s 
 
 45 .EG: 
 BUL 24 
 
 THE GEOLOGY 
 
 OF THE 
 
 SHAETEE SILYEE MINE DISTEICT, 
 
 PRESIDIO COUNTY, TEXAS. 
 
 J. A. TTDDEN, Ph. I>.. 
 
 Professor of Geology, Augustana College, Rock Island, 111.; 
 Special Assistant Iowa Geological Survey. 
 
 BULLETIN OK THE UNIVERSITY OF TEXAS NO. 24. 
 
 - 
 
 
 Entered in the postoffice at Austin, Texas, as second-class mail matter , 
 
 AUSTIN: 
 
 VON BOECKMANN-JONE8 COMPANY, STATE PRINTERS, 
 1904 . 
 
 I 
 
CORRECTION FOR TERLINGUA TOPOGRAPHIC SHEET. 
 
 The words “True North” and “ Magnetic North” on the diagram 
 in the lower left margin of sheet have been reversed by mistake. 
 Correct same by pasting thereon the diagram shown on this slip or 
 by transposing the words “true” and “magnetic.” 
 
 si* 
 
 APPROXIMATE MEAN 
 DECLINATION 1902 
 
J 
 
 299-104-6m. 
 
 THE UNIVERSITY OF TEXAS MINERAL SURVEY. 
 
 BUI^ETIN NO. 8-JUNK, 1904. 
 
 THE GEOLOGY 
 
 OF THE 
 
 SHAETER SILVER MINE DISTRICT, 
 
 PRESIDIO COUNTY, TEXAS. 
 
 BY 
 
 J. A. UDDEN, Ph. D., 
 
 Professor of Geology, Augustana College, Bock Island, 111. ; 
 Special Assistant Iowa Geological Survey. 
 
 BULLETIN OF THE UNIVERSITY OF TEXAS NO. 24. 
 
 Entered in the postoffice at Austin, Texas, as second-class mail matter. 
 
 AUSTIN: 
 
 VON BOECKMANN- JONES COMPANY, STATE PRINTERS, 
 1904. 
 
Digitized by the Internet Archive 
 in 2017 with funding from 
 
 University of Illinois Urbana-Champaign Alternates 
 
 https://archive.org/details/geologyofshafterOOudde 
 
~T~ 2> i ^ 
 
 . ? 
 
 LETTER OF TRANSMITTAL. 
 
 Hon. Wm. L. Prather , President , The University of Texas. 
 
 Dear Sir : I beg herewith to transmit to yon a report on the Geology 
 of the Shafter Silver Mine District, Presidio county, Texas, prepared 
 by Prof. J. A. Udden, Augustana College, Rock Island, 111. Professor 
 Udden spent several months in the field for us last year and the result 
 of his work appears in this Bulletin. 
 
 At Shafter, Presidio county, is situated the only successful silver mine 
 in the State. It has been in continuous operation for nearly twenty years 
 and has yielded to its owners very profitable returns. 
 
 It was thought that a special report on this district would be of value 
 to the further development of the public school lands in that part of the 
 Sate. 
 
 Very respectfully, 
 
 William B. Phillips, 
 
 June, 1904. Director. 
 
ANNOUNCEMENT. 
 
 The University of Texas Mineral Survey is prepared to undertake all 
 kinds of chemical analyses, assays, investigations of technical processes, 
 etc. It has a well-equipped laboratory, furnished with modern scientific 
 appliances, and can make returns promptly. Prices on application. 
 
 Address all communications to 
 
 Dr. Wm. B. Phillips, 
 
 Director, University of Texas Mineral Survey, 
 Austin, Texas. 
 
CONTENTS 
 
 Page. 
 
 Location and limitation of the District 7 
 
 Topography 7 
 
 The Chinati mountains 7 
 
 The uplands of Cibolo and Cienega creeks 8 
 
 The gravel flats of the Rio Grande 9 
 
 Drainage 9 
 
 Stratigraphy. ' 9 
 
 The sedimentary rocks 11 
 
 The Chinati series 11 
 
 The Cieneguita beds 13 
 
 The Alta beds 15 
 
 The dark shales 15 
 
 The yellow sand # 18 
 
 The Cibolo limestones % 18 
 
 The transition beds 18 
 
 The lower brecciated zone 20 
 
 The zone of sponge spicules 21 
 
 The thin-bedded zone — 21 
 
 The yellow limestone 22 
 
 Geographic conditions 23 
 
 Correlations . 23 
 
 The Cretaceous rocks 25 
 
 The Presidio beds 25 
 
 Sections 27 to 30 
 
 General character 29 
 
 Fossils 30 
 
 Correlation 30 
 
 The Shatter beds 30 
 
 Exposures 31-38 
 
 Fossils of the Shatter beds 37 
 
 Correlations 38 
 
 Geographical conditions 38 
 
 The Edwards limestone 39 
 
 The Del Rio clay [?] 39 
 
 The Buda limestone [?] 40 
 
 The land drift 40 
 
 The Rio Grande drift 41 
 
 The igneous rocks 42 
 
 Laccolitic bosses 42 
 
 Dikes and intrusive sheets. . . 43 
 
 Lavas and tuffs .. 43 
 
 Structures 44 
 
 Chinati uplift 44 
 
 The Morita uplift 46 
 
 Dips 46 
 
6 
 
 Contents 
 
 Page. 
 
 Tangential faults 48 
 
 Radiating faults , 50 
 
 Secondary changes in some rocks 52 
 
 Igneous metamorphism 52 
 
 Changes effected by ground moisture 52 
 
 Mineral deposits 54 
 
 Lodes 54 
 
 Chimneys 54 
 
 Contacts 54 
 
 Fissures 55 
 
 Fault-fissures 57 
 
 Fissured zones 59 
 
 Summary of geological events 59 
 
 Acknowledgments. 60 
 
GEOLOGY OF THE SHAFTER SILVER MINE DISTRICT, 
 PRESIDIO COUNTY, TEXAS. 
 
 BY J. A. UDDEN, PH. D. 
 
 LOCATION AND LIMITATION OF THE DISTRICT. 
 
 For purposes of discussion in the present paper we will designate 
 as the Shatter Mine District an area in Presidio county about twenty 
 miles square, with the silver mine at Shatter near the centre. Thus 
 defined, this district includes the greater part of the Chinati mountains 
 to the northwest, it reaches out almost across the flats of the Rio Grande 
 to the southwest, and extends to the Cienega mountain on the east. 
 Geologically it may be said to approximate a unit, the Chinati uplift 
 being the dominant feature which controls its topography as well as its 
 geological structure. The district is 40 to 50 miles south a little west 
 from Marfa, a station on the Southern Pacific Railway, 196 miles 
 southeast of El Paso. 
 
 TOPOGRAPHY. 
 
 The average altitude of the Shatter Mine district is about 4500 feet 
 above sea level. The highest point on the Chinati mountains rises to 
 7730 feet and the elevation of the nearest point on the Rio Grande is 
 about 2575 feet. Thus the extreme amount of relief in the district 
 is nearly 5155 feet. 
 
 Topographically the area may be roughly divided into three parts: 
 the Chinati mountains, the uplands of Cibolo and Cienega creeks, and 
 the flats of the Rio Grande. 
 
 The Chinati mountains occupy an area which is elliptical in outline, 
 with the long axis extending from northwest to southeast, a distance of 
 about fourteen miles. They continue beyond the boundaries of the dis- 
 trict by an extension to the north of some lower peaks and ridges. The 
 north end of the main mountain is highest and is separated from the 
 south end by a sag in the crest caused by an eastward encroachment of 
 the San Antonio Canon. This part of the mountain forms a dissected 
 dome about five miles in diameter. The south end has the greater areal 
 extent, being about eight miles long and five miles wide. It is, strictly 
 speaking, a terraced and dissected mesa, which rises about 2500 feet 
 above the surrounding country. It is built up of several superimposed 
 flows of igneous rock, and all around the mountain these form suc- 
 cessive steps of vertical cliffs separated by intervening talus slopes. 
 The relief of the mesa is still more diversified by incisions of canons 
 from all directions. On the north side these canons are narrow and 
 short, seldom extending more than a mile back from the periphery. On 
 the south side the drainage comes down chiefly through three wide 
 canons, which might rather be described as irregular amphitheaters. 
 These reach back to nearly three-fourths of the distance across the 
 mountain. Several circumstances have contributed to the greater 
 efficiency of the erosion on the south side as compared with that on the 
 
8 
 
 Geology of the Shafter 
 
 north. Draining directly into the Rio Grande, the streams on this side 
 have a much steeper gradient than those on the north side, for the lat- 
 ter join the Cibolo creek at an elevation of about 1500 feet above the 
 river. The disintegration of the south side of the mesa is likewise more 
 rapid, owing to greater extremes of changing temperature on the sunny 
 side. Probably also the rainfall of this region is slightly greater on 
 that side of the mountain. The largest of these canons, or amphithea- 
 ters, is more than 1500 feet deep and is locally known as the Black 
 Canon. The top of the east part of the Chinati mountains is crowned 
 by some small peaks, which are - the last remnants of a lava flow which, 
 like the rest, was formerly continuous across the entire mesa and far 
 beyond. 
 
 The Uplands of Cibolo and Cienega Creeks. Looking away from the 
 Chinati mountains to the east we find an upland of quite varied relief. 
 A succession of alluvial fans and cones extend out from the base of the 
 mountain in an almost continuous belt of varying width. The greater 
 part of this belt is dissected considerably by the present drainage, but 
 the old slopes are still to be seen more or less distinctly. Beyond this 
 belt to the south and in part to the east lie a range of foothills formed 
 by the most indurated members of the sedimentary rocks of the region. 
 These ridges are highest near the south end of the mountain and are 
 there separated from the latter by a valley not much exceeding a mile 
 in width. Turning to the northeast and then to the north the foot hills 
 flatten out and, at the same time, diverge away from the mountains. 
 Outside of these hills there is a belt of varying width, approximately 
 co-extensive with the remaining outcrops of the sedimentary rocks, 
 where low ridges, frequently crossed by small drainage valleys, run par- 
 allel with the foot hills around the mountain on the south and east side. 
 These ridges have long and low slopes on the outer side, in the direc- 
 tion of the dip, and shorter and more abrupt declivities and even verti- 
 cal scarps on the inner side. Over a part of the area of these sediments 
 the ridge structure is somewhat subdued or even entirely concealed by 
 a covering of stream drift-. This topography in part occupies a triangle 
 whose corners lie, one near Spencer’s spring, another at the Cibolo 
 Ranch, and the third at a point a mile below the junction of the Morita 
 and Cibolo creeks. In part it extends as a belt no more than a mile 
 wide, first to the north from the Cibolo ranch and then to the northwest 
 as far north as the Shatter area extends. The remaining part of this 
 topographic division consists of lava-capped mesas and peaks w T ith inter- 
 vening stretches of flat drift plains. The higher peaks exhibit the same 
 terraced slopes as seen in the Chinati mountains. The highest extend 
 in a line from north to south, east of Shatter, rising 1200 feet above 
 Cibolo creek. The Aguja (needle), which stands on the flat top of a 
 wide mesa between the upper branches of the Cibolo, is the most sym- 
 metrical of all the peaks in this region. Three other features add 
 variety to the topography of these uplands : some small areas of granite 
 to the north weather into rounded irregular knobs and hills of small 
 size; a crescent shaped basin is inclosed by highly tilted sediments along 
 the lower course of Morita creek; and east of this the west half of the 
 low dome of the Cienega mountains rises to a flat summit on the east 
 boundary of the district. 
 
 The Gravel Flats of the Rio Grande. These occupy some twenty 
 
Silver Mine District. 
 
 9 
 
 square miles in the southwest. They have an even surface, almost their 
 only relief consisting of a few valleys, or arroyos, that run in a general 
 direction from northeast to southwest. The surface of the plain rises 
 from 2600 feet above the sea at the southwest, where the Rio Grande 
 is only about a mile distant, to 3500 feet and more on the north, ten 
 miles away from the river. This gives it. a slope of about 90 feet to 
 the mile. The upper reaches of these flats are evidently continuous 
 with the drift plains of the higher country and merge into these imper- 
 ceptibly. 
 
 DRAINAGE. 
 
 The present drainage is consequent to the configuration of an older 
 land. The general courses of the two main streams, the Cibolo and the 
 Cienega, were evidently determined by the surface of the old lava 
 flows which once covered the district. There are indications that the 
 Sierra Alta branch of the Cibolo creek may have taken a part of its 
 drainage area from the Cienega system. Usually the major features of 
 the drainage do not show much adjustment to rock structure. On the 
 surface of the sedimentary rocks the streams are clearly superimposed. 
 Adjustment to the structure of these rocks appears, however, in many 
 minor features, as in the location of the ultimate ramifications of the 
 systems and in the occasional widening of the larger drainage valleys 
 when crossing or following strata that are easily disintegrated. In 
 brief, we may say that the larger streams are superimposed while the 
 smaller arroyos are mostly subsequent. 
 
 Though there are no undrained basins, the surface run-off makes a 
 very small percent of the rainfall. This is due not only to the rapid 
 evaporation in a warm and dry climate, but also to an efficient under- 
 ground drainage and to the open texture of the surface deposits. It 
 is only after heavy rains that the channel of Cibolo creek carries a con- 
 tinuous stream. During the dry season its water appears on the sur- 
 face only in a few places above rocky sills. Elsewhere its water sinks 
 into the bed of sand and gravel. Some of the ground water in this 
 basin descends far below the level of its main outlet and must be car- 
 ried away by some other route. The shaft of the Presidio mine is per- 
 fectly dry 300 feet below the bottom of Cibolo creek. This under- 
 ground drainage no doubt follows some of the many joints, faults and 
 disintegrated dikes that traverse the sedimentary rocks in the district. 
 
 STRATIGRAPHY. 
 
 Sedimentary rocks underlie the greater part of the entire district. 
 The oldest of these belong to the Paleozoic group and comprise a single 
 great sedimentary cycle. These will be designated as the Chinati series. 
 They are overlaid by sediments of the lower Cretaceous age, which again 
 are partly covered by the land drift. The greatest total thickness of 
 all the sediments approximates 7,500 feet. They will here be described 
 in the order from below upward. 
 
 The sediments have been displaced and uplifted by deep intrusives, 
 which lie under them, and which have caused the exposure of their tilted 
 edges around the uplifts. On top of all of this we find a series of extru- 
 sive volcanics which have concealed most of the sediments and probably 
 also much of the deep intrusives. The table below exhibits these gen- 
 eral relations of all the rocks in the district. (See also Fig. 1.) 
 

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 11 
 
 TABLE SHOWING THE GENERAL ARRANGEMENT OF THE ROCKS IN THE 
 SHAFTER MINE DISTRICT. 
 
 Group. 
 
 System. 
 
 Series. 
 
 Formation. 
 
 Cenozoic. 
 
 Pleistocene. 
 
 
 Land and stream drift. 
 
 Tertiary. 
 
 Late tertiary. 
 
 Lavas, and dikes. 
 
