BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

CRETACEOUS OF WESTERN TEXAS AND COAHUILA, MEXICO

BY E. T. DUMBLE

(Presented before the Society December 29, 1894')

CONTENTS Page Introduction...... 376 Localities of occurrence, character and relations of the rock...... 376 San Lorenzo section...... 376 Its location and the character of the country...... 376 Details of the section...... 377 Lower ...... 378 Bosque division...... 378 Its members...... 378 Flat Mesa section...... ; ...... 378 K ent section...... 378 Exposures in the Arboles and Burras mountains ...... 379 Episodes of the Bosque period...... 379 Fredericksburg division...... 379 West of the Pecos river...... 379 In the San Lorenzo section...... 379 Caprina crassifibra as a criterion of Fredericksburg age...... 380 The Kent locality...... 380 Washita division...... 380 In the Trans-Pecos area...... 380 At the Kent locality...... 380 Devils River section...... 381 In the San Lorenzo section...... 381 The Finley-Eagle Mountains section...... 382 Comparisons with other localities...... 382 Upper Cretaceous...... 383 Geologic succession and correlations...... 383 Dakota division...... i ...... 384 Colorado division...... : ...... 384 Montana division...... 385 Its importance...... 385 Areas investigated by the author...... 385 General section...... 386 Fossils...... 386 Dikes...... 387 Folds, faults and lava-flows...... 387 LIV—Bum.. Oxoi.. Soc. Am., Vol. 6, 1894. (375) 376 E. T. DUMBLE— CRETACEOUS OP TEXAS AND MEXICO.

I ntroduction . While in its broader features the Cretaceous of western Texas and of the northern portion of the Mexican state of Coahuila corresponds closely with that of the Colorado River section east of it, there are, nevertheless, many important differences in the stratigraphy and faunal relations well worth more detailed study than they have yet received. A few of these differences, which have come under my personal observation during trips made through various parts of the region, are presented as indicating the general character of the formation.

L o c a l it ie s o f O c c u r r e n c e , C h a r a c t e r a n d R e l a t io n s o f t h e R o c k . Only a few remnants of areas are found north of the Texas and Pa­ cific railroad, and that line may well be taken as marking the northern boundary of the Cretaceous deposits of western Texas, since, as a body, they pass north of it only (if at all) under that portion of the road which crosses the Llano Estacado. In Trans-Pecos Texas the basal rocks of this Cretaceous system are best exposed in the vicinity of the railroad, and, as the Rio Grande river is neared in going south from it, beds higher and higher in the section are found. The country, as a whole, slopes rapidly from the north toward the river, and, while in the southern portion of the area the Cretaceous rocks'are found at some of the highest altitudes, along the railroad they occur as the foot-hills of the mountain-blocks, whose cliffs of and limestones tower from 1,000 to 2,000 feet above them, or as detached mesas, buttes or ridges in the wide spread flats which separate these mountains. Much of the limestone of the Lower Cretaceous is metamorphosed as highly as is that of the Carboniferous, and is consequently as well adapted to withstand erosion, yet no trace of it has been observed upon the tops of the ranges north of the railway. Even where it now occupies higher altitudes in this region, as on Sierra Blanca and in the vicinity of Gomez peak, in the northern portion of the Davis or Apache mountains, its posi­ tion is evidently due to the orographic action which formed the moun­ tains. So far as I have observed, while the Carboniferous rests indiscrimi­ nately on various horizons of Algonkian and , the Cretaceous has only been found in contact with the Carboniferous, and Triassie. S a n L o r e n z o S e c t io n .

