These limestones are predominantly sublithographic and pearl gray to woodash gray or white. They commonly contain an irregular internal meshw01k of dolomite that causes the rock to weather in a reticulate pattern. The limestone beds locally contain patches of silica showing prominent sub­concentric "beekite" rings easily mistaken for fossils by the uninitiated. Also occur­ring in the limestone are small excres­cences of subchalcedonic, waxy lustered chert that is colored dark gray to pearl gray or woodash gray to white and weath­ers dull brown. The chert of the dolo­mites is mostly dolomoldic, quartzose, and subgranular to granular, but chert of types more characteristic of the Staen­debach member of the Tanyard occurs locally. Tanyard formation, Staendebach mem­ber.-The contact between the Threadgill and Staendebach members of the Tanyard formation in the Cherokee area is typi­cally marked by a minor but persistent topographic bench (Pl. 31, fig. A; Pl. 32, fig. A) displaying slabs and cobbles of fine grained to very fine grained, nutria to beige or light gray dolomite, as well as thin slabs of the lowest lat­erally persistent porcelaneous and chalce­donic chert in the Ellenburger group in any significant quantity. Both the chert and the dolomite commonly contain angular to subrounded granule-sized frag­ments of different types of chert and at a few places are quite conglomeratic. Although the chert is locally absent for considerable stretches laterally, it is no­tably persistent when considered in an areal sense; and, in its association with the fine grained dolomite, the topographic bench, and the granule conglomerate, it ordinarily makes a good marker, above which the dolomite is generally finer grained and darker colored than it is be­low. Locally, however, several similar dolomites occur on several similar benches, and chalcedonic and porcelane­ous chert occurs lower in the section; so the Threadgill-Staendebach contact may not everywhere be mapped at exactly the same stratigraphic position within the Cherokee area. Lateral variation from limestone to dolomite is common in the Staendebach member in the Cherokee area; but in no instance does the limestone disappear completely, as do the thinner limestones of the Threadgill member. The lower Staendebach is persistently dolomitic and the upper beds are persistently calcitic. It is roughly estimated that the dolo­mitic facies ranges from 100 to 170 feet thick and the calcitic facies from 130 to 200 feet thick. The limestones are mostly sublithographic, pearl gray to woodash gray, and commonly with an internal meshwork of dolomite, that results in a reticulate pattern of weathering. The dol­omites are fine to medium grained, locally very fine grained, and colored light gray to light brownish, pinkish, or yellowish gray to grayish brown, beige, or nutria. Chert is common in both dolomite and limestone, occurring as nodules, concre­tions, plates, and irregular inclusions that are mostly porcelaneous to chalce­donic and commonly oolitic. Such chert is associated with dolomoldic and quartz­ose chert like that of the Threadgill mem­ber, and oolitic laminar cherts are a fea­ture of the upper beds. Gorman formation.-The Tanyard-Gor­man contact in the Cherokee area is not well marked topographically (Pl. 33, fig. A) ; but it does display a vegetation break and is abrupt lithologically at most places, while supporting faunal evidence may ordinarily be found on search. Although the break from the limestones and fine to coarse grained dolomites of the Tanyard to the com­monly microgranular or very fine grained dolomites of the Gorman formation is ordinarily sharp, it is not invariably so; and at least' one locality, a little south­southeast of the J. W. McConnell ranch headquarters, the contact is within a local limestone interval. Bright tones of rose and beige are especially notable in the dolomites of the Gorman formation in the Cherokee area, and the sublitho­graphic, pearl gray to woodash gray lime­stones of this formation less commonly contain an internal meshwork of dolomite than do those of the Tanyard strata. ...... \0 0\ Table 2. Grain sizes of dolomites in the Cherokee Creek section. (A proportionate representation, expressed in sizes determined in the field in March, 1945.) terms of feet of section, of grain Rock unit Micro­gran­ular J.iicro­gran­ular to very fine grained Very fine grained to micro-granular Micro-granular to fine grained Fine grained to micro-granular Very fine grained Fine grained Fine to medium grained Medium to fine grained Medium grained Medium to coarse grained Coarse to medium grained ~ ;:,.. C'l> ~ ~· Honeycut fm. --------------------­ -------­ ------­ 12.5 10 4 6 3.5 ----­ -----­ ---­ ----­ -----­ C'l>..., "' ~- Gorman fm. (calcitic facies) __ 15 16.5 ----­ --­ 21.5 17.5 ------­ -­ --­ -­ c._,__ Gorman fm. (dolomitic facies)___ Total dolomite of Gorman fm.___ 62 77 15.5 15.5 -----­-----­ 23.5 40 5 5 ------­ 4.5 26 22 39.5 13.5 13.5 5 5 -----­--­ ----­----­ ~ C'l> « ;i "' Tanyard fm., Staendehach mem. ( calcitic facies) -----------------------Tanyard fm., Staendehach mem. (dolomitic facies) ____________________ Total dolomite of Staendehach mem. -----------­-----------------­ -------­ ------­---­-----­ --­-----­-­ ------­--­-----­ -----­------­---­ ----­5 5 -------­ -------­84 84 11 35 46 19 ------­19 2 ------­2 --­--­-------­ "ti;:: 0-­,,......... "' ;i,,.. s· ;:; < ~ Tanyard fm., Threadgill mem. (dolomitic facies and total dolomite) ---------------------------­ -------­ -------­ -----­ --­ --­ -­ 71 22 9 25 133 ..... ~ ....... Total dolomite in Tan.yard fm._ -------­ ------­ -­ ----­ ---­ 5 155 68 28 27 133 The Ellenburger Group of Central Texas As elsewhere in the Llano region the local occurrence of sand grains in both dolomite and limestone distinguishes the Gorman formation from the Tanyard. The thickness of the dolomitic facies in the Cherokee area probably varies be­tween 130 and 170 feet, and that of the calcitic facies between 260 and 300 feet. Measurements in the Cherokee Creek sec­tion give 151 and 275 feet respectively for the dolomitic and calcitic facies. Although dolomite is common in the cal­citic facies of the Gorman formation, sig­nificant quantities of limestone do not generally occur in the dolomitic facies. A notable exception to this rule for the dolomitic facies is in the eastern part of the area, north of Rough Creek, east of the San Saba-Chappel road, and west of the Cherokee fault zone. Archaeoscyphia occurs in a thin zone about 225 feet above the base of the Gor­man formation through most of the Cher­okee area, making an important horizon marker. Typical Gorman faunas were obtained at many localities, but fossils are generally less common than in the Tanyard and Honeycut formations. Honeycut formation.-The thinly inter­Ledded dolomites and limestones of the basal Honeycut strata break abruptly from the thickly bedded pure limestone sequence at the top of the Gorman for­mation, and the two formations are easily separated where they are in normal strati­graphic superposition. Throughout much of the area silicified specimens of the brachiopod Xenelasma may be found on the upper surface of the top bed of the Gorman formation, marking the lowest recorded occurrence of that genus as well as the Gorman-Honeycut contact. In the southeast fault block of the Cherokee area, the topographic break from the Gor­man to the Honeycut formation is not well developed, but in the principal block it is very evident. Here, for sev­eral miles, the break from the rough, thickly cedared, vegetationally banded, poor pastureland of the upper limestone of the Gorman formation to the more open pastureland, with fewer cedars and scattered mesquite underlain by rocks of the Honeycut formation, occurs abruptly at the lower margin of an almost bare bench which is so marked that it is fol­lowed for much of its length by old pas­ture roads (Pl. 34, fig. B). This bench is mostly covered, but it is evident from the float that it is underlain by thinly bedded dolomites and limestones. Two to three beds of chert-matrix sandstone, grading laterally to arenaceous dolomite and limestone, occur in the lower 10 to 15 feet of the Honeycut strata along this bench, one being at or very near the base of that formation. The zone of silicified Xenelasma in limestone and the top of the Gorman formation come at the lower margin of this bench. Lithically the dolomites and limestones of the Honeycut formation resemble those of the underlying Gorman strata, except that the dolomites are generally lighter tones of gray or beige and probably not so commonly microgranular. The alter­nation of limestone and dolomite beds is distinctive of the Honeycut. West of the Little Llano River and Cherokee fault zones the top of the Honeycut formation almost coincides with the principal Archaeoscyphia bed and the zone of Ceratopea capuliformis (130 to 140 feet above the base). Rarely the Archaeoscyphia bed is absent because of pre-Mississippian truncation, and for short distances, up to about 25 feet of Honey­cut strata appear above the principal Archaeoscyphia bed and below the Mis­sissippian. Thus the Ellenhurger-Missis­sippian contact is seen to be inconspicu­ously unconformable. Immediately east of the Little Llano River fault zone, how­ever, in the southeastern part of the area, about 60 feet of Honeycut is ex­posed between the zone of Ceratopea capuliformis and the Mississippian; and in this unmapped fault block additional Honeycut strata peel out from under the Mississippian to display 250 to 300 feet of Honeycut strata just west of Chappel. Sand grains have been seen to the top of the Honeycut formation in the Cher­okee area, but, except in the lower 15 feet, are much less common than in the Gorman formation. Although separable without difficulty where in normal stratigraphic succession, The University of Texas Publication No. 4621 painstaking study is ordinarily required to separate Honeycut strata from Gorman in faulted zones. The separation can best be made where diagnostic fossils are found, although the occurrence of sand might suggest the Gorman formation. CARBONIFEROUS The post-Ellenburger beds have been described on pages 42-60, and most geologists will experience little difficulty in distinguishing the characteristically dark, granular, and commonly crinoidal limestone, shales, and sandstones of the Carboniferous from the Ellenburger strata. In the Cherokee area all Mississippian beds are mapped under one symbol and all Pennsylvanian strata under another, the reader being referred to Plummer's map of San Saba County (194.0) for ad­ditional information on the distribution of the formations so mapped. The Mississippian beds in the Cherokee area are, in ascending order: the thin, dis­continuous, brownish, Ives breccia; the thin, discontinuous, gray to olive gray, crinoidal Chappel limestone; and wide­spread, dark, petroliferous shales and minor limestones of the Barnett forma­tion. The two lower formations are too thin and sporadic in occurrence to ex­press themselves topographically or den­drologically, but the outcrop of the shaly Barnett formation is marked by a con­spicuous narrow mesquite-covered bench between the cedar-covered Marble Falls limestone (Pennsylvanian) above and the cedar-supporting limestones and dolo­mites of the Honeycut formation below. DESCRIPTION OF THE CHEROKEE SECTIONS The Cherokee Creek and Kirk ranch sections, described in detail, and the sup­plementary Harris ranch and Salt Branch sections serve to illustrate the develop­ment of the Ellenburger group and re­lated lower Paleozoic rocks in the Cher­okee area. The first three sections are located in the southeastern corner of the area in a large wedge between the north end of the Little Llano River fault zone and the south end of the Cherokee fault zone, and the Salt Branch section is a supplementary section just south of the Cherokee map area. The Cherokee Creek section displays an unbroken sequence of Ellenburger rocks, but the Threadgill member of the Tan· yard formation is entirely dolomitic in this section. Therefore the Kirk ranch section is described, in order to present the calcitic facies of the Threadgill mem· her. The supplementary Harris ranch section furnishes estimates on the thick· nesses of Upper Cambrian strata above the Plectotrophia zone. Although it is in an area of steep dips and minor faults, it is the most favorable locality in the Cherokee area for obtaining esti­mates of the thicknesses of the Pedernales dolomite and San Saba limestone mem· hers of the Wilberns formation. The Salt Branch section, south of the Cherokee map area, was measured in order to ob­tain reliable data on the rocks between the zone of Plectotrophia and the base of the Wilberns formation with which to round out the correlation chart for cen­tral Texas. The sequence and approximate thick· nesses of stratigraphic units displayed by these sections is as follows: Thickness Stiatigiaphic unit in feet Mississippian (about 12 feet of the Barnett formation, 7 inches of the Chappel limestone, and 1 foot of the Ives breccia) Lower Ordovician-Ellenburger group (1128 to 1141 feet) Honeycut formation -----------------------------­142 Gorman formation (426 feet) Calcitic facies -----------------------------------­275 Dolomitic facies -------------------------------­151 Tanyard formation (560 to 573 feet) Staendebach member (300 feet) Calcitic facies ----------------------------------­176 Dolomitic facies --------------------------------124 Threadgill member (260 to 273 feet) Upper dolomhic facies _______ --------------165 Calcitic facies -----------------------------------41.5 Lower dolomitic facies___________________ _ 66.5 Upper Cambrian (545 feet measured) Wilberns formation (545 feet) Pedernales dolomite member________ _ 90 San Saba limestone member________ 140 Pomt Peak shale member ________________ 141 Morgan Creek limestone member__ 165 Welge sandstone member_____ Approximate total thickness of Ellen­burger group and Wilberns for· mation ------------------------------------------------1680 The Ellenburger Group of Central Texas Responsibility for measuring, marking, and describing the Cherokee sections is noted under the individual sections. The Cherokee Creek section is on the S. L. Kirk and T. H. Young ranches, the Kirk ranch section is entirely on the S. L. Kirk ranch, and the Harris ranch sec­tion is on the ranch of Mrs. Julia Calla­han Harris. The locations of the sections are shown on the geologic maps (Pls. 2 and 7) by lines of inverted V's, except that the line of the Harris ranch section is omitted from Plate 7 because of crowding in this part of the map, and the Salt Branch section falls south of the map area. An unnamed supplementary section (marked S on Pl. 2) at the south margin of the mapped area covers the same stratigraphic interval as the Salt Branch section, but exposures are poor and attitudes of beds unreliable. CnEROKEE CREEK SECTION The Cherokee Creek section is the only one known on the basis of detailed map· ping to expose an unbroken sequence of rocks of the Ellenburger group that is complete from the disconformably under­lying Cambrian to the unconformably overlapping Mississippian rocks. Due to thick vegetation and poor exposures fre­quent lateral shifts are required in tra­versing a route that gives satisfactory ex­posures of the entire sequence, but enough recognizable beds were traced laterally to justify reasonable assurance that the sec­tion measured is essentially continuous and unfaulted. Other sections in the Llano uplift offer better exposures, but no sec­tion believed to be reliable is more com­pact. Moreover, the Cherokee Creek sec­tion is especially important to the surface stratigraphy of the Ellenburger group, for it was in the vicinity of this section that the detailed sequence of the rocks and faunas was first worked out; and the formational and member subdivisions de­lineated here were later carried, with modifications, to other parts of the Llano region. Aside from the poor exposures in some intervals, its principal deficiency as a display section is that it exposes only 142 feet of the Honeycut formation, as compared to a maximum known thick­ness (at the surface) of 678 feet in the type section of this formation at Honey­cut Bend on Pedernales River in Blanco County. The Cherokee Creek section was first measured and described by Cloud and R. L. Heller in March 1944. At the same time V. E. Barnes and L. E. Warren ran a plane-table traverse for control, fur­nishing the basis for adjusted measure­ments. Subsequent experience with other sections and improvement in technique made redescription of the section de­sirable; and it was accordingly restudied, remeasured, and redescribed by Cloud in March 1945 using base control taken from the earlier plane-table traverse by Barnes and Warren. The location of the section described is shown on the geologic maps that in­clude this part of the Cherokee area (Pls. 2 and 7) and on a photographic stereo­gram (Pl. 37) by lines of inverted V's whose apexes point up in the section. Lat­eral offsets are indicated by dotted lines except where self-evident or where neces­sarily short and numerous, and in the latter event the line of section is gener­alized on the map. On the ground the line of traverse was originally marked by closely spaced yellow paint spots and arrows (not related to footage), begin­ning at the sixth cattle guard northeast (LJ_,,5 miles by speedometer) from State highway 16 at Cherokee on the road from Cherokee to the San Saba-Chappel road and ending 1.7 airline miles to the northeast on the southeast bank of Chero­kee Creek. On redescription orange spots were painted at 5-foot intervals to facili­tate sampling by those interested in de­tailed studies. The footage is marked at least every 50 feet, more closely in the upper part of the section where the route is difficult, and on all beds that were traced laterally. Alternate lines of tra­verse above the X bed and below the H bed are marked by yellow paint spots only. Painted lines of lateral shift, and the dotted lines on the maps, do not fol­ low the actual datum traced where it fol­lows a circuitous or difficult route. The University of Texas Publication No. 4621 The top of this section is at an alti­tude of 1372 feet, a little southeast of the right bank of Cherokee Creek. It is at a point not quite 0.4 mile airline north-northwest from the eighth cattle guard northeast (6.2 miles by speedom­eter) from Cherokee on the county road from Cherokee to the San Saba-Chappel road. Its base is at an altitude of 1428 feet on a flat between a north-draining draw and the foot of a west-facing scarp. It is about 0.2 mile airline southeast of the sixth cattle guard northeast ( 4.5 miles by speedometer) from Cherokee on the county road from Cherokee to the Sa~ Saba-Chappel road. The Cherokee Creek section as given below includes only strata of the Ellen­burger group restricted. Its top is im­mediately below the Mississippian and its base is immediately above the highest fine to very fine grained, interstitially glauconitic, relatively dark colored Peder­nales dolomite member of the Wilberns formation (uppermost Cambrian). Meas­urement of the Mississippian strata 700 feet south of the top of the Cherokee Creek section, at locality TF-426, showed from base to top, the following succes­sion: Ives breccia, 1 foot; Chappel lime­stone, 7 inches; Barnett formation, about 12 feet. Estimates and measurements of the thicknesses of the Wilberns formation (Upper Cambrian) are given under the Harris ranch and Salt Branch sections. Thickness infeet Inter- Cumu- Feet above Description val lative base Ellenhurger group: 1128 feet thick Honeycut formation: 142 feet thick 1. Limestone, with minor dolomitic inclusions and with 1 to 3.5 3.5 1121..5-1128 2 inches of dolomite at the top (altitude 1372 feet at top of section)-the limestone is suhlithographic, in part a fine pellet limestone; pearl gray to light pinkish gray. The thin dolomite bed at the top is fine grained and nutria to olive brown. Bedding indeterminate. Weathers rough, in part reticulate; medium bluish gray. Silicified fossils present are Archaeoscyphia sp., Ceratopea cf. C. capuliformis Oder, and the siphuncle of cephalopod cf. Mcqueenoceras (TF-95). a brevicone 2. Dolomite-very fine grained to microgranular; olive brown 1.5 5 1123 -1124.5 to yellowish brown and nutria. Seems to be a single ledge. Weathers smooth, brownish gray. 3. Limestone, dolomitic in the lower 3 to 4, feet-the lime­ 21 26 1102 -1123 stone is sublithographic, varying to a fine pellet limestone; pearl gray to woodash gray, in part with irregular yellow­ ish argillaceous films. In the lower 3 to 4 feet fine grained, yellowish gray to brownish olive dolomite forms a matrix for about an equal proportion of limestone inclusions, whereas in the upper beds dolomite is a minor component. Beds 1 to 13 inches thick and poorly exposed. Weathers uneven, reticulate in the lower part; medium gray to blu­ ish gray. Chert occurs as irregular inclusions and nodules at 1107 feet and as scattered partially calcitic cannonballs above 1120 feet. The chert at 1107 feet is semichalcedonic, oolitic to crypto-oolitic; in part pseudospicular and in part actu­ ally showing the spicular structure of the sponge Archaeo· scyphia; brownish to bluish gray to white. Fossils collected along the trace of the top few feet of this interval are Ceratopea capuliformis Oder, Orospira sp., Hormotoma sp., Ophileta sp. (TF-95a). 