 Early tertiary. 
 
 Deep intrusives. 
 
 Mesozoic. 
 
 Cretaceous. 
 
 Lower cretaceous. 
 
 Buda limestone (?). 
 
 Del Rio clay (?). 
 Edwards limestone. 
 Shafter beds. 
 Presidio beds. 
 
 Paleozoic. 
 
 Permian (?) and 
 Carboniferous. 
 
 Chinati series. 
 
 Cibolo beds. 
 
 Alta beds. 
 Cieneguita beds. 
 
 THE SEDIMENTARY ROCKS. 
 
 The Chinati Series. 
 
 The Paleozoic rocks which are found within the Shatter Area form 
 one successive series, changing from conglomeritic rocks below through 
 arenaceous beds and clays to limestones above. They appear to belong 
 to the Upper Carboniferous age and possibly extend up into the Per- 
 mian. They occupy a semicircular belt around the Chinati mountains 
 to the northeast, east, and southeast. This belt varies from one to two 
 miles in width and averages a little less than a mile and a half. Near 
 the silver mine a cusp extends a short distance away from the main belt 
 to the south. At the north it makes a short and sharp detour to the 
 northeast around a small granitic area. To the southwest it disap- 
 pears under the gravels of the Eio Grande flats. An isolated outcrop 
 was also found about six miles to the south-southeast of Shafter, and 
 this exposed the lower members. This is in the basin of Morita creek, 
 and the rocks occur in a narrow belt a mile and a half long, following 
 this stream from northeast to southwest. 
 
 For convenience in description the series will be considered in sev- 
 eral divisions, beginning below, and taking them in ascending order. 
 These divisions will be designated as indicated in the following table: 
 (See also Fig. 2.) 
 
Fig. 2.— General Section of the Chinati Series, about 5 miles north of Shatter. From east to west. A, Presidio beds; B, Cibolo limestone; C, Transition beds; 
 
 D, Alta sandstone; E, Alta shales; F, Oienequita beds; G, Granite near Ojo Bonito. 
 
Silver Mine District. 
 
 13 
 
 Section of the Chinati Series. 
 
 The Cibolo Beds. 
 
 8. The Yellow Limestone 
 
 8. The Thin-bedded Zone 
 
 6. The Zone of Sponge Spicules 
 5. The Lower Brecciated Zone. 
 4. The Transition Beds 
 
 The Alta Beds. 
 
 3. The Yellow Sand 
 
 2. The Dark Shales 
 
 The Cieneguita Beds. 
 
 1. Basal deposits 
 
 Thickness 
 in feet. 
 
 650 
 
 470 
 
 85 
 
 133 
 
 100 
 
 1500 
 
 2000 
 
 1000 
 
 The Cieneguita Beds. 
 
 The lowest of the Carboniferous sediments consist of a series of dark 
 or almost black shales, alternating with heavy lenticular masses of mor- 
 tar rocks 1 , conglomerates, dark limestones and of mixtures of these 
 materials. Some of these masses are upward of thirty feet in thickness 
 and extend for more than a mile horizontally. The shale constitutes the 
 greater part of the division. In places it contains some layers of black 
 chert and in the lower part some greenish [glauconitic?] seams were 
 noted, and also some flattened round concretions containing a dark 
 grayish powdery material. Farthest down, close to a granite boss, 
 against which this shale lies, four granite or gneiss boulders^ consid- 
 erably decayed, were found imbedded in it. They ranged from two to 
 four feet in diameter, and were considerably more micaceous than the 
 intrusive granite which occurs close by, and apparently different from 
 this in texture. At the same place one seam showed an imprint and 
 some carbonaceous material of the flattened stem of a plant. The indu- 
 rated masses range through all possible gradations of conglomerates, 
 sandstones, limestones, shell breccias, and mortar rocks. The latter are 
 the most common. Occasionally they have been crushed and re-ce- 
 mented, forming crush-breccias. Their character can best be presented 
 in some descriptions of individual specimens. 
 
 A sample of the dark limestone was seen to be a breccia of shell frag- 
 ments, the largest of which were about 1-25 of an inch (one mm.)* in 
 thickness, and exhibited a finely reticulated structure in cross section. 
 With these were smaller fragments of shells, joints of crinoid stems, 
 occasional fusulinas, minute calcareous spines and occasional grains of 
 clear quartz mostly angular. All of this was imbedded in a compact 
 calcareous paste. 
 
 Another specimen was a gray mortar rock consisting of sharply angu- 
 lar quartz fragments imbedded in a calcareous matrix of fine texture. 
 
 x This term is used to designate an unaltered clastic rock consisting of an indu. 
 rated mixture of calcareous mud and siliceous fragments of variable sizes and 
 degrees of wear, 
 
 a One inch=25 millimetres, about. 
 
14 
 
 Geology of the Shafter 
 
 The quartz fragments vary in size from pebbles to particles of minute 
 size, being very imperfectly sorted. Some have clearly been fractured 
 in the rock, and the broken pieces lie close together. Most of the quartz 
 is clear, some is opalescent white, and some dark. Mica scales were 
 also observed. A single pebble was seen to be rounded. In some other 
 conglomerate ledges water-worn quartz pebbles are very common. 
 
 Still another ledge is a conglomerate, for the most part calcareous, or 
 breccia of dark gray color. The body of this consists of a calcareous 
 paste, in which unassorted angular quartz grains lie imbedded. 
 Occasionally quartz fragments are fractured and the broken pieces of 
 the grains lie close together, the intervening fissure being filled with a 
 calcareous paste. Most of the quartz is clear, some whitish. In this 
 mass lie larger pebbles of quartz and of limestone, also now and then 
 some mica scales, fragments of feldspar, and pieces of crinoid stems. 
 The limestone pebbles are mostly of a rock entirely different from that 
 in any of the ledges in this part of the section. It is white, very com- 
 pact in texture, free from quartz, and obscurely laminated. These peb- 
 bles range in sizes up to three inches in diameter. Many are plainly 
 faceted in a manner similar to the faceting of small boulders and peb- 
 bles in glacial deposits. These facets vary in number from two to five 
 on the same pebble, they are oriented at angles from 30° to 120°, and 
 are separated by rounded edges. 
 
 The largest of the quartz pebbles in the conglomeritic ledges of this 
 division average an inch in diameter and are usually well water worn. 
 Some consist of white, and some of dark vein quartz. With these a 
 few were seen which consisted of a sort of graphic granite or a peg- 
 matite. The feldspar in this rock is in thin crystals, oriented so as 
 to give simultaneous reflections, when turned in the hand. Most of 
 the calcareous ledges contain angular quartz grains as an original ingre- 
 dient, and most of them also contain some fossil remains. The pres- 
 ence of some large boulders in the lower layers and the nature of the 
 conglomerate with the small faceted cobbles of foreign rock, suggests 
 transportation <and sedimentation by icebergs. 
 
 The following fossils were found in the uppermost conglomeritic 
 ledge of these beds : 
 
 Fusulina cylindrica Fischer. 
 
 Stems of crinoids. 
 
 Fewestella binodata Condra. 
 
 Fistulipora carbonaria Ulr. 
 
 Meekopora prosseri Ulr. 
 
 Stenopora heteropora Condra. 
 
 Seminula, sp. 
 
 Brachiopod shells, in fragments. 
 
 From another mortar ledge, which lies some 300 feet lower down, 
 the following fossils were taken: 
 
 Syringopora multattennuata McChesney. 
 
 Lophophyllum proliferum McChesney. 
 
 Crinoid head, and stems. 
 
 • Dielasma bovidens Morton. 
 
 Productus semireticulatus Martin. 
 
 Pugnax, sp. 
 
Silver Mine District. 
 
 15 
 
 Reticularia perplexa McChesney [ ?] 
 
 Seminula argentea Shepard. 
 
 Spirifer earner atus Morton. 
 
 Macrocheilus anguliferus White [?] 
 
 Pleurotomaria, sp. 
 
 In other ledges, mostly between these two, were noted: 
 
 Fusulina cylindrica Fischer. 
 
 Syringopora multattennuata McChesney. 
 
 Chonetes, sp. 
 
 Productus, sp. 
 
 Seminula ar gen tea Shepard. 
 
 Spirifer cameratus Morton. 
 
 Straparollns, sp. 
 
 Macrocheilus, sp. 
 
 Dentalium, sp. (with fine longitudinal flutings), frequent. 
 
 A fragment of a pelecypod. 
 
 The outcrops of this division lie between Cibolo creek and some of 
 its tributaries, east and southeast of the Ojo Bonito and the Cieneguita 
 ranches. A continuous thickness of 1000 feet was measured, but it was 
 evident that this did not include all, and possibly not one half of its 
 entire development. A continuous section of the entire division could 
 not be found, but the character of the different ledges leaves no doubt 
 that they belong together. 
 
 The Alta Beds. 
 
 Above the conglomerate-bearing beds there are some 3500 feet or more 
 of perfectly sorted arenaceous and argillaceous beds, almost destitute of 
 calcareous material and of fossil remains. These make two divisions, 
 some dark shaly beds below, and some more light colored and sandy beds 
 above. Each is best described separately. 
 
 The Dark Shales. This bed consists of a mixture of silt, clay, and 
 some sand. Its bedding planes are straight and very sharply marked. 
 Throughout there are a number of laminae of coarser and more purely 
 sandy material, varying in thickness from a half inch to one foot. The 
 thinnest are usually less than six inches apart, while such as exceed 
 two inches in thickness are seldom separated by more than two feet of 
 the shale. The latter are sometimes fissile along straight horizontal 
 seams, where mica scales lie imbedded. Calcareous material frequently 
 fills the spaces between the grains in these layers, evidently a secondary 
 deposit from infiltration. Calcareous and ochreous seams also occur, 
 apparently wholly the result of precipitation from percolating ground- 
 water. The sandy layers consist of angular but well assorted grains, 
 the bulk of which range from \ to -J mm. in diameter. The body of 
 the dark shale itself is of the same nature, only finer, and less perfectly 
 sorted, the bulk of the grains ranging from \ to ^ mm. in diameter. 
 Usually it is also very slightly calcareous. 
 
 It may be that a small ledge of limestone, noted on the north side 
 of Sierra Alta hill,* belongs in the lower part of these shales. This 
 
 *This hill lies on the east side of the Sierra Alta creek, about three miles above 
 its junction with the Cibolo creek, and about six miles north of Shafter. 
 
16 
 
 Geology of tile Shafter 
 
 ledge contained : Productus [near Cora] , a Chonetes and a Pleuroto- 
 maria. Otherwise no fossils were noted in these sediments other than 
 some tracks of various sizes and some other impressions on the upper 
 surface of some of the sandy layers. These run in curves, in all 
 instances seen. Some consist of a single series of crescent-shaped 
 impressions lying transverse to the axis of the path, and some are made 
 up of a double series of sigmoid indentations having the same position. 
 The other structures may or may not be of an organic origin. They 
 are flow structures of the kind which have been described as Spirophy- 
 ton in other regions. There are also some tracks consisting of shallow, 
 smooth linear grooves, resembling what are commonly known as fucoid 
 marks. (See Fig. 3.) 
 
Fig. 3.— Tracks of Trilobites (?) in sandy seams in the Alta beds- 
 
18 
 
 Geology of the Shafteb 
 
 The thickness of these shales approximates 2000 feet. 
 
 The Yellow Sand. The dark shales are succeeded above by a sand- 
 stone of tine texture. No exposures appear for a short distance where 
 the change between this and the dark shales and the sandstone occurs. 
 The rock is soft, occasionally almost crumbling, but it contains some 
 indurated ledges measuring a few feet each. Some of the strata of 
 finer texture are bluish gray, and some of the coarser ledges are gently 
 cross-bedded. The body of this sandstone is a coarse silt, composed of 
 well assorted quartz grains, for the most part angular, the bulk of which 
 range from J to mm. in diameter. In the coarser strata the sort- 
 ing is still more perfect and there is mainly only a smaller admixture of 
 grains below g 1 mm. in diameter. The coarse layers are most abun- 
 dant above. Excepting a few track-like imprints in the fine textured 
 bluish layers, no organic vestiges were noted. Thickness: 1550 feet. 
 
 The Dark Shale and the Yellow Sand appear in an arroyo, which 
 runs on the south side of the lava-capped hill known as the Sierra Alta. 
 They dip at high angles, from 20° to 48°, to the east, and the thickness 
 given was obtained from measurements across the strike. The strike 
 is a little west of north. 
 
 The Cibolo Limestones. 
 
 A series of limestones makes up the last of the paleozoic sediments in 
 this area. These limestones differ from those in the lower part of the 
 series in several respects. They contain no large quartz pebbles, though 
 there is sometimes a little sand. They mostly have a lighter color. 
 Fusulinas occur in greater abundance, and they have a much greater 
 development. There are several somewhat different horizons, and these 
 are best described separately. They are most favorably exposed in the 
 east bluff of the Sierra Alta creek, which comes into Cibolo creek from 
 the north, about a half mile below the Cibolo ranch, and three miles 
 north of Shafter. The entire depth of this division is not far from 
 1450 feet. 
 
 At the locality above referred to the different horizons, in ascending 
 order, are as follows : 
 
 1. Transition Beds . — Upward the Alta sandstones give place to 
 a gray marly shale, which contains some lenticular ledges of organic 
 and siliceous sand. One of the lowermost of these measured eighteen 
 feet in thickness and consisted of an almost pure quartz sand below and 
 of an organic sand above, which was composed almost wholly of worn 
 organic fragments. In this division, and mostly in this arenaceous len- 
 ticular mass, the following fossils were noted : 
 
Fig.,4. —1. Stem of a Orinoid; 2, 3 and 4, Siliceous rock containing spicules of sponges. The Crinoid stem is from the base of the lower brecciated 
 zone. The others are from the zone of sponge spicules. All from the Oibolo beds. 
 
20 
 
 Geology of the Sh after 
 
 Fusulina cylindrica Fischer. 
 
 Archseocidaris, sp. [spines and plates]. 
 
 Crinoids [stems and plates]. 
 
 Meekopora prosseri Ulr. 
 
 Stenopora heteropora Condra. 
 
 Fistulipora, sp. 
 
 Meekella occidentalis Newb. 
 
 M. striutocostata [Cox]. 
 
 Productus semireticulatus [Martin]. 
 
 Spirifer cameratus [Morton]. 
 
 Spirifer, nndet. 
 
 Straparollus [two large species, two and three inches wide]. 
 
 Bellerophon crassus Mk. & W. 
 
 Conulccria crustula White. 
 
 Dentalium, sp. 
 
 Myalina, sp. 
 
 These marly beds are seen for a distance of nearly three miles in the 
 foot of the ridge which runs south from the Sierra Alta hill and on the 
 east side of a creek, usually known by the same name. Including the 
 fossil-bearing lenses, they measure about 100 feet. 
 
