. \ 

A GEOLOGIC RECONNAISSANCE IN THE GULF COASTAL PLAIN OF 

TEXAS, NEAR THE RIO GRANDE. 

By A. c.' TROWBRIDGE. 

INTRODUCTION. GEOGRAPHY OF THE REGION. 

During June, July, and . August, 1919, part The region here considered is bordered on 

of June and all of July and August, 1920, and .the south and west by the Rio Grande which 

parts of July and August, 1921, the writer, forms the International ·Boundary, a part of 

assisted by 4-· G. Maddren in 1919 and by its eastern border is the Gulf of Mexico, and on 

W. S. Glock and Lloyd North in 1920, and the northwest and -north it is limited. by the 

associated with L. W. Stephenson, E. W. Berry~ line .of contact between the Cretaceous and 

E. B. Stiles, and Seiior Enrique Diaz Lozano Tertiary systems as mapped by Stephenson.1 

in 1921, carried on reconnaissance .work along Roughly sketched, this line of contact extends 

the Rio Grande in the southern extremity of from a point about 5 miles south of Uvalde, 

Texas. As there seems to be immediate need in Uvalde County, westward and southwestfor a map and a brief description of the for- ward through Pulliam, in the same county, 

mations on the Texas side of the Rio Grande, and thence southwestward and · southward 

to serve as a key for important stratigraphic through Maverick County to . a point on the 

work in progress both in the United States and Rio Grande about 3 miles north of the MaMexico, it has been decided to prepare and pub- verick-Webb county line and about 35 miles 

lish now a preliminary report and map, which downstream from Eagle Pass.' The eastern, 

Will show the general results of the work northeastern,· and northern boundary of . the 

already done. area considered is represented by a line' drawn 

. For aid in the preparation of this report the from a point about 5 miles south of Uvalde 

writer is indebted to Dr. T. W. Vaughan, under nearly southward to Carrizo Springs, in 

whose direction the field. 'stvdies were made Dimmit Courity, thence southeastward to 

and the report was prepared; to Dr. Julia Hebbronville, in Jim Hogg County; th.ence 

Gardner, who identified the molluscan faunas; eastward through Falfurrias, in Brooks County, 

to ·Prof. E. W. Berry, who aided in the collec- and Sarita, iri Willacy County, to the Gulf of 

tion of the floras and identified them; to Dr. Mexico. This line corresponds in part with 

J. A. Udden, who ~indly supplied well logs the south and west boundary of a large area 

from-the 'files of the Bureau of Ecori-omic Geo-. southwest of Brazos River that was surveyed 

logy and Technology of the University of Texas; by Deussen, whose report is now in· pr~ss.2 

to Mr. C. L: Baker, with whom the writer has The counties included in the Rio Grande 

· carried on correspondence and from ·whom he region are extreme south-central Uvalde, 

-hasreceivedinformationandsuggestions;toMr. · western 'Zavalla, eastern Maverick, western 

· G. C. Matson, who has already covered the field Dimmit, and all or most · of Webb, Zapata, 

and wh_ose field notes and maps were available Jim Hogg, Star!, Brooks, Hidalgo, Willacy, for the ·writer's use; to Mr. E. T. Dumble, who and Cameron. The total area considered' is 

freely gave his opinion ori several difficult ques- tions; and toMr.Alexander Deussen, who by oral about 13,500 squareniiles, nearly one twentieth 

conference and by correspondence· gave valuable assistance. Eighty-two published articles 

that bear more or less directly on the problems 

of the region were freely consulted. 

df the State of Texas. 

1 Stephenson, L. W., Th;e Cretaceous-Eocene contact in the Atlantic 

and Gulf Coastal Plain: U.S. Geol. Survey Prof. Paper 90, pl. 15, 1915. 

2 Deussen, Alexander, Geology of the Coastal Plain region of Texas: 

U.S. Geol. Survey Prof. Paper 126. 

85 
86 SHOR.TER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

The region defined above lies wholly within 

the Rio Grande Plain of Hill and V aughan.3 

The Balcones faulted zone, which separates 

this plain from the Edwards Plateau, lies 20 

to 7 5 miles north of the Cretaceous-Eocene 

line. At the landward edge of the territory 

the elevation above the Gulf averages about 

800 feet, and from this elevation there is a 

gradual slope to sea level at the Gulf. Although no part of the region has high relief, 

the part along the Rio Grande from the WebbMaverick county line to Fort Sam Fordyce 

is notably rougher than that farther away from 

the river and that along the Gulf. 

The rough belt near the river is commonly 

spoken of as "the breaks of the Rio ·Grande," 

and its roughness is due to the erosional work 

done by many intermittent tributaries. The 

width of the belt is not the same throughout 

but is determined in any area by the length 

of the tributary streams there. These streams 

average not more than 15 miles in length, but 

as most of them have only shallow valleys in 

their upper stretches the "breaks'' they form 

. are generally less .than 15 miles wide. Although the relief at most places in this belt 

does not exceed 100 feet, there are localities 

where the tops of the highest hills stand more 

than 250 feet above the Rio Grande. In 

general both the relief and the roughness 

decrease with the decrease in average altitude 

downs.tream. ·'It is in this belt that the best 

and the most numerous exposures of the rock 

formations are found . . 

The more nearly flat land east of the 

"breaks" is divided into two parts by a .conspicuous cuesta ~nown as . the Bordas scarp. 

. This scarp and the trench DU t by the Rio 

Grande are the only notable topographic 

featur~s of the region. 

The Bordas scarp crosses the northeast border of the territory mapped at a point 7 miles 

northeast. of Torrecillas, about 38 miles east of 

Laredo and 100 miles from the Gulf, and from 

this point it extends southwestward to Torrecillaf?, thence southward to Ojuelos, and thence 

in a somewhat irregular course, generally southward and a bit eastward, and is cut through by 

the Rio Grande at Rio Grande City. In general the scarp follows the course of the base of 

the Reynosa formation, whose resistant rocks 

3 Hill, R. T., and Vaughan, T. W.1 -Geology oithe Edwards Plateau 

and Rio Gra:qde Plain adjacent to Austin and San Antonio, Tex:: 

U.S. Geol. Survey Eighteenth Ann. Rept., pt. 2, pp. 193-322, 1898. 

produced it. As the land slopes steeply westward and almost imperceptibly eastward from 

its crest this scarp is conspicuous only if viewed 

from the west. 

The area between the br.eaks of the Rio 

Grande and the Bordas scarp is not rough. 

Its gentle southward and eastward (Gulfward) 

slope is broken only by shallow· valleys of the 

heads of tributaries of the Rio Grande, the 

many low hills due to the outcrops of the lenses 

of rock slightly more resistant to subaerial· erosion (which is here of slight effect), and the 

scattered depressions scooped out by·the wind 

and the corresponding slight elevations formed 

by material blown from the depressions. Few 

square miles in this plain have a relief of as 

much as 50 feet, ·and some of them, so far as 

the eye can see, are almost flat. 

Except an area of sand dunes, which covers 

about 2,800 square miles, chiefly in Willacy, 

Brooks, Hidalgo, and Jim Hogg counties, and 

a narrow strip extending for 20 miles along the 

river between Rio Grande City and Sam Fordyce, the part of the region east of the Bordas 

scarp.is notably flat. The areas near the scarp, 

however, are slightly more undulatory than the 

areas near the coast, where the surface generally stands less than 50 feet above sea lev.el 

and where there is no visible 'relief except that 

produced by an occasional ."clay butte" or a 

shallow · stream channel or "resaca." ·. This 

smoothness, however, is broken by the channel of the Rio Colorado, a distributary of· the 

Rio Grande, which in places .is 40 feet deep 

and has vertical walls. 

In the sand belt the relief is measured .by 

the heights of the dunes and the depths of the 

"blow-outs,"but even thereitdoes not exceed25 

feet on an average and 60 feet at a maximum. -. 

The lower Rio Grande region, which . is th'e 

southernmost part of the. United States except 

southern Florida and the Florida Keys,.receives 

much heat through insolation, and its temperature is therefore high. ··The average annual 

temperature is about 72° F., and the range is 

from 20° -in January to 110° in July. The 

sensible heat of summer is greatly reduced by 

constant ea'st Winds of the monsoon type. The 

nights are comfortable, except a ·few during 

which cloudiness ·retards radiation and' the Wind 

dies down.· · The region is semiarid. · There' ·are 

long periods of drc>ught, and when it does rain 

the water falls in great quantities, so ·that dry 

creek beds run full and roads become arroyos. 
A GEOLOGIC RECONNAISSANCE OF TEXAS NEAR THE RIO GRANDE. 87 

The rainfall in 1910 was 12 inches, in 1915 i6 ·and form a great system, but these areas lie 

inches, and in 1916 14 inches. The rainfall at outside of the region here described~ This 

Eagle Pass in 1908 was 15.16 inches and iri 1909 system in Texas has long been well known, 

8.63 inches. At some places there was no rain chiefly through the eariier work of R. T. Hill.4 

at all for three years prior to the fall of 1918. More recently Udden 5 and his assistants have 

The vegetation, which varies in luxuriance published a summary of the g~ology of Texas, · 

with the rainfall, consists of wild grasses, mes- in which the Cretaceous rocks are classified, 

quite, guajill9, catclaw, prickly pear, and other described, and interpreted, and L. W. Stephenforms of cactus, and many other plants, which son is now completing certain work north of the 

together make up what is known as chaparral Cretaceous-Eocene line. · 

or brush. Large trees are folind only along 

the main streams, where pecans grow, and on 

dunes in the sand belt, which bear live oaks. 

The open grassy prairie on the coastal flats is 

broken here and there by derise thickets of 

low live oak brush. 

STRATIGRAPHY. 

GENERAL FEATURES. 

The· beds of rock dip at low angles Gulfward 

from the Cretaceous-Eocene line, and successively younger formations are therefore exposed 

at the surface in wide belts east and southeast 

of that line. The Rio Grande flows in general 

southeastward in a course that is oblique to the 

strike of the formations but that is at some 

places parallel with the strike and at others 

parallel with the dip. It flows from older to 

younger beds, but its gradient is so slightly 

less than the dip of the beds and so nearly 

parallel with the strike for considerable dis-

_tances that the formations change only at intervals of many· miles along tli.e river. 

The formations are of Tertiary and · Quater-· 

nary age. Eocene formations, which lie uncon-

. formably on Cretaceous formatjons along the 

irregular -north boundary, continue at the surface eastward along the Texas-Mexican R~ilrt>ad 

as far as .Torrecillas and southward and south-

-eastward along the Rio Grande . almost to Sam 

Fordyce. Oligocene and Miocene formations, 

which are exposed on the Gulf . Coastal Plain 

farther north, do not outcrop in the lower Rio 

Grande region, the Oligocene probably because 

. they do not exist here and the Miocene because 

. they are covered by a great overlap of Pliocene 

rocks. Pliocene, Pleistocene, and Recent deposits are exposed at the surface over wide 

areas east of the Bordas scarp. (See Pl. 

XXVIII.) 

CRETACEOUS SYSTEM. 

Rocks of Cretaceous age crop out extensively 

in the Edwards Plateau., in the northern part of 

the .Gulf Coastal Plain, and elsewhere in·Texas, 

THE CRETACEOUS-EOCENE. LINE. 

As Stephenson 6 has recently· reported on the 

contact between the rocks of the Cretaceous 

and the Tertiary system in the lower Rio 

Grande region, the writer·gave relatively little 

attention to thjs line of stratigraphic division. 

The line is shown on the map essentially as it 

was drawn by Stephenson, and the contact, at 

least between the Rio Grande · and Nueces 

River, is essentially as he describes it. There is 

no marked . lithologic break at the contact, 

though Stephenson found · evidence of physical 

unconformity in Texas, as did Baker 7 on the 

Mexican side of the border, but the systemic 

significance of such unconformities is difficult 

to recognize, especially in parts of the geologic 

column where locally unconformities are common. The faunal break, however, is great and 

its significance is unquestionable. Not, a single 

species is common to the faunas below . and 

above the break and many genera that oc¢ur in 

the Cretaceous are missing in the Eocene. As 

Vaughan 8 pointed out in 1900, it-is clear _that 

the unconformity at thi~ horizon is really grea~, 

even though lithologically and stratigraphically 

it appears insignificant. In the Rio Grande 

region a long time must have elapsed between 

the end of the recorded Cretaceous and the 

beginning -of the recorded · Eocene. 

TERTIARY SYSTEM. 

EOCENE SERIES. I 

The history of Eocene classification in th~ 

West Gulf Coastal Plain has been long and complicated. Many synonymous and overlappihg 

formation names have been used, and correlations have been made which have later been 

~ Hill, R. T., The Texas section of the American Cretaceous: Am. Jour. 

Sci., 3d ser., vol. 34, pp. 287-309, 1887. . 

6 Udden, J. A., Baker, C. L., and Bose,Emil, Review of the geology of 

Texas: Texas Univ. Bull. 44, 1916. ' 

e Op. cit., pp. 169-181. 

1 Baker, C. L., personal communication. 

s Vaughan, T. W., Reconnaissance in the Rio Grande coal fields ' of 

Texas: U. S. Geol. Survey Bull. 164, pp. 35-36, 1900. , · 
88 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

found to be incorrect. The following classification seems to ·represent the present knowledge 

of the Eocene series in this region: · 

Frio clay · } 

Fayette sandstone (of Jackson age). 

Claiborne group: 

Y egua formation . . 

Cook Mountain formation. 

Mount Selman formation. 

Wilcox group: 

Bigford formation (chiefly contemporaneous with Carrizo sandstone but in part younger). 

Carrizo sandstone. 

Indio formation. 

Midway formation. 

These formations are lithologically so very 

similar that their discrimination as units in 

mapping is extremely difficult, and the lines of 

contact shown on the map should therefore be 

considered only tentative, though there are 

slight differences in the rocks, which, taken 

together with their more significant faunas and 

floras make it possible to separate the formations with some confidence. 

MIDWAY FORMATION. 

and the basal Wilcox . is conglomeratic, the 

pebbles being worn fragments, fossils, and concretions of ' iron · carbonate derived from the 

underlying Midway. 'There is a great unconformity between the Midway and the Reynosa, 

the gravels of the Reynosa having been deposited on a surface which bevels the Midway 

and all other Eocene strata. 

Oharacter.-The Midway formation consists 

chiefly of shale, but includes interbedded lenses 

and layers of sandstone and limestone. At the 

base of the formation in some places there are 

thin and discontinuous beds of conglomerate 

containing abraded fragments of rock and fossils derived from the underlying Escondido formation. Heavy ledges of limestone overlie the 

conglomerate or form the basal beds when there 

is no conglomerate. The limestone is gray, 

compact, and in some places crystalline,. and it 

contains fossils. The beds are at few places 

more than 1! feet thick and are interbedded 

with greenish-gray sand 1 to 2 feet thick. The 

shale is generally dark, and some beds are black, 

but the colors include green, .brown, gray, 

bluish gray, and blue. Some of the beds are 

Distribution.-The formation once known as notably pure, but many are sandy or silty and 

the ''Basal or Wills Point clays," now called gypsiferous. Individual beds do not exceed 

the Midway formation, crops out only in a short 40 feet in thickness. The beds of sandstone 

and narrow belt in southern Maverick County. are gray, yello~sh, brownish, or greenish, and 

Its outcrop is not a continuous belt along the are 'irregularly and thinly bedded. Many are · 

Cretace-ous-Eocene line of contact, for it is micaceous and some are glauconitic. Most of 

overlapped in· southern Uvalde~ western Za- them are rather soft, but some are firm. They 

valla, and northeastern Maverick counties by -vary in .texture but are. genera1ly fine grained. 

· strata of Wilcox age. The outcrop is also ob- Throughout the formation, but chiefly. in the 

scured by patches of gravel belonging to the shale, there are numerous concretions, which 

Reynosa formation, of probable Pliocene age. range in diameter from 6 inches to 10 feet, are 

Relations to adjacent formations.-The Mid- ge:q.erally flat or biscuit-like, and' consist chiefly 

way formation lies disconforrriably on the Es- of limestone. Many are septaria or "turtle 

condido formation of the Cretaceous system. stones." Some of the smaller ones ·are pyriThere is also a great time break at this horizon tized, some are clay ironstones, and some conand another· at the top of the Midway, where sist of iron carbonate. 

it is overlapped. ·by strata of Wilcox age. The A typical section of the . formation on the 

unconformable c~:mtact between the Midway Texas side of the Rio Grande, a mile below the 

below and the Wilcox above is well exposed (1) Blesse. ranch, on the Mexican side, is given 

at the north end of the westward-facing bluff · b 1 eow: 

of the Rio Grande in extreme ·southern Maver-

. ick County, 2 miles north of the Webb County 

line, and (2) on the west wall of the valley of 

Frio River at Bob Evans's ·apiary (Myrick's 

lower apiary of the Uvalde folio), in Uvalde 

County, 12 miles east of the Rio Grande region, 

as mapped in this report. In each of thes·e sections the line between the Midway and Wilcox 

formations is the edge of an erosional surface 

Section of the Midway formation in Texas near the Blesse 

ranch, Mexico. 