ITS LOCATION AND THE CHARACTER OF THE COUNTRY. This section was made in northern Coahuila, beginning at the head of the San Diego river, in that portion of the Burras range known as the DETAILS OF THE SAN LORENZO SECTION. 377

Arboles mountains, and extending eastward and northward to the con­ fluence of El Soro creek and the Rio Grande river, a distance of nearly 60 miles. The uplift which formed the Arboles mountains brought up the basal divisions of the Lower Cretaceous series and gave them a strong tilt to the east and northeast, thus forming a synclinal of Cretaceous rocks in the area between the mountains and the highlands north of Del Rio. Near the river the country is somewhat hilly; but between these hills and the mountains lies an undulating plain, the highest point of which is formed by the crest of a gentle fold of Washita limestone. On reach­ ing the San Diego river the country becomes rugged and mountainous, rising rapidly, and the peak near what is known as the Saddle is 3,700 feet above sealevel, while the plain is only about 1,500 feet.

DETAILS OF THE SECTION. The highest beds, geologically, are found on El Soro creek, and it is at this locality only that beds of the Upper Cretaceous (or Black Prairie series of Hill) are found, and even here they have a very limited extent. The entire area may be said to be Lower Cretaceous. The general section is as follows:

Postr Cretaceous. Feet. Stream gravel and brown silt covering part of flat between Las Vacas and the San Diego river. Reynosa conglomerate. On hilltops and along the San D iego......

Upper Cretaceous. Eagle Ford stage.—Limy clay, shales and flags. Soro creek...... 40

Lower Cretaceous. Vola stage.—Heavy bedded semicrystalline limestone of creamy white color. Void roemeri, Hill. El Soro creek...... 80 Exogyra arietina stage. Blue, yellow and red clays, with bands of sandy flagstones and concretionary limestones. Exogyra arietina, Roem.; E. drakei,* Crag.; Nodosaria texana, C on.; Pecten, sp. undet. Hills near Rio Grande...... 110-140 Washita Limestone stage.—Marly limestone, white to blue in color, semi­ crystalline in places, arenaceous and bituminous at base. Ostrea carinala, Lam. Surface rock of the plain...... 300-400 Caprina stage.—Dark blue or gray semicrystalline limestone, massive or heavy bedded, with much flint. Caprina crassifibra, Roem. ; Qryphxa and sponges. Along the San Diego river and in Arboles mountains ... 600- (?)

* Professor Cragin says of the specimens of E. drakei which I submitted to him : “ The tendency to freer beak than is usually seen in north Texas specimens makes it bear a degree of resemblance to E. arietina, with which, however, it could not be confounded.” 378 E. T. DUMBLE— CRETACEOUS OF TEXAS AND MEXICO. Feet. Comanche Peat and Exogyra texana stages.—Thin bedded, marly lime- Mort.; yellow to blue in color. Exogyra texana,, Roem.; Qryphxapitcheri, stones, Ostrea crenulimargo, Roem., etcetera. Arboles mountains...... 120-150 Glen Rose stage.—Thin bedded, marly limestones underlaid by heavy bedded limestone, with interbedded marly shale. Eequienia texana, Roem.; Pleurocera etrombiformis, Schlot. ( Vicarya branneri, H ill); Natica, Exogyra and Gryphsea of undetermined species. Arboles mountains. Base not seen...... 300- (?)

This is the only Mexican area which I have had an opportunity of ex­ amining. Other details of the section will be given below in connection with those of various Texan localities.

L ower Cretaceous.

