PUBLICATIONS 
OF THE 
UNIVERSI:fY OF TEXAS 
BOARD OF EDITORS 
WILLIAM JAMES BATTLE, Editor-in-Chief. 
HERBERT EUGENE BOLTON, Secretary and Manager. 

KILLIS CAMPBELL, The University Record. 
WILLIAM SPENCER CARTER, Galveston, Medical Series. 
LINDLEY M. KEASBEY, Humanistic Series. 
THOMAS H. MONTGOMERY, JR., Scientific Series. 
PHINEAS L. WINDSOR, General Series. 

The publications of the University of Texas are issued twice a month. For postal purposes they are numbered consecutively as Bulletins without regard to the arrangement in series. With the exception of the Special Numbers any Bulletin will be sent to citizens of Texas free on request. Communications from other institutions in reference to exchange of publications should be addressed to the University of Texas Library. 
I 82-807-lm-899 



BULLETIN 

OF THE 

UNIVERSITY OF TEXAS 

NUMBER 93 
SCIENTIFIC SERIES NO. 11 APRIL 15, 1907 
A Sketch of the Geology of the Chisos 
Country, Brewster County, Texas. 

By 
J. A. UDDEN, Ph. D., F. G. S. A., 
Professor of Geolo&y, Augustans College; formerly Assistant Geologist, University of 
Texas Mineral Survey. 


Entered at th~ Postojjice in Austin, Texas, as second class mail matter under Act of' Congress, pa55ed July I6, I894 
AUSTIN , TEXAS 
Cultivated mind b the guardian genius of de­mocracy. • • • It is-the only dictator that free­men acknowledge and the only security that freemen desire. 
President Mirabeau B. Lamar. 
EDITOR'S NOTE. 
This Bulletin, prepared for the University of Texas Mineral Survey, under .the directi<>n of Dr. William B. Phillips, was ready for publication in the spring Of 1905, but, owing to the discontinuance of the Survey, could not be published at that time. Its high· practical_ and scientific value leads to its issue now as a Bulletin of the.University of Texas. U~ortu­nately, both the drawings and cuts originally prepared to illustrat.e it have been loRt, ~nd it has not bet>n practicable to reproduce them. 


CONTENTS 
PAGE. 
Introductory statement . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . 9 
A. 	General topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 · l. Differential eroaion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1O 
2. 	The .graded plains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1O 
(1) The making of the graded plains.. ... . ... . ... . ....... 10 

(2) Controlling factors .. ................ ....... ......... 11 

(3) Maintenance of the eroded plain ............. .......... 13 

3. 	The high reliefs. . . . . . . . . . . . . 
. . . . . . . . ... . . . . . . . . . . . . . . . . . . . 14 
B. History of the drainage..................................... .. 15 C'. Stratigraphy . . . ... ...... . ... .. . . .. .... .: . . . . . . . . . . . . . . . . . . . . 16 
I. 	The general section.. . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . 17 
(1) The Palaeozoic .................... .............. .'. . 18 

(2) The Ordovician ...............'............. .... .'.... 18 

(3) The Upper Carboniferous ........ .. .. . . . .. , . . . . . . . . . . 20 

(4) 	Rgcks or' the Cretaceous age.......... ......... ......._21 The Lower Cretaceous limestones. . . . . . . . . . . . . • . . . . . . . . . 21 At Persimmon gap ............. .•... ..... ........ 22 On the Altuda mountain ............ .'. . . . . . . . . . . . . 22 On the Cienega mountain·....... ........ ..... ... : . . 23 In Mesa tle Anguila. . . . . . . . . . . ... . . . . . . . . . . . . . . . . . 23 On Christmas mountain .. ........ ...·. . . . . . . . . . . . . . 24 On Mariscal mountain. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 In the Carmen range......................... . ·. . . . . 25 Characteristic horizons .... .... .................·. . . 26 Chemical composition . . . . . . . . . . . . . . ... . . . . . . . . . . . . 27 Characteristic weathering .. ... .... . . : . . . . . . . . . . . . . 28 The Upper Cretaceous series ... ...........·. . . . . . . . . . . . . 29 The Boquill!M! flags ............ ...'. . . . . . . . . . . . . . . . 29 Chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Clastic texture .........·................... ........ 30 Field appearances .......... ·. . . . . . . . . . . . . . . . . . . . . . 30 Resistance to erosion ... . ..... .. .... . .... . .. . . .·. .. . 31 Minor characteristics . . . . . . . . . . . . . . ... . . . . . . . . . . . . 32 Fossils .............. ......... ........... .... .... 32 Correlation . . . ........ ................ , . . . . . . . . . 33 The Terljngua beds . .... . .... : '. . . . . . ... . . . . . . . . . . . . . . . 33 Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Cottonwood creek section . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Section south·of Cuesta Blanca. . . . . ... . . . . . . . . . . . . . 35 Other sootions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Topographic relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 
Contents. 
PAGE. 
Organic remains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Correlationa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 The Rattlesnake beds .. .........-. . . . . . . . . . . . . . . . . . . . . . . 41 The rone of change . ................... ... ......... 41 Thickness . .. . . .. .. .. .. .. .. . .. ... . .. .. .. .. . . . . ... . 42 Sandstones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Finer sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Original calcareous deposits. . . . . . . . .. . . . . . . . . . . . . . . 45 Special sections .............. ....... .". . . . . . . . . . . . . 45 Section near Chi-sos pen . . . . . . . . . . . . . . . . . . . . • . . 46 Section north of Chisos pen. . . . . . . . . . . . . . . . . . . . 47 Section south of Chisos pen . . ..... . ... .. . . .. .. . 47 Foasil wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 48 Vertebrate remains ..... ... .. . .. . .. .. . ..... .. :. . . . . 49 Tu:ffs ....... ...... ..................... ......... 49 Topographic features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Fossils .......... ..... .. .. .. ...... . ......... . . . . 51 Correlation . . . ...... , .. ....... .. ............... ·. . . 54 The Tornillo clays ....... .................. ...... ..... 54 Characteristic coloring .. '. . . . . . . . . . . . . . . . . . . . . . . . . 54 Mechanical composition . . . . . . . . . . . . . . . . . . • . . . . . . . . 55 Chemical composition . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 56 Concretions . . .................... ....., . . . . . . . . . 57 Weathering and creeping.............. :., ...... , .. 57 Geographical distribution . . . ........'. . . . . . . . . . . . . . 5,8 Physiographic character . . . . . . . . . . . . . . . . . . . . . . . . . . 59 T·hickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Fossils ... . .............. .... .. .. . . . . .. . . . .. . ... 60 The Chisos beda. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 The typical rock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Sandstones . . ...... " ..... .....................-. . . . 112 Clays ...... .. .... ... ............... : .. ...... . ... 63 Conglomerates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Descriptions of exposures. . . . . . . . . . . . . . . . . . . . . . . . . . 64 Limits and thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Life and age. .. .. .. .. .. .. .. . .. . . .. . .. .. .. .. .. . . . . 66 The Crown conglomerate .... . .. .... ... .. . .. ... . ... . : . . . 66 The Burro gmvela and tu:ffs. . . . . . . . . . ... . . . . . . . . . . . . . . . 67 The Dugout clays and gravels. . . . . . . . . . . . . . . . . . . . . .. . . . . 68 Recent alluvium and land drift. . . . . . . . . . . . . . . . . . . . . . . . 69 
( 5) 	The igneous rooks. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . 70 The Altuda granite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 
Contents. 7 
PAGE. 
Laccoliths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 
Sills . . ........·........................ .... : .... 72. 
Dikes . .. ... .. . ...... ......... .... ... ... . ........ 73 
Plugs ............. .. .... ......................... 74 
Surface flows .. .. ........ .. . . ...... .. ... ... . ..... 74 

· D. Geological structure .............. , ..... ...................... 75 

E. The Caballos ridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 
F. Overlap of the Cretaceous................................. .... 78 

G. The Rocky Mountain uplift..................... , . ............. 79 

H. The great Terlingua fault .. ... . , ... .. .... ....... .. .. .. ... .. . . . 80 

I. The sunken block. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 
1. Faults .. . ... . ....... .. .. . . . ..... .. .... ....... . .•. ........ 81 

2. Folds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 
3. Christmas mountain fold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 
4. Mariscal mountain fold. . . . . . . . . .............. .'. . . . . . . . . . . . 85 

5. Ohi!IOs mountain fold. : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 
J. Summary of geologic history........... .......... ....., . . . . . . . . 87 

K. Mineral resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 
1. Quicksilver • .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 
2. Coal ... . . ........... . . . .. .. ............. . ................. 94 

3. Silver ... , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 
4. Phosphate .......... ...................................... 100 

5. :Brick materials .. ........ ........... ......................100 


A SKETCH OF THE GEOLOGY OF THE 
CHISOS COUNTRY. 

In studying the geology of the Chisos country, one must con­
stantly bear in mind that the terranes which we now find exposed 
at the surface have been la.id bare from under a covering of over­
lying rocks measuring from two to ten thousand feet iJi depth. 
This great thicki;iess of vanished strata has been removed during a 
long lapse of time, which conimenced with the closing stage of the 
Cretaceous age. We should als9 remember that during the time 
this work of disintegration and removal was going on, and even 
before it began, the whole ground has been subjected to forces 
which have thrown the strata into folds and flexures and broken 
them by great faults into blocks that are extensively tilted. 
As a circumstance which has contributed to the effectiveness of 
erosion in this region, we must regard the fact that it lies on the 
border of the great Cordilleran plateau, where the drainage has 
had more or less direct outlets to the region of the Plains oil the 
east. The descent of the streams has been relat~vely rapid ·and 
transportation correspondingly easy. For the Santiago and Car-: 
meri ranges, which make the ea.st boundary of the Chiws country, 
must be regarded as the southernniost extension of the ·Rocky 
· Mountains in the United States. To the west of these ranges we 
find that the geological structure is more like that of the Great 
Basin and to the east, though. the .superficial aspect is somewhat 

mountain-like, the structure is that of the bordering high plains. 
A. GENE1lAL TOPOGRAPHY. 
The Chisos mountaills constitute the highest · relief in a belt of somewhat interrupted and broken plains, which exten.d nortli.­westward to and beyond Marfa. This belt has ·suffered agreawi­reductiori from the erosive forces than the mountains around it, . owing to the weak resistance of ·the clays and marls, which mi:>stbr constitute the bedrock within its limits. It may be regarded ai;i .a narrow extension of the Howard. bolson* ·described by Prof. R. T. 
*Physiool Geography of the Texas Region., Topographic .Atlas of .th11 United Stateil, United States Geological 'Survey, l!lOO, p. 9. · · 
lO Bulletin of The University of Texas. 
Hill. To the east and to the west of this plain more indurated rocks are now exposed ·and rise in ridges and table-lands. Toward the Rio Grande the plains gradually descend to an elevation of only about 2000 feet above the sea level. From the middle of this de­scending plain the Chisos mountains rise in a low, wide, and dis­sected dome, with long E;purs to the south. 
1. DIFFERENTIAL EROSION. 
'l'he materials subjected to the work of the destructive force-a during a long time in the past' in this region co~ist for the most part of soft sediments, such as clay, deep-sea ooze, and sand. With these are extensive beds of limestone and frequent and variously shaped masses of igneous rock injected into the sediments and in places poured out over them. Almost all of the pronounced relief we meet with here, suoh as hills, ridges, mountains, and table-lands, are due to the _greater resistance of the harder materials. These have been laid bare by the removal of the more readily yielding clays and marls, by which they were originally covered or ·aur­rounded. All the more enduring rocks now rise -above the general surface of the land. In places where the indurated rocks are ab­sent, the ground has been cut ·down to almost perfect plains. The relief of these plains is confined ,to a very gentle general slope towards the drainage channels, low scarps caused by tran-aient ad­justments to new erosion planes, and long, narrow and shallow gullies of the surface drainage. 
2. THE GRADED PLAINS. 
The controlling topographic feature of the country around the Chisos mountains consists of the plains just referred to. It i-a estimated that they make up from two-thirds to three-fourths of the entire area of the country. Their general slope averages a hundred feet to the mile, and varies from seventy-five feet, where it is the most gentle, to two hundred feet, where it is most steep. Only in few places, on some low divides, it is nearly horizontal'. 
(1) Making of the Graded Plains. 
The ongm of the high plains in the southwest, the so-called bolson plains, has been ascribed to different causes by different 
Tke Geology of the Ckisos Country. 
writers. R. T. Hill regards them as constructional detritus plain.a caused by filling of structural valleys.* 
Keyes considers that the bolsons are a part of an upraised pene­plain, practically untoµched by stream action.** It does not. ap­pear that either of these explanations apply well to the plains of the Chisos country. The drift filling fa everywhere of small depth, probably nowhere amounting to as much as two hundred feet. Nor is there any general level indicating a peneplain. If the graded plains here have once been a part of such a plain, this has later suffered extensive destruction by recent erosion. The present re­lief of the plains in this region must be regarded as expressing an equilibrium between recent erosion and transportation on the on(} hand and the resistance of the materials subjected to these forces on.·the other. It is the direct result of general erosion now in progress. This is always accompanied by local accumulations of land drift, owing·to local variations in the factors which determine the effectiveness of the agents at work, and thus the plains are here the result of constructive work as well as destructive. 
It will be perceived that the grade of plains whose existence depends on such nicely balanced conditions will be subject to some change8. Whenever weathering is progressing too slowly to fur­nish as much materials as the transporting agencies are able to re­move, the grade of the plain will, of course, not be maintained, but the resistant rocks will begin to rise above its flat surface. This i: the origin of nearly all of the prominent reliefs rising from the plains, such as ridgffi, butte·s, mesas and mountains, whicl} will be described directly. The sloping plains are developed only where the weathering equals the work of the transportation, which latter depends on several factors. In the main these determining factors are the size of thti transporting current, and the mechanical com­position (the :fineness or the coarseness) of the materials trana­ported. • 
(2) Controlling Factors. 
The current which here consists of what is known as sheet flood, does not vary a great deal. It is true that the rainfall is slightly greater in the mountains and on their outer slopes than farther 
*Reference as above. 
**Keyes, Bolson Plains and the Conditions of their Existence, Am. Geol., Vol. 34, pp. 160-164, September, 1904. 
Bulletin of The University of Te:ta:s. 
out, and this no doubt makes the sheet flood slightly great.er anJ hence more efficient near the mountains, and thus tends to render some of the talus slopes less steep than they otherwise would be. But the difference is probably so slight as to be quite negligible. There is another circumstance which sometimes renders transpor­tation inefficient by reducing the sheet flood. If the 'ground of the plain consists of such coarse material as to allow the rainfall to immediately go down into the ground, there will be no sheet flood to perform any work at all, and transportation will be ~ta standstill, except on comparatively steep slopes where the work . is considerably aided by gravitation. A belt of dissect.ed hills, which extend northwest from the Dugout draw, appears to have been left in this way, rising above the graded plain. They are locally kno:wn as the "White Hills" and consist mostly of gravel and sand. 
The variation in the mechanical composition of th~ eroded rock underlying the graded plains elsewhere is not very great. In fact, these rocks are uniformly so fine in texture as to make their prod­ucts of disintegration 'easily transported even by a slow current. The slopes which . are steep enough to produce such a current in the sheet floods resulting from the present rainfall, is about two feet in a hundred. But there remains on the surface very gen­erally a residue of coarse material, in part derived from concre­tions in the bed rock and in part coming down from indurai;ed rocks in the bordering mountains . . · This constitutes a land drift in a limited sense. Its coarseness has a noticeable relation to the slope. Except in places ·where the wind is at 'work, the most level parts of the graded plain consists of fine sand and silt and no peb­bles appear on the surface. On the moderate slopes and away from the mountains the largest fragments seen seldom measure more than two inches in diameter, while on the higher slopes we find large boulders. · 
The surface of· the graded pla-ins is more or less trenched by arroyos that traverse them in the direction of their general slope. We find these radiating in all directions from the Chisos moun­tains. That the condition;; under which these plains are formed are quite different from those attendant on base leveling is evident from these arroyos. They often run parallel and close together for miles. On the southeast side ·of the Chisos, Glen draw runs side by side with another arroyo for twelve miles, and the two are mostly 
The Geology of the Ohisos Country. 
less than a mjle apart. Such conditions can not exist except where 
. the attitude of the land surface is well adjusted .to asteady rate 
of erosion, and where it is: yet so far above the base level that very 
little side cutting is being. done. Under .. such conditions the dif~ 

"ferent drainage lines make no inroads on ea.ch other. The depth of the arroyos vary from almost nothing to two and three hundred feet. We usually :find them from one.-fourth to one-half a mile apart and · this distance, too, is. proportionate . to the slope, but .in­versely, being. least where the ·alope is most steep, and greatest where the slope is' most fl.at. As we cross over the fiats between th~m we notice that these have a very gentle slope to either side from a middle divide. It is evident that the fl.at~ are being cut down by sheet flood ·erosion, h,owever slow the process· may be. In the~r lower course these long arroyos often have an alluvial valley two or three hundred yards wide with a meandering · channel, but in their upper . course they are V-shaped. . 
(3) • Maintenance of the Eroded Plain. 
W:ith erosion in general progress, it may be asked why the graded plains. are not wholly dissected by the arroyos. .'Dhe present writer believes that the answer. to the questions is to be found in the fact that weathering in this region is very rapid and transportation com,­paratively slow. There is always readyand at hand, wherever the bedrock consists of soft cretaceous ma;rl,s and clays, a surplus of load for the sheet flood. If now. and then this should carve a trench on the surface of the graded plain, this trench will promptly be filled with surplus drift, up to the general grade. Thus there can be no trenching done pn the plain itself. For a simila;r rea­son the backing of the .arroyos and their branches is exceedingly slow. The long arroyos are _overlOaded with debris from the moUJ',l­tains and this is frequently supplied in such quantity as to l;rring about a re-filling of some of their valleys. · About midway between Indianola peak and the old Reed camp the arroyos are now filling to such an extent as to make the beds appear more like alluvial fans .than like alluvial valleys; Ea~h arroyo maintains, as it were, a grade of, its own, depending upon the nature and the quantity of its lOad, and this is irsually so great as to leave but little energy for other work, espeCially in the upper part. of its course,. It ia only farther down that wide cutting takes place. The automatic and 
Bu,lletin of The University of Texas. 
uniform grading oi the plains under a ready load £or the sheet­
:flood current results in the prevention oi the water being gathered 
into currents which might start re-entrant gulleys on its scarps. 
Hence there is a lack of ramifications in the drainage valleys, and 
hence it is also that reduction of old levels often proceed by low 
and long scarp.;; which mark the boundaries of new ' and lower 
grades, wherever these find an 'opportunity to establish themselves. 
'l'o sum up, we may say: tlie plains around the Chisos mountains 
probably mark the location of an old and higher plain. They have 
been cut out of this old plain by a slow process of .erosion urider 
conditions of an overloaded drainage and have long since reached 
an equilibrium oi grade and load to the working current. The gen­
eral direction of the grade i.;; toward the Rio Grande . and this 
stream must have been the prime agent in the destruction of the · old plain. 
3. THE HIGH RELIEFS. 
In a manner, all of the high reliefs in this region may b~. re­garded as due to the greater resistance to erosion by the more in­durated rocks in the faulted and folded complex of formations which have for a long time been subjected to erosion. With the possible exception of the Chisos mountains themselves, no part . of the topography is inherited from the original configuration of the land, although this happens to be reproduced at the present time in several places. For we find that a fold or a fault often brings up the limestones of the Lower Cretaceous age above the surface of the graded plains, and they rise bared or only partially destroyed in their true structural relation;; as ridges or mesas. Thus we have the ridges of the Carmen and the Santiago ranges and the Mariscal mountain ridge extending northwestward· across the Rio Grande toward the Chisos mountains from the southeast. These ridges mark lines of original uplifts. West of Terlingua creek the same rocks form an extensive tilted me.;;a and these again rise into a dome in the Christmas mountains. to the northwest. The Boquillas flags are less resistant but their disintegration never keeps · pace with that of the shales and marls; and thus we find them forming sloping benche.;; bordering the ridges formed by the other lime­stones on which they rest. The sandstones in the Terlingua beda often form long, low, and narrow ridges, where their tilted edges 
The Geology of tke Okisos OO'Untry. 
are expo·sed, but on account of their limited thickness they cause no high reliefs. The igneous rocks, which are usually moat re­sistant, form most of the high reliefs. Pikes and surface flows; as well as intrusives, have weathered out from their matrix of sedi­ments anJ form reliefs of moat varied forms. Ridges protected OD one side by an intrusive sheet and presenting an escarpment of erosion on the other are very common. Domes formed by uplifted sills and loccolites are next in frequency. We also sometimes find walls ·and ridges of dikes and some mesas protected by cappings of surface flows. Many of the igneous intrusive 'bodies are so slightly affected by weathering that they exhibit, almost unchanged, their original molded form. 
The main we;;t body of the Chisos mountains, the so-called "rim rock," consists largely of an intrusive boJy which has once been covered by sediments, and which, so far as the present topography is concerned, must have had the same history as the other intru­1>ives: it has been laid bate by the removal of superincumbent sedi­ments and volcanic flows. Some of the latter are still left in patches on the highest summits, such as Rummel peak, Lost Mine peak, Crow mountain and a plateau south of the Emory peak. The fact that these remain as a cluster around the greatest laccolite in the region ·auggest that the volcanic covering was originally heavier at this point than farther away. 
B. HISTORY OF 'l,'RE DRAINAGE. 
The Rio Grande is the master stream, and it has maintained its course across a number of faults 8.IJ.d folds. With regard to the geological structure, its course is clearly antecedent and was deter­mined before 'the present structures were all developed. Its main tributari~s from the north in this region are 'l'erlingua creek, Tor­nillo creek and Miravillas creek. The first one drains a large ter-· ritory to the west and northwest of the Chisos, and it is clearly subsequent in its character. It has adjusted itself to the larger geological structures encountered. Thus its lower course i8 evi­dently determined py the .great Terlingua fault. Tornillo creek drains the north and east slope of the Chisos and this stream has likewise been influenced by the geological structure, for it follows in the main the line of the outcrop of the softer strata. ·At its mouth 
Bulletm of The Unwersity of Texas. 
it has carved a short canon through an uplifted limestone and, so far, it here retains an antecedent character. The Miravillas if likewise in part antecedent and in part subsequent. The main stream follows the east side of the Santiago and the Carmen . ranges an!f has no doubt been held in place by the great fold of the ranges. But two of its west tributaries cross over from .the west · side of the fold. · These are antecedent. Persimmon gap on the Santiago range mark;i the former crossing of .another tributary from the same direction, whose upper portion has later been cap­tured by Dog creek. It may be that the upper part of the Tornillo creek once ·drained in the same direction, but this can only be a conjecture. . 