 2. Lower Brecciated Zone. This consists of a grayish white lime- 
 stone in heavy ledges, sometimes many feet in thickness, and often 
 thoroughly brecciated. In places the breccia is very coarse, containing 
 blocks six feet in diameter. At one point the middle part of these beds 
 was not brecciated. The rock is sometimes crowded with minute casts 
 of foraminifera, more or less distinct in outline. These lie in a copious 
 compact matrix. In one specimen some round grains were seen to have 
 a white oolitic coating. The rock is more or less fossiliferous through- 
 out. The basal part contains some very large crinoid stems, the largest 
 seen measuring inches in diameter, and with these were some 
 crinoid heads. The following fossils were observed, mainly in the 
 lowermost ledges : 
 
 Fusulina cylindrica Fischer. 
 
 Crinoids, heads, three or four species. 
 
 Archseocidaris, sp. 
 
 Fenestella gracilis Condra. 
 
 Fistulipora carbonaria Ulr. 
 
 Stenopora heteropora Condra. 
 
 Polypora bassleri Condra. 
 
 Meekopora prosseri Ulr. 
 
 Spirifer cameratus Morton. 
 
 S. kentuckensis Shum [?]. 
 
 Spirifer, sp. [very broad form, four or five inches, with high area]. 
 
 Meekella striatocostata Cox. 
 
 Capulus parvus Swallow [ ?] . 
 
 Productus longispinus Sow. 
 
 P. semireticulatus Martin. 
 
 Chonetes, sp. 
 
 Myalina, sp. 
 
 Straparollus, sp. 
 
 Bellerophon, sp. 
 
Silver Mine District. 
 
 21 
 
 Waagenoceras, near cumminisi White. 
 
 The thickness of this member is about 133 feet. 
 
 The Zone of Sponge Spicules . — This consists of more thin-bedded 
 rock, running in even and straight ledges from six inches to one foot 
 in thickness. Below,, these are of pure limestone but above they are 
 siliceous and run into pure sandstone. Some ledges of finer sediments 
 split into very thin straight and smooth laminae from weathering. The 
 lower ledges are sometimes composed of an indurated organic sand or a 
 kind of fine and worn shell-breccia. Sponge spicules are seldom absent 
 at this level and they are sometimes present in great profusion. Some 
 of these were submitted to Dr. J. M. Clarke, who identified them as 
 root-tuft fibres of some hexactine sponge, presumably a Hyalostelia. 
 The longest are usually found lying horizontally on the upper surface 
 of the ledges and measure an inch and a half in length. Frequently 
 cherty layers follow the seams between these ledges and this chert is 
 also full of minute spicules. The siliceous character of the ledges is 
 evidently to some extent due to infiltration. In the calcareous ledges 
 were noted : 
 
 Sponge spicules. 
 
 Crinoid stems. 
 
 Productus, sp. 
 
 A bryozoan. 
 
 Hustedia, sp. 
 
 A cyathophyllid coral. 
 
 Meekella, sp. 
 
 Thickness: 85 feet. 
 
 The Thin-bedded Zone . — This consists of a dark, evenly bedded and 
 compact limestone including some sandy strata. The limestone con- 
 tains a considerable amount of cherty material, which weathers out in 
 rusty edges of plates of irregular shapes on the surface. This is 
 especially the case below. Near the middle the chert frequently 
 assumes the form of spherical shells filled with material like the body 
 of the rock and solidly imbedded in the mass. They range in sizes 
 from three to six inches in diameter. Siliceous material is also found 
 in small solid spherical nodules about half an inch in diameter. Sug- 
 gestions of organic structure — a Pachypora? in one instance — occa- 
 sionally appear on the surface of these. Frequently all the fossils in 
 the ledges are silicified and weather out in perfect form with the rusty 
 color of the chert. Fusulina occurs almost everywhere in this member, 
 excepting in its more arenaceous layers. Under a lens the body of 
 the rock is usually seen to be made up of a paste of minute calcareous 
 fragments mostly less than 0.01 mm. in diameter. In this there are 
 some larger organic particles and fossil remains, mostly such as joints 
 of crinoid stems, spines of brachiopods and bits of bryozoa. Much of 
 this rock is bedded in uniformly thin ledges or seams, measuring about 
 four inches. Another peculiarity of its bedding consists in occasional 
 lenticular thickenings of the ledges, which increase from six inches to 
 two feet and again run out to the same dimensions as before, all in a few 
 feet horizontally. Where least affected by secondary changes a bitu- 
 minous odor is sometimes emitted when this rock is crushed under the 
 hammer. 
 
22 
 
 Geology of the Shafter 
 
 The following fossils were noted in this division: 
 
 Fusulina cylindrica Fischer. 
 
 Hyalostelia, sp. 
 
 Axophyllum rude White and St. J. 
 
 Campophyllum torquium Owen [?]. 
 
 Other cyathophyllids. 
 
 Crinoid stems. 
 
 Fistulipora, sp. 
 
 Other bryozoa. 
 
 Ambocoelia planoconvexa Shum. 
 
 Chonetes, two species. 
 
 Hustedia mormoni Marcon [ ?] a small form. 
 
 Meekella striatocostata Cox. 
 
 Productus costatus Sow. 
 
 P. Mexicanus Shum. 
 
 P. LongiSpinus Sow. 
 
 Pugnax, sp. 
 
 Seminula argentea Shep [small]. 
 
 Spirifer cameratus Morton. 
 
 S. hentuchensiSj sp. 
 
 Bellerophon, sp. 
 
 Aviculopecton, sp. 
 
 Pinna, sp. 
 
 Hungarites, n. sp. 
 
 Horizontally this member varies considerably. To the north of 
 Cibolo creek it becomes sandy below, and to the south it contains a 
 considerable lenticular mass of thick-bedded, brecciated, grayish-white 
 limestone. Fusulina is sometimes present in profusion, while at other 
 points it is absent. Elsewhere the rock is sometimes argillaceous and 
 weathers into slaty laminae. The base is not always clearly limited, and 
 above this division gradually assumes the character of the next mem- 
 ber. As measured in the hills north of Cibolo creek these beds have a 
 thickness of 470 feet. 
 
 The Yellow Limestone. — The highest member consists of a hard yel- 
 low and dolomitic limestone. Its lower part shows bedding planes, 
 like the number below, but higher up the stratification becomes indis- 
 tinct, and single ledges appear which measure twenty or even forty feet 
 in height. The rock is very hard and contains a considerable amount 
 of siliceous material, which probably is of secondary origin. The 
 silica is mostly yellow, like the rock, and is more or less evenly distrib- 
 uted in the mass and usually not forming separate nodules or layers. 
 Indistinct outlines of minute organic fragments are visible under a 
 lens. It is slightly brecciated, especially above, and is frequently tra- 
 versed by minute irregular and branching fissures. Now and then there 
 are larger irregular fissure-like pockets, filled with white quartz. Fos- 
 sils are rather scarce, but the following were noted : 
 
 1. Fusulina cylindrica Fischer. 
 
 2. A fossil with somewhat indistinct texture and form, like that of 
 a Stromatopora. 
 
 3. Crinoid stems. 
 
 4. Crinoid heads. [One very solid block was seen to contain a 
 cluster of crinoid heads, crowded together in a small space.] 
 
Silver Mine District. 
 
 23 
 
 5. Cleiothyris roysii L’Eveille [?]. 
 
 6. Seminula argentea Shepard [?]. 
 
 This member is exposed at only one place; in the hills north of 
 Cibolo creek, a half mile east of the Cibolo ranch. Possibly the upper- 
 most ledges in the hill at the silver mine may belong in this lower part. 
 Elsewhere it has been removed by erosion before the Cretaceous rocks 
 were laid down. The latter are usually found resting on beds which 
 belong farther down. Thickness: 650 feet. 
 
 GEOGRAPHIC CONDITIONS. 
 
 It is clear from the above that the Chinati series comprises the greater 
 part of one single sedimentary cycle. Downward it does not extend 
 through the basal deposits of this cycle. Nor does it include all of the 
 limestones which belong to it above, for the uppermost member is rep- 
 resented merely by a small outlier. When the Cieneguita conglomer- 
 ates were deposited, the shore line was not far distant. Undoubted 
 plant remains occur, and there are ledges of beach-worif pebbles and 
 sand. The granite boulders and the breccia with faceted and appar- 
 ently extraneous pebbles suggest that icebergs may have stranded off 
 this shore, and increased the rate of sedimentation. The shore line 
 later advanced inland and these deposits were then buried under more 
 uniformly fine material, such as is now deposited on the continental 
 shelves beyond the shore, for these are made up of fine sand and 
 silt, usually lacking the finest mud of coast lagoons. This silt has a 
 sharply marked and straight lamination, such as characterizes very 
 rapidly accumulating deposits. As the depth of the sea kept on increas- 
 ing, the sand no longer reached this far out, and a calcareous ooze with 
 fine organic fragments, small foraminiferal casts, slender sponge 
 spicules, and fusulinas kept on accumulating. This stage completed 
 the cycle. Its latest sediments were carried away during the long 
 period of elevation, which preceded the cretaceous subsidence. 
 
 In general the whole series shows a considerable range of variability 
 horizontally, in sections at different places. This is evidently due to 
 diversity of attendant geographical conditions, such as an irregular 
 shore line, a variable depth of the sea, and a diversity of currents in 
 the depositing medium. 
 
 CORRELATIONS. 
 
 The fauna of the Chinati series can not be of an earlier age than the 
 later part of the Upper Carboniferous. It has a general resemblance 
 to the fauna of the Missourian in the Mississippi basin, which is the 
 uppermost and latest division of the Carboniferous age and whose rocks 
 are continuous with the sediments of the Permian on the plains. The 
 present author would have regarded the entire series as belonging to 
 the Upper Carboniferous except for the evident resemblance in its 
 stratigraphic succession to the Guadaloupian series, which lately has 
 been pronounced by Girty to belong to the Permian* and also for the 
 
 *The Upper Permain in Western Texas, by George H. Girty, Am. Jour, of 
 Sci., November, 1902, p. 363. 
 
24 
 
 Geology of the Shafter 
 
 occurrence of two cephalopods which Smith has identified as forms not 
 previously known below the Permian. The Guadaloupian, as described 
 by Girty, consist of some 1700 to 1800 feet of white limestone, under 
 this some 100 feet of dark colored, thinly laminated and foliated lime- 
 stone, then from 2000 to 2500 feet of yellow sandstone, and farthest 
 down a black limestone only partly exposed and still 500 feet thick. 
 This succession suggests an equivalence with the Chinati series, and the 
 occurrence of a Waagenoceras and of a Hungarites in the Cibolo lime- 
 stones emphasizes the suspicion that this part of the series belongs to 
 the Permian age. 
 
 Otherwise the paleontologic evidence is decidedly against the view 
 that the two series are parallels. In a general way the two faunas are 
 quite unlike each other, as appears in the following comparison: 
 
 CHARACTERISTICS OF THE CHINATI AND THE GUADALOUPE FAUNAS 
 
 COMPARED. 
 
 Chinatian. 
 
 Guadaloupian* 
 
 1. Fusulina cylindrica of moderately 
 large size and not of very slender form. 
 
 1. A very large and slender form of fusu- 
 lina. 
 
 2. Entire sponges apparently absent, but 
 with a great abundance of separate spicules 
 of siliceous sponges at one horizon. 
 
 2. Entire [?] sponges abundant, chiefly 
 of the class of calcispongia. 
 
 3. Hydrocorallina absent or very rare. 
 Cyathophyllids common. 
 
 3. Hydrocorallina not rare, other corals 
 rare. 
 
 4. Echinoderms common. Stems of cri- 
 noids most frequent of all fossils. 
 
 4. Echinoderms not observed. 
 
 5. Bryozoa of carboniferous types. 
 
 5. Bryozoa unlike known carboniferous 
 forms. 
 
 6. Brachiopods not specially numerous. 
 
 6. Brachiopods very common. 
 
 7. Pelecypods represented by only three 
 or four fragments in the collection made. 
 
 7. Pelecypods quite numerous. 
 
 8. Gasteropods fairly common, nearly all 
 forms large. 
 
 8. Gasteropods rare, and nearly all of 
 minute size. 
 
 This comparison is based chiefly upon the forms found in the upper 
 limestones in each of the two sections. Some of the differences enum- 
 erated clearly indicate a difference in the attendant geographic condi- 
 tions. The presence in the Cibolo limestones of siliceous sponge remains 
 and of simple forms of cyathopyllid corals, the abundance of remains 
 of echinoderms, and the paucity of pelecypods all indicate the condi- 
 tion of an open sea some distance away from the shore, while in the 
 Guadaloupian fauna the presence of calcispongia, and of hydrocoral- 
 lina, the abundance of - brachiopods and pelecypods and absence of echino- 
 derms all testify to a greater proximity of the shore. The geographic 
 
 ^Characteristics as given by Girty. Reference as above. 
 
Silver Mine District. 
 
 25 
 
 conditions were no doubt different, whether the two limestones were 
 made at the same time or not. It is to be hoped that a more thorough 
 study of the two faunas and more extensive examination of the terranes 
 in the field will throw more light on the question of their relation- 
 ship. With our present knowledge one can not regard it as impossible 
 that the Capitan limestone of Girty may turn out to be an equivalent 
 to the yellow limestone of the Cibolo beds, and that both may be a 
 part of the Permian cap which rests on top of the Upper Carbonifer- 
 ous column to the east of the Cordilleran region in the southern interior 
 of North America. 
 
 THE CRETACEOUS ROCKS. 
 
 At the end of the Paleozoic era the bottom of the sea was elevated 
 and its sediments were submitted to erosion during the Triassic and 
 Jurassic ages. Toward the end of the latter age much of the latest 
 accumulations of the Carboniferous age had already been carried away. 
 Then there came a new subsidence and in the advancing sea the Cre- 
 taceous sediments were laid down. These attained a thickness of some 
 three or four thousand feet. Near the Chinati mountains the later 
 rocks of this age have been carried away by erosion, and only those 
 now remain, which represent the earlier part of the age, the Lower 
 Cretaceous. These naturally fall into five successive divisions, or for- 
 mations, as indicated below. 
 
 SYNOPSIS OE THE SECTION OF LOWER CRETACEOUS ROCKS NEAR SHAFTER. 
 
 Thickness 
 in feet. 
 
 5. Buda limestone [ ?] 70 
 
 4. Del Rio clay [ ?] 80 
 
 3. Edwards limestone 350 
 
 2. Shatter beds 700 
 
 1. Presidio beds 400 
 
 THE PRESIDIO BEDS. 
 
 West of the new shaft of the Presidio Mining Company there is a 
 bluff facing the Chinati mountains, which rises some 250 feet from a 
 valley running nearly north and south and in the south of which the 
 old mining village is located. The west slope of this valley consists of 
 the uncovered upper surfaces of the ore-bearing carboniferous lime- 
 stone. In the bottom and in the east bluff there are exposed nearly 
 400 feet of Cretaceous sediments, beginning with a clay resting directly 
 on the Cibolo limestone and ending with a solid ledge of calcareous 
 mortar rock. This ledge caps several other hills in the immediate 
 vicinity and it is the most conspicuous member in the Cretaceous series 
 of the region, the Edwards limestone excepted. The details of the sec- 
 tion in this bluff are as follows, beginning above. (See also Fig. 5.) 
 