Feet. 

Shale .... "······................................... 7 

Iron concretions . : ..................... ~ . . . . . . . . . . . . . . . 1 

Shale.............................................. 5 

Concr~tionary sandstone ................ ·............ 1 

Blue-black shale with abundant Venericardia smithii. 12 

Concretionary · iron bed .containing 'Venericardia 

smithii . ................................ ~ . . . . . . . . . 1 
,A GEOLOGIC RECONNAISSANCE OF TEXAS NEAR THE RIO GRANDE. 89 

. Feet. 

Dark-gray shale containing disc-shaped iron concre-

. tions and a· few gastropods .. ;_. __ . __ ~ .. _.~~_~_~__ _ _ 25 

Thin and irregularly bedded' s~ndstone, interbedded 

near the top with sandy shale.' ...... ____ ._________ 15 

67 

Thickness.-As the dip of the Tertia~y form~~ tions may not be entirely deform~tional but 

may have been in part original, the usual 

methods of determining thickness from the outcrops are of no great value. In any case, the 

Midway formation is probably nowhere exposed 

in its full thickness because of the Wilcox overlap. 

Only where it has been drilled through for oil 

or water and where its base and top. can be determined from the log or from samples; of the 

core can its true thickness be estirp.ated. Only 

one such boring is known in the Rio Grande 

region-the L. E. Hanchett oil-prospecting 

well, 10 miles west of Carrizo Springs. The 

log of this well shows that. the formation is 194 

feet thick.. The average of all estimates of its 

thickness outside of the Rio Grande region but 

~ear it is 216 feet.,. . 

Fossils.-'rhe fossils collected from the Midway formation of this and neighboring districts 

by the writer and others are listed below: 

A porrhais sp. 

Callocard~a ripleyana (Gabb). 

Calyptrophorus velatus subsp. compressus Aldrich. 

Cerithium penrosei Harris. 

Cerithium? sp. 

Crassatellites gabbi (Safford). 

Cucullaea (macrodonta subsp. ?) texana Gardner. 

Cypraea sp. 

Enclimatoceras vaughani Gardner. 

Fusus ostrarupis Harris. 

Leda milamensis Harris. 

Lithophaga sp. 

Mesalia pumila subsp. wilcoxiana Aldrich~ 

Mesalia sp . . : . 

Modiolus saffordi (Gabb)? 

Natica sp. 

Ostrea crenulimarginata Gabb. 

· Ostrea pulaskensis Harris. 

Plejona limopsis (Conrad). 

Plej ona rugata (Conrad). 

Pseudoliva ostrarupis Harris. 

· Teredo maverickensis Gardner. 

Turris anacona (Harris). 

Turritella alabamiensis Whitfield. 

Turritella humerosa Conrad .. 

Turri~ella mortoni Conrad. 

Turritella nerinexa Harris . . 

Venericardia bulla D~ll. 

Venericar~ia ( alticostata subsp. ?) hesperia Gardner. 

Venericardia smithii Aldrich. 

Venericardia ( alticostata subsp. ?) whitei Gardner. 

Yoldia eborea (Conrad). 

. WILCOX GROUl\ \ c 

SUBDIVISIONS. · 

A thick set of lithologically complex beds 

lies stratigraphically between the top of the 

Midway and the base of the Mount Selman · 

formation. These beds have been v~riously 

known as the '' Lignitic," the "Sabine," the 

:'Sabine River/' and the "Timber Belt," each 

of these names including all strata between the 

top of the "Basal clays" (Midway fornuition) 

and the base of the "Marin~ beds" (Claiborne 

group). Owen 9 first segregated a notably 

sandy phase of the "Lignitic" and called it the 

Carrizo sandstone, from the town of Carrizo 

Springs, where it is characteristically dev~loped 

and well exposed. After the upper sands were 

separated and named 'the underlyingbeds were 

correlated with· the Wilcox of Mississippi and 

called by the ·same name. Vaughan 10 considered the Carrizo formation a . transgressive 

phase of the Wilcox. More recently U dden, 

Baker, and B.ose 11 have tentatively consi.dered 

it the oldest formation of the Claiborne group. 

In a still later ·geologic publication of the 

University of Tex~s Liddle 12 pl~ce4 t~e Carrizo 

of Medi.na County in the. Claiborne gro.up without qual~cation. Deussen 13 found that itlay 

unconformably on Wilcox beds and unconformably beneath the Mount Selman formation 

between Brazos and N ueces rivers, and Baker 1

4 

reported an unconformity between the Carrizo 

and underlying beds of Wilco.~ age on the 

Arroyo del Amole, on the Mexican side of the 

Rio Grande, across the river from the Chupadero ranch. As unconfornii ties are common 

in the Wilcox and as neither the Wilcox nor the 

Carrizo contains fossils other than plants. that 

had not been critically studied, the age of the 

Carrizo sandstone was an open question until . 

1921, when fossil plants were collected under 

the direction of E. W. Berry from. the lower 

beds of the Wil.cox, from the Carrizo, and from 

beds above the Carrizo but below the Mount 

Selman. As a result of these studies of the 

flora and an investigation of the lithology of the 

lignitic beds, the Wilcox is pere r~cognized as 

9 Owen, J ., Report of geologists for southern Texas: Texas Geol. and 

Min. Survey First Rept. Progress, for 1888, pp. 69-74, 1889. 

Ul Vaughan, T. W., Index to the stratigraphy of North America (by 

Bailey Willis and'others): U. S. Geol. Survey Prof. Paper 71, p. 726,1912. 

u Op. cit., pp. 83-84. · 

12 Liddle, R. A., The geology and mineral resources of Medina.Couu.ty: 

Texas U:iliv. BUll. 1860, pp. 87-93, 1918 !1921]. · 

13 Deussen, Alexander, Ge-ology of the Coastal Plain region Of Texas: 

U. S. Geol. Survey Prof. Paper 126 (in press). 14 Baker, C. L., personal communication. 
90 SHORTER CONTRIBUTIONS TO GENERAL GE.OLOGY, 1922 • . 

a group, consisting of thi-ee formations, here 

named, in ascending order, the Indio formation, 

the Carrizo sandstone, and the Bigford formation. INDIO FORMATION. 

Name.-The strata overlying the marine 

Midway formation and underlying the Carrizo 

sandstone are here called the Indio formation·. 

These strata have until now been known as 

the Wilcox formation, for they were believed 

to represent all the deposits of Wilcox age in 

this region. The new formation name is made 

necessary by the fact that the Wilcox as here 

. de:velop~d becomes· a group divisible into three 

formations, of which the Indio is the basal one. 

The Indio formation is named from the old 

Indio ranch, jn Maverick and Dimmi t counties·, 

which includes most of the area of outcrop of 

the formation. 

Oharacter.-Although it is made up of an 

intricate mixture of sediments, the Indio formation consists chiefly of thin-bedded and 

laminated argillaceous sand and arenaceous 

shale but includes some layers of massive clay 

and lenses and layers of sandstone. The clay 

and shale are greenish or bluish gray and light 

chocolate-brown, and most of them are gypsiferous. The sandstone is gray, yellow, green, 

and ·brown, is not notably cross-bedded, and 

is of various textures. It includes also some 

beds of lignite and many calcareous and arenaceous concretions, most of them flat, biscuitshaped, or millstone-shaped bodies. 

Until recently no marine sediments have 

been found in the Wilcox gro-qp except along 

Sabine River, but the discovery of marine 

Foraminifera between 125 and 148 feet from 

the surface and 500 feet above the base of the 

Indio formation in the L. E. Hanchett oil 

drillings Nos. 1 and 2, about 10 miles west of 

Carrizo Springs, in · Dimmit County/5 shows 

that the Gulf · advanced over the low-lying 

lands at least once for a short time during the 

Indio epoch. Beds of Ostrea tasex Gardner 

also are known west of the Hanchett wells and 

at and south of the Glass ranch headquarters, 

in Maverick County, and in the San Pedro 

sheep pasture, 19 miles southwest of Carrizo 

Springs, in Dimmit County. This form is similar to 0 \ ~renulimarginata, which is· of Midway 

age, but is. not identical with it. · The oysters 

15 Well log. and description ·or sil.mples.supplied independently by 

J. A Udden and C. L. Baker. 

and the Foraminifera appear to occur at the 

same horizon. 

The formation is partly marine and partly 

nonmarine and was deposited on both sides of 

a shore line on the landward side of which the 

land was low and on the opposite side of which 

the sea was shallow. The shore line probably 

oscillated .back ~nd forth slowly during Indiotime. 

Distribution.-The Indio formation crops out 

in the Rio Grande region in a belt 10 to 14 miles 

wide in western Dimmit~ southeastern Maverick, and northwestern Webb counties, where 

it is exposed in practically its full thickness~ 

Here it adjoins the Midway on the west and 

the· Carrizo on the east. Farther north, in. 

Maverick County, the Indio overlaps the Midway and rests upon the Cretaceous. Still 

farther north, in western Dimmit and Zavalla. 

counties and eastern and northeastern Maverick County, the Indio is in turn cut out by a. 

great overlap of the Carrizo on the Cretaceous. 

In the extreme n.ortheast corner of the region the 

Indio crops out again between the Cretaceous. 

and the Carrizo where Nueces River has cut 

through the overlying and · overlapping Carrizosandstone. 

Thickness.-The thickness of the Indiorecorded in the Hanchett wells, in Dimmit 

County, is 648 feet, and as· these wells are only 

about a mile west of the eastern boundary of the 

formation at a point where the belt of outcrop 

is widest, the maximum thickness of the Indioexposed probably does not exceed 700 feet. 

Other layers that are covered by the Carrizooverlap may or may not overlie those which 

crop out. 

Fossils.-With the exception of Foraminife:ra, oysters, and a few fossil leaves, the Indio 

formation is faunally and florally barren in theRio Grande region. The protozoans include 

Nodosaria, Textularia, and Globigerina. Themollusks are Ostrea tasex Gardner and Levifusus sp. cf. L. trabeatoides Harris. 

Fossil plants were found (1) on the west wall 

of the valley of N ueces River a mile below Pul- · 

liam ranch, (2) on the east wall of the valley of 

Nueces River at the big bend ll miles above 

the Uvalde-La Pryor road crossing, (3) in the 

clay pit at the end of the unused aerial tram 

south of Elmendorf, Bexar .County, and (4) at 

the Schuddemagen ranch, 10 iniles south of 

Sabinal, Uvalde County. Localiti~s 3 and 4 
A GEOLOGIC RECONNAISSANCE OF TEXAS NEAR THE RIO GRANDE. 91 

are outside the Rio Grande region. A list of 

identified species follows: 

Anona ampla Berry. 

Anona eolignitica Berry. 

Cyperites sp. Hollick. 

Ficus mississippiensis Berry. 

N ectandra sp. 

Oreodaphne obtusifolia Berry. 

Rhamnus coushatta Berry. 

Sabalites grayanus Lesquereux. 

. Sapindus linearifolius Berry. 

CARRIZO SANDSTONE. 

crystallized into a 1 good grade of quartzite,. 

Mai1y of them are highly ferruginous, but some 

are gray or white, colors denoting little iron. 

The sand and the andstone are characteristically cross-bedded. I Owing to inequalities from 

place to place in t~e firmness of ·the . cement 

erosion has in some places produced castellated 

forrns, notably sou h of Chupadero ranch, in 

northwesternmost Webb County. The sandy 

beds vary g~eat~y ip- texture. ~ In the q~arries 

around Carrizo Sprtngs and for some distance 

north of that place f;he rock is fine grained, but 

Distribution.-The base of the Carrizo was in m.any other localfties it is medium and even 

located by Owen 16 at a point 10 miles west of coa:~se grained. . 

Caf!izo Springs, and . the formation, which is The Carrizo ·sandstone includes also thin 

--made up of sandy beds, extended eastward to beds and lenses of clay of various colors and 

the ba~e of the "Marine beds," east of Asher- beds of limestone, yellow on weathered surton, ,occupying a belt about 20 miles wide. faces and light choc late-brown inside, forming 

Whenitjs traced southward, however, the belt concretions, rough ~eds, or lenses, botryoidal 

of .. sands becomes narrower until, at the Rio ·bodies, and irregular masses. Cone-in-cone 

Grt;tnde, it is only 3 miles wide. The strata structure is fairly common ih the beds of fin~ 

tha.t cover it here are not prevailingly sandy texture in which calcareous and argillaceous 

and are what is here called the Bigford forma- ·material are mixed in about equal proportions. 

tion. North of Carrizo Springs the Carrizo 'l'hickness.-The thickness of the Carrizo 

formation spreads northwestward to the Ore- sandstone as estimated by Owen 17 is 200 feet. 

tace_ous ro~ks, overlapping both the Indio and Between a point 10 miles west of Carrizo 

the Midway. Northeast of Carrizo Springs, Springs and a point 3 miles west of Big Wells, 

outside the region shown on the map, the belt however, its thickness is probably more than 

of Carrizo sandstone becomes narrower, the 400 feet, there being at least 200 feet of strata 

Bigford formation here also probabl:y wedging exposed west of Carrizo Springs. At the Rio 

in between the Carrizo and the Mount Selman. Grande, where the sandy belt is narrow, its 

!Relations to adjacent formations.-The Car- thickness is estimated at 118 feet. At another 

rizo sandstone rests unconformably upon the point between the river and Carrizo Springs it 

I:ridio by overlap. In the area east of the out- appears to be about 392 feet. Where the Carcrpp of the contact, however, where the full rizo overlaps the Indio and Midway north of 

tHickness of the Indio is represented, the sim- Carrizo Springs it is doubtless very thin. The 

ilarity of the flora indicates that the relations average of all previous estimates is 250 feet. It 

are conformable. The relation between the evidently varies greatly in thickness, and the 

Carrizo and the Bigford formation is appa- average is now put at 325 feet. 

reintlyconformable. Differences in flora suggest FbssiZs.-Invertebrate and vertebrate fossils 

a brea~ betw:e~n the qarrizo and the Mount hav·e not been found in the Carrizo sandstone!. 

Selman, and there are indications of unconform- Poorly preserved fragments of plants have ofity where the two come together in the railroad ten been noted, but the flora was not critically 

cut 3 miles west of Big Wells, an exposure 15 studied until 1921, when E. W. Berry accommiles east of the Rio Grande region. panjed the. writer to the field and made collecOharacter.-The Carrizo formation is decid- tions. Identifiable leaves were found in a short 

edly more sandy than the other forn1ations of arroyo in the east wall of Nueces River valley 

tHe Wilcox group, the Indio formation below three quarters of a mile below the Uvalde-L_a 

aJd the Bigford formation above, and the beds Pryor . road crossing and in the Bell quarries, · 

oi sandstone in the Carrizo are more firmly southwest of Carrizo Springs, from which Car- : · 

cepwnted. Some of them are cemented and rizo sandstone was taken to build the courthouse and other buildings i~ Carri~o Springs. 1~ Owen J., Report of .geologists for southern Texa8: Texas Geol. and 

Min. Survey First Rept. Progress, for 1888, pp. 7o-74, 1889. 11 Owen, J., idem, p . 72. 
92 SHORTER. CONTR-IBUTIONS TO GE;;ERA~ GEOLOGY, 192·2. 

Th~ li~t g1ve~ below sho~~' acqordip.g tq Berry, 

that the Carrizo is .of Wilcox and not of Claiborile age, . a fa~t which ·is in harinony with the 

lithology an~ stratigraphy of 'the . sandstone 

arid the associated beds·. . · · 

A.crostichum sp. 

Anona·ampla ·Berry. 

Anona _wiiCo:riana Berry. 

Banksia puryearensis Berry. 

.Canavalia eocenica Berry? 

Cassia tennesseensis Berry? 

Cinnamomum vera Berry? 

Dryophyllum tenn~sseensis Berry? 

Eugenia grenadensis Berrv. 

Ficus mississippiensis Le"squ~reux) Berry. 

Gleditsiaphyllum eocenicum Berry. 

Heterocalyx n. sp. 

Mespilodaphne coushatta Berry. 

Myrcia vera Berry. 