BOSQUE DIVISION. Its Members.—The Trinity sands, Glen Rose or Alternating beds and Paluxy sand, which constitute the three members of the Bosque division at the typical locality,* have only been found together in west Texas, in the vicinity of Sierra Blanca Junction. At all other localities one or two of these members are missing. Flat Mesa Section.—The section of Plat mesa, beginning one mile north of the Junction, gives: Feet. Paluxy stage.—Brown quartzitic sandstone...... 35 Silicious conglomerate and grit...... 10 Brown sandstone...... 46 Alternating stage.—Arenaceous limestone, with Actxonella dolium, Roemer, and a small Exogyra...... ■...... 4 Trinity stage.—Brown quartzitic sandstone, exposed...... 100 This section is repeated in the hills just south of the Junction, with interstratal sheets of rhyolite and extrusions of porphyry. Kent Section.—In the section made at Kent (which corresponds in this respect with the sections made at the southeast point of the Llano Esta- cado and at Double mountain) we found no trace of the Alternating beds and only one stratum of sand, which is therefore referred to the Paluxy, because, as Taff has shown, at no place has the Fredericksburg been found resting directly on the Trinity. This sand, at Kent, rests unconformably on a semicrystalline limestone, presumably of Permian age. It contains the largest amount of silicified and opalized wood which I have found in Texas, and it may be that a study of this would decide whether the sand is altogether Paluxy or whether it includes both Paluxy and Trinity.

*Taff: Third Ann. Rep. Geol. Survey of Texas, p. 301. THE BOSQUE AND FREDERICKSBURG DIVISIONS. 379

Exposures in the Arboles and Burras Mountains.—In the Arboles and Burras mountains I found only the Alternating beds, which have there a very considerable thickness, and closely resemble the beds in the Colo­ rado section, both in the character of the rocks and in the fossils, includ­ ing the beds of Requienia texana, Roemer. I did not find that these Glen Rose beds cut through at any point in the mountains visited, but, in all probability, the Trinity sands will be found underlying them, being indicated by the occurrence of springs in the various canyons. Episodes of the Bosque Period.—In the Bosque period, therefore, ^the con­ ditions seem to have been the same over the entire Texan area. The encroachment of the sea was from the south over a gradually subsiding sea-bottom. The advancing shoreline is marked by the deposits of Trinity sands accompanied and followed, in the deepening waters, by the Glen Rose beds, which, while of. considerable thickness toward the south, decrease and finally wedge out toward the north, passing imper­ ceptibly into the Paluxy sands, which are the latest deposits of the pe­ riod, and, with the thin bedded marly limestones of the south, mark the shallowing waters of its close.

FBEDERICKSBVR& DIVISION. West of the Pecos River.—West of the Pecos river the Fredericksburg division has its most meager development along the Texas and Pacific railroad. At Kent it is not more than 50 feet in thickness, and in the vicinity of Sierra Blanca it is only 40 feet thick. Generally the Comanche Peak beds are missing, and the Caprina limestone rests immediately on the Exogyra texana bed.* In the San Lorenzo Section.—In the Arboles mountains there is no ap­ parent break in the sedimentation between the Glen Rose beds and the Exogyra texana, and the only means of determining where the one ends and the other begins is by the disappearance of the comparatively smooth and small form of the Exogyra of the former and the appearance of the better known E. texana, which gives its name to the latter. The Exogyra texana beds and the overlying Comanche Peak beds present little or no variation in the San Lorenzo section from their normal char­ acter. The Caprina limestone, however, has an extraordinary thickness and presents some differences in the distribution of the fossils. The limestones of this stage are massive and cherty, as usual, but the Caprina crassifibra, which at all other localities examined appears in continuous bands, occurs here in nests and at several different horizons. Between horizons of the Caprina limestone, in the lower part of the beds, are bands