Ill general the drainage of the ·Chisos country is in a stage of adjustment to geological structure. . Nearly all of the smaller streams are through this stage, the larger creeks are ·still in it,.and the Rio Grande has har~ly yet entered upon it. 
C. STRATIGRAPHY. 
In orde_r to .understand the geological structure orthis co\Jntry it is essential that we should first know the nature and the thick­neS'S of the strata from which the entire land has been built. It is well known that the bedded rocks, such as limestone, clays, marls, and sandstones have been laid down as sediments on the bottoms of seas and lakes, and lay originally in a horizontal po;iition, The first task of a geologist working in any region is to make out the . general section, or succession from below upward, of the several formations and strata which he finds exposed on the surface. He must measure their thickness and see if this is the same everywhere, or if it differs .at different points. He must make out if all of the 
·. ·strata are present as originally laid down, or ~f any of them have been_remove4 previous to the ·deposition of later beds. When thia is done, he will be able to understand the structural :features of the land. He will see where it has been elevated or where it has been sunken, where the strata have been folded or where they have been . broken, and which strata have been cut away .by erosion or which lie buried below the surface. 
The Geology of the Ghisos Goontry. 
: 1. THE GENERAL SECTION. 
The oldest sedimentary rooks in this region belong to the Palaeo­zoic era. Above these oldest sediments we find the Lower Creta­ceous limestones, which in turn are·covered by various sediments of 
·the Upper Cretaceous. These are overlain, in a few placea, by :flows of lava and by tufl's of a later age and by clays and gravels of the late Tertiary. The most recent formation consists of. the lana drift and the alluvium of the present stream valleys. These overlie all of the other rocks more or less persistently. 
Within a radius of twen~y-five mil~s from Emory peak in the Chisos mountajns no older rocks than the Mesozoic appear at the surface on the north sid.e of _the Rio Grande, but farther north much older sediments cover an area of at least 700 square miles. 
· As these throw important light on the structure of 'the region, be­ing the nearest exposures of rocks which must underlie the entire Chisos country, a brief account of them is included in the gen,eral section, ev~n though this account is necessarily incomplete and the outcrops lie distant from the Chisos mountains. . The divisiona 
•
of the entire section, thus more or less fully described, is as given in the table below. 
Table Exhibiting the Formations Identified in the Cfiisos Country. 
Group.  System.  Serles.  Formation!!.  
- Pleistocene.  Lan!! drift and alluvium.  
Oaenozolc.  Tertiary.  ,oceneandlelocene.  Dng-out clays and fravels. Igneous rocks an bedde!I tuft and gravel!.  
Mesozoic.  Cretaceous.  Upper Cretaceous. Lower Ore~us.  Obl809 beds? Tornlllo beds. Rattlesni.ke beds. Terlln~na beds. Boqnl as flags. Bnda llmest.one and Del Rio clay.Undetermined limestones,shales and conglomorates.  
Palaeozoic.  Oarbonlferons.  Upper . Carboniferous.  Olbolo llmest.one.  
Ordovician.  Middle Ordovician.  Limestones with cherts shales and conglo:norates.  

Bulletin of. The University of Texas. 
(1) The Palaeozoic. 
In the immediate vicinity of the Chisos mountains the Palaeozoic sediments are, as already stated, buried under the Cretaceous rocks; but the Ord and the Santiago ranges mark the western limit of an uplifted area, where the Mesozoic rocks have been removed, and· where the older sediments now are exposed. The nearest outcrop · is in Persimmon gap, where dark shales and sandstones underlie the lower Cretaceous limestones unconformably. The area is ap­parently small and confined to the east end . of the gap. No fossils were noted here and the age of these beds are unknown, except so f!J.r as it can be made out from their position. 
(2) The Ordovician. 
About fourteen miles to the north northwest of Persimmon gap, and nearly due east of Santiago peak, similar shales and sandstones appear again, and from this point, as we travel north, our road is on Palaeozoic rocks · all the way to Marathon, and from there to Altuda. Marathon lies in the center of a triangular area, where the rocks of the Palaeozoic era are now uncovered. This area is bounded on the west by the Ord range, on the north by the Glass mountains, and on the east and south by a less well defined line, extending from the northeast to the southwe<?t, and passing some , distance east . of Haymond. , 
There must be several thousand feet of sediments exposed in this area. They have been folded and faulted and dip, usually at lJ.~gb angles, either to the southea,st or.to the northwest. My trip ·over this country was hurried and no attempt could be made to measure the thickness of these deposits. · In the south part of the area they consist largely of ·aark shales and limestones. These contain heavy ledges of cherty quartz, which rise i'n long ridges running from the northeast fo the southwest. The chert is usually interbedded with ledges of dark limestone, which now and then contain round and loaf-like concretioiis o:f ·chert. · The prevailing color of the chert is white, b{it it'i's sometimes black, green, bluish, or of a bro~n or a pinkish color~·"' ' With' some of the. limestones there are thin seams of . sandstones, ~:qLconglomerates. · The latter . usually consist of wel1-worn :pebbles; mostly of limestone, but also. of liard quartzite. In some shales, which are associated with chert and limestones in· 
The Geology of the Ohisos Country. 
a ridge on the south side of Edward's creek, about ten miles north­east of Santiago peak, there is one stratum whicl;t contains large coiicretionary aggregates of crystals of barite. Some of these ag-· gregates are a focit in diameter, and consist of colu:rnnar crystals radiating from the centre of the concretions. The mineral was seen to follow a stratum about three feet in thickness, and the con­cretions in one place appeared to be present in large quantity. The limestones themselves are frequently bituminous. Some ledges consist of small worn organic fragments. From one of these some fossil · fragments were taken which Dr. Charles Schuchert has iden­tified as the outer portions of the glabella of a· Tri111UCleus. Asso­ciated with this there were also a Plecta;mbowites [seric~f], a Rafinesquina and possibly a Zygospira. A gasteropod of the genus Cyclora was found in another ledge, and in still another limestone a N odosaria was noted. All of these fossils were collected along the wagon road near Ridge spring and at different point> south from this place for a distance of ten miles. The same kind of 
rocks · continue north from this spring as far as to Pe:iia Colorado, 
four miles south of Marathon. North from Pena Colorado we 
find shales, limestones, and thin conglomerates, but no chert b~ds. 
An A thyris, a Fistulipora ( ?) , a Dentalium, and joints of crinoid 
stems were observed in the vicinity of Marathon, where the dip of 
these rocks is quite generally to the northwest. Evidently these 
sediments are younger than the formation which contain the chert 
south of Pena Colorado. 
'rhe thickness of the ancient sediments, which are seen on these 
plains around Marathon, is no doubt several thousand feet, and 
they very likely contain ·formations of more than one age. For 
twenty-five miles the road from the Chisos runs over the edges of 
beds tilted at high angles. The fossils associated with the chert 
show that some of these strata belong to the Ordovician, and Dr. 
Schuchert infers that these are of the Trenton period. In his 
Physical Geography of the Texas region Prof. R. T. Hill expresses 
it as his opinion that the limestones, sha1es, etc., of these plains 
may be of Lower Helderberg age,* and there is no good rea:son to 
doubt that the Silurian rocks are represented in this extensive 
complex of folded strata. · Proceeding to the northwest from Mara­
. *Phy;iical Geography of the Texas Region, R. T. Hill, U. S. Geo!. Surv. Topographical Atlas of t}!e United States, Folio No. 3, p. 4. 
Bulletiln of The University of Teus. 
thon we find that the dips, whieh are prevailingly to the north­
west, grow less steep. We are evidently going away from an axis 
. of uplift and w'e are presumably coming to sediments which are 

later than those encountered nearer to this axis; 
(3) The Upper Oarbonif erous. 
Near Altuda, which is a station on the Southern Pacific Rail­road at the west corner of the Marathon Palaeozoic area, we fin·.i shales and limestones that clearly belong to the Upper Carbonifer­oub series. In the Altuda mountain these rocks form the greater part of the mass arid to the south of it the series outcrop for a · distance of two or three miles. Along a north and south line 
.across this mountain the dip is to the north and increases in steep­neas as we approach a granite area lying two miles south of the mountain. The section of the Carboniferous as seen along this line is about as follows : 
'l'hlck11e11S in feet.. 
A cream-colored limestone almost without bedding planeo and somewhat crystalline in texture, forming a high vertical escarpment all around the mountain. Obscure traces of organic fragments were frequently observed, but no identi­fiable foSBils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 600 
Thin bedded limestone containing shaly and sandy .i!trata. The limestone is ip.ostly in lay~rs from three to six inches thick. ·This member contains Fusulina in -great a~undance at times, but mostly only very minute young individuals. Other fossils noted on the north side of the mountain were : Ohonetes, Pugnax, Productus, Semilnula, Meek.ella, a pygidium of a trilobite, Fistulipora, a spine of a fish, and joints of crinoid stems ............... ;................ 200 
A series of shales, sandstones and limestones, with ~ con­glomerate near middle, probably as much as ....... ......2000 
Fusulina cylindrica of a moderately large size occurs in pro­fusion in 'some limestone ledges in the upper half of the lowest division. Near its middle there is a conglomerate twenty feet in thickness, which contains pebbles as much as three inches in di­ameter. The largest consist for the most part of a fine brown quartzite, while those of smaller size are of white quartz and 
The Geolagy of the Ohisos Country. 
hiack chert. All · aTe well rounded. . The conglomerate is cemented by a dark gray calcareous stony material, which was ·aeen to contain some shell;; of brachiopods, some bryozoa, and a coral. Some hundred feet or ·more under this conglomerate there is a dark limestone which occasionally contains numerous specimens of an ammonoid, and · associated with this were noted 'Some stems of crinoids, some gasteropods, and an Archeocidaris. 
This section is the equivalent of the Cibolo limestone at Shafter in Presidio county. It probably extends down into the Alta beds, which underlie the Cibolo limestone. The thin-bedded limestone described in the section above appears in a ravine to the north­west of Altuda ·mountains and, the face of this outcrop is almost in· itself a proof of the identity of this limestone with the thin­bedded limestone of .the Cibolo formation near Shafter.· Another evidence of the identity of the two is the presence at both places of a peculiar bead-like fossil 'Stem of unknown relationship. 
Whether beds of the Lower Carboniferous and of the Devonian ages will be found between Altuda mo:untain and Peiia Colorado, or whether there is an unconformity between the Upper Carbonif­erous and the older formations i's not yet known. 
(4) Rodes of the Cretaceous Age. 
Some time after the mount11:in ·folds of t4ese older sediments had been leveled down by extensive erosion the region was again submerged and a new succession of sediments were laid down in an open sea, which is known to have had a continuous extension from Canada to Mexico and which covered Western Texas during nearly .the entire Cretaceous age. The sediments in this sea have a great thickness, no less than 8000 feet in Western Texas. They consist of heavy limestones with some shale and ·conglomerate below; flaggy . limestones, chalky rock, marls, and clays in their middle portion; and sandstones, claya and tuffaceous beds above. We will first describe the lower division. 
The Lower Cretaceous Limestones. 
In the Carmen and Santiago ranges, in the Christmas and ·the Mariscal mountains, and in the Mesa de Anguila the Lower Cre­taceous· limestones are extensively exposed, but the whole succes­
Bulletin of The University of Texas. 
sion of all the beds has not been seen. It .is estimated that a 
thickness. of more than 2000 feet appears in the Carmen range. 
"At the mouth of the Santa Helena canon the J.i0wer Cretaceous limestones rise 1500 feet. But at neither of these places is the base of the formation exposed. This comes to view . in Persimmon Gap and elsewhere farther north in the Ord and the Santiago ranges and on the borders of the . Marathon .Palaeozoic area, where only jncomplete measurements have been made .. · 
Persimmon Gap. 
At Pe!simmon gap the lowest 100 feet of the Lower Cretaceous cons1st of shaly calcareous strata, which are not well exposed, 'Above this lies a sharply marked conglomerate, about twenty feet "in · thickneas, consisting of well-worn pebbles, mostly of chert. The ledge may be seen for several miles to the north in the east face of the range. Above it there are two or three hundred ·feet of lime­stone, .some ledges of which were seen fo contain numerous im­bedded tests of Orbitulina texana, Roem. 
Altuda. 
Another place where the basal members of the Lower Cretaceous may be seen is in the high limestone· scarp west of Altuda, about seventy-five mile;; north of the Chisos mountains. At this place it rests on the undisturbed upper surface of the Cibolo limestone of the Chinati section. It consists of gray ledges of limestone only slightly different from the underlying formation. The bounJary between the two is marked in 'SOme places by occasional fillings of yellow sandstone which appear at the base of the forniation in de­pressions in the upper surface of the Cibolo limestone, . and else­where it can be recognized only by a few well-worn pebbles of quartz, which may usually be found at the contact of the two limestones. The conglomerate and the shale seen farther south are absent at this place. The succeeding hundred feet are like the basal layers, consisting of gray limestone. in moderately heavy ledges. These contain a large Pecten. with complex folds, a large Caprotina, some gasteropods, Modiola pe&ernalis and a Cardium. Above this ar~ some forty feet of stronger and heavier ledges, which contain silicified ;;hells of a Radiolithes (like sa'lfvagesii, 
. The Qeology of the Chisos Country. 
D'~rb) in abundance and also a CaprotiM, a large Pecten,. prob~ ably P; duplicosta Roem., and various gasteropods. Again, above this there are some sixty feet of softer• and thinner ledges of ·lime.: stone. These are overlain by several hundred feet of heavy-bedded white limestone, the upper ledges of which are identical with the Edwards limestone of the south central part of the State. Near the base of thi.i division some gasteropods and Gryphaea marcoui Hill & Vaughn were noted . . The uppermost two hundred feet in the Cretaceous section here appear west of Ramsay ·creek and from two to five miles south of Strobel. They consist of more shaly limestones, in which no fossils were noted. In all, this section measures at least 800 feet. There i~ here an overlap on a height of an old land, where the lowermost beds of the Cretaceous ·sys­tem, in which conglomerate appears elsewhere, were never laid down. 
Cienega Mountain. 
The base of the Lower Cretaceous also appears _flanking the dome-like uplift of the Cienaga mountains. The lowermost ledges 
. here consist of sandstone and are overlaid by shales and. shaly limestones, measuring about 150 feet. An impression of the stem -of a plant was seen in the calcareous shales on the west side of the mountain. Above these there is a well developed conglomeritic .sandstone about twenty-five feet in thickness. This contains streaks of pebbles which measure two inches in diameter. Most of the larger pebbles consist of white quartz, but there is also some dark chert. Above this conglomerate there are some 300 feet of somewhat impure limestones with shaly seams. In the lower part <>f these a Caprina., a N erinea, a Radiolithes and a Oardium were noted. On the north side of the mountain this division is over· lain by several hundred feet of massive pure limestone ledges, evi­dently corresponding to the Edwards limestone. The whole series, so far as seen here, has been heated by intrusives and turned into .a fine-grained marble. 
Me~a de Anguila. 
• 
The section in the great east escarpment of the Mesa de Anguila measures about 1500 feet. It is seen to consist of about 700 feet .of massive limestone below, rising in an almost vertical wall, then 
Bulletin of The University of :rexas. 
about 200 feet of ~n impure and more readily disintegrating rock which contains some marly layers, and weathers into a slope1 and highest up 600 feet more of massive ledges of white limestone rise in a nearly vertical wall to the top of the escarpment. Only the lowermost 100 feet were examined by the writer at the mouth of the canon. Just above the water in the river is a dark, almost black, limestone of very compact texture. It i's in straight ledge3 and there are two or three layers of shale interbedded. One -of the limestone ledges was almost filled with a small Gryphaea, probably G. marcoui Hill & Vaughn. A large Pecten and an · Exogyra texana Roem., like one occ-llrring in the Presidio beds 
near Shafter, were also found at this horizon. 
Christmas Mountain. 
In the Christmas mountain there is a thicknes;:; of about 1000 feet of these limestones. The same divisions are indicated here: two heavy bedded aird strong limestones with a somewhat less en­during division between them. But the lowest division is not wholly exposed. At the south end of the mountain the same· Radiolithes occurs that was seen west of .AJtuda. Farther t~ the north and on the west side I found a cast of a large gasteropod, a Globichoncha planata Roemer, a Cerithium proctori Cragin, and als0 another Cerithium. 
Mariscal Mountain. 
To the ;:;outh of the Chisos the Lower Cretaceous has been lifted up and bared in the Mariscal mountain, _which is cut across by the Rio Grande and extends eight miles north of the canon of this river. On the west slope of this ridge just to the north of the west _end of the canon, the contact between the Boquillas flags and these limestones is well exposed and· is very clearly n1ark~d. There is an abrupt change from a heavy bedded strong limestone to the flaggy layers of the upper formation. This is the dividing plane bet~n the upper and the lower Cretac.eous. The upper 100 feet of the lower series were hastily examined. The highest ledge consists of a rock of very compact texture exactly like the Buda limestone 
a:s it appears around Terlingua. Under a lens this rock is · seen to consist o:f a matrix of exceedingly small and indistinct frag­
The Geol<>gy of the Ohisos Country. 
ments in which lie imbedded occasional fragments of shells .of gasteropods and also some shells of a Globigerina. About fifty feet below the uppermost ledge the rock weathers more rapidly and i'B lei3s pure . and compact. This appe.rently repreBents the zone of the Del Rio clay. Some fifty feet still farther dow~ I found a large N erinea, having a spire two inches wide, and a cast of a Lunatia ( ?). Not far from the Sa.me place Kingena. waco.ensis R~er, and Ostrea carinata Lam, were noted, and with these was seen imbedded in one of the ledges some peculiar meandering dark .plates, that on closer examination proved to be a sponge. On the east side of this ridge and from two to three miles south of Lindsey's mine, the uppermost ;compact limestone is about seventy feet thick and weathers out quite distinctly by thE'. removal of more readily crumbling rock below it. This . latter contains layers full -0f fragments of 'Shells and occasional specimens of N odosaria W.C­ana Conrad and of a Gryphaea like G. mucronata Gabb, It will be remembered that these . two fossils occur in the Del Rio l'JO!le~ In the solid ledges below this zone Requie'Tlli,a paUgiq,ta White is iound associated with the same gasteropods that were seen cm the opposite side of the mountain, and some distance ·stil' further down Exogyra texana. Roem. was noted. In the canon of the San Vin­cente mountains the same formations are exposed, but only to a small extent. 
The Carmen Range. 
The Carmen range consists of ridges of limestone of the Lower Cretaceous. In high escarpment on, the east side it appears that they show a thickness of 'Somewhat more than 2000 feet. The Rio Grande cuts across these ridges by the Boquillas canon, and this canon would show the best exposures for a continuous section. But in this survey it was not practical to ·take that route, a~d only a very general idea of the 'Section can be given. It r0£?emhle.<1 the section in the Mesa de Anguila, but it is .heavier. The ledges corresponding 'to the Buda and the Del Rio zones weather out on thP west slope and are only some <iixty or seventy feet in all. Be­low this is a heavy bedded limestone some six hundred feet thick, in the upper part of which I saw Ostrea munsoni Hill. Other­wise ,recognizable fossils in this division are rare. Under this there is a considerable thickness of ledges which weather @omewbat 
Bulletin of The Unwersity of Tezas. 
1Hore readily li:nd which contains Exogyra texa.na Roem:., E. math­
tl'.ft<'niana Conrad, a: 01-yphea, Pecten texanus Roem. (?) and Radi­
&Uthes cf Sauvagesii D10rb. These fossils weri:! seen along th~ 
trail through the Ernst valley, eight' or ten miles north of the R' J 
Grande. In the right bank of Heath creek, where it cuts tl>-. Jugh 
the last of the Carmen ridges, we find the same strata t' _a.t were 
noted at the mouth of the Grand canon: dark limestone with some 
shaly strata and containing Gryphea m0;rcoui Hill & Vaughn. At 
a somewhat higher level in the east face of the escarpment and 
abbl.lt.'tliree miles to the 'south, a Nautilus (near texanus Schum.) 
we.B noted. At the lower mouth of the Boquillas canon the lowest 
rock . seen consists of a very compa,ct and close-grained limestone, 
in which tests of foraminifera are very numerous and in which a 
'G-ryphea and a small Pecten were noted~ This rock is very much 
.fiss'ured arid shows numerous veins ·of a columnar calcite, varying 
ih·.width from two inches to tw:o feet: Above this there is at least 
lf>OO feet of the heavy bed·ded limestone which forms all of the 
Tidges'. in this· range, · and at this place the variations of the ledges 
seen, at different lev'els elsewhere ' are not very apparent. 
• .At the upper mouth of the Boquillas canon just below the old Me±icari village of the same name, . there are a . thousand feet of massive yellowish-gray· limestone with some dark and less heavy ledges below. A very compact arid close-textured gray ledge shows, when examined in a thin section, that it is essentially ffil 
· indurated 	ooze, in which minute particles of organic calcareous materials make up the greater part. Few of these particles are large enough to indicate the 'nature of the organisms from which they are derived, but there are some tests of foraminifera. Such tests are rather common in the entire series except in some ledges :which have undergone secondary changes and have become gran­ular and somewhat crystalline in texture. At this same place they .were noted in great numbers in a ledge lying about 400 feet higher up in the section. 
Characteristic Horizons. 
It was not attempted to make out the succession of separate divisions in the Commanche Series, for this would have required milch more time than was at our disposal, but it may neverthe­
. The Geology· of the Ohisos Country. 
less be worth the while to sum up the observations made bearirig on this subject. 
1. 
. The base of the series consists of sandy or clayey beds con­taining calcareous material also, weathering rather easily. At Cienega mountain these strata have a thickness of about l50 feet. 

2. 
At about 150 feet above the base there is a conglomerate or a pebb~y sandstone measuring some twenty feet. These two· mem­bers,and very likely a part of the succeeding beds above, are ab­sent at Altuda, where they probably never ·were laid down, owing to an overlap. 

3. 
. At an unknown distance a:bove this conglomerate there are some dark ledges of very compact limestone interbedded with some shaly seams, and these are followed upward by several hundred feet of heavy bedded limestones of lighter color. These lowest three divisions are equivalents of the Presidio beds in the Shafter ciection. 

4. 
Then follow two or three hundred feet of more thin-beddel and less pure limestones, which weather more easily and contain e, fauna resembling that found in the Shafter beds. 

5. 
Above this there are several hundred feet of very masi;ive light limestone, which is clearly the equivalent of the Edwards limestone. 

6. 
The last and highest of the series is a compact limestone only some half a hundred feet thick, which .is the equivalent of the Buda. It is separated from the massive beds below by some twenty feet of more easily weathering ledges which are frequently filled with organic fragments and which corresponds to the Del Rio clay. 


At Terlin~a both the Buda arrd the Del Rio zones are much heavier than to the ea-st of the Chisos. These formations clearly thin out or entirely disappear in this direction. 
Chemical Composition. 
The limestones of the Lower Cretaceous are usually quite pure and this is true not only of the most massive ledges, but also of those of le~s development, as is evident from the analysici given below. 
Bulletin of The University of Te:tas. 
Analym of Limest.one from the Buda Hori,zon, One Mile East of Boquillas. 
Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.35 · Alumina . . ........... : . . . . . . . . . . . . . . . . . . . . . . .21 Ferric oxide . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 Lime .......................................53.90 . Magnesia ........... ; . . . . . • . . . . . . . . . . . . .. . . . . . ·.15 Carbonic acid . . ..............................42.23 Water [hygroscopic] . . . . . . . . . . . . . . . . . . . . . . . . . . .18 Water [combined] .......... . ....... ~ ... ~ . . . . . .33 Sulphur . . ...................................trace 
99.59 
Oharacteristi,c Wea.thering. 
The surface of the limestones of the Lower Cretaceous is some­times etched in a manner that seems .to be peculiar to the climatic conditions of the Southwest. On top of the highest ridges of this rock, and elsewhere when these limestones lie bare, we often finJ small hollows or basins, not altogether . unlike so called pot-holes. But they· have clearly not been produced· by any process of wear­ing. They have been made by a unique process of etching, which takes place after showers and rairi-s. At such times water is re­tained in them for a short period and some of the lower algae :flourish in this water, while it lasts. These produce an abundance of carbon dioxide which dissolves the rock. Between rains .this vegetation dries up, and can often be found as a dessicated crust of black mud on the bottom of the b8Bins, ready to take up the work after the next shower. The Spanish name "tinahita" seems an appropriate one for such basins. It is the diminutive of Tinaha, the name of the common waterholes produced by cor­rosion in creeks and canons. 
The Tinahitas' always have a flat or only slightly concave bottom with vertical or even overhanging sides, sometimes beautifully etched. Tpey vary from three inches to 'Six feet in diameter and from half minch to two feet in depth. The common dimensions are a foot in width and three or four inches in depth. 
The Geology of the Chisos CO'Untry. 
Tke Upper Cretaceous Series. 
. At the top of the upper ledges of the Lower Cretaeeous lime­
stones there is quite a marked change :i,n the character of the sedi­
menta. of the age. The making of the heavy-bedded · limestones 
seem tO have come to a sudden end, and beds of this kind were not 
laid. down again during thi'S age in West Texas. There is yet 

"some more li:i;nestone above! but this lies in thin flaggy beds and it contains a noticeaple admixture of fine siliceous sand and. clay. Later on the conditions ·changed so that chalk, marl, and clay were laid down and still later, sand and more clay, and at last, sedi­ments containing a large amount of tuffaceous material. The de­posits of each of these 'Several and successive periods must now be described. 
The Boquillas Flags . 
.In .the vicinity of Boquillas postoffice, all along the west flank of the Car!Jlen range, the beds immediately overlying the Lower Cretaceous limestones are probably exposed tO the . best ad.vantage for study. These beds also lie all around both the Mariscal moun­tain, and on th~ south, east, and north sides of Christmas moun­tain. West of Terlingua creek they extend from C~esta Blanca to the Reed plateau, and ·south of Terlingua, still. farther west be­
. yond the limits of the Chisos region. From two measurements made about three miles east of Terlingua and one measurement taken on the west side of Mariscal mountain, the thickness of these · beds, which we will call the Boquillas flags, is 585 feet or a little less than 600 feet. They 'seem fo, have a quite uniform develop­ment and these three measurements did not vary by more than 50 feet, which perhaps is :within the limit of probable error. 
Chemical Composition. 
The mixed character of the sediments forming the Boquillas flags is believed to be fairly constant, and the analyses given below are from typical specimens of rock, ·except that the ledge from which the first one was taken contained a more than normally high per cent of organic matter. In the specimen from near the Colquitt-Tigne .mine some of the silica has probably been intro­duced secondarily by infiltration, 
BulletVri of The Unwe.rsity of Texa8. 
Analyses of Limestone from the BoquiUas Flags.* 
l.Juesta. Blanca.. Near Uolqultt-Tlgoe Mine. 
Silica .. . . ... .. ... .. . 7.80 per cent. 20.72 per cent. 
Alumina . . . . . . . . . . . . 1.30 per cent. 0.16 per cen~. 
Ferric oxide . . . . . . , . . 1.30 per cent. .45 per cent. 
Lime ... .. ........ .. .49.20 per cent. 43.15 per cent. 