Fig. 5.— Section of the Presidio beds in the bluff west of the shaft of the Presidio Mining Company. 
 
Silver Mine District. 
 
 27 
 
 SECTION IN THE BLUFF DIRECTLY WEST OF THE NEW SHAFT OF THE 
 PRESIDIO MINING COMPANY. 
 
 Thickness 
 in feet. 
 
 27. A strong mortar rock of somewhat variable composition. The 
 imbedded sand is finest, and the matrix is most scanty, 
 below. The middle part is at times almost free from sand 
 and contains Orbitulina texana Roem. The uppermost 
 ledges contain quite coarse sand and often many of the 
 grains have a well-developed enveloping oolitic crust of cal- 
 careous material. Wholly calcareous oolitic spherules also 
 occur. Irregular rusty and siliceous streaks weather out on 
 
 the surface of these ledges, and they contain a large pele- 
 
 cypod with thick valves, resembling a Cardium 30 
 
 26. Not well exposed, but apparently largely an arenaceous and 
 argillaceous soft limestone, with a brecciated ledge below. 
 This number contains Orbitulina texana Roem., Trigonia 
 emoryi Con., and two sea-urchins resembling a Diadema and 
 
 a Toxaster 82 
 
 25. Sandy shell breccia in which the shell fragments are of small 
 
 size 3 
 
 24. Dark gray limestone with imbedded sand, changing into a 
 
 coarse shell breccia above. A large gasteropod like Glaucoma 
 branneri Hill, Pecten n. sp., Cucullcea terminalis Con. (?), 
 
 Lima, sp. (?), Pholadomya texana Con., and a large Ostrea 
 
 were noted 29 
 
 23. Dark gray impure sandy limestone, with Orbitulina texana. . . 12 
 
 22. Sandstone of moderately coarse texture and light gray color. . 15 
 
 21. Cray limestone containing Orbitulina texana 12 
 
 20. Gray limestone in ledges which are not clearly defined. The 
 lowermost ledge contains thin and flat valves of lamelli- 
 branchs of small size. The uppermost ledges have a brecci- 
 ated structure 28 
 
 19. Thin-bedded, marly, soft material, weathering rapidly. Not 
 
 well exposed 10 
 
 18. A grayish white limestone helow, mixed with some fine quartz 
 sand. Somewhat softer than the following number. Above, 
 this runs into, first, an equal mixture of sand and calcareous 
 material and then into a sandstone. The ledges, or laminae, 
 
 measure from seven to eight inches in thickness 29 
 
 17. Dark gray ledges of strong sandstone, in which a considerable 
 
 part of the sand grains are calcareous. 7 
 
 16. Sandy and sometimes cross-bedded conglomerate, containing 
 water-worn pebbles of limestone, chert, and white quartz, 
 ranging up to two inches in diameter. The limestone peb- 
 bles are the largest and are well rounded. The chert is 
 mostly angular and frequently shows fine straight spicules 
 of sponges and silicified fusulinas. The white quartz is more 
 
 commonly in rounded pebbles 14 
 
 15. A gray limestone of fine texture with a small admixture of fine 
 quartz sand. When weathering, it takes on a concretionary 
 appearance 4 
 
28 Geology of the Sh after 
 
 14. Not well exposed but apparently for the most part consisting 
 of an indurated sandy marl below. At about eight feet from 
 the base a ledge of conglomerate was noted. This was seen 
 to contain pebbles of chert, white quartz, quartzitic sand- 
 stone, and limestone. The largest pebble seen was two inches 
 in diameter. A limestone pebble was seen to contain an 
 imbedded fusulina. At eighteen feet above the base a brecci- 
 
 ated limestone of yellow and reddish color was noted 26 
 
 13. Dark limestone of a concretionary structure 1 
 
 12. Fine-grained, gray sandstone 11 
 
 11. Gray, sandy and calcareous shell-breccia of a concretionary 
 
 structure. Dark crystalline calcite noted 14 
 
 10. Light gray stony marl with fine yellow sand in irregular 
 
 laminae 6 
 
 9. Gray laminated shale, interbedded with from one-half to two 
 inch layers of fine sandy and calcareous material. No fos- 
 sils noted 7 
 
 8. Gray mortar rock consisting of calcareous material mixed with 
 fully two-thirds of its bulk of fine sand. This is interlamin- 
 ated with layers of sandstone and shell-breccia, above and 
 below. Spheroidal calcareous concretions and some fossils 
 
 were noted 14 
 
 7. Gray marl 8 
 
 6. Gray, almost compact limestone with a considerable admixture 
 
 of imbedded fine sand and with some angular gravel 2 
 
 5. Gray marl 9 
 
 4. Hard and dark limestone with dark fragments of Ostrea 
 valves of large size and with an Exogyra somewhat like E. 
 
 texana 2 
 
 3. Gray marl changing into a sandy shell-breccia above 2 
 
 2. Gray marl with round concretions of a compact calcareous 
 
 material and measuring from two to six inches in diameter. . 6 
 
 1. Bluish gray clay 17 
 
 When the new shaft of the Presidio Mining Company was sunk at a 
 point some 800 feet to the east of the crest of the bluff where the above 
 section is exposed, a careful record was taken of the strata and typical 
 samples of the rock from each were preserved. These samples were 
 carefully examined by the writer in the office of the company. The sec- 
 tion of the shaft as given in the company’s record and verified by the 
 samples is as below, leaving out the Cibolo limestone. The measure- 
 ments given are those from the record of the shaft, only that these have 
 been corrected for the dip, which it about 15°, so as to represent the 
 actual thickness of the strata on a line vertical to the planes of the 
 bedding. 
 
 SECTION OF THE NEW SHAFT OF THE PRESIDIO MINING COMPANY. 
 
 Thickness 
 in feet. 
 
 19. A mortar rock in which the grains are encrusted with concen- 
 tric layers of calcareous material. Occasional encrusted 
 and rounded shell fragments. Calcareous matrix not very 
 copious. Most sand grains from £ to y 1 ^ mm. in diameter. 34 
 
Silver Mine District. 29 
 
 18. A gray sandstone of uniformly fine texture, the bulk of the 
 
 grains measuring from J to ^ mm. in diameter 17 
 
 17. Yellowish gray sand of very small grains from to ^ 
 
 mm. in diameter, and with some calcareous material 21 
 
 16. A dark shell breccia of more or less worn dark shell frag- 
 ments of all sizes held in a matrix of more light color. 
 
 Ostrea noted, also some pyrite 72 
 
 15. An almost compact limestone containing occasional worn 
 
 organic fragments and some quartz sand 28 
 
 14. A gray sandstone with a sparse calcareous cement 34 
 
 13. Limestone of dark gray and compact texture, with scattered 
 
 angular grains of quartz 11 
 
 12. Yellow .limestone composed of gray rounded organic sand- 
 grains, most from T V to £ mm. in diameter. These are 
 held in a sparse calcareous matrix of ochreous color. Crys- 
 tals of calcite noted 14 
 
 11. Gray mortar rock, with quartz grains mostly from y 1 ^- to \ 
 
 mm. in diameter 2,8 
 
 10. A yellow marl with occasional angular small pebbles of chert. 14 
 
 9. A conglomerate consisting of well rounded pebbles of lime- 
 stone, chert, and vein-quartz, mostly from one to two centi- 
 meters* in diameter, imbedded in a matrix of a fine clastic 
 paste. A layer of crystalline calcite frequently follows the 
 
 surface of the larger pebbles 40 
 
 8. Gray mortar rock with a copious calcareous matrix, sand 
 
 grains mostly from y 1 ^ to ^ mm. in diameter 14 
 
 7. Gray mortar rock with a somewhat sparse calcareous matrix. 
 
 Grains about J mm. in diameter 14 
 
 6. A dark organic breccia composed of shell fragments ranging 
 up to three centimeters in diameter, and containing some 
 
 sand 25 
 
 5. Brownish gray sandstone 2 
 
 4. Mortar rock of dark gray color, containing about one-third 
 
 sand 42 
 
 3. Sandstone of dark gray color with a sparse calcareous matrix 
 which in places is crystalline. An impression of a gastero- 
 
 pod noted 6 
 
 2. An indurated gray marl or clayey limestone, showing a few 
 
 calcareous and organic fragments. Base of the Cretaceous. 26 
 1. Yellow, white, and bluish limestones belonging to the Cibolo 
 formation, were explored by the shaft below this, for a dis- 
 tance of about 200 
 
 General Character. — Although the beds included in the above sec- 
 tions range from conglomerates and clay to limestones and sandstones, 
 they may be regarded as constituting a lithological unit in the sense 
 that they have one character in common in which they differ from the 
 other divisions of the Lower Cretaceous : they are all mixtures of 
 mechanical sediments, such as gravel, sand, and clay, with precipitated 
 carbonate of lime as an original ingredient. The limestones are never 
 
 *One Centimeter=0.39 inch, about. 
 
30 
 
 Geology of the Shafter 
 
 free from sand and the sandstones are hardly ever free from calcareous 
 material. This is not the character of the sediments which overlie the 
 Presidio beds. The oolitic nature of the uppermost ledges heralds a 
 coming change, when the shore of the Cretaceous sea advanced on the 
 existing land and left this region as a more open sea, where the deposits 
 began to be more perfectly sorted. 
 
 Fossils. — Fossil remains are not rare, especially in such ledges as are 
 in part made up of organise fragments. But they are frequently broken 
 up in weathering. The following forms, or forms resembling them, 
 were noted: 
 
 Minute foraminifera. 
 
 Orbitulina texana Roem. 
 
 Astrocoenia [?] sp. 
 
 Enallaster, sp. 
 
 Nerinea, sp. 
 
 Turritella 'planilateris Conrad [?]. 
 
 Cylichna recta Gabb [ ?] . 
 
 Globiconcha planata Roem [ ?] . 
 
 Cerithium [large cast] ? 
 
 Trigonia crenulata Lam. 
 
 Trigonia tajji Cragin. 
 
 Cypricardia texana Roem. 
 
 Ostrea franklini Coq. 
 
 Ostrea [a large form]. 
 
 Pinna, sp. 
 
 Pholadomya texana Con [ ?] . 
 
 Pec ten, n. sp. 
 
 Cucullcea terminates Con [?]. 
 
 Correlation. — Lithologically the Presidio beds are identical with R. 
 T. HilFs Travis Peak beds of the Grand Prairie region and the two 
 are analogous in position.* They constitute what he has called the 
 ff basement sands” of the Lower Cretaceous. Faunally they may be less 
 closely related, but it is significant that a section of the Travis Peak 
 beds which is seen along the Colorado river nearly four hundred miles 
 to the east should present a succession of sediments quite like that 
 of the beds here. In both places there is a strong mortar rock 
 above, which changes downward into sandstone. Under this are some 
 deposits of a finer texture and below these again lie the main conglom- 
 erates. In both places, also, these basement sands extend up to the 
 lowermost limestone ledges which were found to contain Monopleura 
 and Requienia. 
 
 The outcrops of the Presidio beds run in a belt following the outer 
 border of the Carboniferous rocks east and south of the Chinati moun- 
 tains. They are also seen bordering the Carboniferous area on the 
 southeast in the Morita creek basin. 
 
 THE SHAFTER BEDS. 
 
 Upward the Presidio beds give place to a long succession of “alternat- 
 
 *Black and Grand Pariries, Texas, R. T. Hill, U. S.Geol. Surv.,21st An. Rept., 
 Vol. VII, p. 142 et seq. 
 
Silver Mine District. 
 
 31 
 
 ing” sediments, measuring some 700 feet in thickness. These consist 
 of ledges of limestone, stony marls, clays, and fine grained, straight 
 bedded lenticular masses of yellow and brown sandstones, all recurring 
 in frequent alternations. In the entire division certain fossils are 
 characteristic of certain types of rocks, and reappear sometimes in 
 slightly changed form, at successive levels in the same kind of rock. 
 Thus Caprinas are seldom absent from the upper surface of the hardest 
 limestone ledges. Bluish and clayey marly limestones are almost cer- 
 tain to contain Orbitulina texana. Thin ledges of worn shell sand, 
 which sometimes occur, are apt to have spires of Turritellas. White 
 and pure limestone ledges are usually filled with minute foraminifera. 
 The sandstones are not fossiliferous. 
 
 These sediments I shall call the Shatter beds. They are exposed in 
 all the hills immediately around Shatter and extend to the south, south- 
 east, and southwest a distance of several miles from the town of this 
 name, until they disappear beneath the Edwards limestone. A nearly 
 continuous section of these beds was worked out from exposures seen 
 at several different points. (See Fig. 5.) The lowermost, which rest 
 on the Presidio beds, come into view in an arroyo about two-thirds of a 
 mile to the east-south-east of the new shaft of the Presidio Mining Com- 
 pany. These may be described as follows: 
 
 EXPOSURE i. 
 
 Thickness 
 in feet. 
 
 10. Hard gray limestone, in part composed of small organic frag- 
 
 ments and containing Caprina crassifibra Roem 6 
 
 9. Soft gray calcareous rock with Orbitulina texana Roem. in 
 
 great abundance. Porocystis pruniformis Cragin also noted. 7 
 8. Yellowish sandstone 4 
 
 7. Soft marly gray rock containing Orbitulina in its upper part. 39 
 6. Somewhat harder ledges, consisting of organic fragments 
 imbedded in a structureless mass. Small foraminiferal 
 shells and occasional Caprinas’ are imbedded in the mass .... 6 
 
 5. Soft marly materials, not well exposed 15 
 
 4. A calcareous, gray rock, consisting in part of organic frag- 
 mental material, and occasionally containing Caprina cras- 
 sifibra Roem. and a large gasteropod. The upper surface of 
 the ledge is traversed by vertical rusty streaks about one- 
 third of an inch in diameter and these weather out into hol- 
 
 low pits 7 
 
 3. Material like the above, but somewhat softer and more plainly 
 
 laminated 13 
 
 2. Gray limestone, containing shells of small foraminifera and a 
 Monopleura. The latter is silicified and usually stands in a 
 
 vertical position, weathering out with a rusty color 3 
 
 1. Gray soft limestone, not well exposed 2 
 
 In the west bank of Cibolo creek below Shafter a continuous section 
 was made out, of about 260 feet. This includes nearly all of the sec- 
 tion already given. It runs as below. 
 
32 
 
 Geology of the Shafter 
 
 23. 
 
 22 . 
 
 21 . 
 
 20 . 
 
 19. 
 
 18. 
 
 17. 
 
 16. 
 
 15. 
 
 14. 
 
 13. 
 
 12 . 
 
 11 . 
 
 10 . 
 
 9. 
 
 8 . 
 
 7. 
 
 6 . 
 
 5 . 
 
 EXPOSURE II. 
 
 Thickness 
 in feet. 
 