4. Dolomite, grading to calcitic dolomite and minor limestone 11 37 1091 -1102 in the upper 2 feet-very fine grained to microgranular; biege to light grayish yellow, with pinkish mottles; beds The Ellenburger Group of Central Texas Thickness in feet Inter-Cumu-Feet above Description val lative base from less than 1 inch to 18 inches thick. Weathers un­ even, medium gray to pinkish gray; mostly covered in upper 2 feet. 5. Limestone-sublithographic, in part a fine pellet limestone; 1 38 1090 -1091 pearl gray; bedding indistinct. Weathers irregularly, medium bluish gray. Partially to largely chertified cannonballs are abundant in this interval. Fragments of fossils occur in the cannonballs and a small brevicone cephalopod was seen in the limestone. 6. Dolomite, in part calcitic-very fine grained, beige with 3 4.1 1087 -1090 occasional pink streaks, bedding indeterminate. Weathers irregular, medium gray. 7. Limestone--sublithographic; pearl gray to woodash gray, 45 1083 -1087 with irregular greenish to yellowish argillaceous films; beds from a fraction of an inch to a few inches thick. Weath­ers smooth, platy, medium to light bluish gray. Chert concretions and lenses ranging from the size of a walnut to that of a small watermelon are abundant in the lower 3 feet. It is chalcedonic to subporcelaneous; irregu­ larly variegated and in part laminated in tones of bluish gray, brown, and white; and weathers in part to a chalky texture. 8. Limestone, with occasional dolomitic inclusions and in part 28 73 ] 055 -1083 slightly dolomitic-sublithographic, varying to fine grained where dolomitic, in part a fine pellet limestone and locally an intraformational breccia; pearl gray. grading to yellow and rose where dolomitic, and with irregular greenish and yellowish argillaceous films; beds from less than 1 inch to 12 inches thick. Weathers platy, medium gray and blu­ ish gray; mostly covered from 1065 to 1070 feet. Minor chert occurs as nodules, lenses, and angular in­ clusions from 1057 to 1060 and 1065 to 1067 feet. It is subchalcedonic, bluish to brownish gray to white, and weath­ ers shiny white to dull pinkish white. Xenelasma syntrophioides Ulrich and Cooper, Ophileta sp., and !efjersonia sp. were collected (TF-102) from con­ glomeratic chert thought to weather out along the trace of this interval or the beds immediately below it. 9. Limestone--sublithographic; pearl gray to woodash gray, 6.5 79.5 104.S.5-1055 with greenish to yellowish argillaceons films; a markedly thinly bedded interval, with beds varying from a fraction of an inch to only a few inches thick. Weathers smooth, platy, medium gray; poorly exposed in upper part. Chert nodules occur at 1053 feet; being porcelaneous to semichalcedonic, white to light yellowish or brownish gray, in part concentrically banded. ]0. Dolomitic limestone, grading upward to limestone-the 1.9 81.4 1046.6-1048.5 lower foot is comprised of fine grained, grayish yellow dol­omite, with occasional subround to angular inclusions of sublithographic, pearl gray limestone; and the limestone in­creases in proportion upward, being almost pure, except for irregular yellowish argillaceous films, in the top 10 inches. Weathers reticulate, medium bluish gray. Chert is abundant for 1 to 2 inches at the top and bottom of the interval. That at the top occurs as frac· tured lenticles and thin, extensive lenses. It is chalcedonic, in part crypto-oOlitic and pseudospicular; bluish gray to brownish gray and dirty white. That at the base is a thinly irregular layer of calcitic and oolitic chert or a cher· tified fine pellet layer. Fossils seen in the chert were poorly preserved 0 plii­ leta (?) and a low-spired "Euconia.'' The University of Texas Publication No. 4621 Description 11. Dolomite-fine grained; grayish brown and brownish gray, grading toward nutria, with minor lavender streaks; com­posed of one or two thin ledges. Weathers uneven, medium gray. Chert occurs as scattered small nodules. It is subchal­cedonic to chalcedonic, bluish gray and weathers dirty white. ]2. Limestone, in part dolomitic-sublithographic, varying to fine grained where dolomitic; pearl gray to woodash gray, with rose, nutria, and lavender mottles and streaks where dolomitic. Beds from less than 1 inch to 10 inches thick. Weathers solution pitted to reticulate, medium bluish gray with darker gray reticulations. 13. Dolomite-very fine grained; grayish brown to rose gray, with rose and nutria mottles; a single ledge. Weathers hackly to smooth, medium brownish gray to yellowish gray. Subspheroidal, partially chertified, calcitic masses 1 to 8 inches in diameter represent the lowest well-developed "can­non-ball chert" seen in place in this section. In addition rosettes and excrescences of fine grained quartz druse are common in this interval. A brevicone cephalopod and fragments of gastropods were seen in the chert. 14. Dolomite, in part calcitic and grading to dolomitic lime­stone at the base--fine grained; pinkish to yellowish gray to light gray; beds 1 to several inches thick, poorly ex­posed. Weathers platy, medium dark brownish gray. 15. Lime!!tone, in part dolomitic toward top of interval-sub­lithographic, grading to fine grained where dolomitic, in part a fine pellet limestone or an oolite; pearl gray, with streaks and mottles of rose and dull orange where dolo­mitic; beds I to 17 inches thick. Weathers to regular, solu­tion pitted, medium bluish gray ledges. A few partially silicified cannonballs, the lowest recorded from the section, were seen at 1030 to 1031 feet, and chert float occurs in the upper part of the interval. This chert float is partly semichalcedonic and oolitic and partly a granule conglomerate. It is white to light gray in color. Small, rounded, frosted sand grains were seen in some of the chert and limestone float in the upper part of the interval. Fossils were collected from cannon-ball chert weathering out along the trace of this interval or near it. They are Xenelasma syntrophioides Ulrich and Cooper, and a species of Jeflersonia (TF-103). Also a few poor unidentified cal­citic snails were seen in place. 16. Dolomitic limestone and calcitic dolomite-fine grained; light gray to light pink, varying to light greenish gray and light yellow; beds 1 tn 6 inches thick. Weathers platy, me· dium dark brownish gray. 17. Limestone, with irregular dolomitic inclusions and in minor part dolomitic-sublithographic, grading to fine grained where dolomitic, in part a fine pellet limestone; woodash gray to pearl gray, with pinkish mottles and streaks where dolomitic; beds from less than 1 inch to 30 inches thick. Weathers solution pitted, medium bluish gray. Chert occurs as scattered nodules in a thin calcitic dolo­mite bed at 1014 feet; being chalk textured to porcelane­ous, locally oolitic, white, and white weathering. 18. Dolomitic limestone and calcitic dolomite-fine grained, grading to sublithographic where the limestone is pure; brownish gray to light rose pink and pearl gray; beds from Thickness in feet Inter-Cumu,. Feet above val lative base 0.6 82 1046 -1046.6 86 1042 -1046 3 89 1039 -1042 3 92 1036 -1039 . 13 105 1023 -1036 4.5 109.5 1018.5-1023 6 115.5 1012.5-1018.5 3.5 119 1009 -1012.5 The Ellenburger Group of Central Texas Thickness in feet Inter· Cumu· Feet above Description val lative base less than I inch to 32 inches thick. Weathers rough, me­ dium gray. 19. Limestone, in part dolomitic-sublithographic, grading to 7 126 1002 -1009 fine grained where dolomitic; pearl gray to woodash gray, with irregular greenish argillaceous films and with occa­ sional pink streaks where dolomitic; beds from less than 1 inch to 11 inches thick. Weathers solution pitted, in part reticulate, medium bluish gray. 20. Dolomite--microgranular to fine grained; beige to light 10 136 992 -1002 yellowish gray and light pink; beds heavy, weatherihg 1 to 42 inches thick. Weathers smoothly irregular to hackly, medium to dark gray. Chert weathers from the covered basal 2 feet of the in­ terval as loose nodules and excrescences on slabs of dolo­ mite. It is chalcedonic to subchalcedonic, in part oolitic, bluish to brownish gray to brown and white. 21. Limestone, with irregular dolomitic inclusion&-Sublitho-1.5 137.5 990.5-992 graphic, grading to medium grained where dolomitic; pearl gray with pink streaks. A IO-inch bed is exposed at the base, with the remainder of the interval covered. Weathers reticulate, medium bluish gray. · 22. Dolomite, grading to dolomitic limestone in the upper foot 4.5 142 986 -990.5 and going laterally to dolomitic limestone away from the line of section-fine grained to microgranular, light yellow­ish gray to beige. Consists of a single ledge that weathers to thinner beds along the trace of the interval. Weathers rough, in part pitted, medium gray to iron gray. Sandy chert, chert matrix sand, and arenaceous dolomitic limestone occur sporadically along the trace of this inter­ val; but no sand was detected in the line of section. Base of Honeycut formation at base of interval 22. Total thick­ ness of formation 142 feet. A short lateral shift is made on the Gorman-Honeycut contact, moving north to the bank of Cherokee Creek if going up in the section and south to the top of Bee Cave Bluff if going down. Gorman formation: 426 feet thick C alcitic facies: 27 5 feet thick 23. Limestone of remarkable purity-sublithographic to litho-41.5 183.5 944.5-986 graphic, in part a fine pellet limestone; pearl gray to wood-ash gray, with occasional pinkish moltles and streaks, and in part with irregular yellowish to greenish argillaceous films; beds ranging from a fraction of an inch to 7 feet thick. The beds in this inLerval are characteristically mas­sive and the greater part of its thickness is accounted for by beds over 18 inches thick, interrupted by an occasional thin sequence of very thin beds. Weathers smooth to solu­tion pitted, medium bluish gray. Characteristically resistant to non-chemical weathering, these beds form the upper part of Bee Cave Bluff, on the southeast side of Cherokee Creek. The limestone in this interval appears to be the purest as well as the most massively bedded in the Cherokee Creek section, visible dolomite grains being a very minor con­ stituent. Moreover, a corresponding interval of unusually thick bedded and pure limestones averaging 40 to 55 feet thick, and locally with a dolomitic interval near the middle, seems to occur at the top of the Gorman formation through­ out the Llano region. A set of four chemical analyses through interval 23 shows it to average 96.5 per cent CaCOs; 2.27 per cent insolubles; and 1.23 per .:ent Fe.0.1, Al,Os, and unaccounted for. The University of Texas Publication No. 4621 Thickness in feet Inter- Cumu- Feet above Description val lative base Chert was seen in place in the line of section only as minor irregular inclusions at 978 feet, where it is chalce­ donic to subchalcedonic and bluish gray to white. It oc­ curs in other beds along the trace of thi9 interval where they arc better exposed to weathering, hut is generally uncommon. Silicified specimens of Xenelasma syntrophioides Ulrich and Cooper occur on the upper surface of the top ledge of this interval at the top of Bee Cave Bluff (986 feet ahove base of section), and cross sections of Rhombella and Lecano­ spila not uncommonly occur in the same bed along its trace; but no fossils were collected at this horizon in the southeast fault block of the Cherokee area. Several partially silicified specimens of Lecanospira were noted in the lime­ stone at 978 feet but not collected. Lecanospira, Hormotoma, Ophileta, small unidentified gastropods, and a small trilo­ bite were collected from rough chert weathering from the middle of this interval along its trace (TF-96) ; and Lecano­ spira and Rhombella were collected from the limestone 3.5 to 4.5 feet above the base of the interval ( TF-94) . 24.. Dolomite, grading laterally to limestone with irregular dolo­ 7.5 191 937 -944.5 mitic inclusions-the dolomite is fine grained, in part vuggy; light gray to light pinkish gray, locally grading to rose and nutria, The limestone is sublithographic; pearl gray to woodash gray, with irregular yellowish argillaceous films. The beds range from less than 1 inch to 17 inches thick. Weathers rough, in part pitted; medium gray to iron gray where dolomite; reticulate and medium bluish gray where limestone. 25. Limestone, in part with irregular dolomitic inclusions-sub­ 7.5 198.5 929.5-937 lithographic, grading to fine grained where dolomitic, in part a fine pellet limestone; pearl gray to woodash gray, with pinkish ancl brownish streaks where dolomitic, and in part with irregular yellowish argillaceous films; beds from a fraction of an inch to 25 inches thick, being very thin in the top 18 inches. Weathers solution pilted, in part reticulate, medium bluish gray. 26. Dolomite, grading to calcitic dolomite in the top few feet­ 15.5 214. 914 -929.5 very fine grained to medium grained; beige and rose beige to light pinkish, yellowish, and brownish gray; beds from less than 1 inch to 24· inches thick. Weathers smooth, medium gray to light pinkish gray and light pink. Occasional guano-like patches of chalk textured chert were seen from 921 to 923 feet; and minor interstitial chert occurs in some beds. 27. Limestone----sublithographic; pear1 gray, with irregular yel­ 1.5 215.5 912.5-914 lowish argillaceous films; beds from less than 1 inch to 10 inches thick. Weathers solution pitted, medium bluish gray. 28. Dolomite, grading to calcitic dolomite and dolomitic lime­ 3 218.5 909.5-912.5 stone in upper part-fine grained to sublithographic; light pinkish to brownish gray, grading to beige and lavender, with occasional pink streaks. Bedding indeterminate. Weath­ ers rough, medium to dark gray. 29. Limestone----sublithographic, in part a fine pellet limestone; 5 223.5 904.5-909.5 pearl gray, with irregular yellowish and greenish argilla­ ceous inclusions and minor pink streaks; beds up to 10 inches thick. Weathers smooth to solution pitted, medium bluish gray to light gray. 30. Dolomite, with interbeds of calcitic dolomite, dolomitic 12.5 236 392 -904.5 limestone, and limestone-the dolomite is fine grained and The Ellenburger Group of Central Texas Thickness in feet Inter-Cumu-Feet above Description val lative base gray to light pinkish and yellowish gray; whereas the lime­ stone is sublithographic and pearl gray grading to the colors of the dolomite where dolomitic. Beds from less than 1 inch to 24 inches thick. Weathers uneven to reticulate; medium gray to iron gray where dolomitic and medium to light bluish gray in the calcitic portions. Small nodules of chert are common in the lower foot; being semiporcelaneous, micro-oolitic to crypto-oolitic; dull gray to brownish gray and white. Interstitial chert also occurs in some beds. 31. Limestone, with minor irregular dolomitic inclusions-sub­1 237 891 -892 lithographic, grading to fine grained where dolomitic; pearl gray, grading to pink where dolomitic, and with irregular yellowish argillaceous films; a single ledge weath­ering in part slabby. Weathers reticulate, medium bluish gray to gray . .32. Dolomite, grading to calcitic dolomite and dolomitic lime­6 243 885 -891 stone at the base-microgranular to fine grained; light yel­lowish gray to beige, with mottles of darker gray or pink; beds mostly covered in the lower half, bedding indetermi­nate. Weathers to irregular medium gray to iron gray ledges in the upper part. Small nodules of chert are abundant in the upper part of the interval; being semichalcedonic to porcelaneous, dull white to light bluish or brownish gray. 33. Limestone-sublithographic, in part a fine pellet limestone; 25 268 860 -885 pearl gray to woodash gray, grading to light brownish gray and flesh-colored, with occasional pink streaks and small pink specks and locally with abundant irregular green­ish to yellowish argillaceous films; beds seemingly several feet thick in ledges but weathering from less than 1 inch to 18 inches thick. Weathers solution pitted, medium bluish gray. Chert occurs as scattered nodules at the base of the in­ terval; being semichalcedonic and bluish gray to dull white. Small patches of chalk textured guano-like chert were noticed at 874 feet and scattered nodular chert floats into the interval. H bed at 860 feet. Altitude 1366 feet on the sonthcast bank of Cherokee Creek and 1471 feet at its south end, which is about 1400 feet south-southeast of its position in the bluff seg­ment of the section and just west of a fault. Shift 1400 feet north-northwest if going up in the section and south-southeast if going down. (The exact trace of bed I-I is uncerlain for the south 400 feet; but a check measurement made between bed I-I and the sandy interval called bed F from a point 310 feet south-southeast of Cherokee Creek substantiates the approximate position determined by topographic tracing. The arenaceous bed F was traced from the line of section to the check point.) 