# For details see Second Ann. Rep. Cteol. Survey of Texas, p. 717. 380 E. T. DUMBLE— CRETACEOUS Oi1 TEXAS AND MEXICO. containing a Gryphxa in considerable numbers. This occurrence has its only analogue in a band of Exogyra texana, Roem., which occurs in the upper portion of the Caprina limestone on Barton creek, near Austin. Near the top of the Caprina limestone, and above the last bed of Caprina observed in this division, a bed of sponges was found. The only similar occurrence with which I am acquainted is in the Double Mountain sec­ tion, and there the fossils are not so clearly distinguishable. The total thickness of the Caprina, as given by barometric readings, was 650 feet, but it may be found still thicker at other places in the mountains. Caprina crassifibra as a Criterion of Fredericksburg Age.—While the pres­ ence of Caprina crassifibra cannot be taken as conclusive evidence of the Fredericksburg age of the beds in which it is found, since this fossil also occurs abundantly in the Washita, yet in this instance the character of the rocks of the two divisions is so different that there is little danger of mistaking one for the other, the Fredericksburg being much darker in color and more highly metamorphosed than the Washita, which here seems to rest upon it unconformably. The Kent Locality.—The Caprina limestone was not found at Kent, the only member of the Fredericksburg which was positively identified being the Exogyra texana, with such fossils as Ostrea crenidimargo, Roem.; Gry- phssapitcheri, Mort.; Exogyra texana, Roem.; Schloenbachia peruvianus, von Buch. Whether the absence of the Caprina was due to the overlapping of later beds or to erosion prior to their deposition is not known, but it is probably due to the latter cause.

WASHITA DIVISION. In the Trans-Pecos Area.—This division, which in the Colorado River section attains a maximum thickness of 320 feet, finds in the Trans-Pecos area a far greater development, reaching a thickness of nearly 6,000 feet in El Paso county. Here, too, it gives evidence throughout its whole extent of being a deposit laid down in comparatively shallow water on a gradu­ ally subsiding sea-bottom. At the Kent Locality.—At no place is this gradual subsidence more plainly shown than around Kent, where the Fredericksburg and Bosque divisions were evidently subjected to considerable erosion and some dis­ turbance prior to the deposition of the Washita, and we find the lower beds of the Washita resting upon the Fredericksburg, while the higher beds overlap farther and farther on the Bosque sands. The rocks here are principally clays, marls and marly limestones; but in the beds of this division on the northwest side of Gomez peak a bed of limestone was observed containing a considerable quantity of silicious pebbles. The fossils are very abundant and well preserved. SECTIONS OF THE WASHITA DIVISION. 381

The Exogyra arietina clays were not found at this locality, unless they be represented here by the beds containing the very similar form of Exogyra plern, Cragin. In the bed of a stream a few miles southwest of Kent there is an ex­ posure of a fine white marble with reddish spots, which is lithologically similar to the Vola limestone. It contains great numbers of a large fossil which in general shape, size and appearance closely resembled the Vola roemeri of Hill, but they were all so worn that certain identification could not be made. In the upper portion of the beds were two or three bands which were simply masses of Nerinea volana, Crag., beautifully preserved in calcite. A good collection of these was made by Mr W. F. Cummins and myself. The stratigraphic relation of these beds to those of the Kent section was not traced, but beyond doubt they are referable to the Vola limestone. The thickness is much greater here than in the eastern ex­ posures. Deoils River Section.—In the Devils River section, at the base of the Washita, there is a very sandy limestone, which in places carries con­ siderable bituminous matter, especially in its upper portion. This sandy limestone is water-bearing and furnishes many good wells. It is over­ laid by a semicrystalline, heavy bedded limestone, which carries, together with such characteristic fossils as Schloenbachia leonensis, Con., and Kingena ( Terebratula) wacoensis, Roem., a bed of Caprina crassijibra, Roem., which in size of individual fossils greatly exceed any I have seen in the Caprina beds of the Fredericksburg. The exact relationship of these beds is well shown in some of the bluffs of Devils river northwest of Del Rio. One of them, by barometer measurement over 200 feet high, had at its base the large Pecten texana, Roem., of the Washita, about 70 feet above this a band of fossiliferous limestone containing Kingena (Terebratula) wacoensis, Roem., and Schloenbachia leonensis, Con., while at the top of the bluff there is a massive bed of Caprina crassijibra, Roem., with an echinoderm which, although badly weathered, seemed to be a Pyrina. Succeeding the Caprina bed, as shown by exposures southeast of this locality, the limestones are more marly and carry an abundant fauna of species characteristic of the Washita, and are finally succeeded at Del Rio by the Exogyra arietina clays and their capping of the Vola limestone. In the San Lorenzo Section.—In the San Lorenzo section, in Coahuila, the Washita forms the surface rock of a large part of the area, being over­ laid by the Upper Cretaceous only in places very near the river. The Washita has the sandy, water-bearing bed at its base, and this is capped in places by asphaltic limestones. The overlying limestone is semi­ crystalline and very light in color. The only fossil found was Ostrea carinata, Lam. 382 E. T. DUMBLE— CRETACEOUS OP TEXAS AND MEXICO.