 Dark gray limestone, its upper surface with a mottled appear- 
 ance 2 
 
 Gray marl 3 
 
 Gray limestone, compact below and with minute foraminiferal 
 
 casts above 2 
 
 Gray marl 2 
 
 Gray limestone, like number 19 above 3 
 
 Gray, somewhat indurated marl 2 
 
 Gray limestone, like numbers 19 and 17 above 2 
 
 Gray stony marl 2 
 
 Gray limestone weathering into rounded pieces 3 
 
 Gray stony marl 1 
 
 Gray solid limestone in three somewhat indistinct equal ledges. 
 
 Occasionally Monopleuras and Caprinas are seen 8 
 
 Limestone like the previous number. Near the base there is a 
 seam in which Monopleura marcida White, standing verti- 
 cally, is abundant. A large and thick lamellibrauch was also 
 
 noted 7 
 
 Limestone like the above, but full of Orbitulina texana Roem., 
 and with a slightly softer stratum near the middle. Fos- 
 sils noted': a gasteropod, a sea urchin, a small Gryphaea, and 
 a Pecten occidentalis Con 12 
 
 [The numbers which follow, appear some distance further 
 down in the same side of the creek.] 
 
 Gray marly limestone with Orbitulina texana Roem. in abund- 
 ance. Other fossils: Ostrea, sp., Porocystis pruniformis 
 
 Cragin, Toxaster, sp., and some gasteropods. The number 
 contains one thin ledge of more compact texture at about 
 
 seventeen feet below the top 65 
 
 Laminated gray compact limestone, shaly above 3 
 
 Massive gray limestone, weathering yellow, and largely made 
 up of organic fragments, occasionally containing Orbitulina. 
 Fossils: a small Ostrea and Alectryonia diluviana Lam. ( ?) . 7 
 
 Gray stony marl with Orbitulina texana Roem 15 
 
 Sandstone, rather coarse above and changing below into silt and 
 
 clay, very sharply marked off from the limestone above. In 
 an uplifted block, some distance farther down where there is 
 a high bluff on the west side of a bend of the creek, this sand- 
 stone is thirteen feet thick. It breaks into sharply angular 
 blocks and the upper two feet are finely laminated. It is 
 
 also in places obliquely bedded 5 
 
 [The numbers which follow are seen in this uplifted block.] 
 Gray limestone with Orbitulina, hardest below, and containing 
 Nerinea texana Roem., Nerinea, n. sp., Pecten occidentalis 
 Con. and some imperfect specimens of forms resembling 
 
 Avicula, sp., Astarte, sp., and Idonearca, sp.[?] 9 
 
 Fine-grained yellowish sandstone breaking vertically and with 
 a very sharply marked base 20 
 
Silver Mine District. 
 
 33 
 
 4. Gray shale, yellow above, bluish green below, with a calcareous 
 seam 14 inches from top, which has its upper surface liter- 
 ally covered with Orbitulina. The entire shale is finely 
 
 laminated 7 
 
 3. Gray soft marl with Orbitulina texana Roem 30 
 
 2. Gray clayey marl alternating with more indurated bands of 
 compact calcareous material. Near the middle these alternat- 
 . tions are thin and very regular, each from two to three 
 inches in thickness. Orbitulina appears to be absent from 
 this number, but it has white casts of minute foraminifera. 
 
 Above and below the material is somewhat stony 34 
 
 1. Several alternating ledges of gray limestone and stony marl, 
 both of which are made up of minute organic fragments, 
 among which some minute tests of foraminifera may be seen. 17 
 Numbers 6 to 10 in exposure I are the equivalents of numbers 1 to 
 5 in exposure II. It is possible that some ledges have been left out in 
 -exposure I, for the ground was somewhat broken up, but the discrep- 
 ancy can at most involve only a few feet. Evidently the lowest num- 
 ber in exposure II comes down to within a close distance of the upper- 
 most ledges of the Presidio beds. 
 
 In the south slope of the second arroyo which opens into Cibolo creek 
 from the west, south of Shafter, and to the south of the mill belonging 
 to the Presidio Mining Company, I found a section which continues the 
 previous exposure upward. This is as below: 
 
 32. 
 
 31. 
 
 30. 
 
 ■ 29 . 
 
 28. 
 
 27. 
 
 26. 
 
 25. 
 
 24. 
 
 23. 
 
 22 . 
 
 21 . 
 
 20 . 
 
 EXPOSURE III. 
 
 Thickness 
 in feet. 
 
 Hard gray limestone with Caprina crassiftbra Roem 4 
 
 Gray marly rock with Orbitulina texana Roem 20 
 
 Massive gray limestone in one single ledge which breaks into 
 large blocks with rounded edges. Its upper surface shows a 
 
 small Gryphea, and Pecten occidentalis Con 4 
 
 Stony gray marl 11 
 
 Cross bedded yellow sandstone of uniform texture. This ledge 
 
 sometimes runs out suddenly * 12 
 
 Stony gray marl 9 
 
 Gray marly limestone with yellow blotches above 3 
 
 Gray stony marl 4 
 
 Gray limestone with yellow impregnated patches above 2 
 
 Gray stony marl 5 
 
 Gray marly limestone with some small oysters or gryphaeas on 
 its upper surface 3 
 
 Stony gray marl, not well exposed, with some very large 
 moulds of gasteropods and some large lamellibranchs, and 
 Pecten occidentalis Roem. Two thin indurated ledges were 
 noted at 33 and 21 feet above its base. At about 3 feet above 
 its base a ledge of sandstone is seen on the opposite side of 
 
 the arroyo 35 
 
 A gray shell breccia of fine texture weathering into square 
 blocks. The upper surface of this number is very straight. 
 Slender and straight spires of gasteropods are frequent. In 
 

 34 Geology of the Shafter 
 
 places the upper surface is marked by straight and ferrugin- 
 ous crack-like joints. A quarter of a mile further down the 
 creek this ledge is replaced by about 10 feet of brown sand- 
 
 stone 1 
 
 19. Stony gray marl 3 
 
 18. Strong ledge of fine-grained limestone, in part composed of 
 organic fragments and also containing tests of small forum - 
 inifera above. The rock weathers brownish yellow and 
 breaks into square blocks. From its upper surface extends 
 into it a number of irregularly cylindric and sinuous tra- 
 
 versions of oxidized yellow rock 3 
 
 17. Gray limestone containing Monopleura marcida White in pro- 
 fusion and also a Caprina 1 
 
 16. Gray stony marl, almost a shell-breccia near the middle and in 
 
 the upper part 11 
 
 15. Fine compact gray limestone with a very straight obscure lam- 
 ination, weathering into straight rectangular blocks 
 
 14. Shaly gray marl 
 
 13. Gray limestone of fine texture, compact below, and with a 
 profusion of shells of minute foraminifera above. When 
 weathered the upper surface exhibits a structure like that of 
 
 a conglomerate or breccia 
 
 12. An interrupted thin seam of a compact fine-textured lime- 
 stone 
 
 11. Marly gray limestone with imbedded fragments of shells 
 
 10. Limestone of fine texture and gray color, with imbedded tests 
 
 of minute foraminifera 
 
 9. Compact gray limestone 
 
 8. Gray marly limestone 
 
 7. Gray limestone filled with casts of small foraminifera. Sinu- 
 ous yellow tra versions extend downward from the upper sur- 
 face 
 
 6. A thin seam of gray marl 
 
 5. A compact gray limestone containing scattered fragments of 
 
 shells 
 
 4. Gray marly limestone 
 
 3. Gray limestone in two ledges, the uppermost thicker and con- 
 taining a profusion of small foraminifera, the lower more 
 
 compact and with scattered dark organic fragments 
 
 2. Gray limestone : 
 
 1. Gray limestone consisting of a breccia of organic fragments of 
 small size. Casts of minute foraminifera and of Orbitulina 
 
 texana Roem. in considerable numbers 2 
 
 Numbers 16 to 23 in exposure II are identical with numbers 1 to 13 
 in exposure III. These numbers contain a thrice repeated cycle of 
 slightly different calcareous ledges which are clearly shown in several 
 places, always the same, and everywhere easily identified.. The upper 
 ten or twelve numbers in the last named exposure are the beds on 
 which the town of Shafter is built and they are also exposed in the 
 north and east bank of Cibolo creek, on the opposite side from the town. 
 In the hills to the north and east they are overlaid by a series of sedi- 
 ments of very much the same character as those already described, 
 
 to to tOrc|M itOtoli- 1 CO CO^Im CO to to 
 
Silver Mine District. 
 
 35 
 
 including marls, shales, limestones, and sandstones in continuous alter- 
 nations. Below is given a section, which begins in the bed of the creek,, 
 and therefore includes the uppermost of the previous exposure, but 
 which continues some 250 feet higher up. 
 
 33. 
 
 32. 
 
 31. 
 
 30. 
 
 29. 
 
 28. 
 
 27. 
 
 26. 
 
 25. 
 
 24. 
 
 23. 
 
 22 . 
 
 21 . 
 
 20 . 
 
 19. 
 
 18. 
 
 17. 
 
 16. 
 
 15. 
 
 14 . 
 
 EXPOSURE IV. 
 
 Thickness, 
 in feet. 
 
 Compact grayish white limestone 
 
 Marly gray limestone with Modiola pedernalis Roem., Neri- 
 nea, sp., Rostellaria collina Conrad ( ?), Natica texana Con- 
 rad, Exogyra texana Roem., Engonoceras, near complication 
 
 Hyatt, and Toxaster, sp 
 
 Dark yellow limestone consisting of calcareous organic frag- 
 ments in a compact paste 
 
 Marly limestone 
 
 Compact light gray limestone with scattered imbedded grains 
 
 of dark color 
 
 Marly limestone with a more indurated ledge at about four 
 
 feet from the base 
 
 Compact gray limestone filled with casts of minute foramini- 
 fera in the lower part. The upper surface of this ledge 
 shows some irregular oxidized yellow patches which extend 
 
 into the ledge as irregular finger-like traversions 
 
 Gray marl, somewhat stony above 
 
 Sandstone, rather fine-grained^ 'soft below, indurated and 
 
 weathering into straight brown blocks above 
 
 Gray marl 
 
 Straightly laminated gray sandstone, weathering yellow and 
 
 brown 
 
 Gray marl 
 
 Indurated sandstone of uniform texture, cross-bedded, with 
 
 laminae very straight 
 
 Gray marl 
 
 Gray limestone in thin layers alternating with softer nodular 
 calcareous material. In the upper part of this number there 
 are imbedded some dark small nodules, which give the rock 
 a blotchy appearance. A ground specimen shows some 
 imbedded tests of small foraminifera. Some gasteropods 
 
 present 
 
 Gray marl 
 
 Stony marl with a somewhat concretionary structure. A 
 
 very large mold of a pelecypod noted 
 
 Gray shale 
 
 Marly clay below^ with thin seams of somewhat brecciated gray 
 limestone above. A small Ostrea and a small Gryphaea were 
 noted and also casts of one large gasteropod and one large 
 
 pelecypod 
 
 Gray, and yellowish white sandstone, of uniformly fine texture, 
 some with beautiful ripples from two to five inches across. 
 It usually weathers into sharply rectangular blocks which 
 sometimes split up into thin laminae 
 
 4 
 
 28- 
 
 2 
 
 1 
 
 2 
 
 13 
 
 4 
 
 18 
 
 6 
 
 12 
 
 H 
 
 16 
 
 2 
 
 15 
 
 6 
 
 12 
 
 16 
 
 6 
 
 5 
 
 14 
 
36 
 
 Geology of the Shafter 
 
 13. Marly gray limestone with casts of pelecypods, changing into 
 
 clay above 14 
 
 12. Gray limestone with many minute tests of foraminifera, pele- 
 cypods and a Serpula. Orbitulina texana Eoem. also 
 
 noted 3 
 
 11. Clay with a ripple-marked sandy seam at about the middle. 
 Below this seam the clay is dark, while above, it is lighter, 
 
 arenaceous, and calcareous and has some fossils 14 
 
 10. Gray marly limestone, soft above, filled with Orbitulina texana 
 Boem., also sea-urchins and a Pecten, a Gryphsea, and 
 
 Ostrea, and a small gasteropod 20 
 
 9. A compact gray limestone with Caprina crassifibra Roem 3 
 
 8. Marly limestone, bluish gray, weathering into small chips of 
 irregular shape, with some more indurated ledges, especially 
 above. The whole number is almost filled with Orbitulina. 
 Other fossils: Sea urchins, Pecten occidentalis Con., Phola- 
 
 domya, sp 30 
 
 7. Massive gray limestone weathering into large irregular blocks 
 which turn yellow. On the upper surface of the ledge were 
 seen a Cardium, Pecten occidentalis Con., and a small Gry- 
 phaea 4 
 
 6. A gray marl with casts of various pelecypods and gasteropods. 16 
 
 5. Limestone of gray color, with some very large gasteropods and 
 
 Trigonia, sp 2 
 
 4. Nodular semi-indurated marl with casts of several pelecypods. 3 
 
 3. Gray limestone 2 
 
 2. Shaly marl 2 
 
 1. Gray limestone containing frequent imbedded thin and small 
 shells mostly in fragments and also casts of minute fora.- 
 minifera 5 
 
 Up to the highest ledge in this exposure the section is continuous from 
 the top of the Presidio beds and it measures about 625 feet. There is 
 evidently some more of the same kind of material. I estimate that 
 there is at least twenty-five feet more. In the last exposure there is 
 about two hundred feet of beds above the highest orbitulina marl. In 
 a section which is seen under the Edwards limestone at the junction of 
 Morita creek with the Ciholo, about six miles south of Shafter, the 
 highest stratum containing Orbitulina lay about 230 feet below the 
 base of the Edwards limestone. The character of these beds is akin to 
 that of the uppermost in the last exposure [Exposure 4], as may be seen 
 from the description which follows. 
 
 5. 
 
 4. 
 
 3. 
 
 2 . 
 
 EXPOSURE V. 
 
 Thickness 
 in feet. 
 
 Massive limestone [Edwards] 200-300 
 
 Impure marly limestone of light gray color with Exoqyra 
 
 texana Roem. in abundance 75 
 
 Like the above, but with some more indurated ledges, 
 
 Exogyra less frequent 60 
 
 Reddish brown fine sandstone, of very uniform texture, 
 
 occasionally ripple-bedded 
 
 Shaly gray soft limestone 
 
 1 . 
 
 15 
 
 80 
 
Silver Mine District. 
 
 37 
 
 Below No. 1 there is a marly gray rock filed with Orbitulina texana 
 Roem. It seems that most of this exposure is included in the upper 
 part of exposure IV, and if we place the uppermost orbitulina marl at 
 the same level in both, it will be seen that the Edwards limestone should 
 begin about twenty-five feet above No. 33 in exposure IV. This would 
 then place the base of the next division, the Edwards limestone, at an 
 elevation of 680 feet above the top of the Presidio beds and make this 
 figure the thickness of the Shatter beds. The approximate correct- 
 ness of this figure was corroborated by another rough measurement of 
 the same beds at a point three miles southwest of Shatter, where there 
 is a continuous but partly concealed exposure, extending from north to 
 south and made out to include about 700 feet. The latter figure I 
 believe is a conservative estimate of the thickness of the Shatter beds. 
 