34. Limestone~sublithographic; pearl gray to mouse gray, 18.5 286.5 841.5-860 grading to woodash gray, in part with pink tinges and spots and in part with abundant irregular greenish to yel­lowish argillaceous inclusions; beds from a fraction of an inch to 10 inches thick. Weathers solution pitted, in part platy, medium bluish gray. Upper bed of interval is bed I-I. Minor chert occurs as angular inclu5ions al 849 feet; be­ ing subporcelaneous, slightly oolitic, pinkish giay to brown­ ish gray. Scattered small sand grains were seen in the chert and limestone at 849 feet. The University of Texas Publication No. 4621 Thickness in feet Inter-Cumu-Feet above Description val lative base 35. Dolomite, grading to caleitic dolomite in the upper foot­7.5 294 834 -841.5 microgranular to fine grained; mostly light gray, grading to light brownish and yellowish gray. Bedding indistinct, beds as much as 14 inches thick. Weathers smooth, medium gray to iron gray. 36. Limestone, in part dolomitic in the upper 4, feet-sublith­24 318 810 -834 ographic, grading to medium grained where dolomitic; woodash gray to pearl gray, with irregular yellowish to whitish argillaceous films and occasional pink streaks, grad­ing to light rose pink where dolomitic; beds 2 to 10 inches thick. Weathers to discontinuous, solution pitted, medium bluish gray ledges. Chert was observed as occasional small nodules and ex­crescences above 813 feet, being subchalcedonic to chalce­donic, and bluish to brownish gray to white. At 822.5 to 823 and again at 827 feet are thin plates or beds of oolitic chert of a bluish to brownish gray to light brown or dull white color and containing scattered sand grains. Concentrically banded concretionary chert comes in along the trace of the beds at 812 feet and rough ropy chert along the trace of those at 811 feet. Small subrounded to rounded and frosted sand grains are abundant in limestone and oolitic chert from 822.5 to 823 feet, a 2-to 4-inch layer of arenaceous limestone and oolitic sand-bearing chert occurs at 827 feet, and scattered grains of sand were seen in the limestone at 830 feet. G bed at 810 feet; shift 100 feet east-southeast if going up in the section and west-northwest if going down. 37. Limestone, with a 4-inch arenaceous zone at the base-sub­12 330 798 -810 lithographic; woodash gray to pearl gray, with scattered pink streaks and small pink spots, and in part with irreg­ular yellowish to greenish argillaceous films; beds from less than 1 inch to 16 inches thick. Weathers to regular smooth to solution pitted, medium bluish gray ledges. Rough, ropy fossiliferous chert comes into the lower part of this interval along its trace to the north-northwest. The 1·-inch chert matrix sandstone (bed F), which oc­ curs at the base of this interval in the line of section, grades laterally to arenaceous chert, calcareous sandstone, and arenaceous limestone. This arenaceous zone is persistent in the southeast fault block of the Cherokee area and makes a useful datum there. The sand grains are pre­ dominately rounded to subrounded, frosted, and uniformly very small, but large grains are not uncommon and quartz granules occur sporadically. The chert matrix is oolitic, and dull white, pearl gray, flesh-colored, or even rose gray on a fresh surface; weathering to shades of yellowish tan. The limestone matrix is woo-dash gray to pearl gray, or mottled in pearl gray and cinnamon pink; weathering medium to light bluish gray or white. Occasional sand grains also occur in the limestone for a few feet above bed F. Poorly preserved fossils collected from the ropy chert that comes into the lower part of this interval from a point about 600 feet north-northwest of its inferred position in the measured section are H ormotoma, Lecanospira (?), Chepultapecia ( ?) , and unidentified gastropods and ccpha­ lopods (TF-98). Also Hystricurus sp. (a single ccphalon) was collected from the F bed along its trace to the north­ northwest (TF-97). F bed (arenaceous) at 798 feet; shift 150 feet east-southeast if going up in the section and west-northwest if going down. The Ellenburger Group of Central Texas Thickness in feet Inter- Cumu- Feet above Description val lative base 38. Limestone-sublithographic, in part a fine pellet limestone; 11 341 787 - 798 pearl gray to woodash gray, with sporadic pink streaks and small pink specks, and in part with irregular greenish to yellowish argillaceous films; beds from less than 1 inch to 14 inches thick. Weathers to fairly regular, smooth or solution pitted, in part platy, medium bluish gray ledges. Very minor chert occurs as insignificant excrescences on the lowest beds. 39. Limestone-sublithographic, in part a pellet limestone; 3 344 784 - 787 woodash gray to pearl gray, in part with scattered small pink spots and irregular yellowish argillaceous films; beds from less than 1 inch to 8 inches thick. Weathers smooth, medium bluish gray. Chert occurs as scattered irregular patches; being chalk textured to subporcelaneous or semichalcedonic and chalk white to bluish and brownish gray. It weathers dull white and that in the lower part especially has a guano-like appearance. Archaeoscyphia occurs in interval 39 (TF-99) as worn and broken fragments which ordinarily have no distinguish­ ing external characters and are not recognizable as fossil remains until broken and wetted, when the spicular struc­ ture of the sponge wall is evident. This interval is an important datum and was traced across the main fault block of the Cherokee area as well as several minor fault blocks; the guano-like chert serving to call attention to it and the spicular structure of Archaeoscyphia identify­ ing it. 40. Limestone-sublithogra11hic; pearl gray to woodash gray, 16 360 768 - 784 locally with scattered pink streaks and small pink specks, and in part with irregular yellowish to dusty green or white argillaceous films; beds from less than 1 inch to 22 inches thick. Weathers smooth, medium to light bluish gray. Minor chert occurs as nodules and lenticles at 778 feet, as occasional large spheroidal masses at 782 feet, and as float. It is subchalcedonic to subporcelaneous or chalk tex­ tured, bluish gray to bluish white and chalk white, and weathers yellowish white. In dolomite float at the base of the interval occur lenticles of semichalcedonic to porce­ laneous, in part micro-oolitic, bluish gray to white and dull carnelian chert. 4°1. Dolomite-fine grained; light gray to light pinkish gray, 4 364 764 - 768 beige, rose beige and rose; bedding indeterminate. Weath­ ers to discontinuous medium to light gray ledges. Minor interstitial chert was seen. Sand grains of pin point size were seen to he abundant in one piece of dolomite float at the base of the interval. Silicified Lecanospira was seen and the siphuncle of Piloceras resembling P. hornei Ulrich, Foer~te, and Miller was collected 700 feet southeast of the line of section (TF-90), from a limestone which is either the lateral equivalent of interval 41 or belongs to the immediately superjacent or subjacent beds. 42. Limestone, with minor irregular dolomitic inclusions-sub­ 14 378 750 - 764 lithographic, grading to fine grained where dolomitic; in part a pellet limestone; pearl gray to woodash gray, and light brownish gray, in part with irregular whitish argil­ laceous inclusions; beds thin but bedding indeterminate. Weathers to discontinuous, smooth to rough, medium bluish gray ledges. The University of Texas Publication No. 4621 Thickness in feet Inter- Cumu- Feet above Description val lative base Chert occurs as ropy layers, irregular nodules, and rough excrescences; abundant in place in the upper 4, feet and in the float below. It is ehalcedonic to porcelaneous, in part chalk textured, in part crypto-oolitic to oolitic, and in part apparently a chertified pellet limestone with a tex­ ture approaching that of the cannonball chert. Locally it is quartzose; and the color varies through bluish gray, to brownish gray, pearl gray, and china white. Syntrophinclla was seen in place in chert at the top of interval 42, as were also small trilobites and gastropods; but none were collected. E bed at 750 feet; shift 120 feet east-southeast if going up in the section and west-northwest if going down. 43. Dolomite, as judged from float in a largely covered inter­ 20 398 730 - 750 val; grading to calcitic dolomite and dolomitic limestone at the top, and apparently going laterally to limestone in the upper beds-predominantly microgranular, grading to fine or even medium grained in the upper part; mouse gray, yellowish gray, beige, and rose beige, locally grad­ ing to salmon or coral pink. The minor exposures and scattered float cobbles weather smoothly uneven, light to medium gray and tan. Minor chert float in the upper part of the interval is in part a granule breccia. It is subchalcedonic to subgranular, in part with quartzose inclusions; varies through bluish gray, brownish gray, tan, and pearl gray; and weathers dull orange or bluish white. Interstitial chert occurs in some of the dolomite. - 44 Mostly limestone, as judged from float in a largely cov­ 19 417 711 - 730 ered interval; in part with irregular dolomitic inclusions­ most.ly sublithographic, grading to medium grained where dolomitic, in part a pellet limestone; woo dash gray to pearl gray and white, with sporadic small pink spots. Exposed rock weathers smooth to rough and medium bluish gray. Chert is abundant below 715 feet as large blocks, and not uncommon as excrescences, surficial coatings, and angu­ lar blocks above. It is semiporcelaneous to subgranular, commonly oolitic to crypto-oolitic, in part with drusy vugs, commonly finely drusy on broken surfaces, white to brown­ ish gray. Some ooids are white in a brownish gray matrix and some of the so-called oolitic chert may represent chcr­ tified pellet limestone. Base of calcitic facies of Gorman formation at base of in­ terval 44 (altitude about 1485 feet). Thickness of facics 275 feet. Dolomitic facies: 151 feet thick 45. Dolomite, as judged from float and poor exposures in a 33 450 678 - 711 mostly covered interval; grading to calcitic dolomite and dolomitic limestone at top-mostly microgrannlar in lower 22 feet, grading to fine and medium grained in upper 11 feet; rose beige, beige, rose, rose gray, and light yellow­ ish gray to coral pink; bedding indeterminate. Weathers to a mostly covered interval strewn with smooth, light to medium gray and yellowish gray cobbles of dolomite. Scattered to abundant chert weathers from this interval, especially in the lower 20 feet; being porcelaneous to subgranular or chalk textured, with scattered tiny dolo­ molds, in part quartzose, china white to bluish or yellow· ish gray. "Euconia" sp. (TF-69a) was collected ftom chert along the trace of this interval. D bed (arenaceous) at 678 feet; shift 700 feet southeast if going up in the section and northwest if going down. The Ellenburger Group of Central Texas Thickness in feet Inter- Cumu- Feet above Description val lative base 46. Dolomite-mostly microgranular, in part grading to very 23.5 4°73.5 654.5-678 fine grained; light yellowish and pinkish gray to beige and rose beige, with rose mottles; bedding indeterminate. Weathers to discontinuous ledges in a largely covered and rubble strewn surface; the ledges and float cobbles being smooth, medium to light gray and yellowish gray. Scattered angular inclusions, nodules, and plates of chert occur at 662 feet and in the upper 3 feet of the interval. It is subchalcedonic; in part crypto-oolitic and pseudo­spicular; varies through bluish gray, brownish gray, white, rose, and light pink; and weathers shiny white to pink­ish white. Irregular plates and coatings of fine to medium grained quartz druse are common in the upper 3 feet. Porcelaneous, china white, in part finely dolomoldic chert may weather out of the upper part of this interval, but it more probably floats down from above. ­Sand occurs in the upper foot of interval 46 (bed D) ; constituting an arenaceous dolomite in the line of section, but grading laterally to dolomitic sandstone with tiny rounded to subrounded and frosted sand grains which in general are not well sorted within their small size limits. In color it ranges through to_nes of beige; varying to white where bleached or brick red to russet where oxidized, and weathering to medium tones of gray and tan. Bed D is fairly persistent and was traced the breadth of the south­east block of the Cherokee area as an arenaceous zone varying from 5 inches to about 2 feet thick. Occasional sand grains -also occur for about 18 inches below bed D. Fossils were collected from chert float along the trace of the upper part of this interval, but the chert was probably derived from interval 45. They are "Euconia" sp., Ophileta sp., and Raphistomina (?) sp. (TF-69). 47. Dolomite, in minor part grading to calcitic dolomite and 22 495.5 632.5-654.5 dolomitic limestone in the lower 5 feet-mostly fine to medium grained, grading to microgranular from 638 to 643 feet; yellowish gray, grayish yellow, rose gray, and light rose, in part grading Lo medium gray; beds I to 12 inches thick. Weathers to uneven, medium gray ledges. Chert is abundant in the lower 5.5 feet as thin plates, angular inclusions, excrescences, and surficial coatings; vary­ ing from subgranular to subporcelaneous, white, and white weathering chert to dolomoldic and finely drusy chert or films of line grained quartz druse. Interstitial chert is abundant in the dolomite of the lower 5.5 feet and occasional above. 48. Dolomite-mostly microgranulur, but with a few thin beds 27.5 523 605 -632.5 that are medium to fine grained; mostly beige to light yellowish gray, grading to rose beige, light rose, and cin­namon pink; bedding indeterminate. Weathers to discon­tinuous, medium gray ledges alternating with largely cov­ered portions. Minor coatings of fine grained quartz druse were seen in place, and interstitial chert occurs in the medium grained beds. Angular chips of chert float of uncertain derivation occur in fair abundance; being principally porcelaneous to chalk textnred, locally quarlzose, and having scattered tiny dolomolds and occasional cavities lined with dolomolds. It is china white to woodash gray or pinkish white and weath­ ers shiny white to dull tan. C bed at 605 feet; shift about 210 feet north-northwest if going lip in the section and south-southeast if going down. This shift is made on the basis of a zone of billowy surfaced chert lenses or plates an inch or two thick and up to 10 inches across (projected for about 50 feet at the north end of the The University of Texas Publication No. 4621 Thickness in feet Inter- Cumu- Feet above Description val lative base shift) that weather out about 5 feet above the bed marked C. This chert vm ies f1om semichalccdonic to porcelaneous, snb­ granular, and locally chalk textured, and i:; generally crowded with indistinct pin point inclusions. Tt is bluish to brownish gray to chalk white in color, in part concentrically banded, and weathers yellowish white lo tan or white. 49. Dolomite--microgranular to medium grained; consisting of 18 541 587 - 605 microgranular to very fine grained dolomite (tctal of 10 leet) inte1bedded with medium to fine grained dolomite (total of 8 feet). Rose beige, beige, and rose gray, grad­ ing to pinkish and yellowish gray. Bedding indeterminate, beds poorly exposed. Weathers to alternating, thin, medium gray to light yellowish gray or tan ledges and covered portions. Coatings of fine to medium grained quartz drnse are common thronghout the interval, and i nterst.it.ial chert char­ acterizes the medium grained beds. Occasional nodules of chalcedonic to porcelaneous chert also were seen, and along the trace of the lower foot of lhe interval ahnndant angular pieces of chert may be found. This lower chert is mostly porcelaneous, grading to suhgranular and semt­ chalcedonic; white to bluish giay; and weathers shiny white or ydlowish white. 50. Calcitic dolomite, in part grading to dolomitic limest.one­ 5 546 582 - 587 medium grained, light pinkish gray to light gray; bed­ ding indeterminate. Weatherg rough, medium gray, poorly exposed. Interstitial chert and :;mall patches of chalk textured chert are occasional throughoul. Coatings of rough, dark weathering quartz druse were seen near the ba:;e of the interval. 51. Dolomite-microgranular to fine grained, locally with a few 17.5 563.5 564.5-582 scattered vugs; beige to light yellowish gray, varying to rose gray and light grayish yellow, in part. with rose to lavender &t.reah; bedding indeterminate. Weathers to a largely covered interval that is strewn with smooth or slightly pitted, medium to light gray weathering cobbles and has a few medium gray ledges exposed in the lower part. Chert float is abundant in the lower part of the m­ terval and chert was seen in place between 568 and 572 feet as prominent layers, with occasional nodules above. From 570 to 572 feet the chert is chalceclonic to porcelane­ ous; bluish gray, brown, and china white; and weathers mostly shiny white. From 568 to 570 feet it is porcela­ neous to subporcelaneous, with scattered small dolomolds and disseminated quartz, and commonly interlayered with a lesser amount of quartz drnse; it. is china white to woodash gray on a fresh :;urface except that where pene­ trated by oxidation in the drusy layers it is russet to dull brown. Interstitial chert occurs m the dolomite at 570 feet. Rhombella was found m the chert from this interval at locality TF-61. Although sand-hearing chert is fairly common near or at the base of the Gorman formation in parts of the prin­ cipal fault. block of the Che1 okee area, none was noted at this position in or near the base of the Cherokee Creek section. 