The Exogyra arietina clays and shales have a thickness of 140 feet, and are as fossiliferous as usual. They occur only near the Rio Grande. The change from the clays, shales and flags, which constitute these beds, to the massive and somewhat cherty Vola limestone, which overlies them, is abrupt. The Vola has its usual color of creamy white flecked with red, and has a thickness of 80 feet. In places it is almost a saccharoidal marble, and is better developed here than anywhere that I have seen it, except at Kent. Its characteristic fossil, Vola roemeri, Hill, was found in the bed of El Soro creek. Tke Finley-Eagle Mountains Section.—The section between Finley and the Eagle mountains gives the greatest thickness yet observed in the Washita limestone. It is a region of long continued disturbance and volcanic activity, and the entire section has not been found in any one locality, so that the exact relations of some of the beds described are not certain, though the section, as a whole, is approximately correct. Here, too, is shown the greatest variability in the deposits, due to the oscilla­ tion of the sea-bottom. Toward the Eagle mountains the limestones are somewhat marly and carry an abundant fauna, as shown by the collec­ tions of Mr W. H. Streeruwitz. Just south of Sierra Blanca are alter­ nating beds of sandstone and flags, with a thickness of over 4,000 feet, Exogyra amerricana, Marcou, being the only fossil found by me. This is the upper part of the division, and the lower rocks are probably overlain by the later sediments which cover the flat lying between this and the Caprina ridge north of it. In this lower portion, as exposed elsewhere, is repeated the occurrence, noted on Devils river, of the Caprina above what was formerly considered its culminating point in the top of the Fredericksburg, but here it is accompanied by a number of Caprotina forms. The rock-materials: sand, grits, conglomerates, limestones and beds of gypsum still indicate the oscillating character of the sea-bottom. Around Eagle Flat station, and between that point and the Diabolo mountains to the north, the limestones are very arenaceous, and are capped with a sandstone which I have previously referred to the Dakota. Comparisons with other Localities.—From the observations made it would appear that certain fossils which occur at one horizon in the Colorado- Red River section are found at entirely different horizons west of the Pecos. One of these is Exogyra plexa, Crag. In Williamson county this fossil is found in the base of the Washita limestone, if not in the Kiamitia clay, and it is also found at about the same horizon on Duck creek, in Grayson county, while in the Kent section it is more than 300 feet above the base of the limestone and overlies fossils which are found above it in the Colorado section. The recurrence of Caprina and Caprotina forms has already been noted, and the persistence of these forms from the Glen COMPARISON OP VARIOUS WASHITA LOCALITIES. 383

Rose beds into the Washita is additional evidence of the unity of the deposits grouped together as the Lower Cretaceous series. The recur­ rence of so characteristic a fossil as Gryphsea dilatata, of the , in its varietal form of G. tucwncarii, Marcou, in the base of the Washita, at Kent, has also been noticed.* Another case is that of Exogyra arietina, Roem., which, even as far west as the Pecos, overlies the Washita limestone, while in the Sierra Blanca section it is, according to Taff, within 100 feet of the base. In the Malone mountains I found the Nodosaria bed, which is part of the Exogyra arietina beds, underlying a Caprina bed, and Taff found the same relation existing at the south end of the Quitman mountains. Kingrna (Terebratula) wacoensis, Roem., is a form which, in the Austin section, be­ longs immediately below the Arietina. I found a Terebralula, in nowise distinguishable from it, below the Qaprina bed of the Devils River sec­ tion as well as above that bed, and it ranges from bottom to top of the Kent section. It was collected in quantities by Mr Streeruwitz from Devils ridge, just west of the Eagle mountains. While it is easy to dis­ tinguish two or three very different forms, such as an elongate form and another which is nearly as broad as long, these are not confined to separate horizons, but occur in the same bed, and when a sufficient number have been collected all intermediate stages can be readily found in them.