 Fossils of the Shafter Beds. — There are a number of fossils in these 
 beds, some of which have evidently not yet been described. The most 
 conspicuous ones are some casts of large pelecypods and gasteropods. 
 I append the following list of such known forms as seem to resemble 
 those occurring in these beds. 
 
 1. Foraminifera of minute size. These are present in most of the 
 indurated ledges, very often quite abundant. 
 
 2. Orbitulina texana Roem. Present in most of the soft dark gray 
 maids, excepting the upper part of the section. 
 
 3. Porocystis pruniformis Cragin. Present at two horizons: at 
 about 170 feet from the base and near the top of the section. 
 
 4. Diadema texanum Roem. 
 
 5. Holectypus Planatus Roem. 
 
 6. Toxaster texanus Roem. 
 
 7. Serpula, sp. 
 
 8. Lunatia pedernalis Roem. Present mostly in the gray marls. 
 
 9. Natica texana Con. 
 
 10. Nerinea texana Roem. 
 
 11. Nerinea, n. sp. 
 
 12. Rostellaria collina Con [?]. 
 
 13. Rostellaria m texana Con [?]. 
 
 14. Rostellaria, sp. 
 
 15. Turritella, sp. Occurs most in ledges consisting of worn organic 
 sand. 
 
 16. Avicula (?) [a large species]. 
 
 17. Caprina crassifibra Roem. 
 
 18. Caprina occidentalis Con. This and the previous number occur 
 imbedded in the upper face of nearly all of the hard ledges of any con- 
 siderable thickness. 
 
 19. Cardita subtetrica Con [?]. 
 
 20. Cardium mediale Con. 
 
 21. Cucullcea terminalis Con [?]. 
 
 22. Exogyra texana Roem. Common, especially in the upper 200 
 feet. 
 
 23. Lima wacoensis Roem [?]. 
 
 24. Modiola pedernalis Roem. 
 
 25. Monopleura marcida White. Occurring near the base in one 
 ledge and also in two ledges about 250 feet above the base. 
 
 26. Ostrea subovata Shum [?]. 
 
38 
 
 Geology of the Shafter 
 
 27. Ostrea, sp. ind. 
 
 28. Ostrea [like diluviana]. 
 
 29. Pecten occidentalis Con. Is common in the hard ledges. 
 
 30. Pecten mexicanus Roem. 
 
 31. Pholadomya, sp. 
 
 32. Requienia, sp. 
 
 33. Trigonia emoryi Con. 
 
 34. Trigonia texana Con. 
 
 35. Pngonoceras, near complicatum Hyatt. This occurs in the 
 upper 200 feet of the section. 
 
 36. Small hemispherical tooth of a vertebrate. Occurs with a 
 Herinea. 
 
 Correlations. — It is quite' evident that the lower four or five hundred 
 feet of the Shafter beds are to be regarded as the western equivalent 
 of the Glen Rose, as delimited by R. T. Hill in his Geology of the Black 
 and Grand Prairies, Texas.* Here as in the eastern part of the State, 
 Requienia and Caprina suddenly appear in the more indurated ledges 
 overlying the basal sands. Lithologically the Shafter beds and the Pre- 
 sidio beds are quite dissimilar, for while the latter consist of mixtures of 
 calcareous and arenaceous materials, the limestones and the marls of 
 the former seldom if ever contain any admixture of sand, and when 
 sandy beds occur they are free from marly material, and the sand is 
 thoroughly sorted and washed, and frequently ripple-bedded. 
 
 It may be that some of the sandy beds in the upper half of the sec- 
 tion are to be regarded as homotaxial equivalents of the Paluxy sands. 
 But the sandy beds here are individually not of any great horizontal 
 extent and they were evidently formed as submerged bars, probably in 
 deep water. They run out abruptly and are then in many cases 
 replaced by thin ledges of limestone, which are made up of worn organic 
 sand. Though these sandy lenticular masses have their greatest 
 development at from 100 to 300 feet below the top of the formation 
 the lowest ones occur less than 100 feet from its base, and must neces- 
 sarily be classified with that part of the formation which is equivalent 
 to the Glen Rose. The texture and other lithological characters of all 
 of these lenticular masses of sandstone is so uniform throughout that 
 there can be no doubt about the continuance of identical physical con- 
 ditions during the time of their making. This is one of the reasons 
 why the present author regards the Shafter beds as a unity. 
 
 It is nevertheless confidently believed that th£ upper part of these 
 beds will be found to belong to the horizon of the Walnut clays. It lies 
 under the base of the massive ledges of the Edwards limestone, the 
 lower part of which may correspond to the Comanche Peak limestone. 
 The uppermost 200 feet consist of marls that are more white and chalky 
 than the middle and lower part of the Shafter beds, and they contain 
 an Engonoceras. Two specimens of this fossil were found on the hills 
 northeast of Shafter. But decisive evidence of the correctness of this 
 conclusion can not now be given. 
 
 Geographical Conditions. — The Shafter beds were deposited in a more 
 open and perhaps also a deeper sea than the Presidio formation. The 
 
 *21 An. Rep. U. S. Geol. Survey. Part VII. 
 
Silver Mine District. 
 
 39 
 
 bottom of this sea seems to have been subject to frequent oscillations, 
 but its currents were probably more steady and sediments less copious, 
 and thus they were more perfectly sorted. The lesser inequalities of 
 the bottom, which already had a covering of some 400 feet of sediments, 
 naturally aided in making the sorting currents steady and in giving 
 them a greater sway. 
 
 THE. EDWARDS LIMESTONE. 
 
 The Shatter beds are overlaid by a massive white limestone in heavy 
 ledges. This forms the walls of a beautiful canon in Cibolo creek six 
 miles south of Shatter. Its walls rise to a height of 400 feet above the 
 bed of the creek, and consist for the most part of this limestone. The 
 same formation rises to a conspicuous ridge extending to two miles 
 from west to east, three miles south-southwest of Shatter. From the 
 east end of this ridge its outcrop is shown by a more subdued divide, 
 which first trends to the southeast and then to the east, crossing the 
 Cibolo at the canon already referred to. 
 
 A safe estimate of the thickness of this limestone makes it not less 
 than 350 feet. It probably includes the Comanche Peak limestone 
 below and the Georgetown limestone above, but the main and middle 
 part is clearly and without the least doubt the western continuation of 
 the Edwards limestone. My time in the field did not allow of a 
 detailed study of its ledges, but some diagnostic features were noted 
 which were unmistakably those of the Edwards limestone. Seams of 
 flat cakes of gray flint occurred some 100 feet above the base. There 
 were also some white ledges of a crystalline texture such as occur in the 
 Edwards limestone elsewhere. One specimen of the white limestone, 
 when ground and polished, was seen to hold in a transluscent calcar- 
 eous matrix a profusion of white grains, some of which were casts of 
 minute foraminifera, some worn organic fragments, some oolitic accre- 
 tionary spherules and some organic fragments with a white oolitic coat- 
 ing. Here and there in this mass were crystals of calcite with irregular 
 external shape but perfect internal cleavage, measuring from five to ten 
 millimeters in diameter. Two other specimens of a dark gray color 
 had a much more copious and partly finely fragmental matrix, in which 
 were imbedded thin small fragments of flat shells and some entire casts 
 of minute foraminifera. 
 
 THE DEL RIO CLAY ( ?) . 
 
 The Edwards limestone* dips to the south along the Cibolo creek 
 canon and in about a mile and a half south of the north end of the 
 canon, a clayey member covers its upper surface. This clayey member 
 is some eighty feet thick and it contains occasional thin ledges of lime- 
 stone, some of which are crowded with small spires of long and straight 
 gasteropods. During a fifteen minutes search I failed to find Exogyra 
 arietina Roem., which is usually always present in the Del Rio clay. 
 Whether this clayey member is to be referred to the Del Rio or not. 
 must hence for the present be left an open question. It certainly is 
 
 *The term must here be understood in a loose way. designating a limestone 
 which probably includes the Comanche below and the Georgetown above. 
 
40 
 
 Geology of the Shafter 
 
 more calcareous than the latter is near Terlingua in Brewster county, 
 eighty miles to the east. 
 
 THE BUDA LIMESTONE ( ?) . 
 
 Above this clay there is again another limestone about seventy feet in 
 thickness. This consists of strong ledges of a gray color. A single 
 specimen of the rock was examined closely and was seen to consist of 
 a compact or finely fragmental copious matrix with obscure organic 
 fragments. No fossils were looked for, and the indicated stratigraphic 
 equivalence of the rocks is merely an inference from its position. If 
 any of the upper cretaceous series come into view within this area they 
 will be found along Cibolo creek south of this point. Looking at the 
 country from a distance it appeared likely that all the rocks above the 
 last limestone are covered by the land drift. 
 
 THE LAND DRIFT. 
 
 Much of the uplands is covered by a product of land destruction, 
 which may be called a land drift. It consists of a heterogenous mix- 
 ture of angular and rounded boulders, pebbles, sand, and clay, which 
 evidently has been brought together and deposited where it now lies by 
 the same agents of disintegration, erosion, and transportation, that are 
 now at work in this region. When we find this drift, as we sometimes 
 do, from two to three hundred feet deep, it is difficult to conceive of 
 how the present drainage can have laid down such great quantities of 
 material, but when we begin to estimate the quantity of the rock 
 material which has been removed, the hundreds if not thousands of 
 feet of terranes which have slowly been taken away, we readily realize 
 that this deposit is merely an insignificant part, delayed in transition , 
 of a long and slowly creeping train of land drift urged onward by the 
 present drainage. A closer examination of the drift itself corroborates 
 this view. Over extensive flats, where erosion is weak, the surface is 
 covered by sand and silt, which probably accumulates faster than it can 
 be carried away. But where the land has a perceptible slope pebbles 
 and larger fragments appear. As the slope increases, the size of the 
 fragments also increase, showing a control by sheet flood erosion. 
 There are all kinds of mixtures of rounded and angular fragments. 
 Occasionally there are true creek gravels far away from the present 
 streams, showing that these have changed their courses in the past, 
 adjusting themselves to the ground structure. The material is all of 
 local derivation, as may be seen from the table, given below, which 
 fairly represents the composition of the coarse ingredients of the land 
 drift, from which it is largely derived. 
 
Silver Mine District. 
 
 41 
 
 TABLE SHOWING PERCENTAGES OF DIFFERENT KINDS OF PEBBLES AND 
 BOULDERS IN THE ALLUVIAL GRAVELS OF CIBOLO CREEK, NEAR 
 CIBOLO RANCH, SHAFTER, TEXAS. 
 
 Kinds of Rocks. 
 
 Diameter of pebbles and boulders 
 in inches. 
 
 % 
 
 inch. 
 
 1 
 
 inch. 
 
 3 
 
 inches. 
 
 9 
 
 inches. 
 
 Andesite 
 
 21 
 
 31 
 
 47 
 
 50 
 
 
 Undetermined crypto-crystalline volcanic rocks 
 
 26 
 
 32 
 
 24 
 
 8 
 
 Undetermined volcanic rocks of laminated structure.. 
 
 28 
 
 17 
 
 12 
 
 16 
 
 Undetermined siliceous volcanic rocks 
 
 9 
 
 11 
 
 11 
 
 17 
 
 
 Volcanic tuffs 
 
 7 
 
 5 
 
 
 
 
 
 
 Vesicular lava 
 
 
 
 7 
 
 6 
 
 
 
 
 Granite 
 
 2 
 
 2 
 
 
 2 
 
 
 
 Dark shale 
 
 7 
 
 2 
 
 1 
 
 
 
 
 THE RIO GRANDE DRIFT. 
 
 As we approach the fiats of the Rio Grande the bed-rock everywhere 
 disappears under a drift, which at first seems to be a continuation of the 
 land drift, but which, as we come near to the great water way, becomes 
 mingled with material of distant origin. Metamorphic rocks, such as 
 schists and quartzites, begin to appear, and there is a greater abundance 
 of granites among the larger boulders. Still farther out this becomes 
 intermingled with layers of finer alluvial material which has been 
 deposited by that stream before it had cut its changing channel down 
 to its present depth. All of this material may properly be called the 
 Rio Grande drift. For comparison a table is here appended showing 
 the character of the coarse ingredients in the present alluvial deposits 
 of the river. This differs somewhat from that in the river flats, 
 especially some distance away from the stream. The most notable dif- 
 ference is the absence of limestone pebbles. In the highest and oldest 
 drift the limestones have been entirely dissolved away by solution 
 through the agency of the ground water. 
 
42 
 
 Geology of the Shafter 
 
 TABLE SHOWING PERCENTAGES OF DIFFERENT KINDS OF PEBBLES AND 
 BOULDERS IN THE ALLUVIAL GRAVELS OF THE RIO GRANDE 
 RIVER., NEAR PRESIDIO, TEXAS. 
 
 Kinds of Rocks. 
 
 Diameter of pebbles and boulders 
 in inches. 
 
 X 
 
 inch. 
 
 1 
 
 inch. 
 
 3 
 
 inches. 
 
 9 
 
 inches. 
 
 
 71 
 
 63 
 
 79 
 
 50 
 
 
 Vesicular lava 
 
 
 
 1 
 
 18 
 
 
 
 
 Vein quartz 
 
 4 
 
 5 
 
 1 
 
 
 
 
 'Granite 
 
 4 
 
 1 
 
 1 
 
 5 
 
 
 "Schists 
 
 
 
 
 4 
 
 
 
 
 
 Quartzite 
 
 6 
 
 6 
 
 7 
 
 21 
 
 
 Sandstone and conglomerate 
 
 
 
 
 2 
 
 
 
 
 
 CJhert 
 
 2 
 
 2 
 
 1 
 
 
 
 
 Limestone 
 
 13 
 
 28 
 
 11 
 
 
 
 
 THE IGNEOUS ROCKS. 
 
 The igneous rocks of the Shafter area cover about one-third of its 
 entire surface. As to their mode of occurrence they present all the 
 three forms in which igneous rocks appear. 1. Lava flows and tuffs. 
 2. Dikes and intrusive sheets. 3. Laccolitic bosses. It is not 
 intended to discuss here the petrographic features of these terranes, for 
 this is a task that must be left to the specialist in that line. The 
 purpose is merely to present a brief account of my observations of these 
 rocks as they appeared in the field. 
 
 Laccolitic Bosses . — Laccolitic intrusions probably underlie the latter 
 lava flows in the Chinati mountains, occupying the region which is cen- 
 tral to the crescent that marks the outcrops of the sedimentary rocks, 
 but they are now mostly concealed by the later extrusives and by the 
 land drift. To all appearances they were submitted to extensive 
 erosion, and cut down to the same general level as the sediments, before 
 the land was covered by the last great lava flows. We find them 
 exposed now principally only in two places. 
 