Magnesia . ..... .... , . 0.15 per cent. · 0.52 per cent. 
tJarbonic acid ........ 38.50 per cent. 34.20 per cent . 

. Organic matter . . . . . . . 1.10 per cerrt. . . ..... ..... . 
Water (hygroscopic) .. 0.20 per cent. 0.10 per cent. 
Water (combined) . . . . 0.50 per cent. , 0.32 per cent. 
100.05 per cent. 99.62 per cent. 
Clastic Texture. 
The Boquillas flags consist of· a copious matrix of very fine 
calcareous particles, sometimes so :fine as to make the rock appear 
to be without structure, but containing some organic fragmenta up 
to one-half millimeter in diameter. With this some fine grains of 
quartz are usually imbedded, seldom exceeding one-fourth of a 
millimeter in diameter, and there are always present some shells of 
foraminifera. The admixture of sand varies considerably in quan­
tity, and there are some thin layers which may consist of as much 
as one-fourth sand. The shells of the foraminifera are sometimes · crushed flat and .at other times . entire and filled with crystalline 
calcite. Small fragments of t~e shells of InocerMnus are quite 
common, and may. be recognized by their vertical prismatic struc­
ture. 
Field Appeara111Ces. 
In their :field appearance the thin-bedded and flaggy habit is the most constant ·characteristic of · these beds. The ledges of which the formation is built up average from four to eight inches in. thickness. These ledges; or layers, are separated by delicate seams which may not appear in freshly exposed faces. Occa-sion• ally in the lower part of the formation there are ledges which measure a foot through. · As we follow them upward the layerJ 
*0. H. Palm, Analyst. 
1'he Geology of the Chisos Country .. 
become gradually more thin. At certain horizons they have a chalky texture. This is especially true about the uppermost hun­tlred feet, ·Another habit which is quite characteristic is the <man­ner in which the flags have been affected by joints. They always break into blocks with straight faces and sharp angles; . and the three diameters of these blocks are of a somewhat constantly un­equal length. The bloeks are always considerably longer than wide and they have a greater width than thickness. 'These pro~ portions of the three dimensions are the same whether the blocks are large or small. One may find blocks which are four feet long and no more than ejght inches wide and four or five inches thick, 
1h~ color of the beds is quite" variable. A cream-grayish white is the characteristic shade. In the Boquillas region they usually have a faint ferruginous red stain, often quite marked. On Cuesta Blanca, west of Terlingua creek, they are dark and almost bla¢k on fresh fractures in some ledges. The same color was noted north or Christmas mountains in the Corizones and on the north side of the Rosillos mountains. 
Resistance to Erosion .. 
The BoquillaR flags are less resistant to weathering and eroaio~ than the heavy limestone which underlie them. This is no doub.t chiefly due. totheir thinner bedding and more .close jointing, both of which properties cause them to break into sµiall~r blocks that yield more readily to the agencies of disintegration. Around th~ flanks of the uplifted domes and ridges of the underlying lime­stones the tilted edge of this formation usually rises only a part of the way up, leaving the upper ' surface of the older rock bare. 'l'o undergtound waters it is, however, more resistant than the purer rock on .which it rests. On account Of the clay constituent which it .contains, it is more impervious to water and hence it very rarely contains convernous :fissures, and for the same . :i;eason, also, the contact 'between these two formations near Boquillas marks a. line of. hot springs, where waters co:rning from-below are carried out to the nearest point of emergence of the base of the overlying less easily penetrated formation. 
Bulletin of The University of Texas. 
Minor Characteristics. 
One or two other characteristic features should perhaps not be pruised by. On the upper surface of some of the ledges clusters of cubic crystals of limonite are to be seen. These are pseudomorphs after pyrites, which sometimes have formed in the dark ledges. A two-inch seam of amorphous limonitic material was noted in the upper part of the beds south of Cuesta Blanca. 
At several places there were found on some of the layers in these beds apeculiar kind of markings, which consist of straight grooves when on the upper surface of a layer, or straight raised 1ines when on the lower surface. They measure from one-half lo two or three inches in length and are usually connected by similar but diverging structures at one or more points. Sometimes they are crowded t.ogether into a network. These structures have a marked resemblance to some markings observed on a ledge of sand­stone in the Middle Cretaceous . on ~he south side of the Black Hills in South Dakota and described a.a fossil frost cr~ks.* What­ever their origin niay be, ·they are quite peculiar in appearance and certainly different from the structure generally known as mud cracks. Their occurrence at the same horizon in two as widely separated localities as the Black Hills and the Chisos mountains entitles them to our notice. The presen~ writer has· also noted their occurrence in the Boqnillas flags at Ochinaga, in Mexico,. and in Val Verde and Kinney counties in Texaa. 
Fossils. 
Though rich in individuals of a few species, the faun11 of these beds is meager in the number of species. No close search was made for fossils and this circumstance no doubt helps to make the list of forms which were observed very small. It is as below : 
1. 
Globigerina.-Several species were noted in one of four rock specimena which we~e examined, and one or two species were seen in the other three. This 'Shell occurs everywhere throughout the whole formation. 

2. 
Textularia.-This is less abundant than Globigerina but is usually found with it. 


*See Scientific America'!, Vol. LXXII, p. 102, 1895. 
The Geology of the Ohisos Country. 
3. 
Small ammonoid, undetermined. 

4. 
Inoceramus confertim-annulatus Roem.-This is the most common fossil in the formation and occurs throughout its extent, apparently increasing somewhat in size from below upward. 

5. 
Inoeeramus exogyroides Meek. Rare. 

6. 
Inoceramus umbonatus Meek. Infrequent.· 


"t. Orioceras cf. latus Gabb.-This fossil is quite common on the upper surface of one or two somewhat indurated .ledges a little above the middle of the formation. 
8.. A cycloid fish scale, two inches in diameter, was noted on the upper surface of a rather thick ledge of limestone in the bottom of an arroyo near the Colquitt-Tigne mine. 
Correlation. 
. The Boquillas flags are the western equivalent of the Eagle Ford shales in the east part of .the State. Though they are more .calcareous in this region than farther east, the resemblance of the two is quite evident. Both are made up of thin and straight layers, separated by thin clayey seams. Both contain sand and bitumin­ous material. Arid both follow directly on top of the Buda lime­stone. There is, however, a difference in the fauna. In the east part of the State sharks' teeth and large ammonoids chara~terize the Eagle Ford horizon, but these seem to be absent here. The physical co'nditions were evidently not quite the same. In East Texas the formation has a much smaller development. The shore of the middle cretaceou;; sea must have been nearer there and deposition lasted for a shorter period. Indeed, there is some evi­dence of work of submarine currents strong enough to have pre­vented sedimentation for some time after the Buda stage. NIO such conditions were noted in the Chisos country and thus, prob­ably, this formation here reached a greater development and came to have a somewhat different aspect, faunally as well as physically. 
The Terlingua Beds. 
-. 
· There was no very abrupt change attending the physical con­ditions which brought the Boquillas epoch to a close. The upper hundred and fifty feet in the Boquillas beds contain a very consider­able amount of chalky ledges, only slightly more indurated than 
Bulletin of The University of Texas. 
the deposits of the following epoch, which first consist of a yellow­
ish white, indurated, ;;trati:fied chalk. This gradually changes up­
ward to an impure gray marl, which becomes less and less calca­
reous, until it is a true clay. In the uppermost strata of these 
clays there are some thin layers of concretionary limestone and. 
calcareous sandstones. We shall dec;ignate this entire succession 
of somewhat heterogeneous deposit.s "the Terlingua beds," as they 
are well expo~ed along the creek which bears this name. From a 
· structural as well as from a topographic point of view the differ­
ent members of which it is made up are practically a unit. In 

all they comprise a thickneS'S of about 1250 feet. 
Distribution. 
As the Terlingua beds rest on the Boquillas flags, we always find them outside of the latter in going from centers of local up­lifts. The nearest lowlands around the mountains and hills of the Lower Cretaceous limestones are mostly overlaid by readily yield-. ing clays of this formation. This habit is probably best illustrated in the case of Mariscal mountain, southeast of the Chisos, where these beds underlie a belt from one-half to two miles wide, on all sides of the ridge. A 10imilar belt follows around the north end of San Vincente mountain. and then turns around and follows. the west side of the· Boquillas range northward. There is likewiae an irregular belt coming around the Christmas mountain ~n the east. Along the Terlingua creek they underlie a wide and irregular area which is peripheral to a general uplift to the west. A few minor areas are isolated and do not occur with outcrops of the lower beds. One of these lies north of Burro mesa, another just to the east of Paint Gap hill, and still another north -of Laguna inside of the "rim-rock" in the Chisos mountains. 
Cottonwood Creek S ection. 
I 
Along Cottonwood creek there is a: nearly entire section of this division. 'This creek is a branch of Rough Run, and comes in from the east around the north end of Burro mesa, joining Rough Run opposite Dogis mountain. At a place known as Chisos Pen this creek has cut across the long ridge that eriends in a curve north from the north point of Burro mesa. The Terlingua beds 
The Geology of the Chisos Country. 
appear in a continuous section along this creek, extending from the west . side of this ridge to about a mile beyond. They are tilted to the east, which brings the lower part of the section to the surface about a mile out from the ridge. At this point there is a fault, and the section comes to an end just a little above the Boquillas flags. 
The lower one hundred feet consist of thin chalky ledges in every way resembling the Austin chalk.· Some flat concretions of impure limonite lie in the marly layers and perfectly resemble the concretions in the Niobrara chalk in West Kansas. Fragments of Haplascapha, grandis Conrad with attached valves of Ostrea con­gesta (Con.) Hall, are freqµent. Inoceramus undulato-plicatus Roem., another Inoceramus of large ;;ize, measuring nearly two feet &cross, and an Ammonites were noted. These beds dip east at angles of about 10°. They are followed by some four bun.ired feet of soft and gray marly sediments, in which the same fossils appear, and which dip at a higher angle in some places. Going further in the ;;ame direction we find several hundred feet of clays, in which there are some layers of large calcareous concretions, some thin sandy ledges near the top and also a sheet of intrusive rock measuring fifteen feet in thickness. This clay is overlain by a sandstone, ten feet in thickness, which will be classified as belong­ing to the Rattlesnake beds, the next member in the section. The concretions which are com;mon about 100 feet below this sandstone 
·ledge, contain now and then ammonoid shells, among which Pla­centiceras Whitfieldi Hyatt and Nautilus deka.yi Meek were recog­nized. The clays just below this layer of concretio:µs are exten­sively exposed for a mile on each side of the creek, .where they form a. perfectly bare country. They were seen to contain a con­siderable amount of gypsum, · mostly in the form of clear fragments of selenite crystals. Fossils seemed to be wanting. 
A measurement of dips and outcrops makes the thickness of this division here not less than 1280 feet. 
South of Cuesta Blanca. 
On the south side of Cuesta Blanca to the. southwest of Study Butte is another place where the Terlingua beds lie in an entire continuous exposure suitable for measurements. They overlie the 
Bulletin of The University of Texas. 
Boquillas flags which form the table-like hill, called Cuesta Blanca, and they dip to the south at angl~s varying from 25° to 50°. The measurement here falls a little short of 1200 feet. The same divisions may be noted at this place as in the section just de­scribed. The lower calcareous strata are succeeded by more pure clays, and uppermost these contain strata bearing large cdncre­tionary lentils of calcareous material, some eighty feet below the capping sandstone which begins the overlying formation. Hap­loscapha grandis Con., Ostrea congesta (Con.) Hall, and a large Inoceramus occur about 400 feet below the top, and also farther down in the section. 
To the south from here there is an extensive belt where these beds outcrop, reaching almost to the Rio Grande, south of the Rattlesnake mountains. Exogyra costata and E. ponderosa were found on the bare flat surface of a uniformly tilted outcrop of the upper part of this formation east of Dryden's ranch. Each lies in a seam of rock which is slightly indurated, and which could be ·seen on the ground as a straight narrow ridge less than a foot high, but extending for half a mile or perhaps -farther. The two ledges were perhaps 100 feet apart in the section and E. ponderosa was in the lower one of the two. ~ estimate that these ledges lie here about 200 feet below the lowest ledges of sandstone of the next division. 
Other Sections. 
Following the Rio Grande downward we next encounter the for­mation about a mile and a half west of Talley's ranch, where the lower chalky strata appear in the east bank of the creek which comes into the Rio Grande from the north. The exposure reaches out northeastward £or at least two niiles and at a point ·one and two-thirds miles to the east of the ranch the upper clays were found to contain Exogyra costata. . All along the west side of Mariscal mountain the formation underlies a belt varying from one to two miles in width, following the base of the ridge. About five miles north of Talley's ranch and near the U. S. bench mark having the elevation ' of 2217 feet, a hurried measurement was made across the outcrop of the greater part of the formation, leav­ing out one or two hundred feet Of the lower beds. This measure­ment makes the vertical distance from the overlying sandstone to 
The Geology of the Chisos Country. 
the indurated beds below about 1100 feet. Two ledges about fifty feet apart and lying some two or three hundred feet below the top of the formation; consist of a large and sometimes continuous layer of concretions of calcareous material mixed with some sand, and these . contain in some places abundant specimens of large ammonoids~ among which Schloenbachia conensis Conrad ( ?) seemed to be most frequent. North of this place, where the wagon road cro.sses the ridge formed by a sill following the . west side of the mountain, the lower calcareous division of the formation is well exposed in the side of the road and exhibits a number of more or less entire valves of large Inocerami, some of which meas~ ure two feet across. North from •the end of this ridge and at the foot of_ Talley mountain, the concretionary layers in the upper division have reached an unusual development and are well . e>x­posed. Some of the concretions measure four feet in diameter. This same horizon was also noted in the east bank of Tornillo creek at a point miuway between the village of San Vincente and Boquillas postoffice, and the same ammonoid occurs there. Again these strata come into view at Banta Shutup, where the concre­tions consist of an impure clay ironstone and have large shrinkag-e cracks which are often filled with black calcite. The beds here follow the west side of the Stillwell ranch intrusive, dipping to the west. They are also uncovered in the valley around this ranch, where Exogyra ponderosa was again noted and likewise the zone of large septari.a. 
In the valley which lies along the west side of the Ca.rmen range, about three miles east of Stillwell's ranch, there is 3: gray and somewhat indurated marl or soft limestone which belongs in the lower part of this division. · It contains abundant specimens of a few fossils, one of which I think is new. This is a sponge Which consists of a loaf-shaped body of folded plates radiating from a central point. It is undoubtedly closely related to, if not identical with, the sponge already noted in the horizon of the Buda limestone. The plates contain a fine network of hexactine spicules. There was at · the same point numerous specimens of Inoceramus undulato-plicafos Roem., and of Schloenbachi.a leonen.­sis Conrad. Other Inocerami and a Baeulites asper Morton ( ?) were noted. The formation extend·s with some interruptions for 
Bulletin of The University of Texas. 
several miles in a belt northwestward from this place, passin~ M uskhog spring. 
In the country between the Corizones peaks, Box spring, and the Christmas mountains, the Terlingua marls and shales are fre­quently seen, and some of the fossils noted at other places were observed here, but trus region is more broken up by igneous in­trusions and sections suitable for description are le;;s common. 
_, The Laguna is a shallow depression which is caused by the presence of these easily disintegrating beds, a half mile west of the highest peak in the Chisoa mountains. By the folding to which the ground has been subjected here, the shales are drawn out lengthwise and shrunk in other 'directions so as. to be greatly re­duced in thickness. They are likewise more indurated and more fissured by joints. 
Topographi,c Re7ations. 
In their relation to topography the Terlingua beds invariably maintain the habit of appearing on the lowest ground. In the center of the Chisos mountains, where they have been raised by a high fold, they occupy v~lleys among the peaks. On the lowlands they form the :flattest and most desert-like stretches in the entire region. They . have yielded more readily to erosion than any other member of the Upper Cretaceous series. This is due no less to rapid weathering, caused by the weak coherence of the elastic _ele­ments, than to the. promptness with which these are capable of be­ing carrie'd away by the sheet :flood current, owing to their small size. The effectiveness of the sheet flood is increased by the im~ perviousness of the clays, which causes the . rainfall to be shed as from a roof. 
Organiic Remains . . 
The lower part of the formation is largely composed of organic 
sediments. The chalky beds consist almost entirely of a deep sea ·ooze, quite free from mechanical ingredien~. This ooze consists mostly of comminuted fragments of delicate calcareous shells of foraminifera, but entire tests may alway;; be found by proper crush­ing and washing of . the rock. Fragments of the shells of I nocera­mus form an appreciable ingredient in some strata. Including some four hundred feet in this lower division of the formation, ii yielded the following fossils : 
The Geology of the Chisos Country. 
1. 
Globigerina. 

2. Textularia. 



3. 
Other foraminifera. 

4. 
Sponge. 

5. 
· Sea urchin. 

6. 
Radiolithes, like socialis D'Orb. This fossil iB smaller than Hippurites texanus Roemer, which it otherwise somewhat resem­bles. Unlike the latter, it consists of a cluster of several individ­uals which are grown together. It was found in the mads north of Laguna, west of Emory peak. 

7. 
R. austinensis Roem., found in the lowermost ledges. 


8. H aploscapha grandis Conrad. 
9'. Inoceramus undulato-plicatus Roem. There were several smaller I nocerami, which were not identified. 
10. 
Ostr.ea congesta Conrad. 

11. 
Baculites asper Morton. 

12. 
Ammonoids, two undetermined species. 


In the midde division of the formation no fossils were observed. The upper three or four hundred feet contain occasional arena­ceous seams and layers along which calcareous material has gath­ered into large concretions, which are sometimes separate and dis­tant and at other times lie close together, or ·even join, so as to form a continuous bed. In some of these concretions fossil cepha­lopods are' quite plentiful, but only a few forms have been ob­served. Exogyra has not been noted in connection with the con­cretions, but it seems to be1limited in this region to certain indi­vidual layers not far from the horizon in which these occur. 
The forms noted in the upper three hundred feet are as below: 
1. 
Exogyra costata Say. 

2. 
E. ponderosa Roem. 

3. 
Nautilus dekayi (Mort.) Meek. 

4. 
Placenticeras Whitfieldi Hyatt. 

5. 
Schloenbachia leonensis Conrad ( ?) . 


Chemical Composition. 
It will be clear from what has already been said that the com­position of the three divisions in this formation presents two ex­tremes. The lower division is a chalk and the upper part is a clay. 
Bulletin of The University of Texas. 
An analysis of the chalk shows only a small amount of Clayey ma­terial and an analysis of the clay would no doubt show only small quantities of calcareous material. Frequently the clays in· the upper pa.rt of the section give no response to acid. No analysis has been made 'Of these clays but the composition of two specimens of the chalky beds was found to be as below: 
.4.nalyses of Specimens of the Lower Chalky Ledges of the Ter­lingua Beds.* 
West of Ohlsos Pen. South of Ouesta Blanca.. Silica .. ; . . . . . . . . . . 4.54 per cent. 19.34 per cent. Alumina . . . . . . . . . . 0.75 per cent. 3.00 per cent. Ferric ·oxide . . . . . . . . 1.40 per cent. 2.16 per cent. 
Lime . . ......·. . . . . . 52.04 per cent. 39.50 per cent. Magneaia . . . . . . . . . . 0.24 per cent. 1.00 per cent. Carbonic acid . . . . . . 40.70 per cent. 31.50 per cent. Water (hygroscopic) . 0.10 per cent. .30 per cent. Water (combined) . . 0.40 per cent. 2.00 per cent. Sulphur . . ...........·.................. ..... . trace. Organic matter . . . . . . . . . . . . . . . . . . . . .90 per cent. 
lOQ.17 per cent. 99.70 per cent. 
Correlations. 
The lower part of the Terlinqua beds clearly corresponds to the Austin chalk in the. eastern part of the State. The two forma­tions are alike and in their lithological characters the beds are practically of the same kind._ There is a alightly greater admix­ture of elayey material in the western section than in the eastern, especially above. The middle. clays, which are destitute of fossils, are with equal certainty the equivalent of the Taylor marls in the Austin region, and there is no good reason why the ·upper member should not be referred here also. But the upper limit of the Tay­lor marl has so far not been definitely made out for the eastern region; agd until this can· be d<;>ne a· ·more particular comparison would be premature. The upper two hundred feet herald the com­ing change of conditions, by the isolated iirenaceous bands which ·it 
*Ana:Iys'is by :Mr. 0. IL Palm. 
The Geology of the Chisos. Country. 
contains, but its fossils, so far as known, do not indicate any close relationship t9 those which occur in the formation that lies above. 
The Rattlesnake Beds. 
Up to the end of the Terlingua period the sediments of the Cre­taceous age in this region were laid down in open waters that ex­tended beyond the shore belt, but at this time the sea began to be encroached upon by the land, at least 'so far that its waters were more or less hemmed in by inlands and o.ther shore structures. These conditions prevail~d for some time. · -The shore belt evi­dently slowly subsided and thus allowed littoral deposits to ac­cumulate to a considerable thickness. The resulting sediments are entirely different from those of the preceding periods. They consist of sandstones, muddy and peaty clays and silts, now and then some limestone, and occasionally some thin beds of gravel. The littoral conditions were such that vegetable accumulations occasionally were laid down and buried among other sediments. ·These have been changed into coal. · From the nature of the con­ditions attendant upon the making of these deposits they_ are necessarily quite variable in composition and texture,. and indi­vidual ·strata do not persist for any great distance horizontally. 
The Zone of Change . 
• 
· While there is no difficulty in making out the general line of division between the Terlingua and the Rattlesnake formations, as this is well marked in a general way, still the particular level of the change in each local section must be selected somewhat arbi­trarily owing to the frequent variations of the beds in the upper member. Usually the change ill ushered in by a thin stratum of sandstone, which may be overlain by fifty or more feet of clays of the same kind we find below it. Then there may be another ledge · of sandstone with another stratum of clay. Or the sandstones may be replaced . by arenaceous limestone. These beds may truly be said to make a zone of change, since they lead up to the variable deposits above. The sandy and calcareous layers are frequently fossil bearing, and their fauna is clearly related to that above. Yet it is somewhat different. To make close comparisons possible with 
4:2 Bulletin af The · Unwersity of Texas. 
the Cretaceous strata elsewhere by other students, the fossils ob­served in the transition beds are here listed separately : 
1. 
Coral ( ?) . 