52. Dolomile-~fine grained in the line of clesctibed section, but 4.5 568 560 - 564.5 going laterally to microgrannlar dolomile m, may he seen above the Tanyard-Gorman contact in the south segment. of the section and intermil.Lently along 1he trace of this interval. Light yellowish or pinkish gray to beige, with The Ellenburger Group of Central Texas Description darker yellowish streaks. Bedding indelerminat.e. Weathers to a largely covered interval with a few thin medium gray ledges cropping out. Interstitial chert is fairly common, as well as float of blocky weathered chert in part apparently weathering from this interval and in part from above. Lateral tracing in­ dicates that the locally derived chert is predominantly a type composed of irregularly interlayered or interlaring fine quartz druse and a lesser amount of porcelaneons china white chert with scattered small dolomolds. Generally sim­'ilar chert from the interval stratigraphically above coptains typical Gorman fossils. Hystlicurus sp., "Euconia," and a well preserved but un­identified brevicone ccphalopod were collected a foot or two above the base oI this interval and the Gorman for­mation (TF-6.5) as traced beyond the line of section. Base of dolomitic facies of Gorman fo1·mation at base of in­ terval .52. Total thickness of facies 1.51 feet, and of formation 126 feet. Gorman-Tanyard contact at .560 feet-altit.ndp, 1473 feet in line of section southeast of county road and 1443 feet in line of section northwest of county road. Shift 1450 feet northwest if going up in the section and southeast if going down. (Because some of the lower microgranular dolomite of the Gorman for­mation and possibly some of the upper limestone of the Tan­yard formation in the Cherokee area goes laterally to fine to medium grained dolomite alon.e; this contact, it is not a hair­line break as traced in the field; hut evidence from lateral tracing closely limits its position and at no place along the 1450-foot shift are more than 4 or .5 feet of beds in question.) Tanyard formation: 560 feet thick Staendebach member (type section): 300 feet thick Calcitic Jacies: 176 feet thick .53. Limestone; apparently pure from 510 to .556 feet, hut with irregular dolomitic inclusions in the lower 10 feet and the top 4 feet-sublithographic, varying to medium grained where dolomitic, in part a fine pellet limestone; pearl gray to woodash gray, varying to flesh-colored where dolomitic, in part with minor pinkish tinges; beds 1 to 9 inches thick. Weathers uneven, reticulate where dolomitic, medium bluish gray. Chert is abundant in the lower 2..5 feet as layers or extensive lenses and plates strung out parallel to the bed­ding; being suhehalcedonic to porcelaneous, bluish gray to china while, in part handed. Above the basal very cherty zone it occurs as occasional nodules, lenses, plates, and angular inclusions to the top of the interval, and as a thin layer at 339..5 feet; heing chalcedonic to subgran­ular, in part oolitic to erypto-colitic and pseudospicular, locally with qnartzose inclusions and scattered dolomolds. A fresh smface is li~J1t bl11ish or brownish gray to brown­ish orange or white, in part laminated: whereas weathered smfarcs are white to yellowigh gray. The top few feet of the interval are marked by finely drusy and in part do1o­moldic excrescences, vugs, and coatings and by oolitic to oomoldic laminar chert. Ozarlcina was noted at several places along the trace of the upper part oI this interval, locally accompanied by other fossils, hut no fossils were noted or collected from these beds in the line of se('tion. .51.. Limestone, wilh irregular dolomitic inclusions-snhlitho­granhic, grndin!!; to medium grnined when'> dolomitic, locally a fine pellet limestone; gray to light brownish or pinkish Thickness in feet Inter-Cumu-Feet above val lative base 30 .598 .530 -.560 19 617 .511 -.530 The University of Texas Publication No. 4621 Description gray; beds 3 to 22 inches thick. Weathers uneven, locally reticulated; medium bluish gray. Chert was seen in place only in the lower 5 feel where it occurs as occasional platy to irregular inclusions. It is sernichalcedonic to chalccdonic, in part oolitic, bluish gray to white. 55. Dolomite, slightly calcitic and in minor part grading to limestone-medium to fine grained, pearl gray with sporadic light pinkish spots, beds up to at least 30 inches thick. Weathers uneven, in part pitted, medium to medium dark gray. Chert occurs as scattered excrescences and irregular in­clusions; being dolomoldic and finely drusy to semichal· cedonic, locally oolitic, bluish to brownish gray. 56. Dolomitic limestone and calcitic dolomite-sublithographic to coarse grained; pearl gray to woodash gray, grading to light brownish to pinkish gray; bedding inrlctcrminate. Weathers rough, in part reticulate, in part pitted, medium gray to medium bluish gray. Chert occurs as minor thin plates and excrescences, partly oolitic and dirty white. Fossils were collected from a subporcelaneous to granu­lar or chalk textured dirty white chert weathering out near the trace of this interval m· interval 55 about 1400 feet northwest of here (TF-49). They are cystid plates, Ozarkina, cf. 0. typica Ulrich and Bridge, Helicotoma sp., Chepultapecia sp., Ectenoceras sp., unidentified brevicone cephalopods, and Parapletho peltis. 57. Limestone, with irregular dolomitic inclusions-sublitho· graphic to fine grained; woodash gray or pearl gray, grad­ing to light brownish gray; beds from less than 1 inch to 14 inches thick. Weathers reticulate, medium bluish gray. Oolitic laminar chert occurs from tJ87 to 488 feet. Above 488 feet chert is fairly abundant as excrescences, lenses, nodules, and plates; being subchalcedonic to subporcela· neous and locally crypto-oolitic or dolomoldic and quartz­ose, in part concentrically banded, bluish gray to pinkish white and white. 58. Limestone, with minor irregular dolomitic inclusions-sub­lithographic, grading to medium grained where dolomitic; woodash gray to pearl gray; a single ledge. Weathers rough to smooth, medium bluish gray. The minor chert present consists of subpor,celaneous, dolomoldic, dirty white excrescences. Fossils collected from the limestone (TF-93) include Ozarkina cf. 0. cornplanata Ulrich and Bridge, 0. cf. 0. typica Ulrich and Bridge, Ophileta sp., Helicotoma cf. H. uniangulata (Hall), Clarkoceras?, and Paraplethopeltis. 59. Limestone, with abundant irregular dolomitic inclusions­sublithographic, grading to medium grained where dolo­mitic; pearl gray or light brownish gray, grading to light pinkish gray where dolomitic; beds 1 to 14 inches thick. Weathers rough, reticulate; medium bluish gray. Above tJ,74 feet chert is abundant; being mostly oolitic and laminar with holes resembling molds of flat pebbles. It is subgranular to porcclaneous, in part quartwse, break­ing with a fracture like that of paraffin; brownish to bluish gray to dirty white, in part with purplish or pinkish tinges. In the lower beds scattered chelt occurs as irregular nod­ules and ropy excrescences; being subgranular to semichal- Thickness in feet Inter- Cumu- Feet above val lative base 9 626 502 -511 6 632 4,96 -502 16 648 480 - 496 I 649 4,79 -480 13.5 662.5 465.5-479 The l!,'llenburger Group of Central Texas Description cedonic, in part finely drusy, locally crypto-oolitic and pseudospicular, white to gray to purplish gray. Ozarkina cf. 0. typica Ulrich and Bridge was collected from limestone in place 2.5 feet above the base of this interval (TF-93a). Fossils were also seen in chert float at 479 feet, being recorded as Ozarlcina cf. 0. typica Ulrich and Bridge and a small species of Sinuopca, but not col­lected. Fossils were collected from granular, dirty pinkish white, russet weathering chert about on the trace of the upper part of this interval 600 feet west-northwest from the line of section. They arc Ozarkina cf. 0. complanata Ulrich and Bridge, Ophileta sp., Schizopea sp., Ecteno­ccras?, a large brevicone ccphalopod, and Parapletho peltis (TF-47). 60. Limestone and chert; the top 6 to 12 inches being oolitic laminar chert and the remainder limestone with abundant irregular dolomitic inclusions-the limestone is sublitho­graphic, grading to medium grained where dolomitic; pearl gray, grading to woodash gray or light pink where dolo­mitic; poorly exposed. It weathers reticulate, medium blu­ish gray. The chert in the top foot of this interval is subgranu­lar to subporcelaneous, oolitic and laminar, in minor part quartzose; bluish to brownish gray lo white, in part banded. Besides the type described, abundant chert was seen to weather out from the lower beds where traced laterally. This lower chert is semichalcedonic to subgranular or finely granular, commonly resembling fractured paraffin on a freshly broken surface; partly cryplo-oiilitic, dolomoldic, and chalk textured; in part layered with holes resembling molds of fiat pebbles; light gray to dirty white on a fresh surface; and weathers to light shades of russet, tan, or dirty white. Fossils collected from chert at locality TF-29 are thought to be about on the trace of this interval or immediately below it. They include lielicotoma cf. H. uniangulata (Hall), Schizo pea sp., Sinuopea sp., Ophilcta sp., Ecteno­ cews sp., and a larger brevicone cephalopod. R bed at 460 feet (altitude 1420 feet at east end). Shift 680 feet south-southeast if going up in the section and west­northwest if going down. 61. Limestone, with irregular dolomitic inclusions-sublitho­graphic, grading to medium grained where dolomitic; pearl gray to woodash gray, with mino1 pinkish tinges; beds poorly exposed, bedding indeterminate. Weathers reticulate medium bluish gray. ' Much chert weathers out of this interval though little was seen in place. It is irregularly blocky to nodular or concretiona1y with semiconcentrically banded "cabbage-head" slromatolites of the cryptozoon type, and it is especially abundant in the lower half of the interval. It is chalce­donic to porcelaneous, less commonly subgranular wilh a fracture like broken paraffin, locally crypto-oolitic, in minor part slightly dolomoldic, and chalk textured where pene­trated by weathering; light bluish or brownish gray to white, wealhering to tones of dirty white to tan or russet. A bed at 449 feet (altitude 1491 feet at west end). Shift 440 feel east-northeast if going up in the section and west­soulhwest if going down. 62. Limestone, with minor irregular dolomitic inclusions-sub­lithographic, grading in part to a fine pellet limestone; Thickness in feet Inter- Cumu· Feet above val lative base 5.5 668 460 -465.5 11 679 449 -460 11 690 4,33 -449 The University of Texas Publication No. 4621 Description pearl gray to woodash gray; beds 1 to 18 inches thick, poorly exposed in lower part. Wealhers uneven, in part reticulate; medium bluish gray. Oolitic laminar chert weathers out from the lower few feet of this interval as abundant angnlar, russet to yellow­ish weathering blocks. At 447 feet scmichalcedonic, in part crypto-oolitic gray to brownish gray to dirty white chert occurs as irregular nodules and inclusiom. Abundant blocky, white weathering chert floats over the interval hom above. A short lateral shift is made at 438 feet, moving east if going up in the section and west if going down. 63. Dolomite at top and Lottom, grading to limestone in the middle Lhird--medium to fille grained, grading to sub­lithographic where pure limestone; wooclash gray to light pinkish gray; beds 3 to 8 inches thick. W cathers smoothly uneven, reticulate, medium gray to medium bluish gray. Chert occurs as nodules and lenses at 435.5 feet: being scmkhalcedonir to chalk textured, oolitic to cryplo-oolitic, bluish to Lrownish gray to white, and weathering shiny yellowish white. 64. Mostly limestone, with irregular dolomitic inclusions and oceasional thin dolomite beds-sublithographic, grading to medium grained where dolomitic; pearl gray to woodash gray, grading to light yellowish gray; beds 1 to 8 inches thick. Weathers reticulate, platy, medium Lo light bluish gray. Chert is abundant as lenses and nodules from 431 to 432 feet; being chalccdonic to semichalcedonir, banded, bluish gray to brown and white, and weathering shiny brownish white. 65. Dolomite-mostly medium grained, in part vup_gy; light grayish brown, grading to light brownish and pinkish gray; beds 2 to 22 .inches thick. W cathe1s uneven, pitted to smooth, medium gray. Chert is abundant as lenses, nodules, and angular in­clusions from 416.5 Lo 418.5 feel and from 424 to 425 feet. It is chalccdonic to subchalcedonic, in part grading to por­celaneous; locally oolitic to crypto-oolitic; in part wilh minor quartzose inclusions; and bluish or brownish gray to china white. 66. Limestone, with irregular dolomitic inclusions und gradin:i; to calcitic dolomite in upper 4 fect-sublithographic, gracf­ ing to medium grained where dolomitic; pearl gray to yel­ lowish or pinkish gfay; bedding indeterminate. Wcathcrn reticulate, medium gray to medium bluish gray. Chert abundant at base as semironcentrically banded "cab­bage-head" stromatolites of the cryptozoon type. It is sub­chalcedonic to porcelaneous, in part quartzose, brownish or bluish gray to white, and weathers tan tu shiny white. Lenticular chert is abundant from 406 to 407 feet; being suhchalcedonic to chalcedonic, in part crypto-oiilitic, in minor part with quartzose indusiom, bluish gray to white, and mostly weathering shiny white. 67. Limestone, with inegular dolomitic inclusions an•l giad­ing to dolomite in lower foot-sublithographic, grading to medium grained where dolomitic; very light gray to pearl gray or light pinkish gray, grading to light pink and sal­mon colored where dolomitic; beds l to 10 inches thick. Weathers reticulate, platy, medium bluish gray. Z bed at 391 feet; shifL about 150 feet east if going up in the section and west if going down, traveling parallel to and T hir /mess in feet Inter-Cumu-Feet above val lative base 3.5 693.5 434.5-438 8.5 702 426 - 434.5 15 717 411 - 426 10 727 401 - 111 7 734 394, -401 The Ellenburger Group of Central Texas Thickness in feet Inter- Cumu- Feet above Description val lative base ~~~~~~~­ about 70 feet north of the east-west fence between the Staende­ bach survey to the north and one of the Fisher and Miller sur­ veys to the south. 68. Limestone, with irregular dolomitic inclusions and in minor 10 744 384 - 394 part grading to dolomite~sublithographic, grading to me­ dium grained where dolomitic; very light gray to pearl gray, grading to pinkish gray or salmon-coloreige, semiporcelaneous, fossil fragments abundant. Silicified fossils are Ceratopea sp. (Fossils collected about 0.6 mile to the southwest are at 680 feet and are Ceratopea sp. 4, TF-258.) 9. Dolomite---microgranular, yellow beige, one bed. 4 71 674 -678 Chert rare and composed of small rounded objects, pos­ sibly pellets. Sand present as a few scattered grains. 10. Limestone, somewhat dolomitic-suhlithographic, yellowish 6 77 668 -674 gray. Chert cannonballs, semiporcelaneous, brown, common. 11. Dolomite-microgranular, near nutria, beds 1 to 3 inches 1 78 667 -668 thick. 12. Limestone-suhlithographic, old ivory in part mottled yel­ 12 90 655 -667 lowish brown; beds at 655 to 665 feet, 1 to 3 feet thick; 665 to 667 feet, 1 to 2 inches thick. Thickness infeet Inter-Cumu-Feet above Description val lative base ~~~----~----~---~-~-~-~---~~-~--~-~~~ Silicified fossils at 661 to 665 feet are Archaeoscyphia sp., common. (Fossils collected about 0.6 mile to the south­ west are at about 662 feet and are a variety of Ceratopea 10busta Oder and Ceratopea sp. 4? (TF-257). 13. Dolomite-microgranular, beige, poorly exposed but bed-; 15 105 640 - 655 appear to be about 6 inches thick. Quartz nodules common at 642 to 644 feet. 14. Limestone-sublithographic, yellowish brown, beds about l 5 llO 635 - 640 foot thick. Chert skeletal cannonballs, abundant at 638 lo 640 feet. Silicified fossils abundant are Hormotoma spp., "Ophileta" sp., Archaeoscyphia sp., Eopteria sp., Ceratopea capuliformis or young robusta, undetermined cephalopods and gastro­ pods, Euconia aff. E. etna (Billings) (205T-l-17A). 15. Dolomite-microgranular, medium gray to heig,e, bottom 5 l15 630 - 635 portion thinly bedded, top portion :;omewhat more thickly bedded, poorly exposed. 16. Limestone-sublithographic, yellowish brown, one bed. 2 117 628 - 630 Chert cannonballs up to 1 foot in diameter, contain frag­ ments of fossils. 17. Dolomite-microgranular, medium to dark gray, lower 3 7 124 621 - 628 feet paper-thin laminae; upper 4 feet 4­ to 8-inch beds. Silicified fossils at 625 feet collected about 0.6 mile to the southwest are Archacoscyphia ammlata Cullison and Barnesella lewnospiroides Bridge & Cloud (TF-255). 18. Limestonc-sublithographic; medium gray, beige, and brown­ 20 144 601 - 621 ish gray; 601 to 603 feet one bed; the interval is com­ posed of 5 zones, 3 of which arc thickly bedded with beds 1 foot or more thick, and 2 thinly bedded zones with beds 1 to 4, inches thick; exposures are rather poor. Chert, 604 feet, large cannonballs, irregular and in part anastomosing, semiporcelaneous, medium gray, fossil frag­ ments abundant, also cannonballs at 608 and 612 to 614 feet. Chert at 606 feet semiporcelaneous, brown, occurring as a network of compact chert. Chert at 620 feet subchalcedonic, caramel colored, full of spicules, occurring as a I-inch layer. Silicified fossils at 601 to 603 feet are Archaeoscyphia sp., and ccphalopod siphuncles; 604 and 603 feet, fossil frag­ ments in cannonball chert; 608 to 612 feet, Archaeoscyphia sp. abundant. (Fossils at 609 feet collected about 0.6 mile to the southwest are Archaeoscyphia annulata Cullison, Ceratopea cf. C. capulifonnis Oder, Hormotoma sp., un­ identified cephalopods, Rananasus sp., and ]efjersonia sp. (TF-256) ; and at 612 to 614 feet Archaeoscyphia sp., trilobites, and gastropods.) 19. Dolomite-microgranular; variegated beige, browns, and 1 145 600 - 601 purples; appears tu he in about 2­ to 4-inch beds, not well exposed. 20. Limestonc-:;ublithographic; medium gray to brownish gray; 39 184 561 - 600 561 to 567 feet essentially one bed, 567 to 590 feet 2· to 12-inch beds (top few beds nutria), S90 to 600 feet thickly bedded. Chert at 561 to 564 feet small cannonballs; 576 feet por­ celaneous, medium gray, fossiliferous cannonballs, 585 feet cannonball type along bedding plane with abundant fossil fragments; 595 and 598 to 600 feet cannonballs. Chert at S75.5 feet chalcedonic, medium gray, occurring as finger­ sized pieces and nodules. Chert at 588 feet subchalcedonic, medium gray, smooth-fracturing, occurring as a layer of masses 6 to 8 inches thick and 1 to 2 feet long. Quartz nodules numerous at 564 to 569 feet. Silicified fossils at 561 to 5M feet are Archaeoscyphia sp.; 576 feet trilobites and gastropods abundant; 580 feet The Ellenburger Group of Central Texas Thickness in Ieet Inter- Cumu· Feet above Description val lative base cephalopod siphuncles, and indications of Ceratopea; at 595 feet gastropods common; and 598 to 600 feet Archae· oscyphia sp., abundant. Shift east-northeast along base of interval about llOO feet and continue section down drain. 