Nectandra pseudocoriacea Berry. 

Oreodaphne obtusifolia Berry. 

Oreodaphne puryearensis Berry. 

Palmocarpon butlerensis Berry. 

Persea longipetiolatum (Hollick) Berry. 

Sabalites grayanus Lesquereux. 

Sophora wilcoxiana Berry. 

Sterculia wilcoxensis Berry. 

BIGFORD FORMATION. 

Name and distribution.-Most of those who 

· have visited this region have believed that the 

Carrizo sandstone is continuous from the top of 

the Indio formation (the old Wilcox formation) to 

the base of .the Mount Selman, and it appears to 

be continuous at the type section of the Carrizo 

near Carrizo Springs and in that latitude, but 

south of Carrizo Springs the sand and sandstone 

of the Carrizo give place along the strike to 

clay, thin-bedded sandstone, and lignite. To 

these beds, which consist so largely of clay that 

they can not be called Carrizo sandstone, the 

name Bigford is here given, from Bigford ranch, 

which is practically the only habitation in their 

belt of outcrop, where· the beds are well exposed. 

along. the river within the ranch. The outcrop 

widens toward the south from 2 miles at the 

boundary of the Rio Grande region, southeast 

of Carrizo Springs, to about 12 miles at the 

Dimmit-Webb county line. The Rio Grande 

has exposed the beds ·in its north bluffs for .20 

miles. Between Carrizo Springs and Big Wells, 

outside ·of the Rio Grande region, the formation 

is entirely absent. It may wedge in farther 

north, for similar beds appear to occur in well 

logs and in exposures along Nueces River southeast of Crystal City. · 

Oharacter.-Litholog~c~lly the J3igford for~ 

mation consists chiefly of clay of many colors 

and of subordinate quantities of gray, green, 

and brown sandstone, . which is at most places 

not cross-bedded. It contains many beds of 

lignite, the heaviest 20 inches thick, and some 

lens-shaped concretionary masses. It contains 

no paper shales and sands, such as occur 

in the Indio, nor any thick, cross~bedded, and 

commonly quartzitic sands, such as occur in the 

Carrizo. 

Relations to adjacent formations.-Although 

the Bigford formation is at least fairly distinct 

lithologically, it l.s not believed to represent, 

except in its uppermost part, a period of time 

entirely separate from the C~rrizo epoch. The 

greater part of the formation probably grades 

along the strike into beds in the middle and 

upper parts of the Carrizo sandstone, and if so 

is contemporaneous with those beds. At some 

places, however, the uppermost part of the 

Bigford rests with apparent conformity on the 

Carrizo sandstone. The Bigford strata appear 

to represent a lagoon and tidal-flat phase of a 

stage of deposition during which the Carrizo 

sandstone was deposited at or landward from 

the shore line. 

Thickness.-'--The Bigford formation ranges in 

thickness from about 80 to about 4 70 feet. 

Fossils.-The .only fossils found in the Bigford .formation are some leaves of a few species 

of plants, which were collected at a place on 

Concillas Creek a quarter of a mile below the 

point 'Yhere it is crossed by the road from Bigford ranch to Apache ranc}l, about half a mile 

from the Rio Grande, where two sets of beds 

of clay, sandstone, lignite, and lignitic clay 

are separated by an erosional unconformity. 

Leaves occur both below and above the unconformity but were collected only from the upper 

set of beds, in which they are best preserved. 

The forms identified are as follows: 

Anacardites grevilleafolia Berry. 

Banksia puryearensis Berry. 

Canna eocenica Berry . 

Cassia marshallensis Berry. 

Cyperites sp. Hollick. 

Inga wickliffensis Berry? 

J uglans schimperi Lesquereux. 

Mimosites variabilis Berry. 

Mimusops mississippiensis Berry. 

Myrica wilcoxensis Berry. 

Sabalites grayanus Lesquereux. 

Sophora wilcoxiana Berry. · 
A GEOLOGIC RECONNAISSANCE OF TEXAS NEAR T'HE RIO GRANDE. 93 

Berry reports that this is a Wilcox flora, - fonnation. _ It has been mined at Minero, 

one distinctly older than that in the hase of Dolores, Cannel, and Santo Tomas, in Webb 

the Mount Selman formation, which is of County. There are two main producing coal 

Claiborne age. beds and several thinner beds, less pure, which 

cLAIBORNE GROUP. are not worked. The coal beds here are about 

The "Marine beds" of earlier writers in- one-third of the distance from the top to the 

eluded the strata between the top of the ·bottom of the formation, but beds occur 

'' Lignitic" and the base of the y egua. These nearer its base upstream from Pala~ox and 

beds are now separated into two form~tions, almLost at its top near the mouth of Arroyo 

the almost unfossiliferous marine beds. below Santa Isabella, and well logs show that coal 

constituting the Mount Selman formation and beds occur throughout the whole thickness of 

the formation. the highly fossiliferous marine stra,ta above The Mount Selman formation contains constituting the Cook Mountain formation. 

The overlyjng y egua formation, which con- throughout, from bottom to top, many caltains -Claiborne fossils, is also included in the careous concretions, chiefly in the clay and 

Claiborne group. shale. Most of them are composed of compact, fine-grained, pure, almost lithographic MOUNT SELMAN FORMATION. 

Character.-The chief constituent of the 

Mount Selman formation is clay, and exposed 

sections of it are therefore rare except at 

places along drainage lines where thin ledges 

of sandstone and limestone hold up the clay iri 

vertical faces. There are large .flat areas 

where the formation is not exposed -but where 

the shallow surficial drainage channels are so 

universally clayey that the underlying formation must be chiefly if not entirely . clay or 

shale. The sections exposed probably exaggerate the proportion of sandstone in the formation, for they occur only where lenses of the 

more resistant materials are most abundant. 

The clay is gray, black, greenish gray, and 

bluish gray where fresh and yellow or buff 

where weathered. Some of the beds are 

sandy and some are limy, but most of them 

consist chiefly of stiff, compact clay, plastic and 

sticky when wet and hard but with a fracture 

like that of starch when dry. The beds of 

clay contain a large quantity of gypsum in 

lenses, beds, stringers, joint fillings, and 

irregular crystal aggregates. Most of it is the 

transparent platy variety, but some masses of 

bladed crystals are brown. -

The sandstone occurs in layers and lenses, 

some of them 25 or 30 feet thick for short 

distances. The sand is coarse, medium, and 

fine, most of it is micaceous, and some is 

glauconitic. The beds are fairly well consolidated · but are not quartzitic. The formation includes a few thin lenses of gray limestone. 

Coal, both lignitic and bituminous, is a 

common constituent of the Mount Selman 

91048°-23-· 7 

limestone. On the outside they are pale 

yellowish gray or buff; on the inside they are 

light chocolate-brown or gray. Nearly all of 

the:m are septarian, and the fractures are filled 

with calcite. They are of diverse sizes and 

fonns. The smallest are about the size of· a 

pea and the largest are 6 feet in diameter; 

smne are cylindrical, resembling pipe stems, 

smne are biscuit-shaped, and some are nodular 

and extremely irregular; The average concretion is a foot in diameter and spheroidal in 

fonn. 

Distribution.-The surficial distribution of 

the Mount Selman formation ' is shown on 

the map. The formation extends downstream 

along the Rio Grande from a point 6 miles 

above Palafox to the mouth of Sambarieto 

Creek, about 8 miles above Laredo, a distance 

of about 26 miles measured in a straight line. 

The course of the river is irregularly oblique 

to the strike, however, and · this distance is not 

a Ineasure of the .width of the belt of outcrop, 

which is about 16 miles wide_ and of nearly 

uniform width. The western boundary of the 

fonnation, which is the Claiborne-Wilcox line 

of eon tact,-is not sharply defined. In general 

it m.arks the line that separates beds of clay, 

sandstone, and lignite that contain no molluscan fossils and few eoncretions but that carry 

a VVilcox flora (the Bigford formation), from 

beds of gypsiferous, highly concretionary clay 

interbedded with beds of sandstone and limestone carrying unmistakable though few and 

fragmentary marine fossils. T~e Mount Selman-Cook Mountain line, on the east side of . 

the belt, separates the Mount Selman material 
94 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

from the sandy, highly fossilif~rous, redweathering Cook Mountain formation, which 

stratigraphically overlies the Mount Selman. 

The Mount Selman formation, though it 

cont,ains marine rock, is evidently not entirely 

of marine origin. The coal and the sediments 

that carry land plants were doubtless deposited 

in isola:ted coastal basins and lagoons and 

perhaps to some extent on tidal fiats. 

Thickness.-The minimum measured thickness of· the formation is 22.5 feet and the maximum is 707. Deussen assigns to it an average 

thickness of 350 feet in the area next north of 

the Rio Grande region. The average within 

the Rio Grande region is 618 feet. 

Fossils.-The Mount Selman formation is not 

highly fossiliferous, y~t a few poorly preserved 

forms of both animals and plants have been 

recognized. The mollusks so far recognized are 

CornUlina armigera Conrad, Cytherea sp. cf. 

C. trigoniata var. winnensis Harris, Ostrea sp., 

Plejona petrosa (Conrad), Protocardia sp., and 

Venericardia planicosta Lamarck. 

Only two collections of Mount Selman 

plants were made by the writer. They are 

fragmentary and obscure, but Berry has identified them provisionally as Apocynophyllum 

grevilleafolium Berry, Coccolobis claibornensis 

Berry, Ficus newtonensis Berry, Geonomites 

n. sp., and Myrcia n. sp. 

COOK MOUNTAIN FORMATION. 

Distribution and relations.-The Cook Mountain formation is the upper and highly fossiliferous part of what has been known as the 

"Marine beds." Its outcrops lie in a belt 

averaging 13 miles in width where it includes 

the full thickness of the formation, extending 

from the northeast border of the region at and 

west of Encinal, almost due south for about 40 

miles to a point where the base of the formation 

is cut off by a bend in the Rio Grande 8 miles 

above Laredo. From this point for a distance 

of about, 70 miles down the river the strike of 

the formation is almost parallel to the general 

course of the ~iver. The river cuts the top 

of the formation-the Cook Mountain-Y egua 

line-15 miles upstream from Roma. Thus 

the formation is exposed in the "breaks of the 

Rio Grande" for a distance of 7 5 miles. The 

towns included within the belt of outcrop are 

Encinal, Laredo, San Ygnacio, Zapata, and a 

large number of places inhabited by Mexicans. 

• 

This change in the strike from west of south . 

to east of south so as almost to correspond with 

the course of the Rio Grande from Laredo to 

Roma was not known to all previous workers. 

Dumble, 18 for instance, continuing. the west of 

south by east of north strike, draws the Cook 

Mountain-Y egua contact line across the river 

8 miles below Laredo, the Yegua-Fayette line 

about 4 miles above Zapata, and the FayetteFrio line on the Texas side a mile or two below 

Rio Grande City. The true structure was first 

pointed out by V aughan/9 who had collected 

Claiborne fossils far down the Rio Gr~nde and 

understood that they came from the "Marine 

beds." Thus much of what Dumbl~ has considered Fayette was recognized by Vaughan 

as Cook Mountain. This fact explains Dumble's insistence that much if not all of the 

Fayette formation is of Claiborne age. 

Character.-The Cook Mountain formation is 

primarily sandy, and the sand is more or less 

firmly cemented. Most of the rock is medium 

grained, but there are beds of fine-grained and 

coarse-grained sandstone. The beds are brown, 

red, .yellow, green, and gray and are commonly ferruginous, micaceous, and glauconitic. Many of them are cross-bedded and 

ripple m!1rked. Interbedded with the sandstone is some yellowish, bluish, and greenishgray or chocolate-colored clay and a few thin 

lenses of gray limestone. The sandstone and at 

some places the clay contains large, darkgray, hard-centered, crystalline limestone concretions, some of which are fossiliferous. The 

lower two-thirds of the formation weathers 

characteristically into red sandy soils; the 

upper third at most places weathers gray. 

All the soils derived from the Cook Mountain 

formation support vegetation, which is unusually luxuriant. 

Thickness.-The estimates of the thickness 

of the Cook Mountain formation range from 

187 to 668 feet, and average 422 feet. 

Fossils.-The formation everywhere contains abundant fossils, by which it can be disti~guished from the Mount Selman below and 

the Y egua above; indeed it can thus be distinguished from all other formations in the 

region, for its faunas are larger and more 

1s Dumble, E. T., Geology of Southwestern Texas: Am. Inst. Min. 

Eng. Trans., vol. 33, p. 916, fig. 1, 1903. 

19 Vaughan, T. W., Contributions to the geology and paleontology of 

the Canal · Zone, Panama, and geologically related areas in Central 

America and the West Indies: U. S. Nat. Mus. Bull. 103, 1919 . 
A GEOLOGIC RECONNAISSANCE OF TEXAS NEAR THE RIO GRANDE. 95 

diversified than those of any other Tertiary 

formation in Texas. The faunas within the 

formation present four more or less distinct 

facies. 

One of these faunas is found at the base of 

the formation. The complete specific list is as 

follows: 

Anomia ephippoides Gabb. 

Buccinanops sp. cf. B. ellipticum Whitfield. 

Callocardia astartoides Gardner. · 

Callocardia sp. cf. C. bastropensis (Harris). 

Callocardia sp. 

Ceritliium texanum Heilprin. 

Cerithium webbi Harris. 

Corbula (Cuneocorbula) conradi Dall. 

Drillia? sp. 

Egerella sp. 

Epitonium sp. 

Lacinia alveata Conrad? 

Ledasp. 

L'evifusus trabeatoides Harris? 

Levifusus sp. 

Lucina sp.? 

Natica dumblei Heilprin. 

Olivula staminea Conrad. 

Olivula sp. 

Ostrea alabamiensis su bsp. georgiana Co mad (small). 

Polynices arta (Gabb). 

Pseudoliva vetusta Conrad. 

Sin·um declivum Conrad? 

Solen sp. 

Tuba antiquata Conrad. 

Turritella sp. 

Venericardia planicosta (Lamarck). 

Yoldia sp. cf. Y. psammotea Dall. 

A second fauna occurs in the lower part or' 

the Cook Mountain formation but a1bove the 

basal beds. A complete list of species follows: 

Acteon pomilius Conrad. 

Adeorbis? sp. 

Anomia ephippoides Gab b. 

Architectonica sp. aff. A. acuta Conrad. 

Benoistia? sp. 

Cadulus sp. 

Callocardia astartoides Gardner. 

Callocardia bastropensis (Harris). 

Callocardia sp. 

Cancellaria panones Harris. 

Cancellaria sp . . 

Cardiu:r;n (Cerastoderma) harrisi Vaughan? 

·cardium (Cerastoderma) ouachitense Harris? 

Cassis? sp. 

Cerithium sp. 

Cochlispira sp. 

Corbula (Cuneocorbula) conradi Dall. 

Crassatellites sp. 

Crepidula sp. 

Cylichna sp. cf. C. jacksonensis Meyer. 

Drillia nodocarinata (Gabb)? . 

Drillia texacona Harris. 

Egerella sp. 

Epitonium trapaquara (Harris). 

Eucheilodon sp. cf. E. reticulatoides ~arris. 

Eucheilodon? sp. 

Lacinia alveata Gabb. 

Latirus moorei Gabb? 

Leda bastropensis Harris. 

Leda compsa Gabb. 

Lima· harrisiana Aldrich. 

Lucina sp. cf. L. ozarkana Harris. 

Mesalia claibornensis Harris. 

Murex (Odontopolys) sp. 

Natica dumblei Heilprin (large). 

· Natica semilunata Lea. 

N ucula mauricensis Harris. 

Olivula staminea Conrad. 

Ostrea alabamiensis subsp. georgiana Conrad (smali)~ 

Ostrea sellaeformis Conrad? 

Ostrea sp. 

Pholadomya sp. ~ 

Plejona petrosa (Conrad). 

Plejona precursor (Dall)? 

Plejona sp. cf. P. haleiana (Whitfield). , 

Plejona sp. 

Polynices arata (Gabb). · 

Protocardia sp. cf. P. gambrina Gabb. 

Pseudamusium sp. 

Pseudoliva sp. cf: P. vetusta Conrad. 

Pteropsis lapidosa Conrad. 

Pyrula (Fusoficula) sp. cf. P. texana HaiTis. 

Sinum declivum (Conrad). 

Solen sp. 

Surcula gabbi Conrad. 

Teinostoma? sp. 

Tellina mooreana Gab b? 

Tellina sp. A. 

Terebra texagyra Harris. 

Terebra houstonia Harris. 

Tornatina sp. 

Tortoliva texana Conrad? 