 In the San Antonio canon and in the pass which separates the 
 Chinati peak from the south Chinati mountains a dark gray diorite 
 forms the lowermost terrane. This is of a moderately coarse texture and 
 is frequently traversed by some segregation veins measuring from an 
 inch to several feet in width. These consist of a red granite rock, 
 which is more acidic than the diabase and also somewhat more resistant 
 
Silver Mine District. 
 
 43 
 
 to the disintegrating agencies. It frequently forms dike-like ridges on 
 the surface of the country rock and these ridges have an east and west 
 trend. 
 
 To the north and east of Ojo Bonito there is an area underlaid by a 
 red, coarsely crystalline granite, possibly also of a laccolitic nature. It 
 extends for a distance of about three miles from northwest to south- 
 east. It has a width of about a mile at the north, and tapers almost to 
 a point at its southern end. It weathers into small rounded knobs that 
 appear like huge boulders or small hills rising above the land drift. 
 
 The Cienega Mountains consist of a laccolitic dome of a highly acidic 
 intrusive of light color, but its real relations in the field are not known 
 to the present author. 
 
 Dikes and Intrusive Sheets frequently appear in the sedimentary 
 rocks, running in a variety of directions. One of these dikes is seen 
 on a ridge about a quarter of a mile west-northwest from the Presidio 
 Mining Company’s mill. It is fifteen feet wide and trends N. 86° E. 
 Another, which is about twelve feet wide and bears N. 26° W., cuts the 
 Shatter beds about 500 feet north of the town of Shatter on the north 
 side of Cibolo creek. About a half mile to the west-northwest of the 
 new shaft of the Presidio mill a dike four feet wide trends E. 18° FT. 
 (See Fig. 1.) In the basin of Merita creek three miles southeast of 
 Shatter another dike cuts one of the remnants of the lava flows which 
 have been carved into buttes in that direction. In most of these 
 instances the rock is of a. dark basic kind, which easily disintegrates 
 and seldom appears on the surface. In the Presidio mine two of these 
 dikes have been encountered and even four hundred feet below the sur- 
 face they have been found to be thoroughly disintegrated and changed 
 into a yellow clay. The miners call them “clay dikes.” 
 
 Intrusive sheets, which follow the bedding planes of the sedimentary 
 rocks, are hardly less frequent. One comes into view at the base of 
 the east bluff near the junction of the Cibolo and the Sierra Alta creek, 
 and several lie intercalated with the Carboniferous and the Cretaceous 
 strata on the south and the southeast side of the Chinati mountains. 
 Some of these are of a dark gray andesite and are usually quite fresh 
 and unaltered at the surface. Some consist of a light gray rhyolite. 
 One of the latter, which outcrops in an arroyo about two miles north- 
 west from the shaft of the Presidio Mining Company, has a rough 
 columnar structure. 
 
 As we approach the Chinati mountains from the east the dikes and 
 the intrusive sheets become more and more frequent and at the same 
 time heavier. Occasionally they thicken into true laccolites. On the 
 south side some of the ridges in the area of the outcrops of the Carbon- 
 iferous limestone consist of heavy volcanic intrusive sheets, which lie 
 in the same position as the stratified rocks, and next to the mountain 
 the sedimentary rocks and the intrusives are intermingled almost pro- 
 miscuously. Evidently the source of these intrusives lies in that direc- 
 tion. 
 
 Lavas and Tuffs . — Fully one-half of the land is yet covered by lava 
 flows, which no doubt have extended over most, if not all, of its surface 
 at an earlier date. They have since been carried away by erosion. 
 We thus find them mostly absent along the principal drainage lines, 
 as in the direction toward the Rio Grande and along Cibolo creek. 
 
44 
 
 Geology of the Shafter 
 
 But they are invariably present on all high peaks and mesas. These 
 have been carved out usually from several superimposed flows, whose 
 upper surfaces are apt to form stair-like terraces on their sides. 
 The most extensive remnants of lava flows are those which form 
 the body of the Chinati mountains, and whose combined thickness 
 approaches 4000 feet. In all probability the lavas in the hills 
 east of Shafter are remnants of the same flows that form the Chinati 
 mountains and which once were continuous across the Cibolo valley. 
 In texture the lavas vary from coarse and fine porphyries to vesicular 
 and almost glassy lavas. In the mesa known as the Sierra Alta, which 
 is about six miles north of Shafter, the lava sheet which forms the cap 
 is underlaid by several strata of tuffs or volcanic sand, some of a yellow 
 and gray and some of an almost pure white color. The flatter kind 
 appears on the southeast side and shows a thickness of some forty 
 feet. 
 
 The vents from which these lavas came are to be found in the Chinati 
 mountains. 
 
 STRUCTURE. 
 
 The Chinati Uplift . — The controlling structural feature is an east- 
 ern half of the base of a great dome, whose west side lies concealed 
 under the Rio Grande drift and whose north side extends beyond the 
 limits of this district. (See Fig. 6.) The diameter of this uplift 
 approaches some fifteen miles, judging by the curve of the strike of the 
 tilted sediments. Its center lies on the north side of the south half of 
 the Chinati mountains. The sedimentary rocks dip away from this 
 point in all directions, with more or less regularity. The amount of 
 the uplift equals at least the entire thickness of all the sediments 
 exposedj which is about 7000 feet, so that if these had not been removed 
 they would now form an elevation seven thousand feet high, extending 
 for twelve miles east and west and north and south. But the work of 
 erosion has long ago cut down to the base of this dome and reduced it 
 to the general level of the surrounding country. This has exposed the 
 edges of the sediments, which lie in a crescent on the east side. Within 
 this crescent, which probably forms a continuous ring now partly con- 
 cealed by igneous flows and by drift, the erosive forces have probably 
 also cut down the central core of the igneous rocks that caused the 
 uplift. But to what extent this central igneous mass rose above the 
 general level of the land, as it now stands, and to what extent the mass 
 itself may have been cut down, it is not possible to say. It may have 
 had a considerable elevation. The diorite in the San Antonio canon 
 and the granite near the Ojo Bonito is all which at present comes up to 
 our view. 
 
Fig. 6.— General structure of the Shatter area along a line from northwest to southeast from San Antonio canon to south of Morita basin. A, Lava flows; B, Deep 
 
 intrusive diorite; O, Dike; D, Sedimentary rocks. 1— Ohinati uplift. 2— Morita uplift. 
 
46 
 
 Geology of the Shafter 
 
 On top of the uneven plateau of this reduced and leveled dome, the 
 south half of the Chinati mountains as we find them now, are built up 
 from several superimposed flows of extrusive igneous rocks. These 
 flows show no great slope in any direction and the molten lavas must 
 have had a high degree of fluidity. 
 
 The Morita Uplift . — In the basin of Morita creek, about six miles to 
 the southeast of Shafter, is another small and probably circular uplift 
 where the older sediments are raised to the level of the present surface. 
 (See Fig. 6.) From the center of this minor uplift the sediments like- 
 wise dip away, in all directions where they come into view. On its 
 south side the sediments are tilted to an exceptionally high angle, in 
 one place nearly 70°. The north and greater part of this uplift is 
 covered by heavy lava sheets. 
 
 Dips . — These leading features of the structure of this region are 
 shown in the generalized section which extends from the San Antonio 
 canon at the northwest to beyond the Morita basin at the southeast. 
 Returning to examine the details of the peripheral part of the principal 
 uplift we find that while the Chinati dome in a general way is quite 
 symmetric, the dip of the uplifted strata frequently shows minor varia- 
 tions in direction and in amount. Thus on the south side of the Chi- 
 nati mountains, dips ranging from 18° to 40° were measured, the aver- 
 age being perhaps 25° or 30° in the ledges of the Chinati series. 
 Toward the southeast, in the direction of the Presidio mine, the dip is 
 less and possibly does not average much above 20°. (See Plate 1.) 
 Farther north on the east side some dips were noted as high as 48° and 
 the average there is probably not less than 35°. Around the granite 
 area near Ojo Bonito the strike changes frequently and the dip is very 
 irregular and shows a local tendency to arrange itself radially to the 
 granite. This suggests that the granite lies at the center of a minor 
 uplift near the margin of the larger one, and that the granite itself is 
 an intrusion which caused a minor uplift. 
 
 Another structural feature is clearly shown on all sides: the highest 
 dips are nearest the center of the uplift and the lowest dips occur 
 furthest out. This has a bearing on the question of the nature of the 
 unconformity between the Chinati formation and the Cretaceous rocks. 
 The latter seldom have a pitch quite as high as the former, but this 
 circumstance is believed to depend mainly on the fact that the Cre- 
 taceous rocks are now exposed farther out from the main uplift. West 
 and north of Shafter their east dip is mostly less than 15° and often 
 no more than 10°, or even less than this. It is believed that the 
 greater tilting of the Chinati series around the Chinati uplift is not to 
 any great extent the result of pre- Cretaceous displacement, but is chiefly 
 due to the fact that the most highly tilted part of the rocks of the later 
 age has been eroded away and is not now to be seen. Nevertheless it 
 would evidently be wrong to infer that the unconformity between the 
 two series is altogether an unconformity without dip. No doubt there 
 is a difference in the attitude of the strata of the two ages, hut it can 
 not be asserted that this difference has any constant relation to the 
 Chinati uplift. Nor does it appear to be sufficiently marked to be 
 clearly indicated in any exposure within the limits of this area. In 
 places where the Cretaceous strata lie near to the central uplift, their 
 pitch is just as high as that of the older rocks. 
 
Pig. 7. — Illustration of faulting tangential to an uplift in the peripheral region, caused by sinking in of the center due to the greater weight of the higher laud. 
 
 A . normal faults. 
 
48 
 
 Geology of the Shafter 
 
 Tangential Faults . — The peripheral region of the uplift is marked 
 by some fault-lines which run more or less in a tangential direction 
 relative to the circle of the dome-like structure. Near the location 
 known as the Humphrey mine two such faults run parallel, east and 
 west, only 150 feet apart for a distance of about a half mile. (See Fig. 
 11.) They hade* slightly to the north and follow the two sides of a 
 block of limestone which forms a ridge between them, extending in the 
 same direction. This limestone is the same that caps a high ridge to 
 the south from this place, and it has been lowered not less than a thou- 
 sand feet. The faults are normal and the downthrow is to the 
 north. In the hills to the south and west from this place some 
 nearly vertical dikes and fissures trend in the same direction. One such 
 dike is near the diggings locally known as the “Iron Works.” This is 
 a wide dike and it runs east and west. Again, the fissure which has 
 been explored by the Chinati Mining Company trends about 10° north 
 of east. West of the hill on which the first diggings in the Presidio 
 mine were made, there is a fault running about N. 15° E. This hades 
 to the west and has a downthrow in that direction of nearly three hun- 
 dred feet. (See Fig. 10.) About three-fourths of a mile farther north 
 this fault changes into a fold, a small anticline, which can be traced in 
 the arroyos in that direction. In the country west of the Sierra Alta 
 still another fault was seen with the downthrow to the west. 
 
 In all of these instances it will be noticed that the downthrow of the 
 faults is toward the center of the uplift, while the dip of the terranes is 
 in the opposite direction. As volcanic agencies are more or less spas- 
 modic when compared with the steady and unceasing progress of 
 degradation, it is reasonable to suppose that the Chinati uplift rose to 
 its tallest height in the earlier stage of its existence. At first the vol- 
 canic forces raised the dome more rapidly than the work of erosion cut 
 it down, as is usually the case. It is evident that during all the fol- 
 lowing time when the elevated tract was being reduced, it must have 
 had a greater elevation than the surrounding country. This condition 
 of things would account for the nature of some of the faulting just 
 described. The central area being the higher would also be the heavier 
 and some sinking would naturally take place there. By reference to 
 Fig. 7 the above explanation of this kind of faulting will be made more 
 clear. It is a natural result of a slow subsequent settling, that may 
 be looked for near the periphery of any considerable elevation, however 
 formed. But some of the tangential faults seem to be the result of 
 another process. It is evident that if the inner side of a block lying 
 just outside the uplift is elevated, it is apt to a certain extent to be 
 rotated in such a way that its outer side will be lowered. Some of the 
 dislocations seen in Plate 1 were probably caused this way. 
 
 *Hade, deviation from the vertical. See note on page 50. 
 
50 
 
 Geology of the Shatter 
 
 Radiating Faults . — There are some small faults which follow a course 
 that is more or less radial to the principal uplift. Thus we find in the 
 west bluff of the Cibolo, about a mile below Shatter, a block in the 
 Shatter beds that has been elevated about a hundred feet. It is marked 
 off by a fault on its north side running W. 5° 1ST. and by a broken mono- 
 cline on the south side trending in the same direction. (See Fig. 9.) 
 Likewise in the east bank of the Cibolo looking east from Shatter, one 
 sees a number of these small faults cutting the marls and limestone at 
 intervals of a few rods, each with a hade to the south, and with the 
 pitch in the same direction. The downthrow here in no case exceeds 
 thirty feet. (See Fig. 10.) 
 
 Note — T he word “hade” (a contracted form of heald, or heeld) means the in- 
 clination of a vein from the vertical. It is synonymous with underlay and is the 
 complement of the dip, which signifies the deviation from the horizontal. The 
 dip, and consequently the hade, is measured at right angles to the strike, or com- 
 pass bearing. 
 
Fig. 9.— An elevated block Of the Shatter beds seen in the west bluff of Cibolo creek a mile below Shatter. A, a fractured fold; B, a fault; C and D, two fissures 
 
 without displacement. 
 
52 
 
 Geology of the Shafter 
 
 SECONDARY CHANGES IN SOME ROCKS. 
 
 Igneous Metamorphism . — The sedimentary rocks next to the dikes and 
 intrusive sheets which cut through them are sometimes slightly indu- 
 rated from the contact with the fused mass of the intrusive. But the 
 change produced is never very great. Even in the case of the thickest 
 dikes the effect of the heating is not perceptible farther out than four 
 or five feet from the intrusive. In no instance has any crystallization 
 been effected. Very often in case of the thinnest dikes no baking effect 
 whatever is visible. Near the intrusive granite northwest of Sierra 
 Alta the case is different. A limestone which lies in contact with the 
 granite has been changed to a true marble of moderately coarse crystal- 
 line texture. In places the marble is snowy white and elsewhere it is 
 traversed by wavy dark streaks. The limestone which belongs to the 
 Cieneguita beds, contains a small admixture of coarse sand,,* and these 
 grains have been fused into new forms, different from the sand in the 
 unaltered rock. The metamorphism has affected the limestone through 
 a thickness of from twenty to thirty feet and the marble ledge is 
 exposed for a distance of at least a thousand feet. 
 