2. 
Bryozoan, adhering to the outside of a cup-shaped tube, probably from a coral. 

3. 
Gyrodes sp. ( ?) . 


4. 
Natica sp. (?) . 

5. 
RostelUtes texana Conrad ( ?) . 

6. 
Scurria sp. 

7. 
Spironema sp. 

8. 
Volutom01:pha, like poderosa Whitfield. 

9. 
Oamptonectes burlingtanensis Gabb.. ( ?). 


10. Thracia ·sp. 
11. 
Dentalium gracile M. & H. 

12. 
Baculites sp. 

13. 
Lamna, like elegans~ Agaz. 
'. 4. Lamna texana Roem. 



15. Vertebrae of fishes. 
Thickness. 
Including a hundred feet of this zone of change the Rattlesnake formation contains about 600 feet of strata, as delimited in the description which follows. It must be stated, however, that the c;livision between these beds and the. sediments which ovetlie it is much less well defined than the base of the formation. No at­tempt could be made to definitely mark its upper limit. 
The Sandstones of the Rattlesnake Beds. 
The sandstones of the Rattlesnake beds made the most con­spicuous outcrops in these sediments. Single beds measure as much as. fifty and sixty feet in thickness. While they va:ry con-. siderably in color-which may be white, gray, yellow, brown and even bluish-they possess some features which are rather constant. They are, as a rule, quite free from coarse admixtures. Pebbles, even of small size, are quite unknown. These sands hardly ever · contain any mica and they are quite well sorted. In seven sam-· ples which were specially examfoed with regard to texture, there 
The Geology of the Ohisos Country. 
was only one specimen in which the greater part of the material consisted of grains measuring a half millimeter in diameter; in one sample the maximum ingredient consisted of grains measuring one-fourth of a millimeter; in one of grains measuring about one­sixth mm.; in two, about one-eighth; and in one most of the gr~ns were less than one-sixteenth millimeter in diameter. Compared with other sandstones of the same coarseness, the grains are rather little rounded. Some magnetic grains may usually be found. Other­wise the constituent is quartz, of which some is chert, especially among the grains of the largest sizes. With the coarse admix.tures organic calcareous fragments are sometimes also imbedded. Very rarely these form thin seams between finer strata, where tlie sort­ing has been affected by strong currents. Some calcareous ma­
terial is quite generally preaent, and the sandstones grade into al­most pure ledges of limestOne, which, however, are infrequent. 
The bedding is variabl~. In few instances faces of ledges were seen measuring twenty feet and apparently without bedding seams. As a rule, thin bedding prevails. In thin ledges the seams are usually straight and even. But cross bedding is very general, and fine ripple-bedded layers are not uncommon. 
Being the mO"St open in texture of all the sediments in this for­mation the sandstones have been the highways of underground water, and this has often worked secondary changes. The most frequent change is the introduction of interstitial calcareous ma­terial, giving the rock greater hardness and toughness. This in­terstitial material is sometimes a pure calcite with continuous cleavage planes of large size. The change is very generally ac­companied by -development of concretionary structures, also of large 8ize. A frequent form of these is large circular disks, measur­ing sometimes three feet in diameter, and exhibiting the original bedding planes on the edges in weathering. They frequently split along these planes info thin plates, an inch or less in thickness, ex­tending across the entire disk. Spherical concretions have also · developed, the -sizes most commonly seen being from two to six inches in diameter. 
In some of the sandstones, and e;pecially in the lower part of 
, 	the formation, cementing material is present in small quantity only, and mostly confined along the joints, leaving the central part of the delimited blocks soft and ready to crumble under the 
Bulletin of The Unw~r§ity of Texas. 
influence of the weather. This gives rise to a habit of weather­ing which has a very unique appearance and develops some moat irregular cavities on the surface of the stone. ffitimately these. run together and leave bent and irregular plates a~d cusps pro­jecting out from the face of the ledge, in the most fantastic forms and resemblances. 
About three hundred feet above the base of this formation the 
. 	. 
sandstones have :aome peculiar markings, which are the . imprints of the fucoid Halymenites. They can be most readily described ·as cJosely resembling moulds that might be made by pressing the side of a small ear of popcorn into sand. They . usually appear as semi-cylindric depressions on the weathered surface of the sand7 stone, frequently bending and sometimes . branching, varying in width from three-fourths of an inch to nearly twice that width dnd having a length of sometimes as much as six or eight inches. The surface of the impressions i.s studded with shallow pits that measure about three-sixteenths of an inch in diameter and that tend · to be arranged in rows running diagonally across the cylindric de­pression. In cross-section, these pits may be seen to extend all around the cylindric cavities and in a few instances a smooth and straight central core of sandstone was seen to protrude from one end of the otherwise empty mould. In another place the · filling in these moulds had for some reason become more indurated than the matrix around it, and as this weathered away the casts in the original form of the fossil were left bare. In cross and longi­tuJ.inal sections these exhibited a smooth central core of unaltered sandstone. Ne vestige of organic material could be found in any of the specimens examined. In the casts ju1;1t mentioned the sand 
.	wa:s held together by a dark ferruginous cement, wh~ch wa.S lack­ing in the surrounding more rapidly weathering matrix. 
These markings are so abundant in. some sandstone ledges that 
they may be said to fill them. T.hey have been noted in the coal 
bearing beds of the Upper Cretaceous a.t Eagle Pass, and they are 
reported from the Fox Hills sandstone in Eastern Colorado by N. 
H, Darton. · Their real nature must at present be left to con­
. jecture. The cylinders usually lie in a horizontal position. This, as well as their mode of br.anching, suggests that they may be im­pressions of root-stocks of some water plant. Perhaps they are 
·The Geology of the Chisos Country. 
the , burrows of some animal living in the beach sands of this period. ­
Finer Sediments. 
The finer sediments which are interbedded _with these sandstones . 
consist mostly of silts below, and of clay of finer texture higher up. The muddy silts contain mote or less of carbonaceous mate­rial, which is sometimes present in such quantities as tO make the depasits appear perfectly black. Occasionally the silts as well as the, clays are marly and at times we find the "calcareous ingredient gathered into concretion;; with forins of all degrees of regularity. At times these form continuous layers many feet in extent aml from four inches to a foot thick. These usually oxidize to black impure limonite, as the _original . concretionary mass contains a large per cent of carbonate of iron. At · other· places the concre­tions form perfectly circular leaves and disks. The finer clays are usually gray, but baking by igneous intrusions and alao weathering ·.may have turned them yellow, brown, or red. Bluish and greenish 'tints are also ill evidence. The finer sediments constitute the bulk 
of the formation. 
Origina.l Calcar&OUf Deposits. 
Origptal calcareous deposits played a very subordinate part in the sediments of this period. But there are a few ledges of lime­stones which consist of organic fragments, in the form of ill as~ sorted calcareous sand and of mud containing larger shell frag­ments. Some of 'the clays as well as some of the sandstones ah!o · contain a calcareous ingreJ.ient which is present as an original constituent. 
Special Sections. 
A description of some local sections will give a better idea of 
the character of this formation as a whole. In the ridge which 
extends north from the north end of Burro mesa we find the lower 
part exposed. Cottonwood creek crosses this ridge almost exactly 
· at the latitude of 29Q 20' N., and at the east foot of the ridge its 
bed co:µtains a tinaha; near which there is an old camping place 
known as Chisos Pen. The creek crosses the ridge along a line . of 
fracture and the section in the ridge is considerably broken up and 

Bulletiln of The University of Texas. 
obscured. The beds dip to the e~at from 10° to 30° and are essen­tially as follows: 
Section. near Chisos Pen. 
·Thickness In feet. 
13. 	Soft gray ledges of sandstone containing occasional shells of two species of oysters .......... ................140 
12. An intrusive sill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 
11. Silt and clay with some carbonaceous material. . . ..... . . 60 
10. Sandstone, gray .......... . ................... . .... 20 ' 9 . Sand, mixed with clay . and carbonaceous material. . . . . . . 40 
8. . Coal. ....... . .. : ..................... . .... . ...... lf 

7. Silty mud or fir<o clay ..,. ........................... . 2 

6. 	Sandstone containing some mud and in places showing clear lamination, a large oyster present. . . . . . . . . . . . . . 50 
5. Not exposed . . . ................................... . ? 

4. 	Sandstone, irregularly bedded, and weathering to a very irregularly cavernous or pitted surface, .and containing large specime.ns of Inoceramus cumminsi Cragin...... 30 
3. Clayey and muddy sand ......................... . .. . ? 

2. 	Yellow hard sandstone containing fish remains and vari­ous lamellibranchs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 
1. Clays of the 'Terlingua beds. 
About a half mile east of the Chisos Pen aome ledges of soft sandstone rest on clays which contain silicified wood, and concre­tions of clay ironstone, and at one place a layer of a fine grained 
. white sediment which is evidently a tuffaceous deposit. These ledges here lie in a horizontal position, and taking into account the low pitch to the east which shows in the nearest exposures on the west, the sandstone apparently belongs to a horizon about two hundred feet above' the highest member in the section just given. In the sandstone are moulds of various lamellibranehs, among which a Leda, a Corbicula, and a Cardium were noted, and it also contains remains of saurians, fiahes, and turtles and some silicified wood. 
About one and one-half miles farther north the same section is f!een· on the west side of the ridge and may be given thus: 
fhe Geology of the Chisos Country. 
Section north of Chisos Pen. 
Tbicli.ness in feet. 
12. An igneous sill ...........................·...........17 

11. Gray clay or shale ...................................55 

10. An igneous sill......................................20 

9. Olay anJ silt containing Ostrea, cf. glabra M. & H.. ......20 
8. Sandstone . . ......................... : . ............14 

7. Indurated sandy. silt.: .............................. .45 

6. Sandstone with Inoceramus cumminsi Cragin.. ........ . .10 

5. Sandy clay .. ; ......................................28 

4. Sandstone containing many fragments of shells ..........16 

3. Clay .. \.............................................20 

2. 	Hard yellow sandstone with teeth of sharks and shells of lamellibranchs . ·.........................•... . .... · 5 
1. Clays of the Terlingua beds. 
Again, a mile south of Chisos Pen the same beds on the east side of the ridge were measured and described as follows : 
Section south of Chisos Pen. 
"1'hlckness In fPet. 
12. Igneous sill ................................ .. .... 150 

11. Sandy shale ... . . . ................................. 40 

10. Igneous sill . . . . . . . . . . . . . . . . ... . ... . . . . . . . . . . . . . . . . . 10 
9. Sandstone with fragments of Ostrea. glabra M. & H.. . . . . 42 
8. .Coaly shale . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . 8 
7. Sandy shale . . ....· .......... ; . . ... . . . . . . . . . . . . .. 
. . . 40 

6. 	·Soft sandstone containing a large oyster, probably Ostrea contracta Conrad . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . 12 
5. Sandy shale . ................ .'. . . . . . . . . . . . . . . . . . . . . 40 

4. Sandstone. with I noeeramus cumminsi Cragin. . . . . . . . . . 12 
3. Shale ............................................ 30 

2. Yellow hard sandstone with occasional fish remains...... ·~ 
1. Terlingua beds. 
Number 2 and 3 in all these three sections are the same, and they belong to what has already been described as the "transition beds." They were seen in many other places, as in Rough run to the northwest of Maverick mountain, south of the same mountain near Daws~n creek, near the junction of Rough run and Terlingua creek, in several places between this point and the mouth of Ter­
Bulleti?t of The U'Tlliversity of Texas. 
lingua creek, west of Mariscal mountain, and also ou the Tornillo cr~ek south of Boquillas postoffice. At nearly all of these places the sandstone numbered 4, is found lying right above, and it almost invariably cont.aim; the characteristic fossil Inocer(JJJnus cumminsi Cragin. This fossil is quite variable in shape -and size. It varies from three · to four inches to twelve or even fourteen inches in length, and the larger forms are more gibbous than the smaller. Frequently the sandstone is profusely filled with these large shells and their imperfect casts weather out in ·abundance. This ledge frequently presents a surface honeycombed with cavernous hollows that are separated by irregular plates consisting of indurated ma­terial along its joints. For three or four hundred feet above this sandstone the beds are made of variable strata of clayey and sandy silts, in which there are frequently some peaty seams, and occa­sionally these. dev~lop into seams of pure coal. This is the coal­bearing horizon of the Upper Cretaceous in the west part of the State. It is characterized principally by a lamellibranch fauna to which also belongs at leaBt one brachiopod and some gastm-opodt> and a turtle. The forms most frequently observed resemble those named in the following list : 
Lingula cf. rauliniana. d'Orb. 
Mytilus [ ?] sp. 
Veniella cf. conra&i Whitfield. · 
Mactra texana Conr. 
Thracia gracilis M. & H. 
Ostrea cf. glabra M. & H. 
0. contracta Conr. 
Oardium carolinense Conr. 

0. cf. congestum Conr. 
Orassatela cf. obliquata Whitfield. 
Fragment of the carapace of turtles. 
Impressions of plant ( ?) [described above]. 

The coal is bituminous and has. a fine texture. It is also char­acterized by containing small irregular pockets filled with a red­dish resin. 
Fos'sil Wood. 
Above these coal-bearing strata the sediments become less sandy and contain more clay and more calcareous material. This is a 
Tke Geology of the Chisos Country. 
horizon which almost everywhere is characterized by the presence of silici:fi.ed wood, bones of vertebrates, and stray layers of tuffa-. ceous sediments. Fragments of silici:fi.ed wood are almost univer­sally present and entire trunks are not infrequent. Some trunka were seen measuring more than three (in one instance six) feet in diameter and as much as ·forty feet in length. The tree trunks almost invariably occur several together and usually they lie on or under some ledge of 'ilandstone or along some particular layer in the clays. In one instance all . the trunks belonging to one such layer were seen to. be angiospermous exogens, probably all of one species. More than two-thirds of the wood otherwise shows the 
tracheid tiS'Sue of gymnosperms. 
Vertebrate Remains. 
Associated with these tree trunks are the remains of some sau­rians. Many of .these are of large size. Long bones and vertebrae are most common and the centrum sometimes measures llll much as five inches in diameter. Fragments of the carapace of turtles are quite common. At a point about a mile south of the mouth of Dawson creek the vertebrate remains were found to be associated with . a Viviparus, like; V. raynoldsa111US M. & H. The clay in which these vertebrates most frequently occur usually contains a number of rough yellowish concretions, varying fo sizes from a half to two inches in diameter. 
Tuffs. 
In· these clays we often find layers of white impalpable volcanic tuff more or less mixed with other sediments. Usually some fragments of bones may be found near such layers. Instances of this kind were noted on the south side of Dawson creek, two miles 
. 
south of Big Bend postoffice; in the hills a half mile south of Dry-den's ranch on lower Terlingua creek; at a point about four miles west northwest of Talley's ranch on the Rio Grande south of the Chisos; in the bins about two miles east of Neville springs, and on. Cottonwood creek about a mile east of Ohi'ilos Pen. The tuff beds are usually 'only two or three feet thick and often less than this. They thin out, and thicken in· short distances. Their asso­ciation with the vertebrate remains suggest. that the eruptions 
Bulletin of The University of Texas. 
which produced them may occasionally have been destructive to life in the coastal regions of the Cl"(ltaceous sea. 
One of the thickest beds of such tuff was seen in a small sharp­pointed butte, which rises from the plain 'ilbout two miles east of Ohisos Pen. The section which is exposed on all the sides of this butte is as below : 
ThlcknP.ss In reet. 
4. Soft gray sandstone ...................................20 

3. 	A brecciated mixture of sand, gravel, and lumps of clay and sandstone measuring as much as a foot in diameter. Many of the clay lumps are from a white tuff composing the' next number below ..............................30 
2. 	A white compact fine clay which largely consists of an im­palpable volcanic dust. It breaks into angular chips and was seen to contain a fragment of a bone and a piece of pumice. This stratum is separated by a sharply marked line from the number below ..........................38 
1. 	Dark shaly clay with sandy layers. Some silicified wood noted ..............................................60 
On the southwest end of Slickrock mountain the clays and sand­stones of this division are overlain by similar tuff beds and brec­ciated mixtures, · measuring aome seventy-five feet in thickn~s The change from the clays to the tuffs has the appearance of an unconformity. The sharp line of contact between numbers 1 and 2 in the section given above suggests at any rate a great change in conditions. It may very well be that at both these places an ap­parent unconformity might have been produced by currents in the bottom of waters where the volcanic sediments fell. Such cur­rents may very well have been strong enough to tear up and re­arrange loose volcanic mud, especially as the eruptions were prob­ably not far distant. 
About _two miles west •of Ash spring and at a place where an arroyo approaches the south base of a low cuesta, there is a layer of yellow limestone underlying a tough sandstone which belongs to these clays. The calcareous layer is only some twenty inches thick, but it has a very peculiar brecciated appearance, as if the material had been broken up and twisted or kneaded in· some way before hardening into solid rock. a similar layer was noted in the ·same beds a mile east of the Grapevine hills on the east side of the 
The Geology of the Ohisos Country. 
Chisos. This rock seems to be peculiar to the upper part of the Rattlesnake beds, and may help in tracing their extent. 
Topographic Features. 
The Rattlesnake beds have been named from the Rattlesnake mountain, which is a dissected low dome on the east side of lower T~rlingua creek and about four miles south of Big Bend postoffice. These beds are expoi!ed .in an almost continuous but irregular belt encircling the Chisos mountains, and it varies in width from less than a mile to several miles. It is a belt of greater relief than we find where the Terlingua marls constitute the bed rock. In the Rattlesnake mountains it presents its most rugged appearance to­pographically. In most places we find these beds more or less tilted and the exposed edges of the sandstones form long and nar­row ridges, seldom more than a hundred feet high, and mostly 1ess than this. The effect of this unequal resistance to erosion by different strata is frequently increased by the fact that igneous 
· rock sills have been injected between them, and these protect still more effectually the sandstones. This is the case in the Rattle­snake mountain, in Dogis mountain, in Slickrock mountain, in Chili Cortal and Talley mountains, and in the Grapevine hills. Where the strata lie in a horizontal position the sandstones some­times cap low buttei! and mesas · and ·we find a few of these along the Tornillo creek west of McKinney springs and also on the fiats west of Oak caiion. 
Fossils. 
Fossils are usually well preserved in the clays and limestones. In the sandstones they occur mostly as moulds, the original sub­stance of tlie shells having been removed by solution. In such dases identification is difficult and uncertain. A list is appended which includes all the forms believed to have been noted in the formation, including the "transition" beds. 
Plants. 
1.. Impressions of the fucoid Halymenites. 
2. Silicified wood. Fragments and entire trunks of trees lie in the sandstones and in the clays. About fifty out of seventy 
Bulletiln of The Univ!lrsity of Texas. 
specimens from different localities a.re gymnosperms. The other twenty exhibit the trachea!J' tiBsue o:f exogenous angiosperms. 
Invertebrates. 
-3. Bryozoan, a small fragment attached to a probable coral. 
4. 
Coral? ­

5. 
Lingula, cf. rauliniana d'Orb. In a mud layer, ·six .feet under the coal seam in Cottonwood creek. 

6. 
Ostrea cf. contracta Conrad. This is a large oyster of an elongat.ed form. One specimen measured twenty-seven . inches in height and seven j,nches in width. The hinge in one measured nearly five inches from side to side, seven inches in height and was an inch thick. 


7. 0. elegantula Newb. 
8. 0. cf. veleni<vna Con. This is a species which also resem­bles 0. prudentia White, 0. Wyomingensis Meek, and 0. coalvillen­.%8 Meek. It is the most common o:f all oysters in these-beds and varies in form as well as in size. It occurs mostly in carbonaceous, muddy clays. 
9. 
Q. subtrigonalis Ev. and Shum. 

10. 
0. cf. tecticostata Gabb. 

11. 
Lim.a sp. 


12. Inocera.m.us cummilnsi, Cragin. Always in sandstones in the lower part of the formation. Usually much larger than the specimens described as fypes. 
13. 
I . crispii'? var. subcompressus M. & H. 

14. 
Mytilus [ ?] sp. 

15. 
Leda, sp. 

16. 
Crassatela cf. obliquata Whitfield. 

17. 
Cardium carolinense Conrad [ ?]. 


18. 
C. cf. congestum Conrad. 

19. 
Cardium, undet. A species very common in some sand­stones. 


20. 
Corbicula cytheriformis M. & H. 

21. 
Corbicula, sp. 

22. 
Cyprina. 

23. 
V eniella cf. conradi Whitfield. 

24. 
V eniella, sp. 


The Geology of the Chisos Country. 
25. -Thracia graci'lis M. & H. 
26. Mactra texana Conrad [ ?] . 
.27. ·Undetermined lamellibranchs. · 

28. Buccinopsis parryi Conrad [ ?] . 
29. Viviparus cf. raynoldsanus M. & H. This species occurs in association with saurian remains north of the Rattlesnake moun~ tains. 
30, Undetermined gasteropods, 
31. Baculites grandis [ ?] Meek. Only two rather poorly pre­served specimens were observed in one Bingle locality : in the hills west of Dryden'·s ranch. 
Vertebrates. 
[Determined by Dr. S. W. Williston.] 
32. 
Cl,a,osaurus [several limb bones and several vertebrre]. 