Tuba antiquata Conrad? 

Turbonilla? sp. · 

Turricula sp.? 

Turris sp. 

Turritella nasuta Gabb. 

Venericardia mooreana (Conrad). 

Volvula sp. 

Yoldia sp. cf. Y. psammotea'Dall. 

Yoldia sp. 

The middle part of the Cook Mountain for- · 

mation c'arries a third fauna, the names of the 

species of which follow: 

Ancillaria sp. 

Architectonica sp. 

Callocardia astartoides Gardner. 

Callocardia bastropensis Harris. 

Callocardia sp. 

Cancellaria sp. 

Cerithium texanuin Heilprin. 

Corbula (Cuneocorbula) conradi Dall. 

Corbula sp. 

Diplodonta sp. 
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19·22. 

Distorsio septemdentata Gabb. 

Drillia sp. cf. D. enstricina Harris. 

Egerella sp. 

Fusus sp. 

Gaza? sp. 

Lacinia alveata Conrad. 

Latirus moorei Gabb? 

Leda bastropensis Harris. 

Levifusus sp. 

Lucinia? sp. 

Martesia sp. 

Mesalia claibornensis (Conrad)? 

Modiolus (Brachidontes) texanus (Gabb). 

Natica dumblei Heilprin (large), 

Natica dumblei Heilprin (small). 

N atica semilunata Lea. 

0 li vula staminea Conrad? 

Ostrea alabamiensis subsp. georgiana Conrad. 

Ostrea sp. 

Phos sagenus (Conrad)? 

Pinna sp. 

Plejona petrosa (Conrad). 

Plejona sp. 

Polynices arata (Gabb). 

Psammobia? sp. 

Pseudoliva ostrarupis subsp. pauper Harris. 

Pseudoliva vetusta Conrad? 

Pteropsis lapidosa Conrad subsp.? 

Pteropsis sp. 

Pyrula texana Harris? 

Rimella sp. cf. R. texana Harris. 

Siliqua sp. 

Sinum declivum (Conrad)? 

Sphaerella sp. 

Surcula gab hi Conrad? 

Teinostoma? sp. 

Tellina sp. cf. T. mooreana Gabb. 

Tellina sp. d. T. tallecheta Harris. 

Tellina sp. A? 

Trinarcia declivis (Conrad). 

Turris vaughani (Harris). 

Turris sp. 

Tmritella nasuta Gabb. 

Venericardia mooreana Gabb. 

V enericardia sp. 

Finally, a fourth Cook Mountain fauna is 

widely distributed at the top of the formation. 

This fauna marks the Cook Mountain-Y egua 

contact. The most prominent forms are a 

large variety of Natica dumblei, which is 

associated with Ostrea georgiana. Most of this 

fauna is contained in the limestone concretions, · 

not all of 'Yhich, however, carry fossils. Many 

of the shells have been rolled and abraded and 

perhaps concentrated before the concretions 

were formed. A list of the species making up 

this fauna follows: 

Area sp. 

Callocardia astartoides Gardner. 

Callocardia sp. cf. C. trigoniata Lea. 

Calyptrophorus sp. cf. C. valatus Conrad. 

Corhula (Cuneocorbula) conradi Dall. 

Fusus? sp. 

Natica dumblei Heilprin (large). 

Ostrea sp. 

Polynices arata (Gabb ). 

Polynices gibbosa (Lea). 

Solen sp. 

Tellina mooreana Gabb? 

Tellina sp. A? 

Tuba antiquata Lea? 

Tmritella sp. 

Venericardia sp. 

YEGUA FORMATION. 

TheY egua, the youngest of the Claiborne fo::-

mations, lies with some indications of unconformity upon the Cook Mountain and underlies 

the Fayette, of Jackson age. 

pistribution.-The map shows the distribution of the formation at the surface in the lower 

Rio Grande region. The belt of outcrop varies 

in width from 4! to 17 miles, the variation being 

probably due to an irregular overlap of the 

Fayette. The outcrop within the region is 

confined to Webb and Zapata counties and a 

small part of western Starr County. 

Character.-About 90 per cent of the formation in this area appears to be dark gray, black, 

red, pink,. purple, green; a;nd brown. selenitic 

carbonaceous clays. The predominant color is 

dark gray, and the material weathers into darkgray ciay soils. There are also many beds, 

lenses, and seams of soft gray and buff sands 

and sandstones, most of them thin. Dark, 

irregular limestone concretions occur sparingly 

in the clay. 

Thickness.-The average· th,ckness. of the 

strata as.signed to the Yegua outside the Rio 

Grande region is 817 feet, and the range is from 

375 to 1,400 feet, but in this region the average 

thickness of the beds exposed, so far as it can be 

determined, is only about 406 feet. 

Fossils.-Although Dumble 22 reports Tellina 

mooreana Gabb, Turritella houstonia Harris, and 

Natica recurva Aldrich from beds in Mexico 

identified as Yegua, it seems like~ that the beds 

from which they were collected should be included in the Cook Mountain of the Rio Grande 

region. Leaves are also reported to have been 

found in the formation at places outside the 

region, but none was found inside. The only 

fossil seen in the Y egua in the region covered by 

2o Dumble, E. T :' Tertiary deposits of northeastern Mexico: California 

Acad. Sci. Proc., 4th ser., vol. 5, No.6, p. 177, 1915. 
A GEOLOGIC RECONNAFSSANCE OF TE XAS NEAR THE RIO GRANDE. 97 

this report is Ostrea georgiana, . and even this 

occurs more sparingly in the Y egua than in the 

Cook Mountain below or the . Fayette above. 

In the Rio Grande region the deposits classified as Yegua are more closely allied faunally 

with the Jackson than with the Claiborne, but 

elsewhere· the reverse is true~ Possibly only 

the upper part of the Yegua occurs here, and 

this part may be really of Jackson age. Until 

this can be demonstrated, however, the Y egua 

will all be classed as of Claiborne age. 

FAYETTE. SANDSTONE. 

Oharacter.-The materials of the Fayette 

formation are fairly characteristic, although · 

they are lithologically like those of the Cook 

Mountain form~tion, with which they are likely 

to be confused, but the fauna and flora are definite and on the whole the formation is not 

difficult to recognize in the field. 

The Fayette contains more sand and sandstone than any other kind of rock, yet it is not 

so sandy in the Rio Grande region as it is farther 

north and east. The sandstone is exceedingly 

variable-in color almost white, gray, greenish 

gray, buff; in texture fine, medium, and coarse; 

in consolidation ranging from loose sand to 

quartzitic sandstone. The most characteristic 

feature of the sandstone is that it is fossiliferous. 

It is commonly laminated and cross laminated 

in intricate patterns. Interbedded with the 

arenaceous beds there are many beds and 

lenses of sandy and limy greenish-gray, .pink, 

and red shale and clay. Limestone is scarce 

but not entirely absent. Beds of white volcanic ash are found in the formation 'at several places. Large, dark, crystalline liinestone concretions are common. Silicified wood 

occurs abundantly, chiefly in the clays but in 

the sandstones as well. M.ost of the silica is 

opal, but chalcedony is found also. The silicified wood is characteristic of the formation in 

this region. 

Thickness.-The thickness of the formation in 

this region averages not -more than 360 feet, 

about 100 feet less than the average thickness 

elsewhere. 

Fossils.-The formation contains a single 

identifiable fossil, a large variety of Ostrea 

georgiana Conrad. The rich Claiborne fauna 

attributed .to the Fayette by Dumble is Cook 

Mountain, as explained on page 94, but anumber of species of fossil plants are found in the 

ash beds and some in the sandstones. The best 

collections were obtained a.t points 2! and 4t 

miles north of Miraflores ranch, in Zapata ·, 

County. Berry has identified the following 

for1ms: 

Apocynophyllum -2 n. sp. 

Bombacites n. sp. 

Cinnamomum sp. 

Coccolobis n. sp. 

Conocarpus eocenicus Berry. 

Diospyros n. sp. 

Inga n. sp. 

Mespilodaphne n. sp. 

Myristica catahoulensis Berry (?). 

Nectandra n. sp. 

Papilionites n. sp. 

Pisonia n. sp. 

Sabalites vicksburgensis Berry. 

Sapindus den toni Lesq. 

Sapotacites n. sp. 

Spohora claibornensis Berry(?). 

Terminalia phaeocarpoides Berry. 

_Ternstroemites n. sp. 

Berry regards these plants as of middle or 

upper Jackson age and states that they · indicate a warm, probably subtropical climate, 

with local or seasonal aridity, ·probably without general deficiency in rainfall. ·Although 

smne of the formation is marine, much of it,. 

as the 'plants suggest, was doubtless deposited 

sulbaerially on coastal sandy and clayey plains. 

The character of much of the formation in...: 

dieates deposition in shallow water that was 

su'bject to considerable agitation. 

FRIO CLAY. 

In the Frio formation there are 100 to 400 

feet of gray, greenish and yellowish gray, red, 

pink, and blue pure and sandy clays and a very 

few seams of gray sandstone, 1 to 8 inches 

thick. Perhaps the pink color, doubtless due 

to weathering, predominates in the exposures, practically all of which are shallow. 

Nearly everywhere the clay is checked by 

joints in which secondary calcium carbonate 

oceurs in thin plates, and it contains many 

sn:tall calcareous nodules and some small 

masses that appear to be rolled ·balls of calcareous mud. Some of the clay is gypsiferous. .With the exception of an occasional 

oyster bed (Ostrea georgiana) and a very few 

fragments of silicified wood the formation contains no fossils. Some of the beds of volcanic 

ash, notably those at· Rio Grande City and at. 

and in the neighborhood of La Lorna de la Cruz 

east of Rio Grande City, at least one of which 
98 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

is 60 feet thick,· are in the Frio formation 

rather than in th~ Fayette. These ash deposits, however, unlike those in the Fayette, 

carry no fossil plants; but it should be noted 

that not all the ash in the Fayette is leafbearing. 

MIOCENE SERIES. 

OAKVILLE SANDSTONE. 

Deussen 21 describes and maps in the area 

covered by him south and west of Brazos 

River to the border of the lower Rio Grande 

region a thin formation of white and light-gray 

quartzitic sandstone, which he calls the OakVille sandstone. This formation is also mentioned by Dumble,Z2 U dden,Z3 and others. As 

mapped by Deussen, it lies in a belt a mile 

wide along the west foot of the Bordas scarp, 

between the Frio and Reynosa formations, 

east of Torrecillas, in Webb County, at the 

north border of the Rio Grande region, but the 

writer found no certain evidence of the outcrop of this sandstone, although it is doubtless 

overlapped by the Reynosa, which forms the 

Bordas scarp. In the lower Rio Grande region 

the Frio clay seems to continue wen up the west 

slope of the scarp and there to be in contact 

with the Reynosa formation. Where the Rio 

Grande has cut across the Bordas, exposing a 

section, the Reynosa lies directly upon the 

clay and ash of the Frio formation. Nor does 

the OakVille appear between these two formations, in the 15 miles of the exposure of their 

contact in a direction roughly parallel to the 

dip between Rio Grande City and Sam Fordyce. 

In roads, pastures, and fields between Randado 

and the Webb-Zapata shallow oil field and 

elsewhere along and near the Bordas, to the 

west, there is a deep white_or gray sand which 

may have been derived from the Oakville. 

The same kind of sand, however, is commonly 

found east of the Bordas, where it is derived 

from the Reynosa formation. It even appears 

to be derived from the Reynosa west of the 

Bordas, where there are outliers of.the Reynosa. 

There are in the region, however, rather 

thick Miocene sediments, which are overlapped 

by younger formations. Driller's samples 

taken at depths ranging from 4,325 to 4,500 

21 Deussen, Alexander, Geology of the Coastal Plain region of Texas: 

U.S. Geol. Survey Prof. Paper 126 (in press). 

22 Dumble, E. T., Problem of the Texas Tertiary sands: Geol. Soc. 

America Bull., vol. 26, pp. 449 et seq., 1915. 

23 Udden, J. A., Baker, C. L.,and Bose, Emil, Review of the geology of 

Texas: Texas Univ. Bull. 44, p. 97, 1916. 

feet in the Niels Esperson oil test, 15 miles east 

of Brownsville, have furnished many invertebrate fossils, of which Dr. Julia Gardner has 

identified 23 species, all of Miocene age. The 

exact position of these fossils in the Miocene 

can not be determined, but according to Dr. 

Gardner the single fauna represented should 

probably be assigned to a formation near the 

top of the lower Miocene or the base of the 

middle Miocene. 

PLIOCENE SERIES. 

LAPARA SAND AND LAGARTO CLAY. 

The Pliocene Lapara and Lagar.to formations, described by Deussen,24 Dumble, 25 and 

Udden, Baker, and Bose, 26 from areas farther 

north, do not outcrop in the Rio Grande 

region. If they were deposited there they 

have been covered by the overlapping Reynosa formation. 

TERTIARY (?) SYSTEM. 

PLIOCENE' ( ?) SERlES. 

REYNOSA FORMATION. 

Name.-In 1890 Penrose 27 described a deposit of limestone containing many pebbles 

and cobbles under the name "Reynosa limestone," from the town of ~eynosa, Tamaulipas, 

Mexico. This limestone overlies what was 

then called the Fayette sand at Reynosa, 

directly across the Rio Grande from Hidalgo, 

Tex.· Penrose found Recent shells embedded 

in the surfac~ of exposures of this formation, 

and thinking it was Recent, included it in his 

"post-Tertiary formations." In 1891 Hill 28 

described remnants of a formation · that consisted of coarse and fine gravel cemented by a 

calcareous matrix and that occupied terraces 

400 to 1,000 feet above the Rio Grande to the 

north of this region. This he called the 

Uvalde ' formation. Dumble 29 applied the 

name Reynosa division to the series of deposits 

forming the plateau between Nueces and Rio 

Grande, which he called the Reynosa pia teau. 

24 Deussen, Alexander, Geology of the Coastal Plain region of Texas: 

U. S. Geol. Survey Prof. Paper 126 (in press). 

2r; Dumble, E. T., Geology of southwestern Texas: Am. Inst~ Min. 

Eng. Trans., vol. 33, pp. 963-975, 1903. 

26 Udden, J. A., Baker, C. L., and Bose, Emil, op. cit., p. 90. 

21 Penrose, R. A. F., jr., Report of geology for eastern Texas: Texas 

Geol. Survey First Ann. Rept., pp. 57, 58, 63, 1890. 

2s Hill, R. T ., Notes on the geology of the Southwest: Am. Geologist, 

vol. 7, pp. 366--370, 1891. 

29 Dumble, E. T., The Cenozoic deposits of Texas: Jour. Geology, 

vol. 2, p. 560,1894. . 
A GEOLOGIC RECONNAISSANCE OF TEXAS Nl<JAR THE RIO GRANDE. 99 

He stated that the "Reynosa limestone" of 

Penrose formed the top member of his Reynosa 

division, which rested on the Lagarto formation. 

These downstream deposits to which Dumble 

applied the name Reynosa are now known to 

be the same as the upstream remnants to. 

which Hill applied the name Uvalde, and the 

necessity for discarding one of the names has 

become apparent. In view of the fact that 

Reynosa as applied to a part of this formation 

has priority over Uvalde, and that the downstream deposits perhaps afford a better type 

locality, the name Reynosa has been "adopted 

· by the United States Geological Survey and 

"Uvalde" formation has been ab~ndoned. 

The outcrop -of the formation is continuous 

from Torrecillas and Rio Grande City ·eastward. In the area west of the Bordas scarp 

it occurs as remnantal patches, some of them 

doubtless reworked. These materials. were 

included by McGee in his "Lafayette," a 

name no longer recognized as applicable to any 

geologic formation. 

Distribution.-The map shows the distribution of the Reynosa formation in the lower 

. Rio Grande region. Its resistant layers· hold 

up the Frio clay; its base forms t.he Bordas 

scarp, from which it dips at a very low 

angle eastward, outcropping in a north-south 

belt 40 to 65 miles wide. In much of this 

area, however, its outcrops are obscured by 

the wind-blown sands of the sand belt. West 

of the Bordas it occurs only in patches that 

occupy the highest elevations. 

Relations to adJacent formations.-The Reynosa lies unconformably on all older formations. The dip of the formation is much less 

than that of the older formations, and as the 

surface on which it lies is almost flat, it bevels 

the older formations. This structural unconformity, which is shown on the Inap, is 

well seen in the bluffs of the . Rio Grande between Rio Grande -City and Sam -Fordyce. 