21. Limestone and dolomite-in about equal parts occurring as 7 191 554 - 561 a network to lamellar intermixture producing a nodular appearance. This zone weathers recessive and in combina· tion with the overlying massive 6-foot limestone bed forms a distinct scarp. The limestone is sublithographic, medium gray; and the dolomite is fine grained and medium gray. Chert of cannonball texture irregularly distributed as bedding plane layers; and some chert semiporcelaneous, medium gray, compact and smooth weathering. 22. Dolomite-fine grained, medium gray, one poorly defined bed. 1 192 553 - 554 Quartz nodules abundant. 23. Limestone-with abundant network dolomite, irregularly 2 194 551 - 553 bedded. Chert, some flattened cannonballs. 24. Dolomite-microgranular to fine grained, beige, Quartz nodules 1h inch in size, rare. one bed. 5 199 546 - 551 25. Limestone-sublithographic, medium gray, one bed. 1 200 54.S -546 Chert cannonballs, semiporcelaneous, medium gray, abundant. Silicified fossils collected about 0.8 mile to the south· west are Archaeoscyphia annulata Cullison and unidenti­ fied endoceratid cephalopod (TF-254). 26. Dolomite-microgranular to fine grained, light brownish 2 202 543 - 545 gray, one bed. 27. Limestone-sublithographic old ivory to medium gray with 37 239 506 - 543 network dolomite in limestone l!:rading up to dolomite for the portion of interval 514 to 515 feet; bedding 517 to 525 and 538 to 541 feet, 1 inch or less, rest of interval, 2­ to 12-inch beds. Chert at 507 feet subchalcedonic to semiporcelaneous, bluish gray to white, 2-inch layer. Cannonballs at 516 feet porcelaneous, white to dirty white, in part ~tructureless and in part composed of fragments. Chert 526 to 530 feet por· celaneous, beige, structureless, compact, smooth fracturing, in spheres up to 6 inches in size; 536 to 538 feet similar chert, some with concentric banding; and top surface con­ tains nodules and plates of similar chert. Shift east-southeast about 400 feet and continue section down hill. 28. Limestone-sublithographic, old ivory, one bed. 2 241 504 - 506 Chert cannonballs, semiporcelaneous, medium gray; con­ tains many interstices and fossil fragments, about 8 inches in diameter. 29. Limestone-sublithographic, old ivory to light gray, beds 5 246 499 - 504 up to 4 inches thick with most being thinner nodular beds. 30. Dolomite-microgranular, brown and highly weathered, poorly exposed. 3 249 496 - 4,99 31. Limestone-sublithographic, light gray, very thinly bedded, 5 254 491 - 496 poorly exposed. 32. Limestone-sublithographic, old ivory, beds 2 to thick, lower part highly dolomitic from network 12 inches dolomite. 12 266 4,79 -491 Quartz nodules with quartz at outer surface and lime· stone in the centers, rare. 33. Dolomite-microgranular, medium gray to beige with yel­ 9 275 470 - 479 lowish brown and purple streaks and mottles. 34. Limestone-sublithographic medium gray, one bed. and in part pellet limestone, 5 280 465 - ,470 Chert cannonballs semiporcelaneous, beige, 2 to 3 inches in diameter. Some quartz films are present. Thickness in feet Inter· Cumu- Feet above Description val lative base 35. Dolomite-fine grained, medium gray to beige and light 2 282 463 - 465 brown, one bed. Quartz nodules (dolomite in centers) 1f2 inch in size. 36. Dolomite-microgranular, light gray to light brown, beds 6 3 285 460 - 463 to 12 inches thick. 37. Dolomite-microgranular to fine grained, beige, beds 6 3 288 457 - 460 inches to 2 feet thick. Chert semiporcelaneous, light gray, oolitic, occurring as I-inch nodules and plates. 38. Limestone--sublithographic, old ivory, bedding paper-thin 1 289 256 - 457 laminae. 39. Limestone,-sublithographic, old ivory, beds 6 inches to 9 298 44,7 -456 2 feet thick. Chert at 448 feet subchalcedonic to semiporcelaneous, light gray, crowded with pellets. Cannonballs 3 inches in diameter and elongated masses of similar chert at 455 feet semiporcelaneous, brownish gray. Quartz nodules 1 inch in size at 452 feet. Shift downstream and across gully east-southeast about 200 feet along prominent bedding joint and continue section down­ stream. 40. Dolomite-microgranular, beige to medium brown, beds 4 13 311 434 - 447 to 12 inches thick with top 2 feet nodular and thin bedded. Chert at 437 feet semiporcelaneous, light gray, contains sand grains and rounded to flattened 1/s-inch thick pebbles, occurs as a 2-inch layer. Chert cannonballs at 438 feet semiporcelaneous, reddish brown, 2 to 6 inches in diameter. 41. Limestone with dolomite network-limestone sublitho­ 7 318 427 - 434 graphic, old ivory, beds 1 to 2 feet thick. Dolomite net­ work indicative of filled burrows. Chert cannonballs at 431 to 432 feet semiporcelaneous, beige, fossiliferous. Silicified fossils in cannonballs are Xenelasma sp., and gastropods. 42. Dolomite-fine to medium grained, dirty brown to dark 3 321 4.24 -427 gray. Chert subchalcedonic, light gray, occurring as a row of nodules near base of interval. 43. Dolomite-microgranular, light brown, one bed. 1 322 423 - 424 44. Limestone---sublithographic, old ivory, with upper part con­ 2 324 421 - 423 taining network dolomite as if it filled burrows. Sand present in the dolomite. Calcitic fossils are sections of Hormotoma sp. on upper surface. 45. Dolomite-microgranular, medium gray to light brown, one 1 325 420 - 421 bed. Sand is present in bottom 8 inches. The bed retains its characteristics throughout the map area. This is the base of the Honeycut formation. The rest of the Gorman Falls section is measured 3600 feet to the south-southwest where the exposures of the upper part are not so good as in Tie Slide Canyon. The following sup­ plementary section was measured in Tie Slide Canyon on a hare rock surface. a. Limestone--sublithographic, ivory to old ivory; essentially 20 345 400 - 420 one bed, 400 to 405 feet; beds 6 inches to 2 feet thick from 405 to 420 feet. Sand fills what appear to be burrows in top 6 inches. b. Dolomite--microgranular, mediull;J. gray mottled brownish 2 347 398 - 400 yellow, bedding indistinct. Forms a recess around canyon wall. c. Limestone-sublithographic; ivory to old ivory; bedding 33 380 365 - 398 from 365 to 372 feet thin to nodular; 372 to 376 feet a single bed; 376 to 378 feet one bed; 378 to 384 feet The Ellenburger Group of Central Texas Description one bed; and 384 to 398 feet beds 6 inches to 2 feet thick. Silicified fossils are Diaphelasma pennsylvanicum Ulrich and Cooper at 395 feet. d. Dolomite-fine to medium grained, medium gray, beds 4 to 8' inches thick. Somewhat calcareous near middle of interval. e. Dolomite-microgranular, medium gray, mottled brownish yellow, bedding indistinct. Bottom of supplementary section is at water level in pool in bottom of Tie Slide Canyon. Shift along the Honeycut-Gorman boundary to a point 3600 feet airline south-southwest and continue section down slope. Gorman formation: 474 feet thick (420 feet present in Gor· man Falls section) Calcitic facies: 383 feet thick 46. Limestone--sublithographic; ivory; beds 1 to 2 feet thick. Beekite excrescences are present on the surface of beds at 415 and 418 feet. (This interval as measured was 18 feet thick but in view of poorer outcrops it is made to correspond more nearly to the supplementary section in Tie Slide Canyon using the Diaphelasma bed as a tie point.) 47. Dolomite-microgranular, one bed which actually is about 6 inches thick. Some float of limestone containing sand was seen in this vicinity, not found in place. , 48. Limestone--sublithographic, ivory, essentially one bed. Chert present in top surface as a few small rounded nodules. Shift eastward about 300 feet and continue section down slope. 49. Limestone--sublithographic; ivory; beds, 377 to 382 feet, 2 to 6 inches thick; 382 to 384 feet one bed; 384 to 385 feet probably 6-inch beds; 385 to 389 feet one bed; 389 to 393 feet 6-to 12-inch beds; and 393 to 395 feet one bed. Chert excrescences on top surface of interval. Silicified fossils are bryozoans (?) at 384 feet, Diaphe­lasma pennsylvanicum Ulrich and Cooper at 395 feet. Cal­citic fossils are sections of Rhombella sp. and Lecanospira sp. at 393 feet. ( Silicified fossils at 395 feet collected 900 feet to the south-southwest are Diaphelasma pennsylvanicum Ulrich and Cooper and Lecanospira sp. (TF-251). 50. Dolomite-bottom 17 feet microgranular, top 4 feet fine grained; medium gray with yellowish brown mottles and streaks ; beds 2 to 12 inches thick. Chert at 362 to 363 feet subchalcedonic, light bluish gray to white, occurring as rounded nodules up to 6 inches in diameter and as elongated curved plates. Chert at 374, to 375 feet subchalcedonic, light gray, with some pieces con· taining straight spicules, occurring as plates and nodules up to 2 inches thick. 51. Limestone--sublithographic, dark ivory, one bed. Calcitic fossils are Lecanos pira sp. in top surface. 52. Dolomite--microgranular, dark gray mottled and streaked yellowish brown. Chert porcelaneous, white, cloudy from rounded pebble­like objects, occurs as a 4-inch bed near top. Silicified fossils are Rhombella sp. and Lecanospira sp., seen in chert. 53. Limestone and dolomite-intergrading laterally. 54. Limestone-sublithographic; ivory; at 335 to 341 feet beds 1 to 3 feet thick; 341 to 344 feet nodular thin bedded; 344 to 350 feet 1 to 2 feet thick. Chert at 342 to 34.S feet subchalcedonic, bluish gray, abundant as irregular branching nodules and plates. Calcitic fossils at 350 feet are Rhombella sp., Lecano· spira sp., and cephalopods. Thickness in feet Inter-Cumu-Feet above val lative base 8 388 357 -365 3 391 354 -357 19 344 401 -420 1 345 400 -401 5 350 395 -400 18 368 377 -395 21 389 356 -377 2 391 354 -356 2 393 352 -354 2 395 350 -352 15 410 335 -350 Thickness in feet Inter-Cumu- Description val lative 55. Dolomite-microgranular, beige somewhat mottled brownish 2 412 gray. 56. Limestone-sublithographic, ivory, one bed. 4 416 57. Dolomite-fine grained. A few hundred feet east this in· 1 417 terval grades to limestone having a stromatolitic structure. 58. Limestone with network dolomite. 1 418 Shift along bluff downstream about 150 feet east and continue section down bluff. 59. Dolomite-microgranular, beige and brownish gray some· 6 424 what mottled, beds 6 inches to 2 feet thick. Chert porcelaneous, white, chalky weathering, occurs as thin plates in top 2 feet of interval. 60. Limestone-sublithographic; ivory; at 302 to 312 feet beds 19 443 irregular, 1 inch or less thick, having a nodular appearance and with shaly material between; at 312 to 321 feet, 1 to 2 feet thick. Chert at 318 to 319 feel subchalcedonic to semiporcelane· ous, white to bluish gray, occurring as irregular plates, and as a discontinuous network of finger-sized pieces. 61. Dolomite-microgranular, medium gray to nutria, beds 1 to 8 451 2 feet thick. Chert at 294 to 296 feet subchalcedonic, white to bluish gray, contains straight white spicules, very abundant as nodules. 62. Limestone-sublithographic, beige, oolitic, beds 4 inches to 4 455 2 feet thick. 63. Dolomite-microgranular, beige, one bed. 1 456 Chert subchalcedonic, white to brownish gray, oi:ilitic, contains sand grains, occurs as a layer up to 6 inches thick. Sand is disseminated throughout bed. 64. Limestone-sublithographic, ivory, splits into thin layers. 5 461 65. Dolomite-microgranular, light to medium gray, beds 6 21 482 inches to 2 feet thick. Chert at 263 feet subchalcedonic, dull brownish gray, oolitic, sandy, occurs as elongated nodules. Chert at 267 to 270 feet subchalcedonic to semiporcelaneous, medium gray to dirty white, occurs as 1-to 2-inch thick flattened nodules. Chert at 283 to 284 feet subchalcedonic, dark blu­ ish gray, rounded objects possibly oolites or pellets, sandy, occur as 2-inch thick beds. Sand is present at 263 and 283 to 284 feet. Shift downstream about 500 feet along north valley wall of ravine using the chert matrix sand at 259 feet as the principal bed for making the shift. 66. Limestone-sublithographic; ivory; 232 to 235 feel one bed; 31 513 235 to 237 feet beds about 1 inch thick; 237 to 239 feet one bed; and 239 to 263 feet beds 1 to 4 inches thick. Chert at 253 to 255 feet porcelancous, dirty white to medium gray, occurring as bedding plane lenses and layers 1 to 2 inches thick. Chert at 259 feet semiporcelaneous to granular, the granular appearance due to sand grains and other objects, one bed 1 to 2 inches thick. Sand at 259 feet in chert and 2-inch limestone bed above it. 67. Limestone-sublithographic, ivory, with paper-thin bedding. 1 514 Chert dull porcelaneous, brownish while, present as a 2-inch thick bed at top of interval. 68. Dolomite-microgranular, beige, one hed. 515 Chert subchalcedonic to porcelaneous, medium gray and white mottled, one bed at bottom of interval is 2 inches thick. 69. Limestone-sublithographic; ivory; somewhat stylolilic; at 30 545 200 to 202 feet beds 6 to 12 inches thick; 206 to 213 feet Feet above base 333 -335 329 -333 328 -329 327 -328 321 -327 302 -321 294, -302 290 -294 289 -290 284 -289 263 -284 232 -263 231 -232 230 -231 200 -230 The Ellenburger Group of Central Texas Thickness infeet Inter- Cumu- Feet above Description val lative base 2 beds; 213 to 218 feet 1 to 4 inches thick; 218 to 223 feet thickly bedded; 223 to 228 feet 1­ to 1J,.inch beds; and 228 to 230 feet 6-to 8-inch beds. Chert at 202 feet semiporcelaneous, medium gray, has a surface of raised designs, present as an inch-thick layer. Chert at 213 to 216 feet porcclancous, white, occurring as plates. Chert at 223 feet similar but occurring as inch-thick lenses. Shift on top of Archaeoscyphia bed across Colorado River into Lampasas County and downstream to a point about 2700 feet airline southeast and continue section down slope. 70. Limestone-sublitbographic, ivory, beds 1 to 3 feet thick 20 56S 180 - 200 with a massive bed from 192 to 195 feet. Chert at 197 to 200 feet skeletal cannonballs grading into flattened skeletal outlines and finally into compact, por­ celaneous, white, chert layer having a surface of raised designs. Silicified fossils at 183 to 185 feet arc Archaeoscyphia sp., structure in chert nodules; and 197 to 200 feet Archaeo­ scyphia sp., Rhombella cf. R. umbilicata Ulrich and Bridge, undetermined gastropods, and cephalopods in cannonhalls (141T-6-4·3A). West of river and north of ravine north of Gorman Falls, silicified brachiopods from this bed are Syntrophinella (TF-250). 71. Dolomite-microgranular, beige and darker, very poorly ex­ 7 572 173 - 180 posed. Chert porcelaneous, white, with micro-oolitic (?) struc­ ture, occurs as plates in top of interval. 72. Limestone-sublithographic, ivory, irregularly hedded, bed­ 14 586 159 - 173 ding planes 1 to 6 inches apart in lower portion becom­ ing less distinct toward top, with one 5-foot bed at top. 73. Dolomite-medium grained beige, weathers saccharoidal, 7 593 152 - 159 poorly exposed. Chert porcelaneous to subchalcedonic, medium gray to white, oOlitic, occurs in upper part as long masses up to 6 inches thick. 74. Limestone-sublithographic, ivory, one 2-foot bed at bot­ 4 597 148 - 152 tom and 6-inch beds at top. 75. Dolomite-medium grained, bPige, one bed. 3 600 145 - 148 Chert porcelaneous, white, occurs in bottom half of in­ terval as spherical nodules and irregular smooth forms. 76. Limestone-sublithographk, ivory, one 77. Limestone-snblithographic, ivory, beds bed. 14 to 1 inch thick. 3 5 603 608 142 137 -- 145 142 78. Dolomite-microgranular, beige and yellowish brown mot­ 8 616 129 - 137 tled with dull reds in bottom 2 feet and with light gray above. Chert weathered chalky white, occurs near bottom as thin plates. Shift along bluff downstream about 75 feet and continue sec­ tion down side of bluff. 79. Dolomite-fine grained, light gray, pool'ly bedded. 2 618 127 - 129 Chert porcelancous, light gray, abundant as nodules and irregular forms. 80. Limestone and dolomite-a mixture ranging from pure 622 123 - 127 limestone to pure dolomite grading in an erratic manner both vertically and laterally. SI. Limestone-poorly exposed but appears to he thinly bedded, 4 626 119 - 123 I inch or less, weathers recessive. 82. Limestone-sublithographic, ivory, stylolitic, contains some 12 638 107 - 119 dolomite rhombs, beds 1 to 3 feet thick. Chert at 112 feet porcelaneous, white, somewhat dolo­ moldic, occurring a~ a fairly continuous 2­ to 6-inch layer. 83. Dolomite-microgranular lo fine grained, white to light 18 656 89 -107 gray with some beige, beds 6 inches to 3 feet thick. Thickness in feet Inter- Cumu- Feet above Description val lative base 84. Dolomite--fine to medium grained, beige, one bed. 2 658 87 -89 Sand is disseminated in middle foot of interval. 85. Limestone--sublitho!;raphic, ivory, extremely thin bedded 7 665 80 -87 becoming thicker bedded at top. Chert porcelaneous to chalky, occurring in middle portion of interval as ellipsoids. 86. Dolomite-medium grained, beige, one bed. 3 668 77 -80 87. Dolomite-microgranular, beige mottled with yellowish 2 670 75 - 77 brown. Chert chalky, white, occurring as very thin plates. 88. Dolomite-medium grained, beige, beds 4 inches to 3 feet 5 675 70 - 75 thick. 89. Dolomite-microgranular, beige with brown mottles. I 676 69 -70 Shift along bluff downstream about 100 feet and continue sec­ tion down cliff. 90. Dolomite and limestone-medium grained dolomite, brown­ 2 678 67 -69 ish as if stained, grading to limestone below which con­ tains dolomite network. 91. Limestone--£ublithographic, old ivory with numerous brown 30 708 37 - 67 and yellow lined stylolites, beds up to several feet thick. Chert at 61 feet porcelaneous to subchalcedonic, bluish gray to white, occurring as one layer up to :? inches thick. Sand disseminated at 50, 61, and 65 feet. Shift along base of cliff downstream about 150 feet and con· tinne section down slope. Dolomitic facies: 91 feet thick (37 feet present in Gorman Falls section) 92. Dolomite-microgranular; mostly light gray, some beige with 24 732 13 -37 yellowish brown mottles. 93. Dolomite-fine grained, light gray, one bed. 3 735 10 - 13 Sand disseminated in bottom 6 inches of interval. Continue section downstream along edge of flood plain. 94. Dolomite-micrograuular, light gray at bottom becoming 7 742 3 - 10 slightly mottled toward top, bedding indistinct. Chert porcelaneous, white, concentrically banded nodules, occurring in bottom 2 feet of interval. 