33. 
Oeratopsid? [Lower extremity of a tipia.] . 

34. 
Dryptosaurus [several teeth]. 

35. 
Crocodile [not previously known]. 

36. 
Several· turtles [fresh water types]. Below I append a letter from Prof. S. W. Williston of the Dni­vi::rsity of Chicago, who kindly undertook to examine the verte­br.ate fossils from the Rattlesnake beds. CHICAGO, January 30, 1905. DEAR PROFESSOR UDDEN: I am not yet sure of the determina­tion of some of your speoimens. There are a number of limb bone!;! and vertebrre of a moderate-sized herbivorous dinosaur, which clearly belong in the genus Claosaurus, with solid limb bones. A lower extremity of a large tibia I believe to belong with the Cera­topsia, but I find no other indications of that group. The bone is solid and hence can not belong with the Hadrosauridae; it is either a Oeratopsid or a very large · Olaosa.ur of a new form. I find a number of carnivorous teeth, quite like those so common in the Laramie and referable to Dryptasaurus-but they are not of much value for correlation. The large crocodile is new to science, and hence of no value in correlation. The turtles also are of fresh~ 


water type, but their determination is impossible until the material is restored. So far then : the fo'Ssils are of the land and fresh­
Bulletin of The UniverrS'ity of Texas. 
water Cretaceous, and woull have been referred until recently un­
hesitatingly to the Laramie or Judith river. But, these forms also 
occur in the Belly River deposits, which are now known to be con­
temporary with the Ft. Pierre. 
However, I have very little hesitation in saying that the deposits 
are from near the top of the Cretaceous, though the collection lacks 
not a few things characteristic of the Laramie and Judith River 
that I should expect to find in it, and has an undue preponderance 
of Claosaurus-lika dinOBaurs, which are relatively rare in the 
Laramie or Judith River. 
I am inclined to believe that you have either a new vertebrate horizon, or some very decidedly new facies of a known horizon . which will repay thorough investigation. Very truly yours, 
S. W. WILLISTON. 
Correlation. 
Whatever will prove to be the equivalents of the Rattlesnake beds in the Rocky mountain section farther north, I believe that they are certainly identical with the coal-bearing horizon at Eagle Pass, and also with the coal-bearing beds in El Paso county. 
The Tornillo Cl{]J!fs. 
Sandstones become less and less important from the middle part of the Rattlesnake beds upward. The highest part of the forma­tion consists largeJy of clays alone. Parailel with this change is a disappearance of nearly all of its characteristic fossils. There are at least some six hundred feet of such barren clays resting on the Rattlesnake formation, but they can not be separated from it by any sharp limit. As these upper sediments are well exposed for many miles along Tornillo creek, they will here be called the Tornillo clays. 
Characteristic Coloring. 
These clays may usually be recognized by the peculiar sombre colors: gray, dull oliye green, dull blue, dull red, dull yellow, dull purple, dirty brown, and at times black and white, only now and then heightened by the oxidizing influence of heated intrusives to 
The Geology of the Chisos Country. 
brighter shades of red and yellow. Their stratification is ren~ dered conspicuous chiefly by these colors, }Vhich alternate promis-­cuously in different layers, and sometimes also change in the same stratum. On the sides of many bluffs sapped by arroyos and creeks the different strata usually appear a'S color bands varying in width from one to twenty or thirty feet. Otherwise the stratification is not very apparent. All is a fine clay throughout? with only now and then some thin lentils of sandstone. These sandstone ledges are nearly always characterized by great toughness due to cement­ing material, which makes them hard to break. 
Mechanical Composition. 
The clays have a very fine texture. They contain a large ad­mixture of exceedingly fine particles, less than one sixty-fourth of a millimeter in diameter. This renders them quite impervious to water. But they also contain a small ingredient of sand grains, 8ome of which are magnetic iron. Their elastic elements are really not as well sorted as the sediments in most extra-littoral waters. They resemble in this respect more such clays as are laid down in hemmed-in waters, where currents are absent and where all the elements in suspension are thrown down. The small quantity of sand which is mixed in the clay as well a'S the sand which makes up the stony layers, consists of grains that are rather more angular than usual. This fact no less than the presence of magnetite grains and the copious ferruginous ingredient which causes the coloring, suggests that a large part of the sediments have been derived from disintegrated terranes of volcanic origin. 
Calcareous material is scarce. Usually these clays do not effer­vesce on the application of acid. But occasionally a stratum of lime rock may be found, and it is apt to be associated either with sand or gravel and mixed with carbonate of iron. North of Nugent peak a stratum of this kind was seen, which consisted of a mass. of · lenticular grains of carbonate of lime and iron, cemented into a hard rock, in which there were also some quartz pebbles. It had the appearance of a conglomerate of calcareous lumps. These measured from one to four millimeters in diameter. Some had the appearance of being original in the rock, while others were more like true subsequent concretions. The largest had central 
Bulletin ofThe University of Texas. 
radiating shrinkage cracks filled with calCite. This stratum be­
-longs in the upper part of the formation. About a mile east of the Grapevine hills a stratum near the base of these clays contains a lentil of sandstone, twenty feet thick, which is conglomeritic at. intervals. The pebbles in this conglomerate are. well worn 8.n:d round, some measuring an inch in diameter. Among the pebbles in this conglomerate, rocks of different kinds were represented about as follows: 
Limestone [! of Lower Cretaceous age] ....65 per cent. Chert . . . ......................... .-....23 per cent. Silicified wood . . . . . . . . . . . . . . . . . . . . . . . . . 5 per cent. Quartzite . . . . . . . . . . . . . . . . . . . . ....... : . . 3 per cent. · Worn calcaroous concretions. . . . . . . . . . . . . . . 2 per cent. Shale . . . . . . . . . . . . . . . . .. . . . . . .......... , 1 per cent .. Marble . . . . ....................... : .....1 per cent. 
Chemical Composition. 
The variations in color, which these beds exhibit is no . doubt due 
. to their high percentage of iron. Some of this iron is in the form of black magnet~c grains. From this. circumstance and fr9m their rather large content of potash and soda it may be inferred that these clays consist of the finest debris resulting from the disinte­gration of volcanic rock's. Below is an analysis of. a dark cla.y from the hills on the east side of Rough run, about two miles east of Dogis mountain.* . 
Analysis of Tornillo Olay. 
Silica .·. 64.14 Alumina . • . . ...................... .' .. .'. . . . . . 18.81 Ferric oxide . . .......... , . . . . . . . . . . . . . . . . . . . . 6.05 Lime ...•.............. ; ........ . ........ ·.. .74 Magnesia. . . . .. . . . . . . . • . . . . . . . . . . . . . . . . . . . .. . . . .30 PiJtash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . 1.28 Soda.. . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . .. .58 Water [hygro'Scopic] . . . . . . . . . . . . . . . . . . . . . . . . . 6.00 Water [combined].. . . . . . . . . . . . . . . . . . . . . . . . . . 2.50 
100.40 
*0. H. Palin, analyst. 
The Geology of the Chisos Country. 
Concretions. 
Almost at every place where the clay is subjected to rapid 
erosion its surface is covered by small concretions of carbonate of 
lime and of iron, which are left behind as the loose clay· is washed 
away. These concretions are usually of s:rnall size ranging from 
that of a pea to that of an apple. They have a rough exterior and 
are mo·stly of a dark or yellow color. Sometimes they show 
shrinkage cracks inside. In a few places, as south of the mouth 
of pawson creek, there are large concretionary nests of yellow crys­
tals of calcite, which have a radial arrangement. These nests ·measured from six to ten inches in diameter. 
Weathering and Creeping. 
The Tornillo clays weather in a mO'St singular fashion. It has already been noted that they are so fine in texture as to be quite impervious to water. Inversely they will not yield enough moist­ure to enable plants to grow, except where their surface has been mixed with or covered by some land drift. When rain falls the surface of the bare clay swells up into an exceedingly sticky mud, which renders the land practically impassable to man and bea:st. Pools of water will stand on the ground after heavy showers and they will evaporate away by heat and sunshine while only a small part af the moisture filters into the clay. When the ground dries, the clay shrinks and cracks extensively, but as thP, moisture only affects the upper one or two feet, the cracks are limited to the same depth. The clay retains the moisture with such tenacity that the outer layer of a moist lump will warp and break off while the kernel is yet somewhat plastic. As a result, the drying clay breaks up into irregular angular hard lumps, less than an inch in di­ameter. These cover the unweathered strata beneath to depths of from one to three feet on hills and slopes where the clay is bare. They are hard and tough, sometimes wholly separate from each other, and sometimes partly adhering. With every rain the process is repeated, the lumps swell up and are again dried and warped. The swelling as well as the warping produces a small creeping motion among the clay lumps, small in extent but evidently power­ful. On slopes, gravity aids all movements in a doWn.ward direc­
Bulletin of The Univwrsity .of Texas. 
tion and. counteracts all other movements. In the long_run the accumulated effect of this influence results in •cl motion in the direction of the slope. The whole bed of clay lumps thus creeps forward like a glacier. The m'.ovement is evidently very slow, but many of the clay hills show unmistakable indications of its reality in their rounded flowing contours. Such conditions are especially frequent in the hills along Rough run northeast of Maverick moun­tain, along Tornillo creek, northeast of Paint Gap hill, and in the country to the north and east of Rattlesnake mountain. They seem to be the result of some nicely balanced adjustment between clay texture on the one hand and particular climatic conditions on the other, and we must regard this creeping as a very exceptional phenomenon in nature. On Dawson creek, south of Maver-ick mountain, I found a place where the front end of such a creeping body of clay lumps had embraced a dead mesquite tree and was sti.11 holding this in an upright position, after the base of the clay cliff had been washed away by tlie creek. It will be understood from this that the moving body of the clay consisted of a tangle of lumps of such shape and consistency that they adhered suffi­ciently to hang together, when dry, without support. Where two such creeping bodies of clay come down from opposite sides of a small gully they will sometimes bridge the gully and allow the water to escape beneath them. Miniature low natural bridges formed in this way were noted north of the Rattlesnake mountains, along the margins of some of these "clay creeps." At other places small caves have evidently been formed in a similar manner. 
G.eographical Di,stributwn. 
In a general way the Tornillo clays may be said to underlie a circular belt of land just inside that of the Rattlesnake beds, sur­rounding the Chisos mountains. Its most typical outcrops are to be found along the east bank of Rough run, south and east of Dogis .~ountai,n, where they weather into the peculiar rounded con­tours which have just been described. At a poµit northwest of Chisos pen a fresh cut in the left bank of the creek the bedding indicated, clearly, an unconformity. Some of the layers had been cut away and the ·hollow was filled with the same clay or with a clay identical in appearance. The structure was too high up in the 
The Geology of the Chisos Country. 
bank to be accounted for by the action of the present stream. It appeared like an unconformity in the original bedding. No sim­ilar feature having been observed elsewhere, it does not seem likely that there is any general unconformity at this horizon. 
The clays underlie mu~h of the country west of Burro mesa. South of Dawson creek, on a line nearly south of the west end of Maverick mountain; some streaks of tuff are imbedded, and the uppermost part of the section exhibits two seams of peaty material about thirty feet apart, each of which is overlain by a layer of sand­stone from two to four feet thick. Again, we find these clays to the. northeast and to. the east of Rattlesnake mountain, and along Willow creek. North and northwest from Reed'" ,camp they are seen in most arroyos and also over part of the fl.at ~~untry between this place and Lindsey's quicksilver claim. They constitute the bed rock over large areas between and a.'round Talley mountain and Chili Cortal mountain, and around the Dugout wells. From here northward , they underlie some later gravels, appearing north of these in a belt which extends north to the Tornillo creek, on the east side of the Grapevine hills. In the valley of this creek they frequently contain plates of gypsum with a fibrous structur(>. From this point the outcrops follow the south side oi Tornillo crePk-west­ward and appear last on Onion fl.at. 
Physiographk Character. 
From the above account it is clear that the Tornillo clays in­variably occupy the widest stretches of the graded plains in the Chiaos country. The reason for this is clear. The clays are soft and yield promptly to the agents of erosion. Transporting cur­rents most easily hold them in suspension. No stream in this en-· tire region carries water ·so tenaciously. muddy as does the Tor­nillo. Intrusives seem to have avoided these clays, and there are but few igneous sills present . to break the monotony of the. even plains to which they have been reduced. 
Thickness. 
It was not practicable to Sl;ltisfactorily make out the thickness of Tornillo clays. About a mile and a half above the-mouth of Daw­son creek the combined thickness of these clays and Rattlesnake 
Bulletin of The Universi.ty of Texas. 
·..
beds is not far from 1400 feet. In Ash canon on the northwest flank of the Ohisos mountains the dip and the outcrop of the clays alone indicate that they measure at least 800 feet. But the ex­posures here are not continuous, and there may be some faulting. On the east side of the Chisos there seems to be more of these clays. West of the eruptives which surround Stijlwell's ranch and about four miles north of Banta shutup, there js a continuous ex­posure of the tilted edges of the Rattlesnake beds and the Tornillo clays, which dip from 15° to 25° to the west for a horizontal dis­tance in the same direction of nearly a mile. This indicates a thickness of the two formations of at least 1500 feet. But the upper limit.of the Tornillo clays does not -appear. Six or seven hundred feet ~eems a very conservative estimate for this formation. Possibly it is nearer to a thousand feet. 
Fossils. 
Very few organic remains were observed.· Silicified wood is oc­casionally present in the lower strata. About one~half mile north-j east of Rock hut, or about two miles nothwest of Oak spring, frag­ments of ·some large saurian bones with very coarse texture were noted in an exposure of a varicolored clay. A little distance from this place a ledge of sandstone in the same beds was indented on · its upper surface with curving grooves. resembling tracks made by fresh-water clams. They were certainly marks of some organic origin. Theae are the only indications noted of the existence of life during the Tornillo · epoch. There can hardly be any doubt that plants and animals were less abundant in the seas where these clays were laid down than in the waters depositing the Rat­tlesnake beds. Nevertheless, it is believed that a more thorough search in the dark clays may result in the discovery of more fossils. 
The Ohisos Beds. 
The uppermost and latest great formation in this region consists of a heavy deposit of tuffaceous sediments, which I shall call the Chisos beds, as they form by far the greater part of the sediments seen in the Chisos mountains. 
In their make-up the Chisos beds a:e quite unique. They are 
61
Th~ Geology of the Ohisos Country; 
everywhere clearly stratified in thin and well-defined ledges and layers. These measure irom an inch to one and two feet in thick­ness, and persist for long distances. The exposures appear some­what like those of stratified limestones. But the color is usually of a brighter hue than is common in limestones. The great bulk of the strata consist of a bluish gray or white stratified rock, which lie~ in even, thoroughly consolidated ledges. Among these there are oecasional layers of clay and sandstone, and even thin layers of conglomerate. The sandy layers often show cross-bed.ding and occasionally also ripple marks, and the sand is quite well sorted. 
The Typica.Z, Rock. 
Examining the texture more closely we find that the typical 
. rock appears almost structureless to the naked eye. With a hand lens one can distinguish scattered grains of quartz, slightly rounded, imbedded in an obspurely fragmental mass, which con­taills small cavities or vesicles filled with vario.us minerals, evi­dently of secondary origin, such as crystals of quartz, amorphous quartz, or a greenish silicate resembling. serpentine, or more rarely crystalline calcite. Under the microscope 'Small specks of this greenish mineral are seen quite frequently throughout the rock. The sand grains sometimes show round contours, evidently due to wear, and again they exhibit original crystal faces. Worn frag­ments of feldspar may also be observed, and there are even now and then minute grain!$ of magnetite and more -or less well defined 
·specks of limonitic material. The mass or matrix of the rock is usually too fine in texture to enable us fo recognize the individual minerals of which it is composed, even with the microscope. But it is essentially siliceous, for it gives no response to acid. 
By crushing the rock and washing out the sand we may find the 
same minerals: quartz in rounded and angu.lar grains, seldom .ex­
ceeding one-fifth millimeter in diameter, sometimes of a very clear 

· variety and sometimes opalescent: feldspar in clearly worn frag-. · ments or of unabraded crystals; magnetite and limonitic qrains, the fatter evidently a secondary product from the former. V cry rarely we may discover fragments of scoriaceous glass. 
In chemical compasition these sediinents are somewhat variable owing to secondary changes by which they have been affected. A 
Bulletin <)/ The Univ<1J'sity i>f T~as. 
specimen which was taken from the exposures in the hills south of Ward's spring, where the beds appear to be comparatively little altered, was submitted to Mr. 0. H. Palm for analysis. ·His report is as below: 
Analysis of the Ohisos Tuffs. 
Silica . . . .................................... 53.42 
Alumina ..................................... 9.17 
Ferric oxide ... ; ....... , . ·. . . . . . . . . . . . . . . . . . . . . 2.33 
Liine .......... ...... ....... :.......... .......17..15 
lfagnesia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80 
Carbonic acid .......... .. •.... .. .... .. ........16.00

' . 
Water [hygroscopic] . . . .... . ...... ....... .... ; .22 
. Water [combined] ..... . . .. .. ... . .. . ....... ... 1.02 
Sulphur . '. ......................... , .........trace. 

99.51 
Frequently these ledges have been considerably altered, even when not much changed in their superficial appearance. Minute crystals of quartz, rather long and slender, have formed throughout the fine elastic matrix. Small angular spaces of irregular shape have been .filled with chalcedonic quartz, or with minerals re­sembling serpentine and limonite. Or entire ledges have been. af­fected by solution to such an extent that large irregular cavities have been formed, and filled with calcite. In such ledges no mag­netic sand is present, but it seems to be replaced by the limonitic material. 
The Sandstones. · 
The sandstones, which constitute less than one-fifth of these · deposits, are seldom very heavy but consist of mO'Stly isolated ledges a foot or so in thickness, and these rarely are more than two or three in succession. The grains are largely angular quartz, aver­aging perhaps one-fifth millimeter in diameter. But well rounded grains occur also. Feldspar and magnetite grains are present. The interstices between the grains are filled either with some fine tllfl'aceous material or by calcite or by a siliceous deposit, evidently brough~ by the ground water. In one specimen taken south of 
The Geology of the Chisos Country. 
Ward spring magnetite grains were plentiful along some thin Reams following a slanting and curving seam through the ledge. 
The Clays. 
Clayey strata are most frequent and heaviest in the lower part of the formation where they resemble the Tornillo clays. They have a chocolate color and contain small dark calcareous concre­tions. Higher up they become thinner and acquire a dark dirty gray color. On weathering, the higher shaly strata break up into cubical pieces; and concretions, when present, are apt to be larger ·and to have a more smooth surface than in the lower clays. Near Government spring some clay seams in these beds are deep black froni the presence of organic material. An. analysis of a sample from a typical layer of clay from the exposures west of the Chisos 
mountains is as below : 
Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.64 
Alumina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . 12.11 
Ferric oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.45 
Lime ................................... : . . . 1.46 