After the deposition of the Oakville, and doubtless after Lapara and Lagarto time, the strata 

of the region were slightly tilted and a flattish 

surface was developed across their beveled 

edges, and on this surface the Reynosa formation was deposited. In this area it is unconformably overlain by the Beaumont clay. 

Oha~acter.~The Reynosa formation is an 

intricate mixture of gravel cemented by lime 

carbonate, uncemented gravel, limestone in 

which are embedded pebbles and cobbles, 

alnwst gravelless limestone, sand, sandstone, 

gravelly sand, and a relatively small amount of 

clay. · 

About 1 per cent of the pieces of gravel are 

over 4 inches in diameter, and the largest 

measure 8 inches. About a third of them are 

of sizes between 1-! inches and 4 inches; another 

third go through the 1-! inch screen but are 

caught on the. i-inch; and nearly a third goes 

through the i-inch screen. The patches of 

gravel near the Rio Grande are a bit coarser 

than those farther away, and those inland and 

near the Balcones scarp seem to be somewhat 

co2rser than those away from the river but 

nearer the Gulf. The grave~ includes about 77 

per cent of chert, limestone, and vein quartz, · 

materials derivable and - doubtless largely 

derived from the Edwards Plateau,- and about 

23 per cent of igneous rock, most of them 

derived from the western Cordillera. Some 

of the vein quartz may have been derived from 

points west of the plateau, but on the other 

hand son1e of the igneous rocks may have 

been derived from plugs near the Balcones 

scarp. VVhere the gravel overlies the Fayette 

and Frio formations it inch.J-des some silicified 

wood. 1Iost of the pebbles and cobbles are 

well shap~d by abrasion, many of them are 

highly polished, and some show bruises made 

by hard impact during transportation. The 

gravel deposits-are roughly sorted into lenses 

and pockets of different textural grades. _ 

The limestone is gray and sandy and 

weathers into rough, irregular surfaces, due to _ 

irregular concretions and impurities. In places 

it has smficially a tufaceous appearance. Its 

basin, rin1s, terraces, channels, and concentric 

banding . are ·suggestive of deposition by 

springs. The patches south of Espejo r.anch, 

in north-eentral Webb County, and at Carrizo 

Springs are clearly spring deposits, but these are 

not typical, for they contain little or no gravel. 

The sand and the sandstone are gray or 

brown or red and almost universally weather 

into a dark-red sand. The grains are quartz 

coated with red iron oxide, but in places where 

this red coating has been worn off by the wind 

the sand is gray, or even white. 

The few clays are generally sandy, but some 

are almost fat. Below Rio Grande City the 

Reynpsa :formation includes mud balls of Frio 

clay. -

BUREAU OF M\NES LIBRARY 

DENVER 
100 SHOR.TER CONTRIBUTIONS TO GENERAL GE1

0LOGY, 1922. 

The surficial material derived from the 

Reynosa is typically deep-red, pink, or gray 

sand, through which white or gray limestone 

projects at many places. 

Thickness.-The thickness of the formation 

is not definitely known. The patchy deposits 

west of the Bordas do not exceed 30 feet in 

thickness except at a very few places, but at 

the east border of the main belt of outcrop the 

formation may be 500 feet thick or more, and 

around the borders of the Rio Grande region 

it is 200 ·to 600 feet thick. 

Fossils.-N o fossils have been found in the 

Reynosa formation except the remains of 

Recent land snails, crayfish, jackrabbits, and 

a few other animals, whicn have become embedded in the surface as the ·limestone has 

been dissolved and reprecipitated, and except 

the fossils originally deposited in the formations from which the gravel was derived. 

.. Origin.-To attempt at this time a complete explanation of the conditions under 

which this complex formation was deposited 

and cemented would be premature. In the 

main it is of fluvial origin. At least the 

gravel and the interbedded sand and clay 

were laid down by streams. Hill and 

Vaughan 30 gave an excellent description of the 

landward facies of the formation and interpreted it as residual material washed out 

from the Edwards Plateau in late Tertiary 

time and deposited on the down thrown side of 

the Balcones fault in fan-shaped . areas. Perhaps., owing to peculiar climatic conditions, 

- as suggested by Deussen, 31 some of the limestone was deposited by ground water while the 

main mass of the formation was being laid 

down by streams. Some of the limestone also 

appears to have been deposited at the surface 

by springs that rose through the gravel. 

QUATERNARY SYSTEM. 

PLEISTOCENE SERIES. 

LISSIE GRAVEL. 

North of the . Rio Grande region Deussen 

finds in valleys in · the ~eynosa formation a 

deposit of uncemented gravel without limestone, reaching in places a thickness of 900 

feet. These are the " Equus beds" of earlier 

30 Hill, R. T ., and Vaughan, T. W., Geology of the Edwards Plateau 

and Rio Grande ?lain adjacent to Austin and San Antonio, Tex.: U. S. 

Geol. Survey Eighteenth Ann. Rept., pt. 2, pp. 244-247, 254-255, 1898. 

31 Deussen, Alexander, orali nformation. 

writers, so called because they contain the 

remains of Pleistocene horses. This gravel 

was doubtless derived from the Reynosa, 

which was uplifted and eroded before the main 

body of Lissie gravel was deposited. No 

exposure of this material was seen in the Rio 

Grande region, but perhaps it may be hidden 

beneath the sand of the sand belt and the 

Recent delta deposits of the Rio Grande or 

overlapped by the Beaumont clay. 

BEAUMONT CLAY. 

Name and distribution.-Throughout the 

west Gulf Coast province there is a coastal 

belt of clay of varying width, from which 

deposits of silt, sand, and gravel project up 

along the main river valleys in the form of 

terraces. · These terrace deposits are called the 

Leona formation. 

The Beaumont formation has been called the 

"Coast clays" and the Port Hudson clay, but 

it is not known to be the same as the typical 

Port Hlidson formation of Louisiana. It is 

correlated in time with the Columbia formation 

·of the Atlantic coast. The term Beaumont, 

which was first employed by Kennedy, 32 is 

used to designate the formation in this region. 

This formation is covered by Recent . windblown sand for about 50 miles southward from 

the north border of the Rio Grande region, and 

still farther south by the younger clays and 

sands that form the present delta of the Rio 

Grande, but it is exposed at some places in the 

narrow strip north of the sand belt, within the 

sand belt where at places it has been uncovered 

by the migration of dunes, and south of the 

sand belt above high-water mark in the Rio 

Grande. Its areal distribution is shown on the 

map. 

Oharacter.-The Beaumont formation consists of blue and red calcareous clay, weathering yellow, a small number of thin lenses of 

sand, and a few scattered concretions of limestone. It includes also, as shown by well logs, 

a few beds of gravel which are not exposed at 

the surface and do not belong to the underlying 

Lissie and Reynosa formations. That part of 

the formation near the Rio Grande is coarser 

than that elsewhere. Much of this material 

was swept down the Rio Grande and reworked 

32 Hayes, C. W., and Kennedy, William, Oil fields of the Texas-Louisiana Gulf Coastal Plain: U. S. Geol. Survey Bull . . 212, pp. 20, 27, pls. 

1 and 2, 1903. 
A GEOLOGIC RECONNAISSANCE OF TEXAS NEAR THE RIO GRANDE. 101 

more or less by Gulf currents before it was 

deposited. The surficial material of the formation in this belt is chiefly dark-gray clay 

loam overlain by a thin sheet of drifting sand. 

Fossils .. -No fossils were found in the coastal 

belt of this region, but oyster beds are reported 

from the formation at some places farther 

north. 

LEONA FORMATION. 

Distribution.~In the valleys of the Rio 

Grande and of Frio, Leona, and Nueces rivers 

there are a number of terraces, all of them 

lying at levels between the Reynosa upland 

plain and the present flood plains of the 

streams. Such terraces are also found for a 

few miles up the larger tributaries of the Rio 

Grande. Hill and Vaughan 33 first described 

these deposits in the upper Nueces drainage 

basin as the Leona formation. They also men-;: 

tion 34 the Rio Grande terraces and state that 

they are undoubtedly of the same age as the 

Leona formation. Certainly there are no 

greater differences, either in topographic position or in material, than would be expected in 

deposits of this sort and of the same age but in 

different drainage basins. Deussen,35 on the 

other hand, believes that the Leona formation 

· is older than the terraces of the Rio Grande. 

He correlates the Leona with the terraces in 

the basin of San Antonio River at San Antonio, 

and as the remains of vertebrates have been 

found in the terraces at San Antonio which 

identify them in age with the "Equus beds," 

he correlates the Leona formation with the 

Lissie. No vertebrate fossils have been found 

in the deposits on the Rio Grande, and these 

terraces ·appear both to Deussen and to the 

writer to be continuous with the Beaumont 

clay and of the same age. As no formation 

corresponding to Deussen' s Lissie could be 

·found in the lower Rio Grande district, as 

Udden, Baker, and Bose 36 correlate the Lissie 

with the Reynosa and the "Equus beds" with 

the Beauinont, as identical invertebrate fossils 

occur abundantly in both the Rio Grande and 

the Nueces terraces, as in neither drainage 

basin have vertebrate fossils been found, and 

as both the Rio Grande terraces and the Leona 

formation occupy topographic positio:q.s from 

20 to 60 feet above present stream beds and 

l!.'l Op. cit., pp. 253-254, 275-276. 

84 Op. cit., pp. 251-254. 

85 Deussen, Alexander, oral information. 

aa Op. cit., pp. 99 and 102. 

from 75 to 150 feet below the Reynosa plain, 

the terraces in the Rio Grande basin are included, for the present at least, in the Leona 

formation, and the Leona formation is correlated in time with the Beaumont clay. 

Character.-The Rio Grande and Nueces terraces are composed mainly of light-gray and 

buff silt but include some sand and fine gravel. 

The terraces are flat topped and have steep 

riverward slopes. At most places there are 

two or three rather than a single terrace. At 

Palafox, for instance, there are three terraces 

above the present flood plain of the Rio Grande, 

one 21 feet, another 32 feet, and a third 63 feet 

above the level of the river when it was at a 

higher stage than the normal. These terraces 

converge downstream, to the south, becoming 

fewer in number until they merge into a single 

terrace. Hand-level traverses carried 'from 

the river to the top of the main terrace at 

several places g~ve the results shown below. 

The places are listed from north to south, and 

the apparent discrepancies are due to the faot 

that the several traverses were made at times 

when the water was at different stages. 

Elevation of terraces along the Rio Grande above level of water 

in river. 

Feet. 

Mouth of San Lore~zo Creek....................... 50 

Bigford ranch. .. .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . 45 

Apache ranch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 

Minero ......... ·................................. 55 

Laredo.......................................... 60 

Fordyce ........................... ········:······ 61 

Ramerefio ................. : . . . . . . . . . . . . . . . . . . . . . 61 

Chapefio............................. . ............ 42 

Salineno.......................................... 52 

Santa 1\-{argarita.................................. 52 

Three miles below Roma. . . . . . . . . . . . . . . . . . . . . . . . . 34 

Five miles above Rio Grande City. . . . . . . . . . . . . . . . . 36 

Rio Grande City. . . . . . . .. .. .. .. .. .. . . . . . . . . . . . . . . 26 

Garcia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

Fossils.:._The Leona formation has yielded 

several fresh-water and air-breathing mollusks. The specific list includes: 

Bulimulus dealbatus Schiedeanus. 

Helicina tropica Pfeiffer. 

Lampsilis purpuratus Lamarck? 

Planorbis tricarinatus Say. 

Polygyra texasiana Mori. 

Unio tetralasmus var. camptodon. 

Unio tetralasmus vas. manubius Gould. 

These forms are Pleistocene or Recent. The 

heavy dentition on the unios as compared with 

those now living in the Rio Grande and its . 
io2 SHORTE~ CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

tributaries suggests Pleistocene 11ge rather than 

Recent. The fact that the streams have degraded their beds as much as 60 . feet since the 

deposits in which the shells are inclosed were 

laid down also indicates a considerable lapse of 

time. The deposits are correlated with the 

Beaumont, which is classified as Pleistocene. 

The Leona formation is therefore regarded 

as of Pleistocene age. 

RECENT SERIES. 

FLUVIATILE DEPOSITS. 

Delta deposits are still being formed at the 

mouth of the Rio Grande. Below Closner, 

where the normal flood plain merges into the 

delta, large areas are still subject to flood. 

What is known as the "Mission ridge," utilized 

for the railroad, the main automobile road, and 

the "valley" towns-Mission, McAllen, Pharr, 

San Juan, and Donna-stands above flood 

waters and is included within the Beaumont 

formation, but even here the land slopes away 

from and toward the river from the axis of the 

ridge. Other towns in the "valley," including 

Llano Grande, Mercedes, Harlingen, Raymondville, Lyford, Sebastian, and Brownsville, are 

likely to be flooded in years of high water, such 

as 1909, 1919, and 1922. These towns are on 

higher land, however, and are now largely protected by great irrigation canals and their 

borrow pits. Breaks occur in the river banks 

as far upstream as a point above Mission, and 

old river channels or "resacas" are found on 

the widening flood plain. These resacas extend 

down the valley as far as Donna, where they 

concentrate into the definite channel of the 

Rio Colorado, which thus becomes a distributary of the Rio Grande. The channel of the 

Rio Colorado is as much as 40 feet deep where 

·it is deepest, near Harlingen, and 200 feet wide. 

It carries safely the flood waters of the Rio 

Grande except at times of the highest floods, 

when the water breaks through on both sides 

of Mercedes and flows into the basin of Lake 

Tiocano and thence northward toward Raymondville, eastward toward the Gulf (crossing 

the St. Louis, Brownsville & Mexico Railway south of Lyford), and back again 

southeastward to the Rio Colorado near Har.: 

lingen. Much of this water n,ever reaches the 

Gulf, for the surface is so flat and grassy and 

the Gulfward slope is so low that the water 

stands in shallow depressions and behind irrigation canals, and there evaporates or seeps 

into the porous silts and sands. The area that 

is subject to flood is mapped as Recent fluviatile 

deposits. The Rio Grande flood plain above 

Closner is too narrow to be mapped separately 

from the Leona terraces and the Tertiary formations where the Leona is missing. 

Only the surficial part of this deposit is nonmarine. The area having been reclaimed from 

the Gulf by the deposition of sediments brought 

down by the Rio Grande the lower beds are 

marine, for they contain marine Foraminifera, 

which are found where the beds are penetrated 

by wells. The material seen in surface exposures is a mixture of highly calcareous and somewhat selenitic gray, blue, red, and brown clay, 

checked by joints filled with calcium carbonate, 

gray micaceous sand, gray silt, and some 

pebbly sand, the whole impregnated with small 

white pellets of lime carbonate. The clay 

greatly predominates. 

On the surface of the delta, particularly 

along the coast from Raymond ville to Brownsville, there are numerous mounds, 5 to 30 feet 

in height, made of clay and known as "clay 

buttes." ·Though several diverse suggestions 

as to the origin of these mounds have been 

offered, the suggestion made by Coffey,37 that 

they are ·" clay dunes," seems most reasonable. 

Most of them lie on the leeward side of shallow depressions, which are doubtless the source 

of the material of which they were formed. 

Many of these depressions contain salt laglipas. 

·Fresh-water and land shells, bones, logs, 

leaves, and other organic matter, all Recent in 

age, are found in the fluviatile deposits in considerable abundance. 

WIND-BLOWN SAND. 

Recent deposits that are most widely dis..: 

tributed are the sand dunes and wind-drifted 

sandy plains of a large area that extends continuously along the coast southward from 

Baffi.ns Bay to a point east of Raymondville 

and westward for 75 miles in a belt that is 25 

to · 50 miles wide from north to south. This 

area includes practically all of Willacy and 

Brooks counties and the northern parts of 

Hidalgo and eastern Jim Hogg counties. 

Padre Island also consists chiefly 'of sand dunes 

blown·up from the beach of the Gulfward side 

of the island. This is an area of migrating and 

stationary dunes, or "medanos," and of the 

a1 Coffey, G. N., Clay dunes: Jour. Geology, vol. 17, pp. 754-755, 1909. 
A GEOLOGIC RECONNAIS.SANCE OF TEXAS NEAR THE RIO GRANDE. 103 

· ~hallow irregular depressions that are so com- and hasalts, which have been described by 

mon in all dune areas. Cross.:;8 

Near the coast, where the Beaumont clay STRUCTURE. 

u'nderlies the sand, the depressions contain The general dip of the strata in the Rio 

shallow lagu,nas, both salt and fresh. There Grande region is eastward. North of Laredo 

,are also cmisiderable stretches of sandy prairie, the dip is . south of east, between Laredo and 

.and at many places where there is no surficial San Ygnacio it is nearly due east, and below 

-sand the ·Reynosa and Beaumont formations San Ygnacio it is no:rth of east . 