U p p e r C r e t a c e o u s .

GEOLOGIC SUCCESSION AND CORRELATIONS. My first section of the Rio Grande Cretaceous, in “ Notes on the Geology of the middle Rio Grande,” f gives the general succession of beds between Del Rio and thé Cretaceous parting near the Maverick-Webb county-line. The divisions there given were based principally on lithologie grounds, although paléontologie differences were also noted. Beginning at the base, the Val Verde flags, which were found overlying the Vola limestone, were considered the equivalent of the Eagle Ford or Benton shales (no trace of Dakota fossils having been observed). The overlying Pinto limestone was evidently the continuation or the equiva­ lent of the Austin limestone, both lithologically and paleontologically. The Upson clays, corresponding both in material and fauna with the Ponderosa marls, followed, and were in turn succeeded by the San Miguel beds, which were the supposed equivalents of the Glauconitic or Navarro beds of the East Texas section. A subsequent study of a portion of the fossils of the San Miguel beds by Dr C. A. White confirmed this, and he

♦See Am. Geologist, November, 1893, pp. 309-314. f Bull. Geol. Soc. Am., vol. 3, pp. 219-230.

LV—B u m . G k o l. Soo. A m . , V o l . 6, 1894. 384 E. T. DUMBLE— CRETACEOUS OF TEXAS AND MEXICO. decided that they belong to the Ripley. In the East Texas section the Navarro or Glauconitic beds are the highest beds recognized. In the Rio Grande section the corresponding beds are near the middle of the section, being overlaid by nearly 4,000 feet of deposits of Cretaceous age. This 4,000 feet I separated into two stages, the Coal series and the Escondido beds, and grouped all of the beds above the Pinto limestone as the Eagle Pass division. Under the names proposed and used by Dr White * for the divisions of the Upper Cretaceous the section given would be as follows: Montana division = Eagle Pass division. Colorado division == Pinto limestone and Val Verde flags. Dakota division. Not observed here.

It is, of course, entirely possible that the basal portion of the Val Verde flags represent a part of Dakota time. While it may be impossible to draw the line closely between the repre­ sentatives of the Fox Hills and Fort Pierre stages in this region, I have placed it provisionally at the contact of the Coal series and Escondido beds, where we have the final disappearance of the Exogyra costata, Say, which extends in its varieties throughout the beds thus defined as Fort Pierre, and the first appearance of Sphenodiscus lenticidaris, Owen.f This is the characteristic fossil of the Escondido beds in this section and is found in large numbers along the Rio Grande, from just below Eagle Pass to the southern line of Maverick county. Such a division places all of the Texas Cretaceous coals, whose horizons have been determined, in the Fort Pierre stage or subdivision.

DAKOTA DIVISION. My first reference J of certain sandstones in the vicinity of Eagle Flat to this horizon has since been verified by the fossils found in them and extended by Taff in his Carpenter Spring section.§ No other exposures have been recognized. On Soro creek, in northern Coahuila, the Eagle Ford flags rest directly on the Vola limestone.

COLORADO DIVISION. The Eagle Ford clays were found by Taff and the writer in the Eagle mountains, as has been noted in the First and Second Annual Reports of the Texas Survey.