95. Limestone-yellowish brown, contains numerous dolomite I 74.3 2 - 3 rhombs, beds I to 6 inches thick. Chert present as a few thin excrescences. %. Limestone-snblithographic, extremely thin bedded and 2 745 0 - 2 cusped as if ripple marked, splits into very thin layers. The bottom of the Gorman Falls section is located at the edge of a narrow alluvial strip just upstream from a drain on the east side of Colorado River at a poin1 2100 feet southeast of Gorman Falls. DESCRIPTION OF THE SPICEWOOD mation 57 feet and overlaps the Gorman CREEK SECTION section 3 feet. The upper portion of the The Spicewood Creek section receives Staendebach member is poorly exposed its name from Spicewood Creek in south­ almost everywhere in the northeastern eastern San Saba County. The bottom of part of the uplift, but the expqsures on the section is about 600 feet upstream Spicewood Creek are sufficient to allowfrom the mouth of Spicewood Creek and a section to be measured. Faulting iscontinues along the northeast valley-wall present and some of the offsets are ques­for about 0.7 mile. tionable as indicated in the descriptionThis section overlaps the Tanyard sec­of the section. tion 280 feet and displays the upper 400 feet of the Staendebach member of the The Spicewood Creek section was meas­ Tanyard formation. It continues up into ured and described by V. E. Barnes and the dolomitic facies of the Gorman for-L. E. Warren in March 1945. The Ellenburger Group of Central Texas Thickness in feet Inter· Cumu· Feet above Description val lative base Ellenburger group: 1457 feet thick (456 feet present in Spice­ wood Creek section) Gorman formation: 474 feet thick (57 feet present in Spice­ wood Creek section) Dolomitic facies: 91 feet thick (57 feet present in Spice- wood Creek section) The top of the section is located 200 feet north of Spice­ wood Creek 8.9 miles by road south of Bend. To reach the top of the section, follow the road from Bend toward the head of Lake Buchanan 7.9 miles, turn south on a dim cedar cutters' road 0.8 mile to Spicewood Creek roughly following a north-south fence, and thence downstream 0.2 mile. See Plate 8 for exact location. 1. Limestone--containing some dolomite rhombs, grayish brown, 1 1 456 - 457 one bed. Chert similar to skeletal cannonballs is present. 2. Limestone-sublithographic, extremely thin bedded and 1 2 455 - 456 cusped as if ripple marked. Splits into layers 1/16 inch or less in thickness. 3. Dolomite--microgranular, medium gray to beige in part 4 6 451 - 455 mottled with yellowish brown, beds 6 to 12 inches thick. 4. Dolomite-fine grained, mottled dark to medium gray with 4 10 447 - 451 yellowish browns, beds 6 to 12 inches thick. 5. Dolomite-microgranular, medium gray to beige in part 4 14 443 - 447 • mottled with yellowish brown, beds 6 to 12 inches thick. Chert porcelaneous, white, occurs as nodules, rare. 6. Dolomite-fine grained, medium gray, one 7. Dolomite-microgranular, medium gray to bed. beige in part 2 11 16 27 441 430 -- 443 441 mottled with yellowish brown, beds 6 to 12 inches thick. Chert nodules at 436 feet are porcelaneous and white. 8. Dolomite-fine grained and dark gray containing areas 2 29 428 - 430 which are microgranular and medium gray, one bed. 9. Dolomite-microgranular, predominantly light gray and in 2 31 426 - 428 part mottled medium gray with yellowish browns, beds 6 to 12 inches thick. 10. Dolomite-microgranular as in interval 9. 1 32 425 - 426 Quartz druse occurs as a network, and porcelaneous, white dolomoldic chert is common. 11. Dolomite-microgranular as in interval 9, beds 6 to 12 15 4,7 410 - 425 inches thick. Chert porcelaneous, white, occurring as an occasional flat plate with a row of plates at 415 to 416 feet. Sand at 420 feet is disseminated through 2 inches of dolomite. 12. Dolomite-fine grained, medium gray, somewhat mottled, 2 49 408 - 410 one bed. 13. Dolomite-microgranular, mottled medium gray with ye!­ 8 57 400 - 408 lowish hrowns, bedding indistinct, beds 1 foot or more thick. Chert at 403 feet porcelaneous, white, mixed with quartz druse. Chert at 407 to 408 feet porcelaneous to subchalcedonic, white to medium gray, slightly dolomoldic. The bottom of this interval marks the hase of the dolomitic facies of the Gorman formation. Shift about 1000 feet south· east downstream along Gorman-Tanyard contact and continue section down slope to bluff. Tanyard formation: 658 feet thick ( 400 feet present in Spice­ wood Creek section) Staendebach member: 456 feet thick ( 400 feet present in Spicewood Creek section) Calcitic facies: 163 feet thick 14. Limestone-sublithographic, off white, considerable network 11 68 389 - 400 dolomite. Chert at 384 to 386 feet chalcedonic bluish gray, forms an almost continuous layer. The University of Texas Publication No. 4621 Thickness in feet Inter- Cumu· Feet above Description val lative base 15. Limestone-sublithographic, 16. Limestone-sublithographic, off off white, white, one bed. contains considerable 5 36 73 109 384 348 -- 389 384 network dolomite, beds 1 to 3 feet thick. Chert at 358 to 359 feet and 362 feet, porcelaneous, white, occurs as layers of small irregular nodules. Chert at 379 feet subchalcedonic to semiporcelaneous, banded, bluish gray, white. Chert at 384 feet subchalcedonic to semi­ porcelaneous, oolitic, in a layer 4 to 6 inches thick. 17. Limestone---sublithographic, off white, contains small amount of network dolomite, beds 6 inches to 3 feet thick. 9 118 339 - 348 Chert at 339 feet subchalcedonic to semiporcelaneous, white to medium gray, oolitic, forming an almost continu­ ous layer. Similar chert layers but mostly not oolitic arc abundant throughout interval. Shift about 200 feet southeast across two small faults on chert layer and continue section down bluff. 18. Limestone-sublithographic, off white, with abundant net­ 33 151 306 - 339 work coarse grained dolomite, beds 1 to 3 feet thick. Chert at 307 to 309 feet subchalcedonic to semiporce­ laneous, white to medium gray, ocrurs in layers. Chert at .310 feet similar but occurring as nodules. 19. Dolomite-coarse grained, medium gray, beds 1 to 4 feet 27 178 279 - 306 thick. A zone from 297 to 301 feet is calcareous. Chert subchalcedonic to semiporcelaneous, dark gray to white, somewhat quartzose, dolomoldic, occurs as an almost continuous bed from 279 to 281 feet and as masses up to 295 feet. Chert subchalcedonic to semiporcelaneous, white to medium gray, oolitic, occurs as a 6-inch bed at 300 feet. Shift southeastward about 600 feet along bottom of chert across several small faults and continue section down bluff. This shift is not satisfactory since chert of similar character is distributed through about 16 feet of section, and without other recogniz­ uble heds to trace, an error could easily have been made in ~hifting. 20. Limestone-sublithographic, white to light gray, contains 16 194 263 - 279 abundant network of coarse grained dolomite, beds 1 to 2 feet thick. Some of the beds grade rapidly laterally to coarse-grained dolomite. Dolomitic facies: 293 feet thick (237 feet present in Spice· wood Creek section) 21. Dolomite-coarse grained, beige, poorly exposed; a few 26 220 237 - 263 limestone patches surrounded by dolomite are located at 242 to 243 feet. Chert rubble abundant, but none seen in place. 22. Dolomite-medium grained, nutria to yellowish gray, mottled. 7 227 230 - 237 Chert porcelaneons to subchalcedonic, white to bluish gray, occurring as a few nodules near middle of interval. 23. Dolomite-coarse grained, beige, beds about 2 feet thick. 6 233 224. -230 Chert at 224 to 225 feet porcelaneous to subchalcedonic, white to bluish gray, occurring as small nodules. 24. Dolomite-medium grained, medium brown to nutria, beds 14 247 210 - 224 6 to J2 inches thick. Chert particles in bottom 5 feet of interval are dissem­ inated as 0.1­ to 0.3-inch sized particles. Chert at 221 to 224 feet porcelaneous to subchalcedonic, white to blu­ ish gray, abundant as nodules and plates up to 6 inches thick. 25. Dolomite-coarse grained, beige, beds about 2 feet thick. 5 252 205 - 210 Interstitial chert common at 205 to 209 feet. Chert at 209 to 210 feet porcelaneous to subchalccdonic, white to bluish gray, abundant. 26. Dolomite-medium grained, medium brown, beds 6 to 12 2 254 20'3 -205 inches thick. The Ellenburger Group of Central Texas Thickness in feet Inter-Cumu-Feet above Description val lative base ·-~~~~~~~~~~~~~~~~~~~~~~--~~~~~­ Chert subchalcedonic to semiporcelaneous, bluish gray to white, oolitic, very abundant. 27. Dolomite-coarse grained, beige, bedding indistinct but beds probably 1 to 4 feet thick. Chert at 185 to 186 feet semiporcelaneous to quartzose, dirty white to dark gray, dolomoldic. Chert at 194 to 196 feet semipOTcelaneous, dark brownish gray, contains large dolomolds, abundant. A layer of I-inch spherical, subchalcedonic chert nodules is &ituated at 195 feet. 28. Dolomite-medium grained, medium gray to beige, bedding indistinct but beds probably 1 to 2 feet thick. Chert semiporcelaneous to quartzose, dirty white to dark gray, dolomoldic, occurs as masses up to 1 foot thick and as a network, abundant. Considerable quartz druse is present. 29. Dolomite-medium grained, medium gray to beige, bed­ding indistinct but beds probably 1 to 2 feet thick. Chert semiporcelaneous, dirty white, dolomoldie, common. &>me quartz druse is present. Shift downstream southeast about 900 feet along bluff and con­tinue section down bluff. The beds actually followed to make this shift are somewhat beneath the offset and a few feet error in making the shift is possible. 30. Dolomite-medium grained. dark gray mottled beige, beds 2 to 3 feet thick. Chert semiporcelaneous, white, dolomoldic, common as small irregular nodules. A small amount of quartz druse is present. 31. Dolomite-fine to medium grained, dark gray in part mot­tled medium gray, bedding indistinct hut probably about 1 foot thick. Chert semiporcelaneous to subchakedonic, white to me­dium gray, mostly oolitic, occurs as nodules and plates 3 to 6 inches thick and 1 to 3 feet long. A small amount of quartz druse is present. 32. Dolomite-fine to medium grained, dark gray in part mot­tled medium gray, bedding indistinct. Chert quartzose to porcelaneous, white to medium gray, very abundant as an irregularly distributed network. Abun­dant quartz druse. Considerable subchalcedonic to semipor­cclaneom, white to medium gray or bluish gray, dolo­moldic chert is also present, with one bed from 123 to 124 feet nearly continuous. The chert from 125 to 128 feet is in thin, highly dolomoldic, quartzose, bedding plane layers. 33. Dolomite-fine to medium grained, dark gray in part mot­tled medium gray, bedding indistinct. Chert subchalcedonic to semiporcelaneous, white to me­dium gray or bluish gray, dolomoldic, abundant. From 110 to Ill feet forms an almost continuous layer. 34-. Dolomite-fine to medium grained, dark gray in part mot­tled medium gray, bedding indistinct. Chert quartzose to porcelaneous, white to medium gray, highly dolomoldic, very abundant throughout interval as a network. 35. Dolomite-fine to medium grained, dark gray in part mot­tled medium gray, bedding indistinct, but beds probably about 1 foot thick. Chert semiporcelaneous to subchalcedonic, in part slightly quartzose, bluish gray and medium gray to white, dolo­ , moldic, abundant as 6-inch thick lenses up to several feet in length. Quartz drusc at 90 to 91 feet abundant and common elsewhere in interval. 18 20 20 6 11 12 9 7 10 272 292 312 318 329 341 350 357 367 185 165 145 139 128 116 107 100 90 - - - - - - - - -203 185 165 145 139 129 116 107 100 The University of Texas Publication No. 4621 Description 36. Dolomite---finc to medium grained, dark gray in part mot­tled medium gray, bedding indistinct. Chert porcelaneous, white, dolomoldic, abundant as irreg­ular plates and masses. 37. Chert-semiporcelaneous to subchalccdonic, white to me­dium gray or bluish gray, white weathering, lamellar, oolitic, and in part dolomoldic. Forms a continuous bed. This chert is probably the same as in the Tanyard sec­tion from 638 to 640 feet and is used as a tie between the Tanyard section and the Spicewood Creek section. There is considerable latitude for error in this correlation, but to obtain a complete Ellenburger section in this portion of the uplift some such tie has to be made. 38. Dolomite-fine grained, dark gray, mottled medium gray, bedding indistinct. Chert suhchalcedonic to semiporcelaneous, bluish gray to white and in part medium to dark gray, in part oolitic, abundant as nodules. 39. Dolomite---fine grained, brown stained, thick. 4°0. Dolomite-medium grained, dark gray about 1 foot thick. Quartz druse abundant. 41. Dolomite-fine grained, mottled dark gray, beds 4 to 12 inches thick. beds about 1 foot stained brown, beds gray and medium Chert, one 2-inch concentrically handed nodule seen. 42. Dolomite-medium grained, mottled medium gray and dark gray, essentially one bed. 43. Dolomite--fine to medium grained, medium to dark gray with beige mot.ties, beds about 1 foot thick. Chert porcelaneous, in part oolitic, common as plates and angular nodules from 60 to 65 feet. druse abundant at 58 t.o 60 feet. 44. Chert-suhchalcedonic to semiporcelaneous, bluish white, in part micro-oolitic, forming practically a uous bed. 45. Dolomite-fine to medium 46. Dolomite---fine to medium brown, bedding indistinct, thick. Chert at 27 to 28 feet white, occurring as plates. grained, poorly exposed. grained, medium gray 1h-inch Quartz gray to contin­ stained but beds probably up to 4 feet quartzose to semiporcelaneous, Chert at 30 to 32 feet porce­ laneons, white, faintly banded nodule>. Chert at 38 to 39 feet porcelaneous to suhchalcedonic, bluish white, occur­ring as fractured nodules. Quartz druse irregularly distrib­uted at 17 Lo 25 feet; abundant at 48 to 52 feet. 47. Dolomite-medium grained, stained brown, one bed. Chert semiporcelaneous to subchalcedonic, slightly banded, light grayish blue to brown, occurring in masses up to 1 foot in size. Considerable quartz druse present. 48. Dolomite---fine grained, light brown with grayish mottles, beds about 2 feet thick. Quartz druse irregularly distributed throughout interval. 49. Dolomite-medium grained, medium gray with light brown stains, beds 1 to 2 feet thick. Quartz druse, 4 to 8 feet, irregularly distributed. The bottom of the Spicewood Creek section is along the east side of Spicewood Creek 600 feet. upstream from its month. About 20 to 30 feet of fine to medium grained cherty dolomite in sporadic exposures is present beneath the section to lake level. A small fault may be present about 100 feet downstream from the bottom of the section. Thickness in feet Inter- Cumu­ val lative 5 372 2 374 3 377 2 379 3 382 4 386 6 392 8 400 1 401 4 405 35 440 2 442 7 449 8 457 85 83 80 78 75 71 65 57 56 52 17 15 8 0 Feet above base - - - -- - - - -- - -- 90 85 83 80 78 75 71 65 57 56 52 17 15 8 The Ellenburger Group of Central Texas DESCRIPTION OF SMITH RANCH SECTION lithology 22 feet higher in the section. It (SUPPLEMENTARY) appears, therefore, that, using lithologic evidence only at any one place and with­ A supplementary generalized section out impractically detailed lateral tracing, was measured corresponding to the lower the Gorman-Tanyard contact as mappedportion of the Gorman Falls section about would fluctuate through at least 22 feet of 3400 feet northwest of the mouth of Ylancy section. Fossils, of course, if found would Creek, on a bluff on the east side of Colo­ settle the position of the boundary at any rado River. The purpose of this section was to check the tie between the Spice­one locality, but fossils are scarce and for wood Creek and the Gorman Falls sections. the most part the contact was mapped Thickness in feet Inter- Cumu- Feet above Description val lative base I. Dolomite-microgranular. 8 8 88 - 96 Chert at 88 feet porcelaneous, white, in layer s. Sand 2 inches thick at 88 feet and 3 inches of dissem­ inated sand grains at 96 feet. 2. Limestone-fine grained, one bed. 2 10 86 -88 CannonbaII type chert at 86.5 feet. Sand grains at 87 feet. 3. Limestone-sublithographic, paper thin laminae. 1 11 85 -86 4. Dolomite-microgranular. 5 16 80 -85 5. Dolomite-fine to medium grained. 5 21 75 -80 6. Dolomite-microgranular. 2 23 73 -75 7. Dolomite-medium grained. 1 23 72 -73 8. Dolomite-microgranular. 12 36 60 -72 Chert at 69 feet quartzose, dolomoldic, occurs as a net­ work 6 inches thick. 9. Dolomite-medium grained. l 37 59 - 60 10. ll. Dolomite-microgranular. Limestone and dolomite-gradational, dolomite , medium 1 3, 38 1.r 58 55 -- 59 58 grained. 12. Dolomile-microgranular. 1.5 t{.2.5 53.5­ 55 13. Dolomite-medium grained. 1 43.5 52.5­ 53.5 14. Dolomite-microgranular. 23.5 67 29 - .52.5 Chert porcelaneous, white at 34 and 41 feet. Approximate hase of Gorman formation as chosen in the Spice­wood Creek section. 15. Dolomite-medium grained. 1 68 28 -29 16. Dolomite-microgranular. 6 74 22 -28 Chert white, layered at 25 feet. 17. Dolomite-medium grained. 7 81 15 -22 LS. DolomiLe-essentiaily microgranular. 15 % 0 -15 Base of Gorman formation as it would be chosen on basis of lithology. Beneath this section is 4 feet of coarse using intermittently determined lithologic to medium grained dolomite, below which evidence for it. The original reason for is limestone containing chert in which measuring this section was to check the Ozarkina sp. is present. tie between the Spicewood Creek and the When the section was measured the Gorman Falls sections, by checking the contact between the Tanyard and Gorman distance between the base of the Gorman formations was thought to be at the base of formation and the contact between the interval 18, but in tracing the contact sublithographic limestone with paper-thin southeastward along the side of the bluff laminae and the overlying brownish gran­the lower 15 feet of microgranular dolo­ular limestone. In the Spicewood Creek mite appears to grade to limestone. section Lhis point is 56 feet above the Where this interval is limestone the con­supposed base of the Gorman formation, tact would be picked on the basis of whereas it is 86 feet above the point priginally picked, on the basis of lith­Ordovician rocks up to near the top of ology, as the base of the Gorman in the Tanyard formation in an unfaulted the SmiLh ranch supplementary sec­sequence. Unfortunately the section does tion. If the check point is at the same not reach the top of the Tanyard for­stratigraphic level in the two places then mation, but this is not serious since all there is a fluctuation of 30 feet of the the facies of the Tanyard formation are Gorman-Tanyard boundary as mapped well displayed in the section. The rest within the Gorman Falls area. of the formation can be seen about 4 DESCRIPTION OF THE TANYARD SECTION miles up the Colorado in the Spicewood The Tanyard section includes Cambrian Creek section. The Tanyard· section was rocks down to the San Saba limestone measured and described by Barnes and member of the Wilberns formation and L. E. Warren in February 1945. Thickness in feet Inter· Cumu-Feet above Description val lative base Ellenburger group: 1457 feet thick (538 feet present in Tan­ yard section) Tanyard formation: 658 feet thick (538 feet present in Tan­ yard section) Staendebach member: 456 feet thick (336 feet present in Tanyard section) Calcitic facies: 1377 feet thick (17 feet present in Tanyard section) The top of the section is about 1200 feet east of Tanyard Spring. The top 197 feet of the section is measured approxi­mately along the strike in a direction about S. 80° W. 1. Limestone-sublithographic, light gray, with small network 17 17 818 -835 of dolomite having a wide range of colors such as pinks and yellows. The top 4 feet of limestone is not exposed, the surface being made up of angular blocks of chert. Chert subchalcedonic to semiporcelaneous, white to blu­ ish gray, in part compact, in part dolomoldic, and some of it is oolitic, occurring as angular blocks a foot or more in size and very abundant in the top portion of the interval. Note: Somewhat higher beds are present to the east but to include them in the section requires lateral tracing over areas with very poor outcrops and with no recognizable key beds on which to shift. The possibility of error in making a shift makes it inadvisable to· try to place these beds in the section. Furthermore these beds are described in the Spicewood Creek section, located about 4, miles to the north. Dolomitic facies: 319 feet thick 2. Dolomite-coarse grained, medinm gray, beds 1 to 2 feet 23 40 795 -818 thick, not well exposed. Chert porcelaneous, white, clouded, not abundant, occur­ ring as irregular masses. 