. are exposed. · The dunes are sparsely covered . The averages of the dips read for the formawith ,live-oak trees and the prairies are dotted tions are shown below. 

with patches of live-oak brush. There are also 

large areas of wild grasses and "sacahuista." Average dip, in feet per mile, of formations in the Rio Grande 

( region. · Most of the sand is gray or white, but some 

·of it, espeqially where it overlies the Reynosa 

-formation, is red or pink. It has b~en thought 

-that all this sand was blown jnland from the 

<Coast, and perhaps most of it was, although 

there is no greater source of sand here than 

-elsewhere along the .coast, but some of it was 

.derived from the Reynosa formation, on which 

the sand in the western part of the belt lies. 

Here "blowholes" expose the Reynpsa reddish-

-brown sands, and the dunes consist of this sand. 

:Even the gray sands may be deriv~d from the 

:Reynosa, the colored iron coating having been 

:abraded fro_m the qua. rtz grains ring eolian 

transportatiOn. 

COASTAL DEPOSITS. . 

Padre Island is a long dune.-cov~red barrier 

island, which is nowhere more than a mile wide. 

It incloses Laguna Madre, which 1 is 4 to 10 

:miles wide. On the Gulf side of the island 

there are the usual littoral denosits. The 

'beach is sandy and is strewn with innumerable 

sea shells. The water of the laguna is at most 

places so shallow that it can be easily wad~d; 

indeed, at some times and places it is only a 

:mud fiat incrusted with precipitated salts. 

At Point Isa;bel, however, gasolir}e launches 

ply between the mainland and the island, but 

1ong piers are necessary, and the boats drag the 

bottom for considerable distance~ out from 

.either shore. In the laguna mud silt, sand, 

:Shells and fragments of shells, and I salt are all 

'being laid down to form the usual lagoon 

-deposit. 

IGNEOUS ROCKS. 

In southwestern Uvalde County, just outside 

the region and not shown on the map, there are 

Beaumont clay (Pleistocene)....... . . . . . . . . . . . . • . . . . 25 

Reynosa formation (Pliocene?)...................... 17 

Frio clay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 

Fayette sandstone ................................ -. 201 

Y egua formation .. _ .......................... -. -. -. 296 

Cook Mountain formation ............. -............. 435 

Mount Selman formation ............ ~ ............... 180 

Wilcox group (Bigford formation, Carrizo sandstone, 

and Indio formation) .............................. 384 

Midway formation ................................... 155 

These averages are probably too high, however, for the steeper dips are those that are 

most commonly read. At most places the 

strata are so nearly horizontal that the dip 

is not observable. The true average dip of · 

each formation is probably less than half that 

given above. According to the figures given 

the average dip of the Eocene formations is 

273 feet per mile and the Reynosa and Be.aumont together average 21 feet per mile. These 

figures should probably be reduced to 136 and 

10, respectively, or. even .to lower figures. The 

discrepancy between the Eocene and postEocene formations again emphasizes the structural unconforrnjty at the base of the Reynosa. 

The reconnaissance methods employed did 

n~t permit detailed structural investigations, 

but indications of abnormal structure were 

observed or were authentically reported to 

occur at a few places . . According to Baker 39 

there is an anticline about 10 miles west of 

Carrizo Springs, another about 15 miles southeast of Eagle Pass, and probably still another, 

lying chiefly in Mexico, southwest of the old 

Indio ranch house. , The writer saw indications of the first of these. The second is 

described by 'Udden. 40 Along Nueces R1ver 

f · k · th as Cross, Whitman, U.S. Geol. Survey Geol. Atlas, Uvalde folio (No . . several small exposures o Igneous IfOC s In e 64), PP· 3

, 4, 1900. 

form of plugs or necks. Most of them make ag Baker, c. L., personal communication. . 

· d · 11 4o U dden, J. A., Report on a geological survey of the lands belonging to hills, but some are expose In stream va eys the New York & Texas Land co.: Augustana Library Pub. 6, pp. 

<Or on fiat surfaces. The rocks are phonolites 1 88-90, 1907. 

I 
104 SHOR.TER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

below Pulliam, there are anticlines and synclines and faults of small throw. The eastward dip is somewhat accentuated at the 

Webb-Zapata shallow oil field, in the southeast 

corner of Zapata County. There are relatively 

st~ep reversed dips in the vicinity of Roma. 

Probably an anticline here crosses the international boundary. A large anticline has 

been mapped by Crider41 east of Laredo. 

Normal· faults of small· displacement were 

observed on Chaparrosa Creek 11 miles west of 

La Pryor, and half a mile east of the Jones 

ranch, northwest of Crystal City. 

The search for structural features in the 

Rio Grande region is full of difficulties and 

subject to inaccuracies, for formations vary in 

character from place to place, the beds occur 

in lenses and pockets, the formational contacts 

are ill defined, exposures are scarce, wells are 

few, and the logs of most of those which have 

been drilled are inadequate, and false bedding 

planes, which dip at all angles with reference 

to the true bedding, are abundant. 

There is at least one salt dome in the region. 

It is about 5 miles southeast of Falfurrias and 

crosses the Beaumont ..... Reynosa contact. It is 

5 miles long from east to west and 1 to It miles 

wide from north to south. Quaquaversal dips 

are not plainly seen) but a surrounding laguna 

of salt brine, a large body of gypsum on the 

southwest side, and a topographic swell that is 

more than 40 feet high indicate that there is ·a 

salt dome at this place. La Lomita (little hill), 

6t miles south of Mission, on the Rio Grande, 

which stands ·conspicuously above its alluvial 

surroundings and exposes abnormal materials 

and abnormal dips may also be a salt dome, but 

the dips may not represent true bedding planes. 

ECONOMIC GEOLOGY. 

OIL AND GAS. 

More than 45 separate projects are under 

way for the development of oil ' and gas in the 

region, but only three are producing. The 

locations of these projects are shown on the 

map (Pl. XXVIII). 

In 1920 the Webb-Zapata Co. had drilled 45 

wells in the southeast corner of Zapata County 

to an average depth of 160 feet and fo=und some 

oil and a very little gas in the Fayette sandstone. The dip here is normal in direction, a 

few degrees south of east, and is 198 feet per 

41 Crider, A. F., personal communication. 

mile, somewhat more than the general dip m 

the neighborhood. As there are no reversed 

dips, the oil probably occurs i:p_ a lens of sand, 

which plays out and is sealed up westward, for 

the cap rock crops out 1-! miles west of the west 

border of the producing area.. Eight of the wells 

were pumping in 1920, a gasoline engine on 

each. The average yield is between 3 and 4 

barrels a day. As the field is more than 40 

miles from the railroad at Hebbronville, and 

as the road is very sandy, all attempts to market the oil h·ad failed up to the time of the 

writer's last visit. The oil was stored in 

steel tanks at the field. According to recent 

reports it is being marketed. 

Another producing field is 25 miles south of 

Aguilares and 45 miles southeast of Laredo, in 

Zapata County. Gas only is produced. The 

field is developed and operated by the Border 

Gas Co., of Laredo. The gas is conducted 

through pipes to Laredo; where the company 

operates the city gas plant. The outcropping 

formation at the wells is Fayette. A little 

gas is obtained from a depth of 700 feet, but 

the main flow is obtained from a depth 'of 1,200 

to 1,250 feet. The material penetrated in aU 

the wells is almost entirely clay but includes. 

thin seams of sandstone. The main gas s~nd 

lies in the basal part of the Y egua or the upper 

part of the Cook Mountain. The structure 

here also is homoclinal, the gas sand dipping 

eastward at the rate of 30 feet to the mile. 

In the spring of 1921 oil was struck in a well 

of the Mirando Oil Co., in the extreme northeast corner of Zapata County. The well site 

is just west of the Reynosa-Frio contact, and 

practically the full thickness of the Frio clay 

is represented under it. The oil sand was struck 

at 1,430 feet from the surface, in the Cook 

Mountain formation. The first we1l is variously 

reported as having given an initial production 

of 20 to 100 barrels a day. The new field has 

been rapidly developed. In the spring of 1922 

it was credited with 13 producing wells, and 

the estimated possible production ranged from 

1,000 to 2,300 barrelsaday. 42 In addition, more. 

than a score of other wells were being drilled in_ 

other parts of Zapata and Webb counties. 

This field has produced also considerable gas. 

The Caroline-Texas Co.'s test well near Bruni 

is reported as blowing 8,000,000 to 10,000,000 

42 McGreal, P. L., Gulf coast oil operations: · Oil and Gas Jour., vol. 20, 

No. 43, p. 18, March 24, 1922. 
. GEOLOGICAL S R EY 

I 

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f: 

. t

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0 

I 

I ., ........... I 

Tr \ , ) 

' r I\ "' --i--':",- ·-----+-'- .. -- ~ ~ ..... ~ ,. '; , I , ' 

?- i 

EXPLANATION 

SEDIMENTARY ROCKS 

Coastal depo its 

(Mud, silt, sand, ~<hells, and fragments 

of shells and salt) 

Qws 

Wind-blown' and 

(Sand dunes and wind-dri,fwd sandv 

plains) 

Qf"d 

Fluviatile deposits 

(Ghit?.fll/ cla11, kiqhl11 caJco.re&uS and 

.•omewka.tselenitic, ofgraiJ, blue, red, 

and brown colors; checked l>-11 joints 

.filled with calcium carbonate, urav micace&uS sand, urav silt, and sonUl 

pebbl11 sand) 

Qb 

Be~ clay 

>-

~ z 

cr 

, w 

II <{ 

::::> 

0 

Leona formation 

(Chiefly blue and red calcareous cJ.av, weatkcr- ino 11ellow, with some lenses of sand and a few scattered lim.estone ccmcretions) 

UNCONFORMITY 

(Lioht gra71 and buff silt with some Ba?.d and fine oravel ; deposiudonterrace!l. Prob-

<Wlv contamporaneowr with Bcaunwnt _________clAv) _______ ___/ 

Tr 

Reynosa formation 

(Intricate mizture of gravel cemented bv lim.e I 

carbonate, wnce111.81tud gravel, li11Ul8tone with 

embedded pebbles and cobbles, limestone aJ.mostfreefron•uravel, also sand, sandstone, I 

graveUv sand, and a smaJ.J. amount of cJ.av; 

aU intermized) UNCONFORMITY 

Tfl" 

Frio clay 

(Varicolored clav with a few seams of !fret!/ I 

sandstone; t/1.8 claus are 1.earlv evervwkere I 

checked bv joints in which secondarv calcium• 

carbonate occurs in thin plAtes; 80111.8 of the 

clays are ovpsiferous ; some bed3 of volcanic 

ash and manv sn.aJ.l calcareou.s nodule!l are present) 

Tf'a I EXPLANATION 

Fayette sandstone Continued r 

..; Bigford formation Q 

w 

:::J 

(F'ine, medi·um, and coarse urained sandstone I 

ofwhiu, uray, ureenish-way, and buff colors, 

with thin bed3 a?td lenses of weenish-WCtY, 

J)'ink, and rsd sandy and limv shale and clay; some bed3 of volcanic ash; CT!JBtalline lime- atone ccmcretior.s cbmnwn) I 

41 

;3 

c: 

·~ 

c: 

(ChieJl11 cla11 of various colors, with subordi-- z 

UNC<zt;!FO Y? 

I 

T)' 

Yegua forma~ion ( Chitifl.v selenitic a?td carlxma.r.eatUI clays of 

vl\riO'US colors, but predominant/11 dark-grau, l 

with thin bcd3 and lenses of 80-n<l and sandatone; irreoular limestone c<n•cret-wns occur 

aparinolll) UNCONFORMITY 7 

8 

I 

~ ~ 0 I 

naw a1nounts of (fray, green., and ~rown i= 

sandstone a?td man11 bed3 of li!f?liU. La.roelv z conte?nporane&uS with Carrizo sand3tone but 8 in part 1/&unoer) I 

~ s,, Carrizo sandstone 

>-

cr 

<t 

;:::: 

cr 

LrJ 

1--

~ 

0: I 

(Chie.fl11 cro88-bedded sand and sandstone; ~ some of the sandstone cemented and crystal,- ~ lized into vuartzite, and s01ne hivhl11 ferru(Ji- c;;; nou.s; thin bPcU and lenses of variO'USlv colored rla11• and li•ne~tones) 

UNCONFORMITY 

~ ~ 

lt5 

( Varicolor d sand3t<mes, fi ,.,·uoin&uS, mica- ce~nU, and olaucottitic, many of them cross- bedded and ripple marked; int rbedtl d with 

81'bordinate amounts of -..:ar1'colored and- stones and thi1~_l ;:s11s o;:_orav limeston ) 

Tl"rra 

Mount Selm""';n fOrmation 

l(Chi(ff.y clay qf gray, black, oreeni$h, and 

bluU.h colors, with 80me layers and lens s qf 

oo:ndstone and limes ton ; sev raJ b da of liu- nitic and hituminou. coal; calcareous con- cuti{)'llR ahunda••t throughout) I 

I 

I - "~ Indio formation I ~ 1'h:i>t-beci.df'd and laminated aroillaceous sand3 

and arenac~>ou.s shales, wiUt some lavers of maBBive clay a?td tenses a?td lavers of sand-I 

stO?te) I 

I 

Midway formation I 

(Slwle with some inte?·bedded lense~~ and wyers of sandstone and limestone a••d. in places, conglon.eratic bed$ at base) 

GREAT UNCONFORMITY 

\ 

L E 

Oil poOi and wells 

0 

t_ __ j 

Well drilled for oil 

~ 

Clay, sand, and- g ravel pit 

• 

Salt dome I -

~ 

Mine 

ggo 

PROFESSIONAL PAPER 131 PLATE XXVIII 

gs• 

I 

GEOLOGIC MAP 

OF THE 

GULF COASTAL PLAIN OF TEXAS 

ADJACENT TO THE RIO GRANDE 

1(\ F't 0 

J HHF 

Cooe~e:lcu;

lO 

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1 q~ ! J ~~ ' , 

Tr 0 ~--~~---l~ C/r., 1 'JJ/an<·o \. \ 

808 woool" Qws 

...Bf'd(tg.~u 

.OIL eft GAS CDJ 

,/,.,nyo l: 

I g FAL.FURRIAS .Oil. CO. ATLANTIC OIL & 

PRODUCING CO 

.A IU\rhH.A L-.., 

0 c;. G 13 H. () 0 I{ 

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A~UI) ~\I~Vll I 

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K 

I I Qws 

------.L.---,------- 1 

Tr 

IWbbrreou . 

R 

I 

I 

I 

I 

.SIIU1tal-Jenu. / 

1 .c".,,.e-.. 

I 

I 

I 

I 

I 

I 

d.oucrr£0 I OIL CO.' 

I H 

}," GlorJu.. 

ROOJwb 

. J)cl6.uH. 

0 

I NorJIUi" 

: 

--.,--.._, L , 

)'Ruiu:b tu ~" • • fe.su 6 "-i,...).,a 

(Soh n~i'-0- -~ -------

r ~ .c><:J 

gs• 

. j 
A . GEOLOGIC RECONNAISSANCE OF TEXAS NEAR THE RIO GRAND~. 105 

cubic feet and the Laurel Oil Co.'s well No. 1, 

at Mirando, as blowing 40,000,000 cubic feet. 

. According to Lupton 43 the A via tor well, which 

is about halfway between Mirando and the 

Mirando City pools, was brought in witp. an 

estimated production of 40,000,000 cubic feet 

in June, 1922. Well No. 2 of the Schott Oil 

Co. is also a gasser. 

As the first well was not completed when the 

writer was last in the field the structure in the 

Mirando district has not been worked . out. 

Wrather,44 however, states that the producing 

sands in the Mirando and the Jennings :fields 

are at approximately the same horizon. 

The future of the Mirando field can not yet 

be confidently predicted, but the field will brobably be thoroughly tested by the end of 1922. 