* Bulletin 82, U. S. Geological Survey. fC ragin: Fourth Ann. Rep. Geol. Survey of Texas, part ii, p. 245. % First Ann. Rep. Geol. Survey of Texas, p. xlviii. g Second Ann. Rep. Geol. Survey of Texas, p. 734. COLORADO AND MONTANA DIVISIONS. 385 In the middle Rio Grande section they are lime flags, separated by thin bands of laminated clays, and contain Inocerami and fish remains. On Soro creek, west of Del Rio, they are also- flaggy, but somewhat more arenaceous as well as bituminous. Inocerami were the only fossils found here. At this locality the contact between the Yola and the Eagle Ford beds is well shown, and there seems to be absolute conformity between them. The Vola limestone shows no sign of erosion previous to the deposition of the Eagle Ford, but stretches along as a perfectly plain surface through an exposure of several hundred feet. Above the Washita beds, on the northeast side of Gomez peak, near San Martine, there are beds of shaly to flaggy limestones belonging to this stage. At only three localities have I been able to determine the presence of beds equivalent to the Austin limestone. It was found well developed in the middle Rio Grande section, and it was recognized at two localities in the Davis or Apache mountains. One of them was a small remnantal patch near Apache spring, just opposite the ranch-house of the Newman brothers. This was a bed of Gryphsea vesicularis, var. auceUa, Roem., with Ostrea congesta, Con., a large Inoceramus and an unknown oyster. It was also recognized near Gomez peak, at the locality mentioned above, over- lying the Eagle Ford shales.

MONTANA DIVISION. Its Importance.—This division is probably the most valuable, econom­ ically, of all the west Texas Cretaceous, and, when the entire section is made out, it may prove to have as great, if not a greater, thickness than the Washita. Areas investigated by the Author.—The only two areas in which these beds have been examined by the writer are along the middle Rio Grande, the rocks of which were described in the paper referred to above, and a district in Presidio county, lying between the Viejo or Rim Rock moun­ tains and the Rio Grande, which, as it includes the San Carlos coal mines, will be here described as the San Carlos section. This area is situated twenty-eight miles by wagon road south of Chispa and eight miles from the river. The valley is rendered extremely pictur­ esque by its bright colored rocks and the beautiful forms into which they have been carved by the combined effects of faulting and erosion. Four miles to the east of the San Carlos camp the Rim Rock or Viejo moun­ tains rise to a height of 2,700 feet above the valley, the upper 200 or 300 feet being a vertical wall of trap, with columnar structure, in which single columns of 150 feet are clearly defined. The mountains, running north 386 E. T. DUMBLE— CRETACEOUS OP TEXAS AND MEXICO. and south, form a land-locked valley, which is further inclosed by ranges on the Mexican side of the river. All of the sedimentary rocks which are exposed, except possibly num­ ber 2, belong to the Fort Pierre, but to the west are many white hills which may belong either to the Colorado or Fox Hills. General Section.—>The following is a general section from the more de­ tailed one made by Mr Cummins and myself. The measurements are from readings of aneroid barometers and are approximately correct: Feet. 1. Lava-flow; rim rock of mountain...... 200-300 2. Interbedded sandstones of various colors with calcareous clays and vol­ canic ash...... 550 3. Conglomerate, resting unconformably on 4...... 1- 16 4. Lava-flow; apparently conformable on 5...... 50 5. Interbedded brown and red sands, purple shales and yellow quartzitic sandstone...... 500 6. Gray and purple shales with thin strata of sandstone...... 200 7. Coal shales with beds of laminated sands and two seams of coal; highly fossiliferous between and below the coal seams...... 800 8. Interbedded sands and sandstones, some highly calcareous; fossilif­ erous...... 250 9. Shales with concretions of clayey limestone containing fossils...... 175 This gives a total thickness of over 2,800 feet. Fossils.—The fossils contained in numbers 7, 8 and 9 are very abun­ dant and well preserved. From the large collections made a suite was sent Mr T. W. Stanton, of the United States Geological Survey, for deter­ mination. He reports the following species: 1. Nautilus dekayi, Morton. 2. ScMombachm deUmarerms, Morton. 3. Schloenbaekia, (?) n. sp. 4. Baculites asper, Morton. 5. Baculites ovatus, Say. 6. Placenticeras guadalupse, Roemer. 7. Heterocerm, sp. undet. 8. Hamites, two species, possibly new. 9. Turitella trilira, Con. var. 10. Streptidura interrupta, Con. (?) 11. Gyrodes, sp. undet., cf. G. inframrinata, Gabb. 12. Volutomorpha, n. sp. (?) 13. Ostrea elegantula, Newberry. 14. Exogyra costata, Say var. This particular variety, intermediate between typical E. costata and E. ponderosa, occurs in the Eutaw group and in the lower part of the Ripley, in Alabama, always at a lower horizon than true E. costata. 15. Pinna, sp. undet. Resembles P. loquata, Con., of the Ripley. 16. Inoceramus cnpsi, Mantell. 17. Trigoma thoradca, Morton. FOSSILS, DIKES, FOLDS, FAULTS AND LAVA-FLOWS. 387