3. Dolomite-medium grained, dark gray, not well exposed. 2 42 793 -795 Chert porcelaneous, brown, oolitic, occurring as irregular masses. 4. Dolomite-medium to coarse grained, medium gray, not 3 45 790 -793 well exposed. Chert porcelaneous, white, occurring as irregular masses. 5. Dolomite---coarse grained, medium gray, not well exposed. 4 49 786 -790 6. Dolomite-medium grained, dark gray, I-foot beds. 3 52 783 -786 7. Dolomite-coarse grained, medium gray, 1-and 2-foot beds. 3 55 780 -783 Chert porcelaneous, dolomoldic, occurring as irregular masses. Considerable quartz druse is also present. 8. Dolomite-medium grained, beige, in I-foot beds. 12 67 768 -780 7Position of calcitic and dolomitic contact varies i consequently a different value is given for the Sp1cewood .Cieek section. The Ellenburger Group of Central Texas Thickness in feet Inter· Description val 9. Dolomite-medium grained, dark gray, one bed. 2 Quartz druse abundant, and some porcelaneous highly dolomoldic chert is also present. 10. Dolomite-medium grained, beige to dark gray, beds about 4 1 foot thick. Chert porcelaneous, white, occurring as irregular nodules and masses. 11. Dolomite-fine grained, dark gray to brown, 6-inch beds. 2 Chert porcelaneous, white, occurring as irregular nodules and masses. 12. Dolomite-medium grained, beige to yellow beige and some 8 nutria, beds 6 to 12 inches thick. Quartz druse as a network is associated with highly dolomoldic, porcelaneous chert, very abundant. 13. Dolomite-medium grained, nutria at bottom ranging to 11 beige at top, beds about 1 foot thick. Chert porcelaneous, white, somewhat dolomoldic, occur­ring mostly as small, angular fragments, sparse. H. Dolomite-fine grained, beige with pink mottles, one bed. 1 15. Dolomite-medium grained, nutria, beds about 6 inches 5 thick, not well exposed in line of section. Chert at 740 feet subchalcedonic to semiporcelaneous, light gray, occurring as small irregular nodules. A small amount of quartz druse is present. 16. Covered-probably medium grained dolomite. 3 17. Dolomite-medium grained, beige to medium brown, beds 2 about 1 foot thick. A chert network, highly dolomoldic, occurs as a few irreg­ular masses. 12. Dolomite-medium to coarse grained, medium gray to rose, 6 beds 1 to 2 feet thick. Chert porcelaneous, white, dolomoldic, occurring as irreg­ular masses. 19. Dolomite-medium grained, medium gray, beds 1 to 4 feet 29 thick. Quartz druse is abundant as an irregularly distributed net· work, and a minor amount of porcelaneous, white, dolo­moldic chert is present. 20. Dolomite-medium grained, beige to medium gray and light 8 gray, beds 1 to 2 feet thick. Chert and quartz druse are about equally common. Chert porcelaneous, white, dolomoldic. 21. Dolomite-fine grained, beige to dark gray. 2 Chert porcelaneous, white, occurring as ellipsoids up to 2 inches thick. 22. Dolomite-medium grained, dark gray, beds about 2 feet 7 thick. Quartz druse is abundant as a network and along bed­ding planes. In top foot chert is porcelaneous, white, dolo­moldic, and layered. 23. Dolomite-medium grained, light brown, beds 1 to 2 feet 13 thick. Chert in bottom foot porcelaneous, white, dolomoldic to oomoldic and oolitic, up to a 6-inch thick layer. Quartz druse m~sses are irregularly distributed throughout rest of interval, and some highly dolomoldic, porcelaneous chert is also present. Cumu- Feet above lative base 69 766 -768 73 762 -766 75 760 -762 83 752 -760 94 741 -752 95 740 -741 100 735 -740 103 732 -735 105 730 -732 111 724· -730 140 • 695 -724 148 687 -695 150 685 -687 157 678 -685 170 665 -678 Thickness in feet Inter-Cumu­val lative Description 24.. Dolomite-medium grained, beige to medium brown, in beds 2 to 3 feet thick. Quartz druse very abundant along bedding planes and as network masses, and some porous, porcelaneous, dolo­moldic network chert IS associated with the quartz druse. 25. Dolomite-medium grained, dark gray, beds about 2 feet thick. Quartz druse, a small amount, and a few nodules of porcelancous white chert are present. 26. Chert-porcelaneous, lamellar, oolitic. Shift along this distinctive chert bed southward up ravines and around spurs to a point about 3000 feet distant in a direction about S. 10° E. and continue section down hill in a direction about S. 60° W. 27. Dolomite-fine grained, dailc gray to beige, beds 4, to 6 inches thick. Chert varied with some porcelaneous, white, in part dolomoldic plates, and some highly masses. Quartz druse occurs from 637 28. Dolomite-medium grained, medium to part with purplish splotches, beds about Some quartz druse is present. 29. Dolomite-fine grained, dark gray with porous, dolomoldic to 638 feet. dark gray and in I foot thick. purplish splotches. Chert porcelaneous, white, dolomoldic, occurring as nodules and plates. 30. Dolomite-medium grained, dark gray, beds 6 to 12 inches thick. Quartz druse is present in bottom foot. 31. Dolomite-fine grained, dark gray, beds about 1 foot thick. Quartz druse abundant as a network. Chert in top foot dolomoldic and lamellar with occasional plates of slightly oolitic, white porcelaneous chert. 32. Dolomite-fine grained, dark gray to beige, beds 6 to 12 inches thick. Quartz druse present from 611 to 615 feet, with some indication of its presence in the rest of the interval. Chert in top 2 to 3 feet porcelaneous, somewhat quartz­ose and oolitic, occurring as flat plates, suggesting lamellar structure_ 33. Dolomite-medium grained, medium gray to beige, about 1 foot thick, weathers pitted. Quartz druse from 599 to 600 and 605 to 611 feet. 31. Dolomite-fine to medium grained, dark gray, 6-inch 35. Dolomite-fine to medium grained, beige, one bed. beds beds. Chert porcelaneous and dolomoldic associated with abun­dant quartz druse. 36. Dolomite-fine grained, dark gray, 6-inch beds. 37. Dolomite-medium grained, medium gray, beds 1 to 2 feet thick. Chert porcelaneous and dolomoldic associated with con­siderable quartz druse. 38. Dolomite-fine grained, beige, beds 6 to 12 inches thick. 39. Dolomite-medium grained, medium to dark gray, one bed. Chert porcelaneous and dolomoldic associated with consid­erable quartz druse. 40. Dolomite-fine grained, beige, one bed. 41. Dolomite-medium grained, medium to dark gray, one bed. Chert porcelaneous and dolomoldic associated with consid­erable quartz druse. Feet above base 11 14 2 4 1 4 5 9 12 2 3 1 5 2 2 1 1 181 195 197 201 205 206 210 215 224 236 238 24.1 242 247 249 251 252 253 654 640 638 634 630 629 625 620 61I 599 597 594 593 588 586 584 583 582 - - - - ---- - - -- - - -- 665 654 640 638 634 630 629 625 620 611 599 597 594, 593 588 586 584 583 The Ellenburger Group of Central Texas Thickness in feet Inter- Cumu- Feet above Description val lative base 42. Dolomite-microgranular to fine grained, medium gray with 4 257 578 - 582 very light purplish splotches, beds 6 to 12 inches thick. 43. Dolomite-medium grained, medium gray, one bed. 2 259 576 - 578 Chert porcelaneous and dolomoldic associated with consid­ able quartz druse. 44. Dolomite-medium grained, medium gray, beds about 2 feet 5 264 571 - 576 thick. 45. Dolomite-fine grained, mottled purple or light gray and 3 267 568 - 571 beige, beds 6 to 12 inches thick. 46. Dolomite-medium grained, beige, two beds. 3 270 565 - 568 47. Dolomite-fine grained, beige with purplish splotches, beds 3 273 562 - 565 6 to 12 inches thick. 48. Dolomite-medium grained, beige, one bed. 2 275 560 - 562 Chert porcelaneous, white, dolomoldic, associated with abundant quartz druse. 49. Dolomite-fine grained, purplish gray, one bed. 1 276 559 - 56() 50. Dolomite-medium grained, medium gray, one bed. 2 278 557 - 559 Chert porcelaneous, white, dolomoldic, associated with abundant quartz druse. 51. Dolomite-fine grained, light beige, one bed. 1 279 556 - 557 52. Dolomite-medium grained, beige, beds up to 2 feet thick. 5 284 551 - 556 Chert porcelaneous, white, dolomoldic, associated with abundant quartz druse. 53. Dolomite-fine grained, purplish gray, two beds. 1 285 550 - 551 54. Dolomite-medium grained, beige, one bed. 2 287 548 - 550 Chert porcelaneous, white, dolomoldic, associated with abundant quartz druse. fi5. Dolomite-fine grained, beige, beds 6 inches thick. 2 289 546 - 548 56. Dolomite-medium feet thick. to coarse grained, beige, beds about 2 6 295 540 - 54.() Chert porcelaneous, white, dolomoldic, associated with abundant quartz druse. 57. Dolomite-medium grained, medium gray, beds about 2 feet thick. 6 301 534 - 540 58. Dolomite-fine grained, poorly exposed. beige, beds 2 to 6 inches thick, 7 308 527 - 534 Chert porcelaneous, white, finely dolomoldic, occurring as thin plates along bedding, rare. 59. Dolomite-fine to 12 inches thick. medium grained, pinkish gray, beds 6 to 2 310 525 - 527 Chert porcelaneous, white, dolomoldic, associated with abundant quartz drnse, occurring as a network. 60. Dolomite-coarse grained, medium gray, essentially weathers pitted. one bed, 5 315 520 - 525 61. Dolomite-medium grained, light gray. 13 328 507 - 520 Chert semiporcelaneous, highly dolomoldic, associated with a preponderance of quartz druse forming network masses a foot or more in thickness, irregularly distributed but abundant in bottom 2 feet of interval, and occurring as abundant somewhat more continuous layers from 514 to 520 feet. It is present in the intervening portion of the interval but is not abundant. 62. Dolomite-coarse grained, medium gray, beds 1 to 2 feet 7 335 500 - 507 thick, weathers with a pitted surface. 63. Dolomite-coarse grained, medium gray, one bed, weathers 1 336 499 - 500 with a pitted surface. Chert porcelaneous, white, dolomoldic, occurring as irreg­ ular nodules. Shift southeastward around hillside about .300 feet and con­tinue section southward down slope. Thicknes8 in feet Inter· Cumu­val lative Description Threadgill member: 202 feet thick Upper dolomitic facies: 41 feet thick8 M. Dolomite-coarse grained, medium gray, beds 2 to 12 inches thick. 65. Dolomite--medium grained, medium gray, one bed. 66. Dolomite-coarse grained, light gray, essentially one bed. 67. Dolomite-fine grained, mottled purplish gray to brownish gray, one bed. Shift westward 100 feet along side of gully and continue sec­tion down slope southward to bottom of gully. 68. Dolomite--coarse grained, light gray, beds 1 to 1. feet thick, weathers with a pitted surface. Limestone f acies: 138 feet thick 69. Limestone-sublithographic, very light ivory, abundant net­work and bedding plane, coarse grained dolomite, one or two beds. 70. Dolomite-coarse grained, light gray, one bed. 71. Limestonc-sublithographic, very light ivory, abundant net· work and bedding plane, coarse grained dolomite, essentially one bed. 72. Dolomite--coarse grained, light gray, one bed. Chert in top subchalcedonic, cloudy, light brown, weath· ering dirty white, occurring as irregular masses along bed­ding plane. 73. Limestone--sublithographic, very light ivory, abundant net­work and bedding plane, coarse grained dolomite, beds about 3 feet thick. 74. Dolomite-microgranular, purplish gray to beige, one bed. Laterally to the south this bed changes by degrees to coarse grained dolomite and then to limestone. Chert porcelaneous, white, occurring as nodules, rare. Shift from bottom of gully southward about 150 feet along liillside and continue section southwestward down slope. 75. Limestone-sublithographic, white to very light ivory, beds about 3 feet thick alternating with zones having beds 1 to 3 inches thick. The thin bedded zones are about 3 feet thick in lower part of interval becoming thicker upward, with top 13 feet having no beds over l foot thick and mostly ranging between 1 to 6 inches in thickness. Coarse grained dolomite as a network and along bedding planes common throughout interval. Chert minutely quartzose and smoky gray, sparingly present at a few places in interval. None seen is persistent for an appreciable distance along the strike. Fossils as cross sections on bedding surfaces are com­mon. No collections were made hut fossils can be broken out of the limestone. 76. Limestone--sublithographic, white, one bed, contains net­work of coarse grained dolomite. Chert minutely quaitzose, dark smoky gray, occurring as irregular pieces up to I by 6 inches in size, rare. 77. Limestone-sublithographic, white, beds 2 to 4 inches thick. 78. Limestone-sublithographic, white, beds up to 2 feet thick. Beekite excrescences ~ inch in size are present but scarce. 79. Limestone-sublithographic, white, beds up to 5 feet thick with a zone from 333 to 334 feet composed of 1-to 2-inch beds. Coarse grained dolomite is present as a discontinuous network and in places along bedding. Fossils are round·headed trilobites at 335 feet. Shift along bedding plane southward for 75 feet and continue down slope to Cambrian boundary. Note laterally gradational boundary between the limestone and dolomite. Feet above base 18 1 4 1 17 8 5 8 2 13 2 55 6 3 4 32 354 355 359 360 377 385 390 398 400 413 415 470 476 479 483 515 481 480 476 475 458 450 445 437 435 422 420 365 359 356 352 320 - --- -- - -- -- - - -- - 499 421 480 476 475 458 450 445 437 435 422 420 365 359 356 352 8In both directions from the line of section the entire Threadgill member is dolomite. See Plate 8. The Ellenburger Group of Central Texas Thickness in feet Inter· Cumu-Description val lative Lower dolomitic f acies: 23 feet thick 80. Dolomite-coarse grained, light gray with red, purple and 23 . 538 yellow weathering stains, beds 1 to 4 feet thick, weathers with a pitted surface. Chert quartzose and black, occurs sparingly in bottom foot of interval. Shift along Cambrian-Ordovician contact near top of hluff about 700 feet to the southeast and continue section S. 30° W. using a 6° dip in line of section. Actual dip is about 9° to the north. Upper Cambrian Wilberns formation: 297 feet measured to bottom of section Pedernales dolomite member: 277 feet thick 81. Dolomite--~fine to very fine grained, pinkish gray, platy, 10 548 in beds 2 to 6 inches thick. 82. Dolomite-fine to very fine grained, mottled dark gray to 17 565 light brown and nutria, beds 4 to 12 inches thick. Chert semiporcelaneous, minutely quartzose, dolomoldic to compact, light brown weathering dirty white, abundant as network aggregates. 83. Dolomite-fine to very fine grained, light brown to dark 9 574 gray and nutria, beds 6 to 12 inches thick. Chert very scarce. 84. Dolomite-fine to very fine grained, nutria to light brown 45 619 and dark gray and from 250 to 261 feet mostly beige, beds 6 inches to 4, feet thick. Chert at 216 to 245 feet common and at 245 to 261 feet abundant as qua1tzose, highly porous, dirty white lay­ ers along the bedding. Chert oolitic, semiporcelaneous, light brown and 6 inches thick is present at 227 feet. Conical fossils on bedding surface at 223 feet. 85. Chert-semiporcelaneous, white, faintly micro-oolitic, some-2 621 what dolomoldic, occurs as a rather continuous layer. 86. Dolomite-fine to very fine grained, light brown to dark 14 635 gray, somewhat mottled, beds 6 inches to 2 feet thick. Chert quartzose, highly porous, dirty white, very abun­ dant as layers along the bedding. Shift along top of coarse grained dolomite along bluff about 500 feet and continue section down spur to the south. 87. Dolomite-coarse grained, light gray, seldom with mottles 75 710 having a very light purplish tint, beds 1 to 4 feet thick weathers pitted. ' Chert at 139 feet to 155 feet common; at 155 to 175 feet. scattered and in smaller pieces, chalky textured to sem1porcelaneous and minutely quartzose, in part chalcedonic. 88. Dolomite-coarse grained, mottled medium to dark gray 11 721 and beige, beds about 2 feet thick, weathers pitted. Chert chalky textured to semiporcelaneous and minutely quartzose, in part chalcedonic, occurring as irregular finger­ like to flattened nodules. 89. Dolomite-medium grained, beige with some medium gray 3 724 mottles. Chert at 111 to 113 feet semiporcelaneous, minutely quartzose, slightly dolomoldic, occurring as irregular %-inch sized pieces. 90. Dolomite-very fine to fine grained, mostly purplish with 19 743 dark gray and beige mottlings, beds 6 to 12 inches thick, slightly pitted on weathered surfaces. Feet above base 297 -320 287 -297 270 -287 261 -270 216 -261 214. -216 200 -214 125 -200 111. -125 111 -114 92 -111 250 The University of Texas Publication No. 4621 Thickness in feet Inter-Cumu-Feet above Description val lative base Shift across drain and along hillside about 400 feet to the south. This shift is unsatisfactory. The chert does not hold a constant level and nothing could be traced without question. The dip of the rocks in this area averages 10° and this value was used along the flood plain to help establish the tie point. A median point was chosen between the points obtained by lateral tracing and by instrument. The error in this shift is probably within 10 feet. 91. Dolomite-microgranular to fine grained, mottled and 32 775 60 -92 streaked mostly light purplish and light olive green to beige, beds irregular and about 1 to 2 feet thick. Chert porcelaneous to semiporcelaneous, in part quartzose, highly dolomoldic, dirty white, and occurring as network excrescences on weathered surface which cannot be seen on freshly broken surfaces. In upper part much of the chert is in curved layers as if deposited with or concen­ trated along growth surfaces of stromatolites. Fossils are silicified Scaevogyra seen at 78 and 81 feet. 92. Dolomite-microgranular to very fine grained, somewhat 15 790 45 -60 mottled beige and medium gray with some yellowish and purplish streaks and mottles, beds 1 to 2 feet thick. 