 Changes Effected by Ground Moisture . — The changes wrought by 
 igneous metamorphism in this region are on the whole not as important 
 as the effects produced by the action of ground water. These are 
 almost everywhere apparent. The ground water has frequently cut 
 caverns in the limestone and again filled these with various mineral 
 and mechanical deposits. In the underground work of the Presidio 
 mine several empty caverns have been explored, and it is beyond ques- 
 tion that some of the ore deposits are fillings in such ancient caverns 
 in the carboniferous limestone. In the base of the Cibolo limestone, 
 three miles north of Shafter, there is a brecciated deposit which fills an 
 old collapsed cavern. It consists of fragments of all sizes, some angular 
 and some water-worn, blocks of shale from the lower part of the 
 Chinati series, cherts from the upper beds, pieces of glassy and spheru- 
 litic lavas, and some other rocks not known in outcrops near by. 
 
 The carboniferous limestone is quite free from impurities and has 
 been more readily dissolved by the ground water than any other forma- 
 tion. Small fissure-like passages were noted in many places on the sur- 
 face, but they were sometimes collapsed and sometimes filled with clayey 
 or ochreous deposits, or with pure and crystalline calcite. In the lime- 
 stone wedge near the Humphrey’s mine there was a solid body of cal- 
 cite three feet in diameter which had been followed to a depth of sev- 
 eral feet in one of the prospect holes. The upper part of the fissure 
 explored in the Chinati mine has clearly been opened up into cavernous 
 expansions by the ground water, and these were later filled by the ore 
 bodies. 
 
 Another effect produced by the ground water consists in a silicifica- 
 tion of the limestones. In some places this silicification ceased in its 
 incipiency and the silica has been deposited chiefly in the organic bodies 
 which the limestones contain. The fossils which such ledges carry fre- 
 quently weather out conspicuously on the surface. This is the condi- 
 tion of the thin-bedded member of the Cibolo beds north of Shafter. 
 Elsewhere the body of the rock is also affected and the entire mass is 
 
Silver Mine District. 
 
 53 
 
 charged with more or less silica. Such is often the Zone of Sponge- 
 Spicules in the Cibolo beds. South of the Chinati mountains and 
 north of the diggings already referred to as the “Iron Works” some 
 of the thin-hedded limestones have been altered to a wholly siliceous 
 white chert which yet perfectly retains the characteristic bedding planes 
 of these ledges. The thickened lenses, which are characteristic of 
 the horizon, are unmistakably present, although every trace of fossil 
 remains seems to have been destroyed. In general the silicifica- 
 tion of the limestones of the Chinati series is more pronounced 
 on the south side of the mountains than to the east and north. On 
 the surface of the Cibolo limestones one often finds rusty patches 
 impregnated with ferruginous and siliceous material. These patches 
 nearly always extend down into the bed-rock along irregular fis- 
 sures. Quite often they seem to partake of the nature of “chimneys,” 
 in that they go down more or less vertically without following any of 
 the joints in the rock. Many of them have been explored by prospect 
 holes and found to contain limonitic siliceous ore, that not infrequently 
 have shown a trace of silver and occasionally of other minerals. They 
 are most frequent about the diggings on the hill near the silver mine 
 and some were the surface outcrops of the ore in that mine. Some 
 appear on the bluff of the Cibolo limestone which follows the east bank 
 of Sierra Alta creek. About a mile and a half north from the junction 
 of this creek with the Cibolo there are two pointed buttes rising from 
 the lower slope of this bluff. These consist of a hard and dark brown 
 rock of fine texture which in the smaller butte is very much like a fine 
 sandstone, but which in the larger one contain minute crystals of some 
 feldspar. On the north side of this butte a layer of siliceous tufa, 
 clearly a precipitate, lies in close contact with the harder rock and 
 apparently changes into this and is continuous with it. I am inclined 
 to believe that these two “necks” are related to the rusty impregnations 
 in the limestone elsewhere, and that they are all due to the activity of 
 some underground water which has followed irregular lines of fissure 
 and cavernous passages in the limestone. When we consider that the 
 igneous intrusions are not very far away and that siliceous tufa usually 
 is a deposit from heated waters, the suggestion is at hand that the 
 ground moisture in this case was in a heated state. As feldspar has 
 been produced artificially in a wet way, under pressure ,it is not alto- 
 gether impossible that the rock in the necks is an underground product 
 of a hot spring, now laid bare by erosion of the containing terrane. 
 
 Even the igneous rocks show the effects of the action of ground water. 
 Most of the dikes have been thoroughly disintegrated and in many 
 places where explored in excavations they have been found to be reduced 
 to a yellow soft clay. In the Presidio mine the rock in one of these 
 dikes is altered to a pure white and structureless kaolin. 
 
 In the diorite near the Ellsworth mine in San Antonio canon there 
 is indubitable evidence of the action of heated moisture or gases in a 
 subterranean fissure. On one side of a dike-like ledge which cuts the 
 diorite, there are stalactitic structures of a greenish silicate that show 
 all degrees of irregularity and symmetric perfection of form. They 
 are from one-half to one and one-half inch in diameter and exhibit thin 
 concentric laminae as well as a radial structure in cross section. To all 
 
54 
 
 Geology of the Shafter 
 
 appearances these structures are related to the stalactites described by 
 E. S. Dana from Hawaii and which he found had been made by super- 
 heated vapors in underground fissures. 
 
 MINERAL DEPOSITS. 
 
 With the above account of the geology of the district a few notes on 
 the mineral occurrences will perhaps not be out of place. 
 
 At the outset it may be stated that the mineral deposits in this dis- 
 trict are limited to the older sediments and to the deep intrusives. The 
 Cretaceous rocks and the latest extrusives have suffered comparatively 
 little change from the mineralizing processes. They sometimes exhibit 
 fissures filled with calcite and occasionally the igneous flows have been 
 altered to a soft green chloritic rock or are otherwise decomposed, but 
 they are not known to contain any mineral deposits of importance. 
 Such deposits occur here in five forms, viz. : as lodes resembling fillings 
 in ancient caverns, as impregnations in small fissures of irregular form, 
 as contact deposits, as fissure veins, as fault-fissures, and as fissure 
 zones. 
 
 Lode . — To the first one of these categories belong the ores of the 
 Presidio mine. It lies in irregular pockets or lodes from a few feet to 
 nearly a hundred feet in length. These are connected by more narrow 
 leads, where the ore body is often more sharply marked off from the 
 unaltered ledges of the country rock. (See Plate 1.) The body of the 
 ore is mostly a yellow, somewhat porous rock-like mass of siliceous mate- 
 rial in which crystals of calcite and galena are not seldom seen. In the 
 average run, there is only about 3 per cent of carbonate of lime and 
 from 45 to 50 per cent of silica. The percentage of calcareous material 
 increases toward the wall of the ore bodies and it usually also increases 
 with the amount of galena which is present. The bulk of the silver is 
 in the form of a chloride dispersed through the body of the ore. But 
 there are also pockets of galena rich in silver. This is sometimes 
 altered to cerussite. Sphalerite, quartz and malachite are sometimes 
 found with the other minerals. While the Presidio mine is the prin- 
 cipal instance of this kind of ore deposits, other lodes of a similar 
 nature have been found in the same limestone to the west. The lodes 
 occur only in the most pure strata of the Cibolo limestone, and in the 
 Presidio mine they reach lower and lower levels in this formation as 
 they are followed to the east, descending more rapidly than the actual 
 dip of the sediments. 
 
 Chimneys . — The lodes in the Presidio mine are undoubtedly geneti- 
 cally related to the siliceous impregnations which frequently appear on 
 the outcrops of the Carboniferous rocks and which have already been 
 described as a result of the activity of the ground water at an earlier 
 period. None of these chimneys have as yet proved to be rich enough 
 to be profitably worked and they probably mark some less open passage- 
 ways of the solutions which deposited the ore in the Chinati mine. 
 Even some of these are clearly fillings in old caverns, whose etched 
 walls are plainly exposed in some prospect holes. Galena, calcite, and 
 hematite occur in the siliceous mass which now fills them. 
 
 Contacts . — At the contact of the granite with the Cieneguita beds 
 
Silver Mine District. 
 
 55 
 
 near Ojo Bonito masses of hematite and limonite lie against the side 
 of the intrusive, in some places near the exposure of the marble pre- 
 viously described. Farther north some pieces of float of crystalline 
 magnetite was taken, and it is reported that larger bodies of this 
 mineral lie along the same contact beyond the limits of this district. 
 To the same class of deposits we must also refer the hematite and the 
 magnetite masses which have been explored on the south side of the 
 Chinati mountains at the diggings known as the “Iron Works.” These 
 ore bodies are perhaps primarily the result of fusion by contact with the 
 molten mass of the intrusives at each place, but no doubt they have later 
 been altered and affected by solvent agencies following the zone of con- 
 tact. 
 
 About one mile south of Spencer’s spring, on the south side of the 
 mountains, two explorations have been made on another contact of 
 igneous intrusives. The uppermost part of one of these shows a cavern- 
 wall which is covered with a stalactitic calcareous deposit. Busty 
 cavern “box work,” such as is seen in the Great Wind cave in the Black 
 Hills, covers another wall. Limonite, galena and cerussite were the 
 principal minerals in this location and the latter occurs in a vein in the 
 bed of an arroyo near by to the south. 
 
 Fissures . — The main explorations of the Chinati mine, which is 
 located about two miles west of the Presidio mine, have been made on 
 a fissure vein bearing E. 10° N. (See Fig. 10.) This fissure is in the 
 Cibolo limestone and its hade is about 37° to the north. It shows on 
 the surface as a slight sag, running several hundred feet. About a hun- 
 dred feet to the south another fissure runs on a parallel course and this 
 has 'also been found to contain some ore. Hear the surface the upper 
 vein, where excavated, shows several cavernous expansions, but farther 
 down it is a well defined fissure with stringers of galena and calcite, 
 mostly next the hanging wall. It has been followed for about five hun- 
 dred feet horizontally at three levels about one hundred feet apart. 
 
Fig. 10.— Section at t^e Cftinatl mine, from N, tq 4. mineralized fissures; B, Presidio beds; 0, Cibolo limestone and shales; D, Igneous rocks of tfce Chinatl M^s. 
 
Geology of the Shafter 
 
 57 
 
 In the hills of the same limestone, from one to two miles southwest 
 of Humphrey’s location, and about a half mile west of the “Iron 
 Works,” are three small fissure veins which run north and south. These 
 carry carbonates and silicates of copper in the form of chrysocolla, 
 azurite, and malachite. In some small excavations which have been 
 made, these veins show a thickness of only a few inches. One hades to 
 the east and another to the west at an angle exceeding 45° from the 
 vertical. The limestone near the two veins which are farthest to the 
 west, has been changed to a marble. 
 
 Fault-Fissures . — In the Humphrey mine, on section 9, some four hun- 
 dred feet of tunneling has been done on a fault vein which follows the 
 north side of a wedge of Cibolo limestone that has been let down several 
 hundred feet as already described. The ore which has been brought up 
 is limonitic and siliceous and has been found to contain lead [galena 
 and cerussite] as well as silver. The quality of the ore is believed to war- 
 rant more extensive exploration. This ore lode has in one place been 
 followed into a cavernous extension in the limestone block on its south 
 side. 
 
F)g. 11.— Section N — S, at the Humphrey mine. A, Humphrey shafts on ai 
 
Silver Mine District. 
 
 59 
 
 Fissured Zones . — The Ellsworth mine in the San Antonio canon is 
 in what appears to be a fissured zone which cuts the diorite already 
 described, and also the overlying lavas. Some rich pockets of native 
 silver are reported from this claim and galena ore is also found. The 
 trend of the Assuring is somewhat south of east. 
 
 SUMMARY OF GEOLOGICAL EVENTS. 
 
 The earliest event recorded in this locality is the encroachment of the 
 sea during the later part of the Carboniferous age and the deposition 
 in the marginal region of this sea of some sandy and conglomeratic 
 shales and limestones. Floating ice possibly brought some boulders and 
 pebbles which were added to the finer sediments, and stray fragments 
 of vegetation sometimes drifted out from the shore and became 
 imbedded in the accumulating mud of the Cieneguita formation. The 
 bottom of the sea kept on sinking and the shore receded until littoral 
 sediments no longer were brought this far out and the clearer waters 
 gave a habitat to a deep-water marine fauna of cup-corals, crinoids, 
 foraminifera, and reticulate sponges, which are imbedded in the Cibolo 
 limestones. This condition of things lasted until near the end of the 
 Paleozoic, at the time of the Appalachian revolution. Then, or at some 
 time during the early Mesozoic, the bottom of the sea was elevated and 
 became land. This land was subjected to erosion and destruction and 
 hundreds or perhaps thousands of feet of the latest sediments were again 
 carried away to other seas. In this locality the land thus carved out 
 from the earlier sediments had reliefs of several hundred feet. Pos- 
 sibly there was also some folding and tilting of the ground, but this 
 can not have been very extensive. At the end of the Jurassic the land 
 again sank and the sea advanced once more. Over the hills and in the 
 valleys of the Jurassic lands the Cretaceous sea laid down first lagoon 
 muds, beach gravels, and sand, and later on more open sea sediments, 
 such as marly silts, limestones, and deep-sea ooze. This sea remained 
 until the be, ginning of the Tertiary or possibly somewhat later. Then 
 its bottom was elevated in the great uplift which resulted in the mak- 
 ing of the Eocky mountains. This region lies in the trend of that 
 great uplift and was part of it. The Tertiary age was a time of great 
 volcanic activity and the deep intrusives of the Chinati mountains were 
 now forced in under the sedimentary rocks and lifted them up into the 
 Chinati and the Morita domes, which rose high above the rest of the 
 land. No doubt this did not happen all at once hut gradually and by 
 repeated intrusions. In the meantime the elevations suffered continued 
 degradation, and the relief of the land was reduced until the uplifted 
 domes were lowered to about the same level as the laud around them. 
 Then again volcanic disturbances gained the ascendency. They had by 
 this time begun to reach the surface and they poured out liquid lavas. 
 These spread over the land in thick flows that must have extended out 
 for perhaps twenty or thirty miles from the vents in the Chinati moun- 
 tains. This activity continued for a long time, but at last it came to 
 a standstill. Probably it was not continued far into the Pleistocene 
 age, for the heavy igneous overflows have already since then had time 
 themselves to be extensively invaded by the destructive elements. The 
 
60 
 
 Geology of the Shafter 
 
 most extensive erosion of the earliest glacial deposits in the Mississippi 
 valley dwindles into insignificance when compared with the post-plntonic 
 erosion in this territory. The latest volcanic flows have contributed the 
 largest quota to the detrital accumulations of the land drift. This 
 drift is the most recent product of constructive agencies in this region. 
 
 ACKNOWLEDGMENTS. 
 
 The author wishes to express his obligations to Dr. Wm. B. Phillips, 
 the Director of the Survey, for much help and advice in all of the work 
 done ; to Mr. B. F. Hill for aid in the field ; to Prof. F. W. Sardeson for 
 aid in the identification of some bryozoa and corals ; to Prof. J. W. Beede 
 for examining the Paleozoic pelecypods ; to Dr. T. W. Stanton for deter- 
 mining some fossils of the Cretaceous rocks ; to Prof. J as. Perrin Smith 
 for the identification of two cephalopods, and to Dr. G. H. Girty for ex- 
 amining some Carboniferous fossils. 
 
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