18. Cardium alabamehse, Gabb. = C. multistriatum, Gabb. This species was also collected by Professor Cummins in 1889 across the Rio Grande river from Presidio. 19. Cardium. tenuistriatum, Whitfield (?) 20. Oyprimma, n. sp. (?) 21. Liopistha, sp. undet. Related to L. protexta,, Con. 22. Liopistha (OymeUa), sp. undet. 23. Pholadomya, sp. undet. Resembles P. roemeri, Whit. 24. Teredo (?) tubes. 25. An undescribed coral. Mr Stanton adds the following note: “ While the fauna as a whole is closely related to the Ripley (Navarro) fauna, several of the species, and especially the ammonites, are not known to range so high elsewhere. I regard the Exogyra, the Placenticeras guadalupse, and the two species of Sckloenbachia as especially important in determining the horizon as lower than Ripley. The preponderance of evidence in the collection indicates that it comes from a somewhat lower horizon lying somewhere between the Navarro beds and the Austin limestone—that is, within the Ponderosa marls and probably pretty well down in them.” A good specimen of the tooth of Ptychodus mortoni, Ag., was also found. Dikes.—The valley is intersected by two series of dikes, one striking north and south, the other and later east and west. They probably rep­ resent the fissures through which the two lava-flows of the mountain welled up to the surface, although we found no immediate connection between them. In one place where the coal seam is cut through by a dike the coal is coked, but usually the metamorphism of the rocks along their courses extends only a small distance on either side. Folds, Faults and Lava-flows.—Monoclinal folds are beautifully devel­ oped in this valley, with the same strike as the later dikes. The great San Carlos fault, west of the camp, has a throw of over 2.300 feet, bring­ ing the upper lava-flow nearly to the level of the valley. The strike of the fault is north and south and its downthrow is to the west. There seems to be an approximate synchronism between these lava- flows and those of the Shumard knobs. The volcano of Pilot knob, south of Austin, was active during the later stages of the Austin lime­ stone, and probably continued well into the Ponderosa, and since no erosion was observed in the bed immediately underlying the first lava- flow, this lava of the Viejo mountain is also seemingly of Ponderosa age. From the San Carlos and Balcones faults it is evident that these lines of eruption continued to be lines of structural weakness long after the dikes and lavas themselves had cooled. Thus the main Balcones fault 388 E. T. DUMBLE— CRETACEOUS OF TEXAS AND MEXICO. has a direction closely approaching tha:t of the later of the systems of dikes, and the nearly north-and-south direction of the Cretaceous-Tertiary contact in Maverick county is probably due to a monoclinal fold of the Cretaceous similar to those observed west of it. This gave rise to an embayment in the Tertiary which stretched northward nearly or quite to Uvalde, which may well be called the Nueces embayment, since that stream now drains the entire area of the present surface exposure of these deposits.