Gas has been found at several other places, 

notably at Reiser, 2! miles west of Agui1ares, 

in Webb County; .in the Spurrier wells Nos. 1 

and 3, in western Starr County; in the Higgins 

Oil & Gas Co.'s wells, 15 miles southeast of 

Eagle Pass, in Maverick County; and in the 

well of the International Refining Co., 7 miles 

northeast of La Pryor, in Zavalla County. At 

Reiser, which is only a few miles northwt1st of 

the northern border of the Mirando field, the 

gas is obtained ·from the Y egua and Cook 

Mountain formations at depths ranging from 

150 to 1,650 feet. The Spurrier wells also contain gas, though in small quantities, fron1 the 

Y egua. The gas in the Higgins wells is obtained from Cretaceous beds. The well of the 

International Refining Co._ is reporte-d to have 

struck a 50,000,000 cubic foot gas blow at a 

depth of 1,200 feet from the surface. , The 

outcropping formation is the Indio, and pre-. 

sumably this gas also comes from tJretaeeous 

beds. The first well at Pulliam, on the UvaldeZavalla county line, is reported to have produced about 5 barrels of oil a day. ~ 

So far as known the results of all other tests 

have been negative. However, outside of the 

Mirando field few of the deep test wells have 

been completed, and oil may have been struck 

in wells unknown to the writer. Several of 

the projects, notably those of the Riggin ;, International, and Empire companies and the 

Hanchett project, have been carried on in a 

serious and intelligent manner, but t~e operators 

ta Lupton, C. T., oral communication to L. W. Stephenson. I 44 Wrather, W; E., The Mirando Oil Co. well, Zapata Count;y, Tex.: 

Am. Assoc. Petroleum Geologists Bull., vol. 5, p. 625, 1921. 

drilling many of the wells have shown neither 

intelligence nor diligence . 

The search for oil and gas in paying quantities will be continued only with great diffi- . 

culty and with large risk of. money. The work 

of locating and mapping areas in which the 

structure is favorable is difficult, and even a 

well-developed and accurately mapped anticline may contain no oil-bearing sand. The 

possibility that the San Miguel formation (correlated with part of the Taylor marl . and the 

Anacacho limestone) of the Upper Cretaceous, 

may be petroliferous is not unpromising. The 

Cook Mountain formation gives some indication of being a fairly persistent producing sand 

where the structure is favorable. Both oil and 

gas have been found in this formation in the 

Mirando field and in smaller quantities elsewhere. It appears to contain indigenous petroleum at Oil City, Nacogdoches County, and 

at Crowther, McMullen County. This formation should be tested wherever drilling is done. 

The expenses of development will be high, for a 

large part of the region is not easily accessible 

to railroads or seaports, and even after development there is still the problem of getting the 

product to market. 

Probably the most promising structural 

features to look for and to test are salt domes. 

About 50 per cent of the known Texas salt 

domes have produced oil and some of them 

have produced it in large quantities. The salt 

dome southeast of Falfurrias was tested some 

years ago by the Texas Co., but with negative 

results. The hill south of Mission, known as 

La Lomita, which may be a salt dome, is now 

being tested by the La Lomita Oil Syndicate. 

Other salt domes will doubtless be discovered 

in the region, and petroleum may be found ·in 

them. 

On the whole the prospects of the ultimate 

discovery of one or more highly productive oil 

or gas fields in this region are not discouraging, 

but the preliminary work should be done with 

great care and the money required will be 

expended at considerable risk. 

COAL. 

According to Ashley/5 the coal which has 

been extensively mined at Minero, Dolores, 

Cannel, and Santo Tomas, in Webb County, 

4o Ashley, ·a. H., The Santo Tomas cannel coal: U. S. Geol. Survey -

Bull. 691, pp. 251-270, 1919. 
106 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

and which is in the Mount Selman formation, on the underground waters o{ the whole of the 

is probably the largest body of cannel coal of southern part of the Texas Coastal Plain. The 

bituminous rank in the United States, if not in countie8 mentioned are close to the lower Riothe world. Johnson & Coleman are taking. Grande region and the conditions that prevail 

out enough of this coal at their mine, 4 miles in them extend into this region. 

east of Palafox, to run their pumping plants oh 

the river. Coal from the same formation and BRICK-MAKING MATERIAL. 

' probably of about the same quality, is found on Few of the clays of the lower Rio Grande 

Espada Creek, in Webb County, near the base region &re suitable for making good brick. 

of the Mount Selman formation. A 14-inch There is an abandoned brick plant at Reiser. 

bed of cqal is also reported to outcrop near the The . pit is in uppermost Yegua or lowest 

top of the Mount Selman formation on the Rio Fayette, and the clay is fairly pure. Facing 

Grande at low water at and near the mouth of brick of good grade was made here. The plant 

Arroyo Santa Isabella. was abandoned when the Border Gas Co. 

Beds of coal, probably of poorer quality, may shut down its gas wells at Reiser, thus shutbe obtained from the Indio and Bigford forma- ting off the gas used as fuel. At no other 

tions at many places where these formations place in the region has facing brick been manuoutcrop. Lignite is found at the surface and at factured. 

shallow depths at places east and southeast of Along the lower Rio Grande and its main 

Dentonio and at the Mangrum and Rutledge delta distributaries there are numerous pits 

ranches, west and south of La Pryor. It is from which material is taken by Mexicans for 

also penetrated in wells at La Pryor. An 18 to making crude adobe brick. The largest and 

20 inch bed of hard, brittle, and bright coal most efficient plant of this kind is at Los 

outcrops 1! miles above the mouth of San Ebanos, ~bout 2 miles southwest · of Sam 

Lorenzo Creek, in Webb County, in the Big- Fordyce. Beaumont terrace silts, or recent 

ford formation. There is also an exposure of fluvial silts, are mixed with waterin the pit and 

coal at the mouth of the first creek south of the worked into plastic masses having the conApache ranch house. sistency of dough. This material is thrown . 

At Pilotes ranch, about 10 miles north of into trays containing six brick-sized compartPalafox, as many as 11 seams of coal and bone ments, which are sprinkled with dry sand to 

were struck in a test drilling at depths ranging prevent it from sticking. The clay-filled tray 

from 74 to 236 feet. The surface formation . is then carried by hand to a sunny flat near the 

here is the Mount Selman, but the carbonace- pit and inverted, the wet bricks being left for a 

ous beds are in the Bigford formation. Coal in time to dry. · When they have dried out suffithis same formation outcrops on Concillos ciently ·· they are stacked or "ricked" in open 

Creek near its mouth and on Nueces River 2! piles for further drying by sun and wind. 

miles east of the Asherton-Big Wells road, a Finally they are placed in rough kilns made of 

few miles outside the Rio Grande region. adobe bri,gks and baked, branches of roesCoal is reported by Judge A. F. Spohn, of quite being used as fuel. The brick is poor, 

Zapata, t? occur just north of Zapata and in but it stands fairly well the pressure of the 

the valley of Dolores Creek near the Webb- common Mexican one-story house. 

Zapata county line. This locality is in the GRAVEL. 

outcrop of the Cook Mountain formation. No The gr:avel supplies of the region are plenti- coal was seen, however, in the many sections of ful, widely distributed, and of good quality. the Cook Mountain formation exam_ined. The Reynosa formation furnishes gravel of 

WA',rER RESOURCES. high grade for road ballast and concrete work 

practically wherever it is exposed, whether in 

Deussen and Dole 46 have investigated the the main outcrop of the formation or west of 

underground water resources of La Salle and 

McMullen counties, and Deussen47 has reported 

46 Deussen, Alexander, and Dole, R. B., Ground water in La Salle and 

McMullen counties, Tex.: U.S. Geol. Survey Water-Supply Paper 375, 

pp. 141-177, 1916. 

the Bordas scarp, where only patches of Rey47 Deussen, Alexander, Geology and underground waters of the southelj.Stern part of the Texas Coastal Plain: U. S. Geol. Survey WaterSupply Paper 335, 1914; Geology of the Coastal Plain region of Texas: 

U.S. Geol. Survey Prof. Paper 126 (in press). 
A GEOLOGIC RECONNAISSANCE OF TEXAS NEAR THE RiO GRANDE. 107 

also, but neither is this produced. The only 

sand used commercially to-day is in the Beaumont delt.a-like deposits at Mission and McAllen, where 25-foot lenses of almost pure-wh~te 

sand, underlie clay and silt. The sand obtained at Mission sells for $2.25 .a cubic yard 

at the pit. 

GYPSUM. 

The sediments of the region inclose a large 

amount of gypsum in thin beds, seams, plates, 

crystal aggregates, and larger bodies, but it is 

nowhere utilized. 

nosa are found. As only 1 per cent of the 

gravel is over 4 inches in diamet~r, as threefourths of the pebbles and cobbles are composed 

of chert or other siliceous material, as the 

pieces. are well rounded and smooth, and as 

the material is practically nowhere so firmly 

cemented as to require blasting in its excavation, this deposit makes excellent commercial 

gravel. · There are half a dozen or more 

pits along the river between Sam Fordyce and 

Penitas. ~n some the loosely cemented and 

unconsolidated gravel is taken out with steam 

shovels, and in others it is removed by hand. 

At some plants it is sorted by · power-driven ROAD MATERIAL. 

screens, and at others by hand screening_. The quartzites, sandstones, limestones, conMost of the gravel is used in the valley, but cretions, and conglomerates of the region 

some is shipped as far as Ho~ston. At would make good material for building roads, 

Green's, in Webb County, on the International but as few roads have been built they have not 

& Great Northern Railway, where there is an yet been so utilized. Argillaceous and caloutlier of Reynosa, a large pit has long been careous materials from the 'Reynosa formation 

under excavation. Most of the gravel is loaded have been used to harden an excellent road 

by. steam shovels into cars and used as ballast · through the sand belt from Falfurrias to Edinon the railroad. Some of it has been used to burg, permitting automobile traffic into the 

surface a section of ·the San Antonio-Laredo valley from coastal points farther north. 

wagon and automobile road between Green's Shallow pits are opened along the road every 

and Webb. mile or two and the material is laid on a graded 

SAND. 

Good sand is not so abundantly produced, as 

gravel, although there is plenty of it. The 

sand lenses in the Reynosa would furnish 

some, but they do so only where the sand is 

mixed with the finest grades of gravel. The 

sand sorted out of the Reynosfl. by the wind. 

and piled into dunes in the main sand belt 

and elsewhere is generally pure, fine-grained 

quartz sand, but it is riot used commercially. 

An abundance of sand is derived frpm the 

coastal beaches and lagoons in the sand belt 

surface of sand. This road lacked about 10 

miles of being completed in 1920. The same 

material is being used in the same· way on the 

sandy road to Rio Grande City from Hebbronville. · CEMENT. 

It is possible that a proper combination of 

limestone, clay or shale, and gypsum might be · 

found for the manufacture of Portland cement, 

coal, oil, or gas being used for fuel, but no 

commercial concern has yet found this combination. 

·. 

NEW SPECIES OF MOLLUSCA FROM THE EOCENE DEPOSITS OF 

SOUTH"jESTERN TEXAS. · . 

I By J I LIA GARDNER. 

The new species and subspecies ~hose n jmes 

are used in the preceding paper by .J C. 

Trowbridge are described below. One species 

that had already been described and th 1 t is 

here treated as a subspecies of Ostrea alabamiensis is considered in detail because of its 

abundance in the middle and .upper Eot1 ene 

deposits in the lower Rio Grande region. 

The letters after some of the station num ers 

have the following significance: (a) abun 

(c) common, (p) present, (r) rare. 

PELECYPODA. 

Family NUCULIDAE. 

Genus NUCULA Lamarck; 

Nucula sp. 

Plate XXIX, figure 3. 

The rock in the area a little south of Larfdo, 

in Webb County, is made up'largely of a Nuaula, 

which is probably distinct from any spJcies 

described, though. it is very close to .N. mag~~ca Conrad. It is about 3 centimeters high jand 

broad and is sculptured with prominent gr9wth 

lines and a uniformly developed, subcutaneous 

radial threading. No hinges or perfect ~teriors have been found, but the pearly :£

1

-ragments of the thick sP,ell pack the matrix. 

Occurrence: Cook Mountain formation. ~tation 7492 (p), 1 mile east of Laredo; st8jtion 

8818 (a), 5 miles southeast of Laredo, Webb 

I County. 1 

Family OSTREIDAE. 

Genus OSTREA (Linnaeus) Lamarck. 

Ostrea .tasex Gardner, n. sp. 

Plate XXXI. 

?Ostrea.multilirata Conrad, U. S. and Mex. Boundaryj Survey Rept., vol. 1, pt. 2, p. 157, pl. 12, figs., a-d, 

1857. 

Shell of only moderate dimensions for the 

genus, subequivalve, ovate-trigonal in outline, 

generally somewhat produced ant~riorly ; '!surface layer decorticated; both the right an left 

valves radiately sculptured with 15 to 20 tine, 

approximately uniform, rippling riblets, s~ilar 

to those developed upon the attached val e of 

0. crenulimarginata Gabb; ligament area rnod-

. 91048°-23--8 . j ' 

erately large, somew~at flattened in the right 

valve, the n1e.dial d pression relatively wide, ., 

broadly but not d eply excavated; growth 

striae . obvious; late al margins very finely 

crenate, the crenae. Pf_rsistent in .some individuals around the entir, Inner margin; basal margin often fluted in h~rmony with the' external · 

sculpture; adductor uscle scar crescentic, lying 

below the median ho izontal, ~oderately large, 

nof very deeply exca1 ated; pedal scar obscure. 

Dimensions: Right valve, altitude 80 millimeters, latitude 65 . millimeters; left valve, 

altitude 98 Inillimete [

1

s, latitude 72 mi~limeters. 

Type locality: StaFon .8925, a quarter of a 

mile northeast of G~ass ranch headquarters, 

Dimmit County, TexJ 

Ostrea· tasex is the ~nalogue in southwestern 

Texas of Ostr~a crenu~imarginata of the Midway 

of Alabama. In the Al~bama species, however, the right valv~ is smooth, but in those 

found along the Rio Grande the two valves are 

similarly sculptured. j Toward the umbones the 

radials are sharply elevated, though they tone 

down into an even fluting toward the outer 

margm. j 

Ostrea tase:-c is probably identical with Ostrea 

mv.Ztilirata Conrad, cdllected from the so-called 

Cretaceous of "Dry c}eek, Mexico." . Conrad's 

types are in the Nj tional Museum and are 

fairly well preserved. . Nothing of the. kind has 

been reported from the Cretaceous by later investigators who hav, collected extensively in 

the Rio Grande area:!, However, Ostrea cortex, 

. collected at the saillje locality, is certainly a 

Cretaceous species, a : d until "Dry Creek" can 

be definitely located and the confusion in the 

stratigraphic relations cleared the Wilcox form 

may well be kept ap rt. 

Stations in Indio formation. 

8927 (c). 4 miles south 1f RQckdale, Milam County. 

6278 (a). 11 miles south 1of Sabi:q.al, Uvalde Coun:ty. 

3187 (r?). East side of fhe Rio Grande, in Maverick 

County, about 3! miles above the mouth of San Ambrosia 

Creek. I . . - 9173 (c). 17 miles wes of Carrizo Springs, Dimmit 

County. 

109 

t I 
110 SHORTER CONTRIBUT'IONS TO GENERAL GEOLOGY, 1922. 

8925 (c). A quarter of a mile northeast of Glass ranch 

headquarters, Dimmit County. 

1/67 (c). 3 miles north by west of San Pedro ranch, on 

the Maverick-Dimmit County line. 

Ostrea .alabamiensis subsp. georgiana Conrad. 

Plate XXIX, figures 6, 7; Plate XXX. 

Ostrea georgiana Conrad, A cad. Nat. Sci. Philadelphia 

Jour., 1st ser., vol. 7, p. 156, 1834. 

Dana, Manual of geology, 1st ed., p. 519, fig. 811, 1863. 

, Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, 

p. 683, 1895. 

Ostrea titan Conrad, Acad. Nat. Sci. Philadelphia Proc., 

1st ser., vol. 6, p. 199, 1854; Pacific R. R. Rept., 

vol. 6, pt. 2, p. 72, pl. 4, fig. 17a; pl. 5, fig. 17a, 1857. 

Ostrea contracta Conrad, U. S. and Mex. Boundary Survey 

Rept., vol. 1, pt. 2, p. 160, pl. 18, fig. 1 a-d, 1857. 

Ostrea alabamiensis Conrad var. contracta Harris, Bull. Am. 

Paleontology, vol. 6, p. 9, pls. 3, 4, pl. 5, fig. 2, 1919. 

. Type localities: "Orange burgh, S. C.; Shell 

Bluff and near Milledgeville, Ga.;'-Conrad, 

1834. 

Ostrea alabamiensis subsp. georgiana is the 

most conspicuous species in the later Eocene 

of Texas, and in many localities it is the only 

fossil collected. Its range of variation, like 

that of other oysters, is wide. Only a very 

small percentage of individuals retain the fine 

epidermal sculpture, the one constant feature, 

and one identical with that of 0. alabamiensis. 