Magne·sia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 
Potash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.40 
'Soda . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 1.70 
Water [hygroscopic] . . . . . . . . . . . . . . . . . . . . . . . . . . 2.60 
Water [combined] . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.60 
100.21 
The Conglomerates. 
Congl_?meritic seams are sometimes present, especially in the middle and upper part of these sediments. But they consti­tute an insignificant fraction of the whole. Some of the conglom­eritic seams are associated with the sandy ledges, and their pebbles, which consist mostly of well-worn limestone, average from one­fourth or a half inch in diameter to two or three inches. Else­where we :find well-worn and rounded small boulders of limestone as well as igneous rock from three to six inches in diameter, im­
•bedded in the stony ledges of finer .texture. The limestone boul­
Bulletin of The University of Texas. 
ders are sometimes partly silici:fied on the surface, so that this is protected, as it were, by a more or less continuous crust of chert. 
Description of Exposures. 
South of Oak Canon and about two and a half miles to the southeast of Rock Hut there is a high cliff of the Chisos beds, which here lie in a nearly horizontal position. Some 600 feet are seen and the exposure shows two igneous sheets, one of which is about thirty. feet thick. The other is much thinner and in places exhibits some vesicles like those in a lava flow. One sandy ledge ccmtains rounded limestone pebbles. The lower part of the elifi · expo8es conBidera:ble clay but the upper part consists mostly of sandy tuffaceous sediments with thin strata of clay. 
South of Ward spring the beds dip toward the south at angles from 5° to 35° and in the ridge between this spring and Blue creek, in all some 1200 feet are in view, consisting of rapid alter­nations of the main type of rock with Bandy and clayey layers. Some boulders ·and streaks of gravel occur . in the uppE;r part of this section. In the Blue creek valley the formation rises in the hills on both sides to several hundred feet, anQ. they have been carved into beautiful pillars and towers which adorn especially the lower part of the slopes. Sandy layers are frequently in evidence and. occasionally these contain pebbles. In the country which lies south of the mountains between this valley and Juniper creek the Chisos beds mostly constitute the bed. rock, dipping usually at low angles. In the bed of the arroyo which runs to the east about two miles south of Lower Juniper Spring, sandy ledges are rather fre­quent. 'The more fine-grained ledges have here been altered by the ground water to BUCh an extent .that they are Sometimes· diffi­cult to recognize. Calcareous material and quartz has been gath­ered into pockets and concretions which stud the surface of many of the ledges in odd forms. The upper part of Juniper valley is eroded from the strata of this formation, and they form the lower slopes of Crow mountain. At the base of the easternmost spur of this mountain a ledge of a coarse and well-worn conglomerate was interbedded between the white siliceous strata. Rummel peak and Nugent mountain consist of . the same strata capped by igneous 
flows. An estimate of their thickness as shown in the east slope ­
The Geology of the Ohisos. Country. 
of Rummel peak makes them at least 2000 feet and possibly 250Q feet at this place. On the north side of the mountain they appear in the lower part of· the slopes and cover the territory .between Lone mountain and Panther spring on the east, and Moss well and Government spring to the west. In brief, we may hence say that the Ohisos beds underlie the east half of the mountains, and extend in a narrow crescentic belt around them on their west side. They also appear in several places in the country .southwest of the mountain, so far out as 13outh of the Mule Ear pe-aks. 
Limits and Thickness. 
The dividing plane between the Tornillo clays and the Ohisos beds it was not found practicable to accurately :fix. Judging from appearances there is a gradual change in two hundred feet, from variously colored clays through . yellow and gray clays and to white tuffaceous sands and stratified tuffs. This horizon of change may be seen two miles south of Oak spring and also near the . east base of Nugent mountain. At both of these places some irregularities in dips were noted, but it could not. be satisfactorily ascertained whether these were due to an unconformity or to some orogenic displacements. The evidence seemed to favor the latter view. The uppel: limit of the Chisos beds is still more uncertain. Near 9ak spring and Ward spring the upper part of the formation has been removed by erosion. West of the Lower JUn.iper spring the Chisos beds are overlain by an igneous sheet, which lies in a hori­zontal position, while the strata below dip to the east. . In Rummel peak the sediments seem to contain some lava flows · farthest up in the section, but there. was no opportunity to make this point cer­tain. The general statement seems alone warranted at the present time that the Chisos beds consist of at least two · thousand feet of strata consisting of highly tuffaceous sediments. 
Topography. 
The .Chisos tu:ffs are more resistant to destructive agencies ' than the , underlying. 'l'ornillo clays and they weather less pTomptly. Gullies and arroyos which drain their surface are yet far fl'oin hav­ing reached the limit of their transporting power, and the greater 
Bulletin of The Univarsity of Texas. 
part of their energy is spent in the corrosion of their channels. And so these tu:ffs have a very broken topography, their surface rising and falling in ridges and slopes that often curve up into vertical cliffs on the outer sides of the Chisos mountains. The area of their outcrop 1s the most picturesque part of the entire landscape. 
Life and Age. 
No fossil remains of any kind were found in the various phases of the Chisos beds. But the considerable quantity of lime present in some of the strata and the rare occurrence of some ·dark car­bonaceous clay seams indicates a proba.bility of the existence of such remains in some ledges. Under the circumstances a deter­mination of their age is not possible, and their reference to the Cretaceous is only provisional and tentative. But it is probably correct. There is no clear evidence of the existence of an uncon­formity with the clays below, but there appears to be a gradual transition or change from the lower to the higher beds. This change consists chiefly in a gradual increase of the volcanic debris upw!J.rd. 
. The Grown Conglomerate. 
In the base of Crown peak at the east end of Green gulch the 
uppermost sediments consist of three conglomerates, which are in­
terbedded with strata resembling the typical Chisos beds. These 
conglomerates measure each some ten or twenty feet in thickneas# 
and a great many of the pebbles and boulders are well rounded. 
Most of them consist of various kinds of lavas and trap rocks, but 
limestones are also well represented and some of these contain 
fossils of Lower Cretaceous age. The strata dip here about 45° 
to the north northeast. 
Conglomerates of the same kind are frequently seen in large blocks which have come down on the slopes of the hills on the east side of Oak creek north of Oak spring. We also find them in Ash ·canon to the west of the "Rim Rock." At these places the greater part of the pebbles and boulders in the conglomerate consist of limestone. ~.11 are thoroughly worn and rounded, even the boulders which measure eight or ten inches in diameter. Carboniferous fos­
The Geology of the Ohisos Country. 
, ails 'were noted in some of the boulders, but the Lower Cretaceous lilliestone8 ·are also represented. Again~ this conglomerate was seen at Burro spring on the west side of Burro mesa, where it dips to the east in the hill north of the spring and is it least forty feet thick. All the boulders are well worn and some measure a foot in diameter. Probably the greater ·number are limestone. Here, as at the other places, the limestone boulders have been silicified or sometimes otherwise al­tered to a depth of about one-fourth of an inch from their surface. , Boulders and pebbles of this kind are seen in several places along the road between Castillon peak and Reed's camp, and these must have come from exposures of similar gravels in the region between this road and the mountains. From the fact that these gravels are interbedded with sediments quite identical in appearance with the Chisos boos, and since they overlie these and have the same dips, we must suppose that both are of the same age, and th~t the Crown. gravels represent the latest episl?de in the Chisos period. The further fact ·that the Chisos beds have occasional layers of the same kinds of gravels, contain­ing rounded pebbles of limestone and of igneous rocks, emphasizes this supposition. The gravels were probably made during the time of the last emergence of the land from the Cretaceous sea. 
The Burro Gravels and Tuffs.. 
S~me gravels, breccias, sandstones, and tuffs are of a later but unknown age. Such are the sediments which directly underlie the lava flows that cap the Burro mesa, and which were noted under a lava sheet about a mile and a half 'Simtheast of the mouth of Daw­son creek. The east escarpment of Burro mesa exposes near Rock Hut several strata of this kind, which dip to the south at quite high angles. In the arroyos that drain the mesa west of this place, there are heavy beds of sandy tuffs associated with the lavas. In the hills southeast of the mouth of Dawson creek, at the locality already mentioned, a well-worn conglomerate is seen filling a small arroyo or stream bed, cut in tuffs and again covered with tuffs. Upward and ontward on both sides of the buried dimunitive chan­nel the conglomerate runs into the sand. In the base of the west side of Crown mountain the edges of the dripping strata of the 
BulletVn of The Univorsi,ty of Texas. 
Crown conglomerate are buried under tuffs which underlie hori­zontally the igneous cap of the molll\tain. Other conglomerates and sandy tuffs of this class were noted in the hill which rises to the southwest from Ward spring .and in Burro .mesa. along Smug­gler's canon. 
· All of these deposits a.re closely associated with lava flows and some are apparently interbedde.i. with them. Their general field appearance is quite different from that of the Crown conglomerate and of the Chisos beds. . They are less regularly . bedded and they vary much more in composition and in texture. The tuffs are ;(re­quently yellow . or even red and sometimes filled with concretionary structures entirely unlike anything · seen in the lower beds, from which they are. separated by an unconformity; If we regard the . igneous rocks as OI Tertiary. age, it follows that the inter bedded sediments :should be of this age also. 
The Dugout OT,ays arid Gra.vels. 
On the west "side of Tornillo creek and northwest of Boqtiillas there i~ an oval area ·about twelve mile8 long and four miles Wide with its longer axi~ extending from northwest to the southeast, where the sediments of the Cretaceous age are covered by from one to ·three hundred feet of clays, silts, saii.ds, and gravels of a reddish color. The finer sediments constitute the greater bulk of the formation~ as 'one sees it along the road to the Dugout w'ells from Boquillas. In Ric~s canon, a few miles-to the north, there are mostly gravels. The formation rises in a fl.at ridge, .:which extends northwestward from the Dugout wells. Its finer se.ii­ments resemble the Equus beds of the Great Plains and l!lSo the Pleistocene ri~er silts in the valley of the Rio Grande. N·o fossils were found, but as the formation rests on a topography which has been developed long after the cessation of volcanic actiVities in this region, it must, be Of quite recent age as compared ·With the igneous rocks of the Ohisos mountal.ns. But it is no doubt older than the alluVia:l deposits in the valley of the Rio Grande, for it lies at a higher level and has itself suffered extensive erosio~. . Possibly the Dugout clays and gravels are an old alluVial drift laid down bv 
Tornillo creek. . . . 
\ 
The Geology of the Ohisos Country. 
Recent Alluvium and Land Drift. 
The alluvial deposits consist of clay, sand, and gravel that cover 
the lower flats of the streams and at times rise into level branches 
on the sides of the valleys. It is nowhere very deep. Along the 
Rio Grande it is usually a loose sandy silt, but there are also finer 
clays of a darker and brownish color. Thes~ are deepest just above 
the canons which the river is cutting through the mountain ridge& 
it traverses, and at points where tributaries come in · from either 
Bide. At the former places the alluvium is silty and has evidently 
been laid down during great floods, when the water has backed up 
against the narrow canons and has stood as high as seventy feet 
above low water, or even higher. Neat the junctions of tributaries 
with the main stream the alluvium is. usually gravel, mostly 
brought down by the. smaller stream. A considerable deposit of 
1mch gravel is seen alon~ the lower course of Fresnal creek near 
Solis ranch. As a rule, the alluvial deposits are not very heavy, . 
indicating a condition of comparatively active corrosion. 
The land drift, which co;vers the greater part of the plains, is 
u&ually also thin. On Burro mesa it may reach a depth of two 
hundred feet in some places, but this is an old drift, which is now 
being removed by erosion and which must have -been laid down by 
an earlier drainage quite different from the present. In the ar­
royos the land drift consists of gravel and sand, which is alternately 
laid up in banks and again torn down and rolled by the angry 
water which fills their channels after heavy showers. Away 
from these smaller streams it consists of. silt and sand, :fine enough 
to be carri~d along by the sheet flood, and with this is an a·J.mix­
ture of pebble·s, .which are small enough to be slowly. rolled by the 
eame agent. -The size of these pebbles seems to bear a pretty con­
st:m.t relation to the slope of the plain. On the average slope they 
seldom exceed an inch and a half in diameter. Sometimes there 
are no pebbles, and in su~h places the drift is apparently accumu­
lating. Elsewhere they are so plentiful as to form a thin continu­
ous pavement, and this usually occurs on slopes where erosion is . quite active. 
I 
Bulletin of The Univwrsity of Texas: 
5. THE IGNEOUS ROCKS. 
No more than a mention of the most important occurrences of igneous rocks in this territory can be attempted in this paper. My field "Studies would not warrant anything further. Observations on the volcanics were made only so far ·as they had a bearing on the general geological str}lcture. Notes on some thin sections have .been furnished by Mr. B. F. Hill, who k:in~ly· examined. these and made. such determinations as were possible on short notice. It ii:; . believed, nevertheless, t~at the brief descriptions based on his notes and on such o'bservations as were made in the field, may help to give a general idea of the nature of the volcanics in this region and may be of same service to future observers. 
Igneous rocks are frequently exposed and appear as deep in­trusives, as laccolites, as sills, as dikes, as .Plugs and as surface flows. 
The A.ltuda Gra.nite. 
An intrusive granite boss lies uncovered over an area of some­what more than a square mile, four miles north of Mt. Ord, south­west of Altuda. It is a moderately coarse-grained rock of reddish gray color, and it weathers into large blocks, sometimes twenty feet in length. · It rises in a hill several hundred feet high. The Carboniferous sediments which surround this hill dip away from ­it and have evidently at one time formed a continuous cover over the whole area. 
Laccoliths. 
Some laccolites of the dome-shaped type occur in the region 
north and east of Terlingua. Maverick mountain·is a conspicuous 
instance. It is a mass of intrusive rock rising 800 feet from the 
plain and covering an oval tract extending two miles east and west 
and about one. mile north and south. 
The R.-Osillos mountains must likewise be regard.ed as a laccolite 
broadly oval in form and with diameters measuring respectively 
about four an'd six miles. This rock consi'Bts of a feldspathic 
porphyry occasionally almost granitic in texture. The feldspar is 
largely orthoclase and is most copious in the ground mass, which 
also contains some magnetite. Augite and mica · are present in 
The Geology of the Ohisos Country. 
visible crystals. The dull red tint which this mountain shows 
when seen from a distance is due to the presence of large spheroid 
kernels in the main rock, which are more resistant to weatheting 
than the main mass and hence are left covering the surface of the 
hills. These kernels measure from a few inches to several feet in 
diameter and they weather to a dull red. 
The mountains around Stillwell's ranch north of Banta shutup consist of another laccolitic intrusion less regular in form than that just described, but of a somewhat similar rock. In a specimen taken from near the ranch there was a decided porphyritic struc­ture. The ground mass. consisted of feldspar, quartz, pyroxene and magnetite. The phoenocrysts were mostly augite. Olivine was also noticed. In places this intrusion is cut by dike-like traver­sions of a lighter rock, apparently a quartz diorite. These ·dike;; run in a northwest-southeast direction and are usually only two or three feet thick. In weathering, the main body of the rock in these hills has the same ha.bit as that in the Rosillos mountains. · 
The northeast projection of the Chisos mountains contains a partly uncovered laccolite, which extends over several square miles in the vicinity of Panther springs. This consists of a porphyritic andesite in which the ground mass has been extensively silicified, and whose phoenocrysts appear to be chiefly plagioclase and augite. The rock is fissured by joints which run N. 35° E.-S. 35° W., and which in some places are only an inch apart, thus cutting the rock up into · thin vertical slabs. 
A rock of nearly the same character forms the laccolitic in­trusives known as the Corazone peaks, but a specimen of the ma­terial in one of these peaks from near the Marlay wells was seen to contain some orthoclase crystals and some original quartz. One of these masses rises in a sharp peak, which is a thousand feet high . . 
The largest intrusive mass in this entire region is the so-called "rim-rock" of the Chisos mountains. This rises to a height of more than two thousand feet and covers eight or ten square miles of territory. A specimen from the main mass of this intrusive, taken above the falls in ·Oak canon, may be called a quartz por­phyry, in which the groundmass as well as the phoenocrysts con­sist of quartz and feldspar, with a preponderance of the former mineral in the groundmass. ~he gro_undmass was somewhat de­
Bulletin of 'fhe Univetrsity of Texas. 
composed and there was evidence of silici:fication by infiltration. The joints in the rock · are close together and they have nearly the f:ame bearing here as at Panther spring, about N. 30° E.-S. 30° W. 
Sills. 
There may be some other igneous.masses of a. laccolitic nature, but those already mentioned are the most important. Igneous sills are even more common. These are thinner sheets that have been injected as molten ma.tenal between the ledges of the bedded rocks and which run parallel with these ledges, sometim~s for many miles. 
There are several different types of rock among these sills. On the east side of Mariscal mountain at a point about two and one­half miles west and a half mile south of Solis' ranch, there are two sills in the Boquillas flags, each measuring about five feet in 
·thickness. They are separated by about forty feet of the flags. 
The lower sill consists of a dioritic rock. The upper one is a 
greenish black obsidian which breaks into more or less rectangular · 
blocks. It is plainly intrusive, but along the contacts of the sheet 
with the sediments there is hardly a suggestion of change in the 
latter by baking. The obsidian is almost a perfect glass except 
for a few spherulites, which may be seen in a thin section, and for 
some few crystals of orthoclase and augite, some of which are large 
enough to be visible to the unaided eye. 
The greater number of sills consist of either of two types of 
rocks. They are andesitic or dioritic.. A sill . of andesitic rock 
is exposed in a line of hills extending to the southeast from the 
east side of Terlingua creek opposite Dryden's ranch. This sill 
is about seventy feet thick. It consists of greenish dark and heavy 
rock, of moderately fine texture, fissured vertically by joints which 
are only two or three inches apart, and weathering into a multitude 
of round boulders, from half . an inch to three inches in diameter. 
In a. thin section one finds that it is porpyhritic in structure, with 
a very fine groundmass. The latter consists of pyroxene [augite?], 
plagioclase, and much magnetite. The phoenocrysts are augite and 
plagioelase, with a few crystals of mica. Perhaps this rock may be 
regarded as intermediate between andesite and basalt. Other sheets 
like this, or extensions of the same sill, appears in several ridges 
The Geology <;>f the (!hisos Country. 
tojh~ west1 to the south, and to the east of this place and also in the Rattlesnake ·mountains and on the east side of Willow creek. 
,Another ·instance was noted in the Chisos Pen ridge and in the Dogi{m.ountain·. West of the Chisos mountains, sins of this rock have quite generally been injected among the strata of the Rattle­snake beds. 
· A much more extensive sill of dioritic rock follows the Boquillas flags on. the east side of the mountains. This sheet is particularly conspicuous on the west side of the Boquillas ranges north of Still­·well's ranch and about · three miles east of Mailbox tank. · At a point about four, miles east and one mile north of McKinney's spring this sill is some fifty feet thick. It is a dark, coarse-grained rock with nearly one-half of its bulk consisting of augite. The feldspar is· prevailingly orthoclase. Magnetite is abundant, and the sand ·in the arroyos which cut the sheet is often black from the abundance of its magnetite . grains. Pyrite is also present, especially in the lower two feet of the sheet, when freshly exposed. Olivine was noted, and in a thin section it was seen to be changing 'to serpentine. Apatite is present in a sheet of the same character 
. and in the same beds high up in the Chisos mountains about a mile west of Emory peak. Along the west ·side of Mariscal moun­tain we have apparently the same sill, but here it is often consider-. ably altered. ' The olivine is mostly changed to serpentine and the magnetite to hematite or limonite. A sheet which covers a hill lying about three · miles . northwest of Dugout Wells ·and northeast of Nugent mountain consists of a more acidic rock which lacks distinct groundmass and contains much magnetite. The feldspar is mostly orthoclase and there is some quartz. The sills in Lone mountain and in Grapevine hills are of the same nature. 
Dilees. 
Three specimens may be taken as types of the dike-f~rming rocks. One is from a dike· about four miles east-southeast of Reed's camp .on the Rio G;rande, south of the Chisos. ~his is a porphyry of dark color. The phoenocrysts consist of a few but 
· large feldspar crystals, inO'Stly orthoclase, and a mineral resembling acamite. There are also crystals of augite. The groundmass is 
Bulletin of The Univ6rsity (}f Texas. 
of ophitic texture, partly of the .same minerals as the larger crystals, but containing also apatite and magnetite. ' 
Another specimen is from the system of dikes which radiate toward the southeast from Christmas . mountains. This niay be called a basic andesite. It is a dark rock with a fine, almost felsitic groundmass, consisting mastly of plagioclase and magnetite. Its phoenocrysts are quite large and consist of plagioclase, augite, and magnetite. 
The third specimen is from one of three dikes near Ward spring and nmy be called a quartz porphyry. There are several dikes of this kind in the foothills of J;he Chisos mountains. Three cut the Chisos bed·s to the southeast from Rock . Hut and extend toward Ward spring. This .specimen is a highly :siliceous rock, quartz ap­pearing in the groundmass as well as in the larger crystals. In another 'Specimen from the same dike quartz and feldspar was seen to be present in about equal proportion. Several dikes of the same nature follow the same trend to the southeast of the mountains. 
Plugs. 
A sharp-pointed butte locally known ·as the "black peak," which rises from the plain about four miles northeast of Box spring, is ·clearly a remnant of a vertical plug-like intrusion. It may or may 
· not have been connected with a volcanic vent. It is an almost perfectly black porphyry, which in many respects resembles the rock of the dioritic sill intruded ·among the Boquillas flags east of Tornillo creek. It consists almost entirely of plagioclase, mag­netite, and augite, and these minerals make up the visible crystals as well as in the groundmass. Olivine is an accessory mineral. 
Surface Flows. 
In the upper part of the Chisos beds there are some igneous sheets which evidently are interbeMed with the strata of the for­mation. They may have been flo~s on the shores of the waters in which these beds were deposited. These sheets lie· parallel with the strata of the sediments and at the same time exhibit vesicular structure. The heavy sheet in the high bench east of Ash spring and north of Oak spring appears to be of this kind. · A specimen 
The Geology of the Chisos Country. 
of this rook was seen to have a porphyritic structure with 'Some 
very large slat-like crystals of plagioclase in a fine grained an.l 
ev.en.· felsitic ·groundmass. Magnetite· is present in considerable 
quantity and there is a filling of a green structureless mineral not 
unlike chlorite, which lies in many interstices among the other 
minerals, especially in lhe basal part of the sheet. This rock for~s 
a sheet more than a hundred feet in thickness, and it extends as a 
vertical cliff a distance of at least ·a half mile in the side of the 
mountain; Between Oak spring and Ward spring other thinner 
flows with vesicular 'Structure lie in the white ledges of the tuff­
·aceous sedimentS. 
'rhe lava sheet which formg the peak of Mouut Emery is a quartz porph)TY with a matrix which is occasionally almost glassy, ex­hibiting spherulites. The specimen examined had evidently been silicified thTough secondary changes. The jlow which 'Surmounts the high mesa to the southeast of this peak is of the ·same nature. ~here are other flows of still greater extent in Burro mesa. In the north point of this mesa no less than five flows appear in the north escarpment, and these dip to the 'South. Tuffs are inter­. bedded with these, some of which have a fine compact texture, while ·others grade thr9ugh volcanic· sands into breccias. At a point about a mile southwest of Rock Hut one lava sheet appears, which is only a few feet thick. Its upper surface is checked by cracks, an inch wide and from six to seven inches apart, and these are filled · with volcanic sand. In Smuggler's caiion on the west side of this mesa, the sheet which is expO'Sed in the brink of the falls is a hard felsite, and this rests on a soft and yellow volcanic sand. A h~lf mile east of this place some lavas contain occasional small 
streaks of black obsidian with inclO'Sed large crystals of feldspar. 
Various other surface flows are to be seen southwest of the 
Ohisos mountains. One was observed dipping at a high angle with 
the sediments in the north . part of Green gulch. 
D. G:IDOLOGICAL STRUCTURE.. 
The country traversed lies between the Southern Pacific Railroad and the Rio. Grande river in the longitude of the OhisO'S mountains. Only ·a few days were spent in that part of this tract which lies north of 29° 30' north latitude. But here I found structural fea­
Bttlletin of The Univ6rsity of Texas. 
tures of such unusual interest that I can not pass them by without 
mention. These have been in part described by R. T. Hill before 
this. The area ·as a whole is traversed diagonally froi;n north 
northwest to south .southeast by the southernmost extension in the 
United · States of the great Rocky mountain fold. This mighty 
system of mountains is here represented by a na:rtrow and sharp 
and for the most part single fold, less than two miles wide. It is 
known at the north as Altuda mountain, at the south as the Car-­
men range, and . between these two as the Ord and the Santiago 
ranges. East of the line of these ranges the great plains have 
their beginning, though they here more resemble · a plateau. On 
the west si.de we meet with the typical structure of the Basin region, 
and this is continued westward beyond the limits of the State. 
North of Altuda the structures representing the Rocky mountains 
flare out and apparently soon have a width of many times three 
miles. With the beginning of the Carmen range on the south 
there is again an abrupt increase in the width of the continent~l 
''backbone'' and this increase is continued into Mexico. 
The question at once suggests itself: what caused this principal 
crest of the great mountain range to contract· to such insignificant 
diinensions between Altuda and the Carmen range? I believe that 
the right answer to this question is to be found in the fact that 
the Rocky mountain axis here crosses, almost at right -angles, an­
other and much more ancient mountain system represented by the 
Caballos ridges. · 
E. THE CABALLOS RIDGES. 
The town of :Marathon lies near the center of a triangular plain, 
which extends for about fifty miles to the northeast from the Ord 
range. Against this range the plain abruptly terminates on the 
west. On this plain the Cretaceous sediments have been entirely 
removed and the ancient floor, on "!'hich these were laid down and · which they once covered, is now bared. This floor consists of 
sharply :folded and highly tilted strata of Palaeozoic sediments. 
All around the plain ·the slightly raised edges of later sediments 
form a well-marked ~arpment. The :folds of the older strata 
trend in a northwest-southeast ·direction and extend the whole 
length of the plain. They exhibit such regularity and persistency 
The Geology of the Ohisos Country. : 
of dir.ection that they have all the appearance of being a small part or an extensive system. Their axes point on the one hand straight to the Solitario uplift, which is forty mile.s· distant to the south­west, .and which exposes similar 'Sediments, underlying Cretaceous strata;, folded in nearly the Bame trend. . In the opposite direc­tion .the folds point toward the Ouachita system of mountains in Oklahoma. Prof. R. T. Hill, who has described these ridges,* be­lieves that this folding was produced contemporaneously with the great Appalachian movements in the eastern part of the continent. His observations are given in the following words: "These (the Ca:ba:llos ridges) are low ridges of Palaeozoic rocks rising from the :floor of the Marathon plain south of the Comanche mountain·s. They are composed of the degraded vertical edges of Palaeozoic limestones, shales, and cherts occurring in closely folded, buckled ridges, trending northea'Bt and southwest. The cherts are often white in color and over 100 feet thick, and form the backbone of long, .low ridges, .such as the Pe:iia Colorado, occupying·the low area between.the Santiago Sierra on the west, the Glass mountains on the no.rth, the Maravillas Plateau on the south, and the Stockton Plateau cm the eaBt. daballos Sandia, a beautiful >hill of white 11hert about 15 miles south of Marathon is the highest of these ~ummit'B, and rises about 1000 feet above the plain to an altitude of about 5000 feet. · 
''The Caballos and Glass mountain are exposures of ancient post­:Pal!:!-eozoic structures of Appalachian type and age, which have been revealed by .the erosion of the Cretaceous sediments that prob­ably once embedded them." .· 
The same author says of the Ouachita mountains** that these 
,"are in general old and represent the remnants of once more lofty 
atid extensive ranges, which have undergone degradation since early 
Mesozoic:tiine." While' there is probably no continuity between the · 
Caballos. folds and the OuaG!hita system there seems to be good 
ground for the 'belief that they were made at the same time. The 
Cabalios ridges were made and cut down by erosion during the time 
whi'ch elapsed: between some period (probably the late Carbonifer~ 
ous) of th~ l~ter half of the Palaeozdic era and the begi11ningbf 
.. . ' . . . . 
*R. T. Hill, Physical Geography of the Texas .Region, 'U. S. GeOI. Surv. Topographic Atlaii of the U. S., Folio 3, p. 4~ · · · · 
.*~fere~ce .a& above. · 
Bulletin of The University of Texas. 
the Cretaceous ·age. They no doubt furnished a part of the ma­terial in the making of the Triassic and Jurassic deposits else­where. 
It was after the Ca.hallos ridges had been cut down to almo;;t a peneplain and after this peneplain had been buried under all but the latest Cretaceous sediments; that the movements began which resulted in the making of the Rocky mountains. Folds and faults were made, which crossed diagonally the axis of these earlier fl.ex­ures. Now it seems clear that where a fold is developing along an axis which crosses a belt that has been previously compressed and folded in a direction more or less vertical to the axis of the later disturbance, the belt so affected will less readily yield to the forces at work than the country on either side. For on either side of such a system of folds the strata lie more nearly horizontal, while they stand edgewise in the folds. One can easily bend a block of paper in a direction which is vertical to the plane of the sheets, but ,not in a direction which is parallel with this. . Rather than to bend in this direction the sheets will 'break. Between Altuda and the Carmen range the rigidity of the folded Palaeozoic strata caused them to break in a single sharp dislocation forming the Santiago and the Ord ranges, rather than bend into several fold~ or fissures .by several smaller faults as seems to have happened on either side of this belt of the ancient mountain structure. 
F. OVERLAP OF THE CRETACEOUS. 
Another structural feature due to pre-Cretaceous mountain making in this region . is the evidence of an overlap in the basal strata of the Cretaceous formation. The nature and extent of the overlap is not known, but there is no doubt that it exists. The base of the Lower Cretaceous at Altuda does not include a con­
•
glomerate which is a pronounced feature of Persimmon gap and at the Cienega mountain. The same conglomerate is also absent in the Solitario, which, as already stated, lies on the axis of the Caballos uplift but it reappears at Shafter, which is to the west of this. It is evident that the old mountain axis still stood ·somewhat higher than the country on either side of it, when the Cretaceous sea was ad-..::ancing, and in this way its lowermost sediments are present on the flanks of the uplift but not on its highest elevations . 
. 
The Geology of the Ohisos Country. 
G. THE ROCKY MOUN'TAIN UPLIFT. 
As already stated, the structure which represents the Rocky mountain uplift in this region consists of the Ord, the Santiago and the Carmen ranges. In general the line marked by these ranges is a line or belt of displacement, to the west of which the ground has sunk, and to the east of which it has been elevated. Farthest north the displacement is most gentle. West of Altuda.the limestones of the Carboniferous age and the Lower Cretaceous limestones dip to 
. the west at angles of most frequently about · 15 °, until in about three miles they disappear under the eruptives. Farther south the dip west of the face of the Ord range becomes more steep and prob­ably changs to a fault. At Persimmon gap in the Santiago range the main Lower Cretaceous limestone is thrown into an abrupt fold with a nearly vertical dip on the west side and a fault on the east side with the downthrow to the east. Evidently the range was here originally a sharp anticline, which fractured on the east side. On the whole, the range here involves a displacement, along which the terranes lying on the west side have been let down at least 2500 feet more than the terranes on the east, while-the nar­row block which forms the ridge has been elevated some 1500 feet more than the latter. At Dog canon, which cuts across the range at a point about five miles :farther south, the range is clearly a compressed fold, in which the ledges of the Lower Cretaceous lime­stones now stand in a vertical position. The arch of the fold has been fractured and removed 'by erosion and its base exhibits even a fan structure due to lateral compression. Eight miles south oi this point .the Carmen range begins. This is clearly a continua­tion of the dislocation we find in the ranges to the north, but the lateral thrust has here given rise to a number of ·folds of more gentle build. The downthrow to the west is quite as great but .less abrupt, and the faulting is more frequent, but nowhere is vio­lent. This condition gives a hint as to the location of the pres­ent course of the Rio Grande. A stream which backs across the lowest sag of a developing moutain crest will naturally out­strip other streams in capturing tributaries. It has less cutting to do at the start, and after a larger drainage area has once been acquired, the greater volume of water is likely to insure the re­
Bulletin of The Univarsity of Texas. 
tention of the advantage gained, even if conditions at first existing should be reversed, as they are in this case. At present the Carmen range is higher than the south part of the Santiago range, and it is several times as wide, but it must have been the lowest part of the Rocky mountain fold in this region at the beginning. 
H. THE GREAT TERLINGUA FAULT. 
At the mouth of Teriingua creek the Rio Grande crosses a great fault, which runs a course about N. 36° W. for some ten miles . north of the river and at least as far to the .south of the river. At this point the total displacement is about three thousand feet. This decreases as we follow the fa.ult northward and it increases for some distance when followed into Mexico. The fault is marked by a high escarpment, facing the east. This is caused. by .a rise of. the Lower Cretaceous limestone above the present plain of erosion of the more readily yielding clays and marls of the Upper Cretaceous sediments. This fa.ult clearly illustrates a feature re­cently described by Keyes.* The friction against the rising block has been sufficient to abruptly bend upward the edge of the more flexible strata on the side of the downthrow. On the face of the great escarpment south of the mouth of the Gr1md canon a thin block of the limestone measuring several hundred feet in length, is rotated in the plane of the fault in such a manner as to suggest that one end of the block has been held down by the sunken block while the other has been more free to follow with the great wall from which it has been disengaged by a fracture. · 
I. THE SUNKEN BLOCK. 
If we limit the Chisos country to the region lying between the ranges representing the Rocky mountains on the east and the great fault at the mouth of Terlingua creek on the west and regard it as extending from the Rio Grande river on the south to the Corazones and the Rosillos mountains on the north, we· may say that it covers a part of a sunken block, which measures about thirty-nine miles from e.ast to west and which has settled from four to six thousand feet pelow the level of the terranes on either side. 
*Journal of Geology, Vol. XIII, No. 1, p. 68. 
The GeolQg1J of the Ohisos Country. 
While this block clearly m~tain:S the nature of a. structural· 'Qµit, it haB itself suffered extensive and in places violent deforma­tion by faulting and folding. These deformations are so numerous that a detailed account of them is out of the question at this time, though there is no doubt that it would throw important light on the nature and the distribution in time of the orogenic forces which have been at work in this region. But it has been necessary to limit attention to a few of the more salient features. 
1. FAULTS. 
To the northeast of the Rattlesnake mountains several nearly 
vertical faults cut the almost horizontal strata into narrow blocks. 
Two of-these are on either side of an elevated 'block which appears 
in the north bank of Terlingua creek about a mile south of the 
mouth of Dawson creek. The downthrow on either side is at least 
two hundred feet. The creek turns and follows the east side of 
this block for nearly a half mile above this point. Both faults are 
vertical and both show the edges of the strata on either side turned 
up against the fault plane as by friction against the raised walls. 
Their cpurse is about N. 22° W. and the distance between them is 
about 900 feet. Within a mile, as we go east, are two more faults, 
parallel with these. These have the downthrow to the east and in 
the case .of the farther one this amounts to several hundred feet. 
They bear about N. 14° W., and apparently continue for several 
miles. These four faults are clearly genetically related, lying in 
one parallel system, and this system continues for at least a few 
miles south and two or three miles north . 
. The east side of Burro mesa i\another fault-line, which extends 
from Rock Hut to south of Blue creek in a direction about N. 5° ·w., with the downthrow to the weBt. This mesa is a block which has settled deeper than any other part of the great sunken Ohisos country, and as a consequence the lower members of the great lava complex which once covered the entire country has so far escaped destruction by erosion. It has let these down sonie two thousand feet below their former level on either side, and southwest of Ward spring they are seen to lie against the Ohisos beds. The west side 
oi the mesa is a monocline where the Ohisos beds rapidly rise from 
under, as we proceed in that direction. Between the north end of 
Bulletin of The U'niv&sity <>f Texas. 
·this mesa and the south end of the Christmas mountain is a belt 
where disturbances are frequent. At a point about three and a 
half miles south and one mile east of Christmas spring, an arroyo 
locally known as Cottonwood creek crosses a fault which runs a 
course nearly due north and south and which has a down.throw to 
the west of not far from two thousand feet. It brings the Tornillo 
clays down to the level of the base of the Terlingua beds, and it can 
be traced for nearly two miles, but it seems to turn to the west 
farther north and to become continuous with a great fault wliich 
follows the west side of the Christmas mountains. 
To the southeast of the Chisos mountains and crossing the Rio Grande there are several faults which bear nearly due north and south. Thus between Mariscal and San Vincente mountains there is a block about one and one-fourth mile wide, which has been · dropped down from one to two thousand feet. It is marked off on the west by a vertical faulted wall forming the east side of Mariscal mountain and on the east by a fault which cro~ses the river di­rectly east of the Solis ranch. Another fault crosses the river to the east of the village of San Vincente and from there it runs almost due north, passing on the west side of the U. S. benchmark, having an elevation of 1881 feet. Three miles farther north it passes just to the east of the benchmark with the elevation of 1925 feet. The downthrown in this case is on the west side, and it ,measures somewhere from 500 to 1000 feet, decreasing to the north. North and northeast of the Chisos is a group of faults which 
have the same general trend as those on lower Terlingua creek. 
One of these is the Muskhog spring fault. It runs S. 23° E. and 
points straight in a continuous line with the west line of the first 
fold of the Carmen range. At4'Muskhog spring the Rattlesnake 
beds have been lowered by this fault to the level of the upper ledges 
of the Boquillas · flags, against which they now abut. The down.­
throw is to the .west. Thi_s fault is one of the best developed de­
formations of its kind in the country, and it was traced for several 
miles. Two small abrupt folds with the same trend follow it in 
the vicinity of the spring, at a distance of some two hundred yards 
to the east. These folds are next' to another smaller fault which 
brings up the lower ledges of the Boquillas flags. Several faults 
with this trend cut the ground in the mountains around the Still­
The Geology of the Ohisos Country. 
· well ranch and the Rosillos mountains are rent by at least three: one to the east •and one to the west of Stroud's ranch and one in the arroyo of Cottonwood spring. The latter, which is rather a :fissure, extends southeast across Tornillo creek and through the Grapevine hills, where it clearly is compound and has let down a small block, cuiting the sill which caps these hills. 
It will be noted that the trend of the axes of the displacements so far described is more or less parallel with the sraes of the sunken Ohisos block. Some faults have been noted which run a course that is vertical to this direction. At a point about one-half mile north of Muskhog spring, three faults, which are not two hundred feet apart, b~r W. 10° S.-E. 10° N. A thin block is raised some three hundred feet between two of these faults, and the third one [the one farthest south] has the down throw on the north side. To the southwest of McKinney's spring a fault cuts acwss the igneous sill at its north end. An unusual form of faults was seen on the north side of Cuesta Blanca. Here the Rattlesnake beds have been let down to the level of the Boquillas flags along a line which is concave to the north but otherwise has a general direction from e:ast to west. On the south side of the Rosillos mountains an east­west fault runs about ·a mile south of Stroud's ranch. But the most singular of all faulting in this entire region was noted in Grace canon about half a mile northeast from the McKinney and Parker mine. A block, about 300 feet square, has been let down some 500 feet, so that the Rattlesnake beds, which compose it, lie against the lower part of the Terlingua beds. The fault marking the outside of the·block is distinct enough to be traced with the point of a knife. 
In general, it is true that the faults whose trend is vertical to the direction of the axes of the principal dislocations have an irregular development. This may be taken as an indication that the making of some of these was incidentally the result of the forces which produced the faults trending N. N. W.-S. S. E. 
2. FOLDS. 
But a very small part of the sunken Ohis0s block, if any, bas not been affected by folding. The sediments only exceptionally lie horizontal. Even when apparently undisturbed, they are usually . 
Bulletin· of The University of Texas. 
found to be tilted a few degrees in some ·direction. The ·most fre­quent dips are from 10° to 30°. A detailed :tepresentation of these deformations would show a number of synclines, anticlines, and monoclines of varied length, mostly trending in a northwest­southeast direction. 
Among the disturbances whose trend is parallel with ·the great structures of this region we find an anticline west of the Rattle­snake mountains. It is fully three miles wid~. The west side of Burro mesa exposes a belt of Upper Cretaceous sediments which dip a little north of east. Along a line that points about 10° east of south from Indianola peak runs the axis of another anticlinal fold. Several folds of unknown extent are in the country between the Chisos mountains and Boquillas postoffice, and all appear. to have a northwest-southeast trend. An extensive syncline with the same trend has 1ts axis about midway between ..McKinney's spring and Neville spring. Banta Shutup is on the southwest periphery of the south half of a quaquaversal, and this may be the south end of an anticline extending northward. For three miles to the southeast of Maverick mountain the dip-is quite uniformly to the south and southeast. The fault on the east side of Burro mesa marks an axis from which tbe strata dip away on either side. This anticlinal feature of the fault appears at a point west-southwest from Ward spring in the north side of a high butte. A similar anticlinal arrangement is seen in Cottonwood creek, a mile west of Chisos Pen. The block on the west side dips west at a low angle, and the block on the east is tilted away from the f~ult first gently and farther out more rapidly. 
Many folds trend in various other directions. Hayes ridge, which forms the easternmost spur of the Ohisos mountains, is an . anticline trending nearly east and west. North and northeast of Moss wells the Chisos beds dips to the north, and south of Paint Gap hill the Rattlesnake beds often pitch to the south. Changes in direction and amount of pitch are frequent and often rapid. The entire sunken block is broken, folded, and twisted in an almost bewildering complexity and the manifold attitudes of the sedimen­tary strata defy brief detailed description. 
If we divide the great sunken block into two halves by a straight line extended midway between the Rocky mountain fold and the 
The Geology of the Chisos Country. 
face of the great Terlingua escarpment, and nearly parallel with the latter, we will find that this line connects the three greatest deformations affecting the_block: These are the Christmas moun tains at the north, the Chisos mountain uplift in the middle and the Mar'iscal mountain farthest south. 
3. CHRISTMAS MOUNTAIN FOLD. 
As seen from the south end, this mountain is an elliptic dome­shaped uplift a little more than three miles long and not quite two miles wide, with the longer axis extending N. 36° W.-S. 36° 
E. The Lower Cretaceous limestones are raised 2200 feet above . the surrounding plairi and are lifted 3500 feet above their position under the Upper Cretaceous sediments on the surrounding plains. 'The dome is fractured on the west side by several faults and has here an abrupt slope, which in places is a vertical scarp. On all · _sides it has suffered considerably by erosion, but the form of the dome is still quite apparent. In all probability the uplift is due to an intrusive mass which is nowhere exposed .but lies concealed under the folded limestone strata that yet forms the roof of the dome. At the south end the principal fault, ~which is marked by a vertical cliff, is associated with an extensive fissuring of the lime­stone, and the fissures are filled with large quantities of calcite. These fissures have the same trend as the fault. 
4. THE MARISCAL MOUNTAIN FOLD. 
From the top of Mount Emery two long mountain ridges may be seen to the southeast, extending north across the Rio Grande from Mexico. One of these, lying farthest east, is called the San Vincente mountain. It stops nearly short of the international boundary and the river has just begun to cut into its north foot­hills. The other is known as the Mariscal mountain and this pushes its north · end across the Rio Grande nine miles into Texas. The . river has cut a canon diagonally across this ridge and this canon is 1400 feet deep at the crest of the ridge. The ridge is an unsymmetrical fold, whose west limb has in places been tilted into a vertical position and whose east limb is much less inclined but is abruptly faulted. at its east end, farthest to the south. Several 
Bulletin of The UnivtArSity of Texas_. 
minor faults cut the limbs of the fold. The ridge comes to a cus­pidate point at its north end. At its widest place the ridge meas­ures about three and a half miles across. It consists entirely of the Lower Cretaceous limestones, but the Upper Cretaceous beds have also been involved in the uplift, as their dips adjust them­selves to that of the fold over a belt from· one to three miles wide on all sides. 
5. THE Omsos MouNTAINs FoLD. 
The most violent of the deformations of the great sunken block is an abrupt folq caused by the intrusions of the "rim-rock" of the Chisos, and this may be regarded either as an enormous plug or ab an unusually high laccolite. The evidence is inconclusive in regard to which it most resembles. On the west side the Ohisos beds lie nearly horizontal. As we follow them up to the great igneous mass, we find them thrown into small but abrupt folds close to the vertical wall of the mountain. On the south side the same relation is maintained: the sediments run up abruptly against the igneous mass. But on the east and on the north sides it is different. Here the sediments have been elevated and they rise from the east in the f~>rm of a half arch, which bends over the top of the "rim-rock,'' and from the north they rise along a sharp contact, which stands at an angle of 75° from the horizontal. This attitude of. the ·sedimentary rocks on the east and north ·sides is decidedly like their habit around laccolitic intrusions, but their position to the south and west, where there is no regular tilting, is much more like the behavior of stratified terranes around plugs or near dike-like intrusions. .The fact that thiS igneous mass has been seen to take on a brecciated structure in one or two placea 
[as at. a point about one mile west of Laguna] favors the hypoth-. 
esis of a volcanic vent. Such a theory would also explain the pres­
ent physiographic feature of the mountains, for a volcanic cone 
would naturally effectively retard the erosion of the country it 
covered and cause this to stand higher than the surrounding coun­
try for ·a long time after the cone itself were carried away by 
erosion. But even on this theory the ·south "rim-rock" may be 
regarded as to some extent laccolitic in its nature. For it is very 
well known that molten magmas may stop short of reaching the 
top of their vent and may find their way out far ,down under the 
The Geology of the Ohisos Country. 
surface aild form intrusive masses more oi less distant from the source of the supply. If a vent should form over a sharp fold the underground stresses might very well be such as to favor the ac­cumulation of an intrusion near the vent itself and close to the fold, or even within it. The :fissure through which the molten material escaped may have been under the west side of the "rim­rock." At the south it was then forced in under the Middle Cre­taceous sediments and these were elevated, but at the north the intrusion which forms the north "rim-rock" apparently found an outle! at a 'higher level and perhaps on the concave north and upper side of the flexure produce-d by the uplift to the south. This intrusion, or other intrusive flows from the same source, appear to have been forced far out under the latest Cretaceous sediments to the north and east. Everything considered, this seems to the present writer the most probable structure of the Chisos moun­tains. The whole round cluster of peaks, including an area ten miles in diameter, is a remnant· pedestal under a volcanic pile, protected from erosive destruction by the endurance of the ancient cone, · which once covered it and is now all but wholly removed. The two "riin-rocks" and other scattered laccolitic masses are the roots of the cone itself, or, to carry the comparison farther, they are tuberous swellings on these roots, now more or less uncovered by erosion. What we find in the Chisos mountains today is then the structure developed at a depth of several thousand feet under the surface of ancient lavas. 'These lavas were at least in part forced up through :fissures in the strata which are now exposed. 
J. SUMMARY OF GEOLOGICAL HISTORY. 
Tlie earliest record known is that of an open sea, during the Trenton age, on the 'bottom of which sea calcareous sediments and fine mud were deposited. Marine conditions continued, whether with or without interruptions is not known, until near the end of the Carboniferous age or possibly somewhat later. Then a dis­turbance occurred which resulted in the making of a folded moun­tain range, whose axis extended in a northeast-southwest direction an unknown distance beyond the limits of the ridged plains now surrounding Marathon. During the time which elapsed after the making of th~se moutains and until the beginning· of the Ore­
~8 Bulletin of The University of Texas. 
taceous age-essentially during the Triassic and the Jurassic ages -this mountain range was reduced almost to a peneplain, so that when the Cretaceous sea advanced the old folds did not rise much above the rest of the land, probably not more than a few hundred feet. . The Cretaceous sea endured for a long tilne. At first on open and wide ocean held its sway and on its bottom heavy deposiU! of calcareous mud and ooze settled. In tilne the waters became less deep and the sediments gave i.ndications of a slowly approach­ing shore line, as they changed to marls and clays. pistant vol-· canoes on the yet distant land oecasionally mingled their ashes with the clays of the sea. Later on the shore line came near. Olay and sand and sometimes even gravels and vegetal accumulations were laid down. An equilibrium was maintained for some time between the forces tending to elevate and those tending to sin:k the land. The latter prevailed so far as to make possible a practically continuous addition of littoral deposits, throughout the Rattlesnake and the Tornillo periods. These beds do sometimes exhibit minor 
. unconforrnities without tilting, due perhaps to the making and un­making of low bars and islands. They can hardly have any other significance. Volcanic eruptions became more frequent and also less distant in place, as shown by the greater frequency and thick­ness of occasional ash beds. · At last these materials became copious and largely exceeded the other ingredients of the deposits of the sea. Land was evidently near, if not right here. But the bottom of the sea continued to sink under the weight of the Chisos tuffs. Meanwhile many new volcanic vents were opened and they some­times emptied their contents under the waters of the sea as weil as on the near lands. New mountain folds were already forming, involving the earlier limestone formations of the age, for worn pebbles of these are found in the Chisos beds. The·making of the Rocky mountains had now begun. · 
At the end of the Cretaceous age, or perhaps duriug the early part of the Tertiary age, the sea receded and an extensive land rose to take its place. There were volcanoes and mountain folds. Molten magmas were injected among the sediments and poured out over their surface. Blocks of sediments whose, dimensions are measured in miles, adjusted themselves to orogenic fore~ into most varied positions, and were folded and fractured p~omiscuously. 
Th~ Geology of the Ohisos Country. 
This mountain-making period continued through the greater part of the Tertiary age. With the gradual cessation of volcanic dis­turbances; destructive forces attained to the ascendency and these have since reduced the formerly much higher mountains and plateaus to their present condition. 
K. MINERAL RESOURCES. 
So far, cinnabar is the only material in this region which has proved to be of economic importance. The mining of quicksilver is now an established industry in Brewster county. But silver and coal have also been found. Attempts at the mining of silYer haYe not been entirely discouraging and the mining of coal on a small scale, to supply the local demand for fuel, may prove profitable. 
1. · QUICKSILVER. 
It is proposed to here briefly state such general conclusions re­garding the occurrence of the quicksilver ore as now seem to be clear. 
The Big Bend mine explorations now (1904) show that the ore occurs in a dike of andesitic rock as well as in the fissures in the Cretacequs shales and limestones surrounding the intrusiYe. The dike is, in common with other such structures when injected into soft 'Sediments, irregularly developed, running out in a short dis­tance when followeJ to the west and changing into a sill aboYe. It is a filling in an irregularly developed fault and the ore is in a place where the ground water or the mineralizing solution follow­ing the dike upward would be impeded, consequent upon ·a change of route. 
It will be remembered from earlier reports of this survey that in the Marfa and Mariposa mine and · in the old Terlingua mine much of the ore has come from brecciated fissures in the upper surface of the Edwards limestone. This was originally directly covered by the next to impervious Del Rio clay, which at the present time is mostly removed. Evidently in this case, also, the ore was pre­cipitated from rising solutions at a level where these were hemmeil in by the impervious cover of the clay. As cinnabar has been found in this mine also in a porphyry, the ore is in that respect in 
Bulletin of The UnivtJrsity of Texas. 
a situation quite similar to that at Big Bend. If the mineralized solution followed the course of a body of an intrusive in the lime­stone it would be likely to have a comparatively open and unim­peded passage along this intrusive, as long as this traversed the limestone. For in limestones dikes are quite uniform in their development. But reaching the overlying clay the solution would find the dike less regular and the clay also would hinder its ascen­sion. · The solution would. naturally follow the upper surface of the limestone and precipitates would find lodgment in its fissures un­der the clay. 
This theory finds verification in alomst every instance of known occurrence of ore in this territory. In the region of the Colquitt­Tigne mine, . where the ore occurs in fissures in the Boquillas flags, it has· been observed that' the veins are occasionally enriched im­mediately under the more argillaceous strata which these flags con­tain. In Christmas mountain, where cinnabar was. discovered last summer, it is likewise associated with a dike of andesite and is in a fissure in the uppermost ledges of the Lower Cretaceous limestone near the summit in the dome. There can be no doubt that this limestone was covered by the more impervious sediments of the Middle Cretaceous at the time the ore was deposited. Thus the con­ditions here would 'be like those at the Marfa and the Mariposa mine and the old Terlingua mine. The rising solutions deposited ore on encountering a more impervious overlying formation. Again in Mariscal mountain, at Lindsey's claim, we find the ore at the same horizon, and likewise on the crest of a fold. At this place, as on· Christmas mountain, the Del Rio clay is absent, but the base of the Boquillas flags is still left and ore occurs in seams between some of the ledges of this formation. In one of the openings made a small offshoot of an amlesitic dike was followed by a thin stringer of cinnabar on its upper surface. These occurrences show that the fracture produced by the offshoot has served as a conduit of the mineralized solution and that precipitation has occurred at a place where it met with resistance in its Blow upward percolation along th€· small "stringers" of the dike and i~ the less pervious sediments by which the stringers of the dike are surrounded. In one of the openings in the Boquillas flags on the north end of the mountain there were three seams of cinnabar about one-fourth inch thick ·and 
The Geology of the Ohisos Country. 
twenty inches apart, following joints which cut the sediments run­ning transversely to the axis of the mountain. · 
Another circumstance points to the correctness of the same view. This is the fact that the principal ore deposits have been discovered on crests of anticlines or else on lines of changing dips, where the fiexures are convex upward. The deposits on Christmas mountain and on the north end of Mariscal mountain are examples of the former ,instance, and the ore at the Marfa and Mariposa mine, that· in the Chisos mine, and that on the east side of Mariscal mountain, illustrate the latter mode of occurrence. This last mentioned oc­currence is at a point about three miles west and one mile south of Solis' ranch. The cinnabar is here, as elsewhere, frequently in and near a dike. The dike in this instance cuts the lower edges of the Boquillas flags. 
According to this view of the origin of the ore, we shoud expect the principal ·deposits to lie in places toward which ascending cur­rents have converged, as on the ''hips" and crests of anticlines. The pres~nce of cinnabar in the intrusives l.s due to impregnation, which has introduced the cinnabar as a secondary mineral. We have seen that profound changes have been effected in this manner in other volcanics of this region. There is all the more reason to regard the quicksilver ore as a secondary deposit, for we always find the rock altered in a peculiar way in the vicinity of the ore deposits. The exact nature of this alteration is not known, but it causes the dikes to weather with a characteristic bleached yellowish color, different from that which characterizes the same weathereJ rock in its usual condition. This bleached appearance of the igneous rock happens to be especially well shown near the two min­eral focalities on the east side of Mariscal mountain. It is notice­able in some places where cinnabar is not known to exist. One instance of this sort was seen at a µoint about three miles west and one mile north of Mailbox tank, at the east foot of the Rosillos mountains. An intrusive here weathers to a resemblance of the dike associated with the cinnabar in the Christmas mountains. But an analysis of the rock failed to show the presence of any trace of the mineral. This analysis is as below: 
Bulletin of The University of Texa$. 
Analysi,s of an altered igneous rock from the ea$t side of the 
Rosillos Mountains.* 