93. Dolomite--microgranular to very fine grained; mottled 15 805 30 -45 medium gray to dark gray, beige and purplish gray; beds I to 4 feet thick, weathers pitted. Some calcite is present in the rocks as bodies about as large as the surface pits. 94. Dolomite--microgranular to very fine grained, medium to 10 815 20 -30 dark gray with some yellow and purple streaks; beds in bottom 7 feet, 6 inches to 2 feet thick; in top 3 feet, 2 to 4 inches thick; weathers blocky and smooth. San Saba limestone member: 20 feet measured to bottom of section 95. Limestone somewhat dolomitic-mottled light to medium 20 835 0 -20 and dark gray, sparingly dolomitic at bottom becoming pro­gressively more dolomitic upward until only the girva­nellas remain limestone. The dolomitic portion of the rock forms a raised network on weathered surfaces. The bottom 9 feet is essentially one bed as is also the portion of the interval from 11 to 17 feet. From 9 to 11 feet 1-to 2-inch beds and from 17 to 20 feet 4-to 18-inch beds. On gentler slopes the bedding joints are more closely spaced, forming parallel sided slabs. This section was measured when the water in Lake Buchanan was low. When the lake is full a portion of the bottom part of the section as measured will be under water. The bottom of the Tanyard section is about 6000 feet south of Tanyard Spring and about 2800 feet downstream from Wolf Spring. DESCRIPTION OF SUPPLEMENTARY SEC· TIONS IN THE T ANYARD AREA An attempt was made to estimate the interval between the base of the Tan­yard section and the alate-Billingsella bed. The following section is described from the v1c1mty of a small tributary of Jim John Creek about 2200 feet due west of the mouth of Jim John Creek (Cedar Hollow). The top of the section is about 1500 feet north of Jim John Creek and 400 feet east of the tributary. Thiekness in feet Inter-Cumu-Feet above Description val lative base L Dolomite-very fine grained to microg;ranular, poorly ex-14 14 101 -115 posed. Chert of the type described in Tanyard section, interval 91, very abundant. Fossils are Scaevogyra sweezeyi Whitfield, S. elevata Whit­ field, Sinuopea sp., and Proplina or Archinacella. Silicified girvanellas are abundant (205T-l-11A). 2. Dolomite-very fine grained to microgranular, beds range 42 56 59 -101 up to 2 feet thick. 3. Dolomite~limestone transition. Ints. 1---3 = Pedernales dol. 16 72 43 -59 Shift from the upper boundary of an inlier of San Saha lime­stone downstream and continue section southward. Shifting on the boundary between limestone and dolomite is not satisfac­tory since this boundary fluctuates. The amount of error in this shift cannot be estimated on the ground but it appears by comparison with another section that about 16 feet of beds was not measured. 4. Limestone-mottled, medium gray with yellow and brown 28.5 100.5 14.5-43 wavy streaks, beds 6 to 18 inches thick. San Saba ls. 5. Shale-with thin beds of fine grained, medium gray, dolo-10 ll0.5 4.5-14.5 mitic limestone. 6. Limestone-oolitic, brown, g;lauconitic, one bed. 3 113.5 1.5-4.5 Fossils are alate Billingsella on the top surface of the bed. 7. Limestone-oolitic, white, contains pebbles. The next bed 1.5 115 0-1.5 beneath contains Plectotrophia. Ints. 5-7 =Point Peak sh. One thousand feet northwest of the mouth of Jim John Creek, 68 feet of sec­tion was measured between the alate­Billingsella bed and the bottom of the Pedernales dolomite. The base of the dolomite in this section is roughly 29 feet higher than it is 1700 feet to the west-southwest. About 1000 feet west of Lake Buchanan and about 4200 feet north-northeast of Tanyard Spring the base of the dolomite is 57 feet above the alate-Billingsella bed and the first chert is 47 feet up in the dolomite. The latter figure is apt to be in error since the dips of the rock and the slope of the surface of the ground are both gentle. The best interval measured is located about 6000 feet north-northeast of Tan­yard Spring, along an east-west fence about 200 feet east of the lake. The alate-Billingsella was not found but the same sequence of beds as measured near Jim John Creek was present with Plecto­trophia just below a 1.5-foot white peb­bly oolitc, above which was a 3-foot brown oolite, the top of which should con­tain the alate' Billingsella. The interval be­tween the postulated alatc-Billingsella bed and the first chert in the Pedernales dol­omite member of the Wilberns formation is 97 feet. The contact between the San Saba limestone and the Pedernales dolo­mite is in a covered zone about 25 feet above the postulated alate-Billingsella bed. Using this section for comparison on the assumption that it is the best and that the chert holds a constant horizon, then there should be 37 feet of beds beneath the Tanyard section to the alate-Billings­ella bed. These measurements show a marked though not unusual fluctuation ( 43 feet) of the boundary between the San Saba limestone and the Pedernales dolomite memhers of the Wilberns formation in a relatively short distance. THE MOORE HOLLOW AND WARREN SPRINGS AREAS, RILEY MOUN­ TAINS, LLANO COUNTY INTRODUCTORY STATEMENT The Moore Hollow and Warren Springs areas lie on the south and north limbs, respectively, of the faulted Honey Creek basin in southeastern Llano County, 10 to 14 miles airline southeast to south­southeast of the town of Llano. Both are reached via the Llano-Round Mountain road, on the turnoff to Click, and the south edge of the Moore Hollow area is only a little north of Click. Altogether about 7 square miles was mapped geolog­ically between the latitudes of 30° 34' to 30° 39' north and longitude of 98° 32' to 98° 34'. Plate 10 shows the geology of the areas involved at a scale of I :10,000, and figure B of Plate 33 is a stereogram, at the approximate scale of 1 :20,000, of part of the Moore Hollow area. Control for the base maps of the Moore Hollow and Warren Springs areas was taken from tape and Brunton compass traverses along the Llano-Click road by Cloud and local residents. The base was plotted and adjusted by L. E. Warren from data recorded by Cloud on aerial photographs of the U.S. Department of Agriculture. On its east side the Honey Creek basin is dropped by the Riley Mountain fault against the pre-Cambrian Packsad­dle schist, which underlies the eastern half of the Moore Hollow and Warren Springs areas. In fact, the presence of Ellenburger rocks in this part of the Llano uplift is due to the fact that the Honey Creek basin is structurally the deepest part of the region, the Pennsyl­vanian Smithwick shale being faulted against the Packsaddle schist at the mid­dle of the basin. Due to subsidiary east-west faulting associated with the structural adjustment of the basin, it was not possible to ob­tain a complete section of Ellenburger rocks on either limb of the Honey Creek basin. Strata below the Gorman forma­tion were therefore measured and de­scribed from the Moore Hollow area, whereas the Gorman and Honeycut for­mations were measured and described from the Warren Springs area. Rocks mapped and described include a complete sequence of the rocks known to occur be­tween the pre-Cambrian and the Penn­sylvanian of the Llano uplift, except for Devonian strata and the basal Missis­sippian Ives breccia. Unusually steep dips for the Llano region combine with topog­raphy to make possible the observation of a thick sequence of strata in a rela­tively short horizontal distance in the sections here described. With local exceptions, and disregard­ing artificial clearings, the Riley Moun­tains are thickly wooded throughout. Cedar is the dominant type of tree and its growth seems to be more closely re­lated to topography than to geology, be­ing very dense on north slopes and more scattered on south slopes of hills. Vege­tational banding is prominent and useful in the Cambrian and Carboniferous rocks, and in spite of control being primarily topographic the dolomites of the Tan­yard formation are relatively sparsely wooded. Dendrologic patterns, however, are less helpful to mapping in the out­crop area of the Ellenburger group of the Riley Mountains than in areas of more gentle topography. The Warren Springs, Moore Hollow, and East Canyon sections, comprising the composite Riley Mountain section, are 1e­scribed on pages 260-287 and diagram­matically represented on Plate 14. Table 3 presents a summary of physical and chemical data for samples from the War­ren Springs and Moore Hollow sections, prepared by Barnes from data furnished by Dr. S. S. Goldich and Mr. E. B. Par­melee of the Argicultural and Mechani­cal College of Texas. ,On balancing accessibility, complete­ness, compactness, percentage of exposure, and "normality" of expression, the com­posite Riley Mountain section is seen to include the best section of Ellenburger rocks now known or likely to be dis­covered. Adding the work of Goldich and Parmelee to that of the present authors, it is also the most intensively studied. For these reasons the Lower Ordovician part of the combined Moore Hollow and War· ren Springs sections is here designated the standard section for the Ellenburger group. The composite Tanyard-Gorman sect10n (pp. 230-249), adjoining the Ellenburger Hills 25 miles north-north­east from here, is a preferable type sec­tion, but its widespread and relative in­accessibility make it unsuitable as a standard. The Ellenburger Group of Central Texas UPPER CAMBRIAN RILEY FORMATION In upward succession the three mem­bers of the Riley formation are the Hick­ory sandst9ne, the Cap Mountain lime­stone, and the Lion Mountain sandstone members. They are easily mapped in the Riley Mountains because of their charac­teristic topographic and photographic ex­pression (Pl. 33, fig. B). The total thick­ness of the Riley formation in the East Canyon section, which may be taken as representative of the type area, was com­puted as 780 feet. Hickory sandstone member.-The Hick­ory sandstone member rests nonconform­ably on the varied but mostly dark and schistose metasedimentary rocks of the pre-Cambrian Packsaddle schist. At or near its base through most of the Moore Hoilow area is a conspicuous breccia bed a foot to 30 inches thick and com­prised of subangular to angular pheno­cl asts of white vein-quartz in a sandstone matrix. Upward it consists predominantly of noncalcareous, nonglauconitic, largely cross-bedded sandstones wherein the grains are an?ular to subrounded, poorly sorted, and display a wide range in size. Its thickness varies but is about 335 feet in the East Canyon section. Fossils were not observed in the Hickory sandstone of the Moore Hollow area. For the most part the Hickory sand­stone me~ber i~ _topographically and dendrolog1ca1ly d1stmct. It rises in low bluffs above the Packsaddle schist and is separat~d ~rom the bluffs of the Cap Mountam limestone member above by a be~ch on the dip-slope of its upper beds. This bench is the site of residual and colluvial accumulations and is so promi­nen~ that it was once locally cleared and cultivated to form a landmark known as "halfmoon field" (PL 33, fig. B, south­west corner) . The Hickory sandstone supports a growth of deciduous oaks ~hcreas mesquite and bee-brush are prom'. ment on the Packsaddle schist below and cedar on the Cap Mountain limestone above. Clumps of live oak are a feature of the bench at the top of the Hickory sandstone. Cap Mountain limestone member.-The Cap Mountain limestone member is are­naceous in the lower part and slightly so in the upper part, grading to the Hick· ory sandstone member below and the Lion Mountain sandstone above, but topographically and dendrologically dis­tinct from both. Characteristically it is a granular, brownish gray to grayish brown or brown, markedly glauconitic limestone. It is about 420 feet thick in the East Canyon section. Trilobites and acrotretid hrachiopods were noted locally, but none were collected. The Cap Mountain limestone member expresses itself as a cedar-covered, south· facing scarp between the broad flat at the Lop of the Hickory sandstone and the narrow but conspicuous flat underlain by the Lion Mountain sandstone member. In the Moore Hollow area the lithologic boundary between the Cap Mountain lime· stone and Lion Mountain sandstone mem­tiers of the Riley formation was placed at the top of the highest of several prominent. limesto~e beds containing many wh1te-weathermg Lingulella. This boundary splits the Aphelaspis zone and includes the lower sparsely'vegetated part of the "Lion Mountain bench" in the Cap Mountain limestone. Lion Mountain sandstone member.-At the top of the Riley formation is the highly glauconitic and locally calcareous Lion Mountain sandstone member. Weath­ering of this 24,-foot sandstone has pro· duced a narrow but prominent bench at ~he t?p of the Riley formation, separat· mg it topographically from the scarp­forming_ W elge sandstone and Morgan Creek limestone members of the Wilberns formation above. Lenses of granular limestone that weather out of the Lion Mountain sandstone are crowded with A phelaspis and other trilobites. The lower part of the Aphclaspis zone, however, and the more sparsely vegetated part of the "~ion Mountain bench" were mapped w1th the Cap Mountain limestone in the Moore Hollow area. W1LBERNS FORMATION All five named members of the Wil­berns formation are represented in the Moore Hollow area, and the total thick­ness of the formation in the East Canyon and Moore Hollow sections is about 560 feet. Welge sandstone member.-The basal member of the Wilberns formation is a sparingly glauconitic, brown to brownish yellow sandstone about 17 feet thick that forms a low scarp adjacent to the Lion Mountain flat and grades upward into the Morgan Creek limestone member. A single specimen of Lingulepis was found in the W elge sandstone of the Moore Hollow area by Dr. Leo Hendricks (TF-414). Morgan Creek limestone member.-The Morgan Creek limestone member consists of granular, ordinarily glauconitic lime­stone that is pink to reddish brown at the base and greenish gray to gray above. The basal few feet are arenaceous, grading to the Welge sandstone below, and the manner of weathering of the beds above the Eoorthis zone suggests local shaly partings in the upper half of the member. In the East Canyon section the Morgan Creek limestone is 110 feet thick, with Eoorthis texana Walcott 48 feet above its base. Pterocephalia, Elt>inia, and other trilobites are abundant imme­diately below Eoorthis, and Billingsella is very abundant above it, in both cal­citic and silicic preservation. Huenella occurs through 50 feet of beds above Eoorthis, in calcitic preservation. In the western part of its outcrop in the Moore Hollow area the Morgan Creek limestone member is wholly within a prominent, south-facing scarp separated from that of the Cap Mountain limestone member of the Riley formation below hv the Lion Mountain flat and distinct fro~ it in being steeper and in having a thick growth of black persimmon interspersed with the cedar. In the eastern part of its outcrop, however, the Eoorthis zone is at the crest of the scarp, with the upper beds of the Morgan Creek limestone coming in on the steep north-facing dip-slope that forms the south slope of Moore Hollow. Point Peak shale member.-The Point Peak shale member, 216 feet thick in the Moore Hollow area, lies above the Mor­ gan Creek limestone member. It consists of green shale with interbedded granular glauconitic limestone in the lower half and mostly sublithographic, stromatolitic limestones with local shale partings in the upper half. On the north limb of the Honey Creek basin, a little over 4 miles north of Moore Hollow and ahout l.3 miles north of Warren Springs, the Point Peak shale member consists mostly of green shales with interbedded granu­lar limestones, and the stromatolitic limestones are reduced to local thin beds. Although this difference in facies was recognized from the beginning of work in the area, and the upper 115 feet of mostly stromatolitic limestones in the Moore Hollow area were actually mapped as a separate unit, they are included with the San Saba limestone member on Plate 10. This resulted from an application of member lithologies that is defensible on some grounds but which obscures the actual stratigraphic associations of the zones of stromatolitic bioherms with the ,upper Point Peak shale in many parts of the Llano region. A restudy of the problem of the bioherm associations by , the authors in January 1946, after the geologic map had gone to the engravers, resulted in a decision to include the bio­hermal or biostromal limestones at this position with the Point Peak shale member, except where they are mapped as a separate unit or can be shown to go laterally into limestones like those of the San Saba limestone member. This interpretation for the Moore Hollow area is shown on Plate 33, figure B. The Point Peak shale member is sep­arated from the Morgan Creek limestone member below on the basis of the first appearance of the crinkly·surfaced, platy, green, micaceous limestones characteris­tically associated with the more shaly zones of the Point Peak member; and the same type of crinkly-surfaced limestones occur in its uppermost shaly zone be­tween the biohermal limestones at its top and the San Saba limestone member. The lower shaly part of the Point Peak shale member is mostly on the south slope of Moore Hollow and is more thickly wooded than the Morgan Creek The Ellenburger Group of Central Texas limestone below; whereas the upper stro· matolitic limestones are on the north slope of Moore Hollow and are sparsely cedared or almost barren, in contrast to the thicker growth of cedar on the San Saba limestone above. Characteristic fossils of the Point Peak shale member are species of silicified Billingsella, Plectotrophia, and occasional trilobites. Plectotrophia, which occurs in association with a characteristic large alate-Billingsella about 30 feet below the top of the normal facies of the upper Point Peak shale in the north limb of tpe Honey Creek basin, probably occurs 20 to 25 feet below the top of the stro­matolitic facies in the south limb (p. 285). San Saba limestone member.-In the Moore Hollow area the San Saba lime­stone member is sublithographic to medium grained, being generally less granular than in other parts of the Llano region. It is brownish to greenish gray, grading to pearl gray, with yellowish to buff mottles and streaks. Thin oolite beds occur locally near the base and top of the member, but glauconite is scarce except in the upper part. The San Saba limestone weathers to a darker tone of gray and is darker on a fresh surface than the Ellenburger limestones above it. It grades laterally and vertically to the Pedernales dolomite member, with con­sequent variation in thickness from the 144 feet measured in the Moore Hollow section. Locally it is difficult to be sure whether apparent lateral transition between the Pedernales dolomite and San Saba lime­ stone members is