Silica . . . ............ ........... ....62~00 per cent. Alumina ... . . .. ... ... ... . .. ........ . 15.49 per cent. Ferric oxide . . . . . . . . . . . . . . . . . . . . . . . . . 6.00 per cent. Ferrous oxide . . . . . . . . . . . . . . . . . . . . . . . . 1.08 per cent. Magnesia .... .. .. .... .. .... .... .. . . . 0.10 per cent. Lime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.78 per cent. Soda ............. . .. . ... .. .. . .. .. , . 5.10 per cent. PotM1Sh . . . . ..... ; . . . . . . . . . . . . . . . . . . . 3.68 per cent. Water above 105-110 C ... ..... . . ..... .. 2.95 per cent. Water below 105-110 C. . . . . . . . . . . . . . . . 0.80 per cent. Carbonic acid . . .... . . ............ , . . . 0.88 per cent. Titanium dioxide . . . . . . . . . . . . . . . . . . . . . 0.86 per cent. Phosphorus pentoxide . .......... ....... .. ......trace. Manganous oxide . . .. ..... ... . . .. .... . ........trace. 
·In some parts of the world quicksilver deposits are found in close association. with hot springs.** While the Brewster county ore can not be said to now have very close associationa of this kind, it is interesting to note the presence of several hot springs on the Rio Grande river in the Carmen range, only ten miles distant from a known occurrence of cinna­bar. Three of these springs emerge from near the zone sepa­rating the Boquillas flags from the heavy bedded limestones of the Lower Cretaceous, and thus demonstrate the fact that the more clayey overlying flags may cause the ground water to follow their under-surface in seeking a higher outlet. Some of the, :fis­sures which these springs have followed are now filled with ma­terials that must have be~n deposited from their solutions, and they resemble in every respect the _gangue of some of the cinnabar leads in the limestone at Terlingua, except that the ore is absent as is also often the case at Terlingua. It consists of crystalline calcite and of amorphous carbonate of lime, in which fragments of the country rock are imbedded. In its chemical composition it re­sembles the tufa which is now forming around the hot springs. 
*0 . H. Palm, analyst. **Beeker, Qui.cksilver Deposits of the Pacific Slope, U. S. ~ol. Surv., Monograph XIII, p. 52. 
Th8 Geology of the Ohisos Country. 
Analysis of Precipitations from hot springs in the Garmen rang6 on Rio Grande riv.er: 
Deposit In a. fls;ure ot an Tufa from a. hot spring
old spring near mouth near the mouth of 
ot Tornlllo ;}!"eek. Tornlllo creek. 
Silica .......·....... ~ 19.26 per cent. 19.00 per cent. Alumina ............. 1.00 per sent. · 4.02 per cent. Ferric oxide . . . . . . . . . . 1.30 per cent. 1.60 per cent. Lime ................ 42:95 per cent. 40.60 per cent. Magnesia .......... ~ ·. 0.60 per cent. 1.10 per cent. Sulphur trioxide .. ." .. , 0.10 per cent. trace. Carbonic acid . . .......32.80 per cent. 30.16 per cent. Water· [hygroscopic] . . . 0.20 per cent. 0.80 I.>er cent. Water [in combination]. 1.35 per cent. 2.00 per cent. 
99.56 per cent. 99.28 per cent. The knowledge we now possess of the cinnabar deposits in Brew­ster county seem indeed to bear out. the conclusions reached by Geo. F. Becker in the study of similar deposits elsewhere, who ascribes them to . mineralized ( and probably hot rising solutions. He says:* "The mineral associations in which cinnabar is found" (in dif­ferent parts ol the world) "seem to show conclusively that it has been deposited from solutions. A very large part of the known deposits of cinnabar are extremely similar in character, a fact which seems indicative of a similar origin. It is certain that some 
of the deposits are due to precipitation from hot volcanic springs, and it may fairly be inferred that many of them were found in this 
• 	manner. The diversity of the country rocks in which the deposits occur is evidence that only a part of thern can have derived their metallic contents from their own wall rocks ; the remainder must owe their cinnabar to some source between the point at which the waters acquired their heat and the surface. Between the depth at which volcanic foci lie and the surface of the earth there must be substances of world-wide distribution which frequently contain mercury in some form as an original ingredient. · These substances 
*Reference as above, p. 55. 
Bulletin of The University of Texas. 
are probably massive rocks, and the only known rock of corre­spondingly wide distribution is granite." 
On this theory the significance of the association of the quick­silver with volcanic rocks in this region is also quite clear. The Rills and especially the dikes traverse the sediments to great depths and are frequently so placed that they must have furnished in the past, as they do at the present, the most direct passage ways for ur.derground waters which traverse the formations from below up­ward. Intrusive sheets and dikes cause several springs and seeps in the country today. Indeed, it was on the basis of the associa­tion of the igneous rocks with the Lower Cretaceous limestones that the director of the ·survey, Dr. Wm. B. Phillips, was able to predict the discovery of cinnabar east of the Chisos three years ago. 
2. COAL. 
'The existence of coal in the Chisos country has been known for i:;ome time, but its field relations, its quality and general develop­ment have been matters of uncertainty. 
From the stratigraphic data already set forth, it will be remem­bered that the Rattlesnake beds contain the coal. 
The several places at which coal has been observed extend from the Rosillos mountains to the mouth of the Terlingua and from the west side of the Carmen range to near the Cigar mountain. Be­low some of the localities are enumerated: 
1. In the north side of a hill on the south side of the road leading southwest from Stroud's ranch and about two miles from this ranch, the face of this coal has been laid bare by three short drifts, but it is at present concealed. The seam is reported to have• measured eighteen inches. Over the coal are some sandy and bituminous dark silts, and under it are first some three feet of clay and then a 'black carbonaceous sandstone. Some silicifieJ trunks of trees were noted close by the explorations. The strata dip about ten feet to the south. The intrusives of the Rosillo;; mountains are not far distant and have evidently baked the coal to some extent. Some of the coal taken out from the pits was left on the ground, and this exhibited a cubic fracture, an unusually bright luster and a very compact texture. A few pieces were picked up which had these characteristics in a superlative degree, 
Tho. Geology ~f the Ghisos Country. 
and in which no flaws could be seen, although they had been ex­posed to the weather clearly for a long time. These have the ap­pearance of jet, and have great crushing strength. Some show very faint but unmistakable evidence of woody structure on a cylindrical outer surface. A small block, which was shown to the writer by Mr. Carruthers, of Alpine, probably came from this place. It measured nearly four inches in diameter. It was clearly a coal formed from some species of wood. A well known gem expert, who examined some of this material for the writer, says that, if found in blocks of sufficient size, this mineral may be valuable as a sub­stitute for jet. It resembles jet in its luster, but it has the com­position of anthracite, as is shown by the following analyses: 
Analyses of coal from two miles southwest of Stroud's ranch.* 
Anthracite with jet
Anthracite from two luster from two 
miles southwest of miles southwest 
Stroud's ranch. ot Stroud's ranch. Moisture ... . ............ . 2.441............ 1.47 Ash .................... . 4.231... . . . . . . . . . 1.93 Volatile combustible matter .. 15.381 .... . ..... .. 13.07 Fixed carbon . . . .......... . 77.951 ............ 83.53 
-I 
100.00, . . . . .. .. . ...100.00 Sulphur . . . . . . . . . . . . . . . . . . .93 . . . . . . . . . . . . 1.26 
The jet-like anthracite represents entire tree trunks ~hich form a part of the coal seam. The coal is quite different from the other seams in the region in having 'SO small a percentage of ash. Its high content of fixed carbon is clearly due to baking by the intrusives in the Rosillos mountains. 
2. 
On the north side of Tomillo creek, north from the Grape­vine hills, a small seam of coal was uncovered in a surface pit some years ago. Peaty and bituminou'S shales have been observed in several places in the west part of the Tomillo fiats. 

3. 
In the country immediately west of the Slickrock mountain two pits have been made in which coal was found. In neither place the seam measured quite twenty inches in thickness. 

4. 
The Kimble pits are some openings made on a seam which i;, expOBed on the south side of a hill at a point two miles north of the 


*Analyst, Mr. 0 . H. Palm. 
Bulletin of The University of Texas. 
Chisos pen north of Rough run. This seam of coal measures about twenty inches. It has a considerable dip and the ground is somewhat broken. It is associated with bituminous and sandy shales or clays. An analysis of this coal shows that it contains: 
Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.68 Ash.. . ................................... 16.60 Volatile combustible matter. . . . . . . . . . . . . . . . . . . . 24.20 Fixed carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.52 
100.00 Sulphur .. .88 
5. What is probably the same seam as that seen in the Kimble pits is exposed in the bottom of Cottonwood creek at Chisos pen, where it measures eighteen inches. The creek follows a line of dis­placement at this place and the coal also appears about two hun­dred yards away from the arroyo on the north side, in a gully, where it has about the same development. In a piece of this coal which was more closely examined, some pitted Tracheids of gym­nospermous wood could be discerned. Some very small grains of marcasite· were seen along some horizontal seams. Above as well as below this coal we find carbonaceous and bituminous silts and clays. Its composition resembles that of the coal in the Kimble pits, with somewhat less ash and somewhat more of volati!e ma­terial, viz. : 
Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12 Ash .· ........... ; . . . . . . . . . . . . . . . . . . . . . . . . . 14.42. Volatile combustible material. . . . . . . . . . . . . . . . . . . 34. 72 Fixed carbon ................................ 44.74 
100;00 Sulphur . . 1.32 
6. In the .flats which lie about one and one-fourth mile south­east of the ·southeast side of Maverick mountain there are some exposures of a thin coal seam. One of these which appeared in the east bank of a creek showed eight inches of good coal with nearly two feet of "bone" or coaly shale below. An analysis of this coal is as below : 
TheJ Geology of the Chisos Country. 
Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.46 Ash ....................................... 25.90 Volatile combustible matter . . . , . . ...............34.88 Fixed carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.76 · 
100.00 Sulphur .. 1.00 
This coal contains frequent ·small grains or pockets of a yellow 
resin, from the size of a mustard seed to that of a bean. They are 
reddish-brown in color, very irregular in shape, ·and usually frac­
. tured by weathering. 	Mr. 0. H. Palm, who has examined them, finds that ''they easily melt and burn if brought near the flame. They are soluble in chloroform, ether, and alkalies. Their melt­ing point is between 205° and 210° 0." 
7. 
Some coal seams were noted in the hills on the west side of Terlingua creek about three and a half miles south of Study butte. No)le of these seem to be more than a foot in thickness, but the coal is associated with a "bony'' shale, which is highly bituminous. Grains of resin are present in the ·shales as well as in the coal. 

8. 
The same seam of coal appears again in the hills on the opposite side of the creek about · a mile to the south, where it dips at a high angle. 

9. 
In the low ridges of the flats between Terlingua Abaja and the mouth of Terlingua cxeek [about three-fourthB of a mile to the south. and a little west of the village] a coal seam crops out, which is eigliteen inches thick. It dips 30° to the southwest. This coal is associated with some sandstone ledges and it is about in the same horizon aB the Ohisos pen coal. A piece taken from the weathered face of this outcrop contained:* 


Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.10 Ash . . . ........... ... ..... . . ...... , .......·. 21.50 Volatile combustible matter. . . . . . . . . . . . . . . . . . . . 37.38 
·Fixed carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.02 
. 100.00 Sulphur .. 0.90 
•o. H, Palm. analyst. 
Bulletin of The University of Texas; 
10. 
On the north side of the Rio Grande, about seven miles west of Mariscal mountain, the Rattlesnake beds contain some bituminous clays, that have thin seams of coal. 

11. 
Apparent~y trustworthy reports say that coal has been seen in some of the arroyos north of Tal~ey's ranch and southwest of the north end of Mariscal mountain and also in the country northwest of Banta Shutup. In both of these localities there are extensive outcrops of the Rattlesnake beds. 


The writer believes that the Rattlesnake beds and their included coals are the near equivalents of the coal-bearing beds at Eagle. Pass. Both.overlie the marine clays of the Middle Cretaceous, the Upson clays and Terlingua beds. The coals are somewhat 
·alike in composition and they both contain small grains of resin. Some very characteristic impressions of pla~ts are common to both formations, as shown on a previous page, and some fossils arc common to both. Still there is a difference in the fauna as well as in the nature of the sediments themselves. The latter are somewhat more variable in the Chisos country, where the sand­stones are slightly coarser and have a more open texture. The sandstones are here. also less persistent when traced horizontally. All this indicates less open waters in which sedimentation took place in the Chisos region. This view is corroborated also by the difference in the two faunas. At Eagle Pass ammonoids are quite common, while in the Chisos country these are rare and the oyster family is much more frequently in evidence. This has a bearing on the economic features of the coal seams. We ·should not ex­pect to find these as constant in the more variable sediments in the 
Chisos country as they have proved to be in the Eagle Pass region. They were made neartlr to the shore in the western country and will naturally be more variable and uncertain. On the other hand, it has been shown that the Rattlesnake beds belong to the true coal-bearing horizon of the Upper Cretaceous series in the West, and that they will bear exploration. They outcrop in a circular belt around the Chisos mountains, and this belt can not yet be rngarded as sufficiently explored. It is true that all the outcrops which have been examined, show only thin seams. The variability oi the beds proves that local and sudden changes are to be expected. Such variations may be for the better as well as for the worse. 
The Geology of the Chisos Country. 
At Jmy rate, some thin seams are already known, and in a country where fuel is as scarce as here, some of these might prove profitable to work on a small scale under present conditions. 
3. SILVER. 
On the north side of Altuda mountain some silver-bearing galena has been mined. The workings are on both sides of an arroyo, which comes from the west and runs on the north side of the moun­tain. The workings consist of some three hundred feet of drifts and shafts, some of which follow the contact of the ore-bearing rock and the Cibolo limestone, which forms the country rock. On the south side of the arroyo there rises from this limestone . a wedge-shaped peak of partly silicified material, which clearly at some time was a filling in a cavern in the limestone above which it now rises. This peak consists of an altered conglomerate or breccia, containing pebbles, boulders, and large blocks of S\lndstone, of various kinds of igneous rocks, of granit~, and of limestone from the Cretaceous as well as from the Carboniferous strata. The infiltration. of siliceous material has rendered this mass some­what resistant to the agencies of weathering and erosion, and so the country rock has suffered more rapid removal by these agencies while the more enduring filling remains in part intact. Most ·of the ore occurred in irregular leads near the contact of this filling with the limestone in the wall of the ancient cavern. On the north side of the arroyo · a siliceous tufa, probably a precipitate 
· from underground hot solutions, is largely in evidence ·over a part d the surface of a large hill. Its contact with the limestone can be traced in several places, and occasionally it takes on the ap­pearance of an igneous intrusive. Some very ~ich ore has been taken out from one of the explorations near the contact at this deposit with the limestone. 
In the mind of the writer there is no doubt that the mineral deposits a~ this place, whatever their value, have essentially the same origin and history as the l~des in the Shafter mine near the Chinati mountains.* The material filling the ancient caverns is 
*See the Geology of the Shafter Silver Mine District, Presidio county, Texas, Bulletin The University of Texas Mineral Survey, No. 8, p. 54 et seq. 
Bulletin of The University of Texas. 
much more extensive here at Altuda, and it possibly consists in part of intrusive material. What bearing this may have on the quantity of ore no one can say. Lodes vary from barrenness to the richest deposits known. They are always expensive to explore. The explorations at Altuda are as yet not extensive enough to give an estimate of the quantity of the ore, which in a pocket on one of the drifts was very rich. 
4. PHOSPHATE. 
Some of the rock samples taken from the strata of the Trenton age near Ridge ·spring, south of Pena Colora:do, were found to have specimens of the small gasteropod Cyclora. This fossil is common in the phosphate-bearing ledges of the Trenton horizon in Ten­nessee, and its existence here suggested the desirability of having a1; analysis made in order to see if the limestone would contain phosphate in this western :field. Two small specimens of the lot brought from the field were analyzed and one yielded 6.03 and the other 1.70 per cent of phosphorus pentoxide. This is clearly in excess of the usual phosphate content of limeston!J's, and suggests the possibility of workable phosphate rock. The author had no suspicion of the existence of any phosphate in these rocks at the time he was in the :field, and the specimens analyzed were collected ar> types of the rock and taken for the fossils they contained. The regularity and persistence of the bedding of the Palaeozoic IJ(:diments will render exploration for phosphate-bearing rock easy, and should any strata prove to be of sufficiently high grade there fa little doubt that they will be extensive. The :fiel'd that should be explored is coextensive with the Caballos ridges between Warwicks and Del Norte gap and for some distance to the northeast of War­wicks. In appearance the phosphate-bearing rocks do not differ much from other limestone. 
5. BRICK MATERIALS. 
''J.'he reduction of the quicksilver in the Terlingua mines has created an unsteady market for brick. This market has been ·sup­plied by Mr. John Dryden, who finds a suitable clay on lower Terlingua creek. His clay consists of the upper three or four feet 
Ths Geology of the Chisos Country. 
of wash, or alluvium, near this stream. The clay is tempered by 
. the addition of a "sand,'' which is really the soil formed from a thoroughly disintegrated intrusive andesitic rock. The brick is of excellent quality and has proved to stand a white heat for sixty hours. It is free from cracks and has a dirty gray or cream color. lt is hand-made, sun-dried, and burnt in square up-draught kilns with thick adobe walls. The total output up to this time ( 1904) is 1,250,000 bricks. The brickyard and kilns are located a short distance north of Dryden's ranch. 










BULLETIN 

OF THE 

UNIVERSITY OF TEXAS 

SCIENTIFIC SERIES 

VOLUME ONE 
AUSTIN , TEXAS 
1905-1907 

CONTENTS 
Bulletins 1, 2, 3 and 4 are reprints properly belonging in the Reprint Series, and are therefore not to be bound with this volume. 
5. 	Test of a Vertical Triple Expansion High-Duty Pumping Engine in Operation at the Water Works, San Antonio, 
~Texas, by A. C. Scott. 52 p., illus. pl. 1905. 
6. 	
Vegetation in the Sotol Country in Texas, by W. L. Bray 24 p., pl. 1905. . 

7. 	
Ol}servations on the RalJits of Some Solitary Wasps of Texas, · by Carl Hartman. 72 p., pl. 1905. 


8 . . The Protection of Our Native Birds, by T. H. Montgomery, Jr. 30 P·. 1906. 
9. 	The Austin Electri'c Railway System, by members of the senior class in Electrical Engineering, 1906. 123 · p., illns. pl. 1906. 
10. Distribution and Adaptation of the Vegetation of Texas, by 
W. L. 	Bray. 108 p., pl. map. 1906. 
11. 	A Sketch of the Geology of the Ohisos Country, by J. A. Udden. 101 p. 1907. 

THE UNIVERSITY OF TEXAS BULLETIN 
GENERAL SERIES 
1. 
The University of Tea:as Record, vol. v, no. 3, Ma.rch, 1904. 

2. 
Alumni Notes. 13 p., Ma.rch, 1904. 


3. 	
Bome Wholesome Educati-Onal Statistics, by W. S. Sutton. 12 p., illus. :March, 1904. 10 cents. 

4. 
Oourses of Study in Law Pursued in The University of Te1ea8, by J. 


C. Townes. 16 p. March, 1904. o. p. 
5. 	Notes Ooncerning the Progress of the University, by Wilson Williams, Register. 3 p., 1904. o. p. 
6. 
The University of Tea:as Record, vol. v, no. 4, July, 1904. 

7. 	
The C~nsolidation of Rural Schools, by Una Bedechek and G. T. Bas­kett. 38 p., illus. October, 1904. 25 cents. 


8. 	
The Pride of Texans and Their University, by T. H. Mont~omery, Jr. 5 p. Nov.ember, 1904. o. p. 

9. 	
Letter to Alumni Regarding the Proposed Law School Building. 2 p. December, 1904. o. p. 


10. Views of The University of Tea:as. 42 p., illus., n. d. 20 cents. 
11. 	What Should be Done by Universities to Foster the Professional Edu­cation of Teachers? by W. S. Sutton. 24 p. 1905. 15 cents. 
12. 
The University of Tea:as Record, vol. vi, no. 1, February, 1905. 

13. 	
Bchool Buildings, by A. C. Ellis and Hugo Kuehne. 119 p., illu... pl. June, 1905. 30 cents. 

14. 
*The University of Texas Record, vol. vi, no. 2, September, 1905. 

15. 	
The Teachlng of Agriculture in the Public Schools, by A. C. Ellis. '56 p., illus. December, 1906. 25 cents. 

16. 	
A Study in School Swpervision, by Carl Hartman. 180 p. 1907. 50 cents. 


HUMANISTIC SERIES 
1. 	
The Trans-Isthmian Oanal: a Study in American Diplomatic History ( 1825-1904), by C.H. Huberich. 31 p. March, 1904. 25 cents. 

2. 	
The Evolution of "Causa" in the Contractual Obligations of the OiviZ Law, by Samuel Peterson. 24 p. January, 1905. 25 cents. 


3. De Witt's Colony, by Ethel Z. Rather. 99 p., 4 maps. 1905. 35 cents. 
4. 	Some Fundamental Political Principles Applied to Municipal Govern­ment, by Samuel Peterson; and Evans University Prize Orati<>n8 by A. D. Robertson, K. S. Dargan, Jr., Edward Crane, R. J. Chan­nel. 39 p. June, 1905. 15 cents. 
6. 	The <hotesque in the Poetry oft Robert Browning, by Lily B. Camp­bell. 41 p. April, 1907. 25 cents. 
-The Record is no longer issued in this series, but bear'il its volume and number, as heretofore. 
THE UNIVERSITY OF TEXAS BULLETIN 
SCIENTIFIC SERIES 
I. 	Contributions from the Zoological Laboratory of The University of Texas. Reprints from various journals. April, 1904. o. p. 
2. 	
Contributions from the Zoological Laboratory of The University of Texas. Reprints from various journals. April, 1904. o. p. 

3. 	
Contributions from the Zoological Laboratory of The University of Texas. Reprints from various journals. April, 1904. o. p. 

4. 	
*Uontributions from the Zoological Laboratory of The University of Texas .. Reprints from various journals. May, 1905. o. p. 

5. 	
Test of a Vertical Triple Expansion High-Duty Pumping Engine iti Operatio-n at the Water Works, San Antomo, Texas, by A. C. Scott. 52 p., illus. pl. June, 1905. 35 cents. 


!!. 	Vegetation in the Sotol Country in Texas, by W. L. Bray. 24 p., pl. June, 1905. 25 cents. 
7. 	Observations on the Habits of Some Solitary Wasps of Tea;a8, by Carl Hairtman. 72 p., pl. July, 1905. 25 cents. 
8. 	
The Protection of Our Native Birds, by T. H. Montgomery, Jr. 30 p. October, 190!1. 25 cents. 

9. 	
The Austin Electric Railway System, by members of the senior class in Electrical Engineering, 1906. 123 p., illus. pl. 190!1. 50 cents. 


10. 	
Distribution and Adaptation of the Vegetation of Texas, by W. L. Bray. 108 p., pl. map. November, 1906. 35 cents. 

11. 
A 	Sketch of the Geology of the Ohisos Country, by J. A. Udden. In 


press. 
REPRINT SERIES 

I. 	A Semantic Study of the Inda-Iranian Nasal Verbs, by E. W. Fay. From the American Journal of Philology, 25: 309-389 and 26: 172­203, 377-408. March, 1906. o. p. 
2. 	Oontribution8 from the Zoological Laboratory 07; the University of Texas. From various journals. May, 1906.. o. p. 
MEDICAL SERIES 
I. 	Yellow Fever: a Popular Lecture, by James Carroll. 32 p. ,rune, 1905. 15 cents. 
2. The Oare of the Insane, by D . M. L. Graves. 16 p. 1905. 15 cents. 
3. 	The 1903 Epidemic of Yellow Fever in Texas, and the Lesson to be Learned from it, by Dr. G. R. Tabor. 22 p. June, 1905. 15 ct 
In addition to the bulletins named above are the following: 
a. 	
The Official Series, which includes catalogues, Regents' Reports, and administrative bulletins. 

b. 	
About 35 bulletins issued ,before March, 1904, when the division into series began. An important group among these are the nine Uni­versity of Texas Mineral Survey bulletins. 

c. 	
The University of Texas Record, formerly, but no IQlllger, included in the General Series. Numbers of the Record have been issued from two to four times a year since December, 1898, and it is now in its 7th volume. A general index to the first six volumes may be found in volume 6. 


*Future numbers of Contributions, first published elsewhere, will appear in the Reprint series.