However, in the large mass of material available for study-several hundred individualscertain differences in the average individual 

developed at the different horizons can be detected. None of the huge, very much elongated and massive shells so common in the 

Fayette near Romain Starr County have been 

found ~ the Cook Mountain formation farther 

north. The forms occurring at this lower 

horizon are of only moderate dimensions, 

usually relatively thin and broadening at the 

base, and, as a rule, conspicuously laminar. 

This decided laminosity is one of the more persistent features of the Cook Mountain 0. georgiana. The Cook Mountain form grades into 0. 

alabamiensis though the hinge is usually heavier 

~nd more elongated. The very large, massive, 

narrow, and elongate individuals described by 

Conrad under 0. contracta occur in considerable 

numbers in the Y egua, and in the Fayette this 

type has shut out almost entirely the smaller 

and thinner race of the Cook Mountain. 

Stations in Cook Mountain formation. 

6650 (c), 6657 (c), 6652 (c). Environs of Cotulla, La Salle 

County. · 

8816 (p). 100 feet south of windmill, Marguerita ranch, 

6.4 miles west of Webb Station, Webb County. 

8785 (p). 2.1 miles east of Webb, Webb County. 

8841 (p). Half a mile west of triangulation station 

Dolores, 2 miles north of Dolores, Webb County. 

8842 (p). Three-fourths mile west of triangulation station Dolores, Webb County. 

8830 (p): 10 miles northwest of Laredo, Webb County. 

2113 (c). 5 miles northeast of Laredo, Webb County. 

8765 (r). 2.3 miles northeast of Laredo, at crossing of 

San Antonio road with International & Great Northern 

Railway, Webb County. 

7492 (p). 1 mile east of Laredo, Webb County. 

6058 (c) (old series). Laredo, Webb County. 

8835 (a). 6 miles southeast of Laredo, Webb County. 

6436 (r). La Perla ranch, below Laredo, Webb County. 

8840 (c). North bank of Salado Creek, Zapata County. 

9116 (c). Soledad, Zapata County. 

9112 (c). 2 miles down the river from Zapata, Zapata 

County . 

9109 (p). Rio Grande prospect, 4.8 miles southeast of 

Zapata, on Roma road, 2.5 miles northeast of main road, 

Zapata County 

9110 (p). 4.8 miles.southeast of Zapata, Zapata County. 

9113 (c). 5! miles southeast of Zapata, Zapata County. 

9175 (p). 14.2 miles below Zapata, Zapata County. 

6432 (r). Arroyo Dolores, Zapata County. 

9106 (r). Zapata County. 

Stations in Yegua formation. 

8784 (a). 3.4 miles east of Cactus Station, Webb County. 

8809 (c). 12 miles northeast of Laredo, Webb County. 

8849 (p). 7 miles northeast of Laredo, Webb County. 

8061 (c). 7 miles east of Laredo, Webb County. 

8834 (a). 8 miles east of Laredo, Webb County. 

9119 (p). 2~ miles southwest ·of Alejandrefias ranch, 

Zapata County. 

7741 (r). 1 mile northwest of Pedernal ranch headquarters, Zapata County. 

9182 (a). 3.2 miles northeast of Lopefio, Zapata County. 

8843 (a). 23~ miles northwest of Rio Grande City, Zapata 

County. 

9187 (p). 2! miles south of Romirefio, Zapata County. 

9188 (p). 3 miles south of Romirefio, Zapata County. 

Stations in Fayette sandstone. 

6147 (a). C. T. Tom ranch, l mile south of Campbellron, 

Atascosa County. 

6144 (a) 6 miles east of Campbellton, Atascosa County. 

6146 (p). 3 miles southeast of Campbellton, Atascos~ 

Countv. 

360i (r). Winchester, Fayette Co'unty. 

9094 (p). Reiser, Webb County. 

9098 (c). 1.3 miles southwest of Webb-Zapata shallow 

oil field, Zapata County. 

7740 (a). Charco Redondo ranch, Zapata County. 

8060 (p). Charco Redondo ranch, Zapata County. 

9179 (p). 3.6 miles northeast of Palo Blanco ranch, Zapata County. 

9181 (a). 3 miles northeast of La Presa ranch house, 

Zapata County. 

9096 (c). 3.6 miles south of Huisatche ranch, Jim Hogg 

County. 

9192 (p). 19 miles north of Roma, Starr County. 
. NEW SPECIES OF MOLLUSCA FROM SOUTHWES~RN TEXAS. 111 

9191 (c). 15 miles north of Roma, Starr County. I tions of th.e shell bu absent upon the posterior 

9189 (r). 7 miles above Roma, Starr County. third, narrower tha those :upon the right valve 

9193 (c). 1.7 miles northeast of Agua Dulce ranch, north and separated by interspaces of a width almost 

of Roma, Starr County. 1 t th d' 1 ' 1 t f t · 7493 (p ). 121 miles below Arroyo Tigre on road to Roma; e9.ua 0 e ra Ia s; scu P ure o pos er10r area 

Starr County. similar in the two alves; incremental sculpStations in Frio clay. ture pronounced, particularly toward the base; 

6666 (r). 9 miles north of Tilden, McMullen County. hinge area of mode ate width for the genus, 

6665 (a). One-fourth mile east of post No. 7, west of obliquely grooved; hinge very imperfectly preTilden, McMullen County. served; very narro~ medially with short ver3093 (p). Fort Ringgold, Starr County. · 1 h b 1 'd · h ~I?a te~t , a rupt y WI erring ~ear t e extremFamily PARALLELODONTIDAE. Ities; distal teeth c arse, few In number, and 

Genus CUCULLAEA Lamarck. rudely parallel to thf hinge margin; characters 

Oucullaea is one of the more conspicuous of interior not kno~. . 

fossils in the Midway of Texas. Apparently Dimensions: Altitude,42.0millimeters; length 

there is but a single species, thou~ .. h th.e small of hin,ge, 39.0 milli* eters; maximum latitude, 

and ·poorly preserved northern forms may 4 7.5 millimeters; d~ f.meter, 40.0 ± millimeters. 

prove ~o be distinct from those of the Rio Type locality: Wi te Bluff, Rio Grande, Ma vGrande province. This species is ertainly not erick County, Tex. Type, U. S. National 

identical with 0. macrodonta, f~und farther Museum: 

·east, though the differences may ot be of spe- The specimens fr~m Kaufman County, in the 

cific rank. Its relation to 0. s ~ordi Gabb, northern part of t~e State, are smaller than 

described from a lost type two- enths of an t~ose from ~oints alpng the .Rio Grande. They 

inch in latitude, is even less defini e. differ, too, In the !fore uniform development 

Cucullaea (macrodonta subsp.?) texana .Gardner, n. of a radial sculpture upon the posterior area. 

subsp. Oucullaea (macro onta subsp. ~) texana varies 

Plate XXIX, figures 4, 5. widely in outline and .to a lesser degree in 

Cucullaea macrodonta Harris (part), Bull. Am. Paleontol- detail of scplpture. , The shell runs larger and 

ogy, vol. 1, No.4, p. 165, 1896. heavier than the tY[pical 0. macrodonta Whit.;. 

Cucullaea saffordi Harris (part), Bull. A · Paleontology, field, and the sculpture of the left valve, though 

vol. 1' No.

4

' P· 165, 

1896· discTepant, is regulf r and differs from that of 

Shell large, heavy, inflated, the ingle valves the right only in thl narrower ribs, wider interrudely quadrate ,in outline, the. d uble valves spac~s, and in its evelopment over a more 

cordate in cross section through le umbones; restricted portion f the shell. These . differumbones prominent, inflated, ove hanging the ences, though sligh , are fairly constant in the 

cardinal area, the tips incurved nd prosogy- imperfect n1aterial i hand. Oucullaea saffordi 

rate, slightly in advance of the me ian vertical; Gabb was describe from a shell 0.2 inch in 

length of hinge approximately fiv -sixths that latitude. This typ , according to Harris, has 

of the shell; anterior extremity ·a gulated dor- been lost. Until t~e young have been corresally, rounding broadly into the entral mar-: lated with adults a} the type locality and the 

gin; base line approximately hori on tal medi- species adequately ~escribed from such mateally; post,erior extremity obliqu y truncate; rial the characters · f 0. saffordi Gabb must be 

posterior area flattened, delimited by a narrow open to question. · here is need for uncommon 

but obtuse ridge with a shallows lcus behind caution in Hardej1an County, as the Upper 

it; valves differ.entially sculpturtd, t. he . right Cretaceous and Eo ene outcrops are very near 

valve exhibiting ab'out 30 low, road radials to one another andl were confused in some of 

separated by linear interspaces a d persisting the earlier eollectiol s. 

to the posterior keel, where they bruptly dis- 1 

· 

fin l Stations i Midway formation. appear; a very e, . c ose, and u~iform radial 

cording developed upon the posthrior area in 2440 (c). 4 miles east f Kemp, Kaufman County. 

some individuals, but commonly bsent, as in 2439 (r). 1 mile east o Webberville, Travis County. 

?6584 (r). D'Hanis-Y 1ncey road, about 7! miles east of the type, or evanescent toward the margin. south of D'Hanis, Medi a County. 

Left valve exhibiting only abo t 24 radials 3180 (c). Bluff on Fri River, half a mile below Myrick's 

distributed over the medial and nterior por- ·(Evans's) apiary, Uvald County. 
112 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922, 

583 (r). 18 miles southeast of Eagle Paas·; Maverick 

County. · . 

4398 (p). 18 miles southeast of Eagle Pass, Maverick 

County. 

6583 (p). Biboro Creek, just below Biboro tank, about 

18 miles southeast of Eagle Pass, Maverick County. 

6575 (c). White Bluff on Rio Grande, land of Indio Cattle Co., about 4! miles west of south of Jacal ranch house, 

in southeastern part of Maverick County. 

Family CARDITIDAE. 

Genus VENERICARDIA Lamarck. 

Venericardia (alticostata subsp.?) whitei Gardner, n. 

subsp. 

Plate XXXII, figure 3. 

Venericardia alticostata Harri~ (part), Bull. Am. Paleontology, vol. 1, No. 4, p. 171, pl. 5, :fig. 3, 1896. 

Not Venericardia perantiqua Conrad, Am. Jour. Conchology, vol. 1, p. 8, 1865. 

Shell small but thick and heavy, tranversely 

elongated in outline, rather compressed; umbones low, broadly rounded, for the most part 

strongly anterior in position; lunule minute; 

anterior extremity bowed in front of the lunule; 

posterior extremity obliquely produced and 

broadly rounded; base line feebly arcuate; surface coarsely sculptured radially; radials 

usually 19 to 21 in number, broad, crowded and 

heavily corrugated upon the anterior and medial portions of the shell; narrow and less 

closely spaced posteriorly; ligament characters 

not known; dentition apparently normal, very 

. poorly preserved in all the material available 

for study; adductor scars very distinct; pallial 

line simple; inner margins very coarsely crenate. 

Dimensions: Altitude, 25 millimeters; latitude, 31 millimeters; semidiameter, 13.5 millimeters. 

Type locality: Station 3180, Bluff on Frio 

River half a mile below Myrick's apiary, Uvalde 

County. 

The new subspecies differt5 from V. alticostata 

s. s. and from the subspecies hesperia in the 

simple radials. V. smithii is larger and relatively higher as a rule, with normally over 30 

radials instead of less than 25, as in whitei. 

Venericardia perantiq_ua was described under._ 

the name Gardita subq_uadrata by Gabb from 

the Eocene of New Jersey. The casts are very 

imperfect, but they indicate a much compressed, 

trigonal to subquadrate f<;>rm quite unlike anything reported from the Eocene of Texas or of 

the Gulf. 

The form is named in honor of Dr. Charles A. 

White, one of the foremost of the earlier paleontologists and one of the first to collect from the 

Texas Tertiary formations. 

Stations in .Midway formation. 

3180 (p). Bluff on Frio River half a mile below Myrick's 

(Evans's) apiary, Uvalde County. - · 

6279 (r). 11 miles south of Sabinal, a few hundred yards 

south of the junction of Elm Creek with Sabinal Creek, 

Uvalde County. . · 

4398 (r). 18 miles southeast of Eagle Pass, Maverick 

County. 

V enericardia alticostata subsp. hesperia Gardner, n. subsp. 

Plate XXXII, figures 1, 2. 

Venericardia alticostata Harris (part), Bull. Am. Paleontology, vol. 1, No.4, p. 171, 1896. 

Not Cardita alticostata Conrad, Am. Jour. Sci., 1st ser., 

vol. 23, p. 342, 1833. · 

Shell of moderate dimensions, thick, porcellanous, inflated-cordate in cross section; umbones prominent, well rounded, slightly anterior in position; lunule .very small, deep; anterior extremity strongly and evenly bowed in 

front of the lunule; posterior dorsal margin 

gently sloping, the posterior lateral margin obliquely truncate; base line feebly arcuate, 

strongly upcurved anteriorly, abruptly rounded 

posteriorly; gen,eral character of surface sculpture similar to that of V. alticostata s. s.; radials not far from 20 in number, normally tripartite on the anterior and medial portions of 

the shell as in V. alticostata~·· crest of :radials 

narrow and sharply serrate; lateral "terraces" 

well defined; inter-radial channels broadly Ushaped;· incremental sculpture very fine and 

sharp; heavy growth stages rarely developed; 

hinge and ligament characters poorly preserved; 

hinge ·plate thick, heavy; adductor scars impressed, the anterior excavated; pallial line 

simple; inner margins deeply crenate. 

Dimensions: Altitude, 38 millimeters; latitude, 37 millimeters; diameter, 44 millimeters. 

Type locality: Station 3180, Bluff on Frio 

River half a mile below Myrick's apiary, 

Uvalde County. 

The subspecies hesperia is shorter and wider 

than ·v. alticostata and more inflated in the umbonal region. In sculptural characters, however, the two forms are similar. 

Venericardia bulla Dall is smaller and relatively higher and has a higher and relatively 

heavier hinge plate and less nuinerous radials. 

In both V. smithii Aldrich and V. alticostata 

subsp. whitei the radials are simple. 

Stations in Midway formation. 

1/128 (p). 1! miles northwest of New Fountain,. Medina 

County. 

3180 (p). Bluff on Frio River half a mile below Myrick's . 

(Evans's) apiary, Uvalde County. 

J 
NEW SPECIES OF MOLLUSCA FROM SOUTHWESTERN T'EXAS. 113 

3181 (p). Frio River just above waterhole opposite 

apiary, below Englemann's ranch, Uvalde County. 

?6279 (r) (No. 274c). · 11 miles south of Sabinal, a few hundred yards south of the junction of Elm Creek with 

Sabinal Creek; Uvalde County. , 

?4398 (r). 18 miles southeast of Eagle Pass, Maverick 

County. 

6583 (p). Biboro Creek, just below Biboro tank, about 18 

miles southeast of Eagle Pass, Maverick County. 

6575 (p). White Bluff on Rio Grande, land of Indio Cattle Co., about 4-! miles west of south of Jacal ranch house, 

in the southeastern part of Maverick County. 

Family VENERIDAE. 

Genus CALLOCARDIA. 

Callocardia astartoides Gardner, n. sp. 

Plate XXXII, figures 4-7. 

Shell small for the genus, thick shelled, trigonal, ovate in outline, astartifortn, moderately 

compressed; umbones not conspicuous, the tips 

proximate and prosogyrate, slightly anterior; 

lunule rather wide, cordate, slightly depressed, 

and delineated by an incised line; escutcheon 

not defined; anterior extremity bowed slightly 

in front of the lunule; posterior end of shell 

obliquely truncate and feebly arcuate from the 

umbones to the basal margin; base line arcuate, more abruptly rounded posteriorly than 

anteriorly; external surface finely and evenly 

threaded concentrically; resting stages conspicuous, usually one strongly defined near 

the umbones and several not quite so prominent 

toward the base; a fine and regular radial lineation visible upon weathered surfaces; external 

ligament mounted upon a rather heavy nymph 

produced more than one-third of the length of 

the posterior dorsal margin; dentition robust; 

the laminar anterior cardinal of the right valve 

broken away; medial cardinal rather slender, 

cuneate; posterior cardinal produced; left anterior and medial cardinals united under the 

umbones to form an asymmetric V, the anterior 

cardinal slender, the medial cardinal relatively 

heavy and deltoid; posterior left cardinal 

broken away, doubtless very thin and laminar; 

a very s4ort obtuse lateral tooth developed in 

the left valve, received in a corresponding 

socket in the right; adductor scars distinct, the 

anterior the more prominent; pallial sinus produced almost to the median vertical, obtusely 

trigonal; inner margins entire. 

Dimensions: Right valv.e, altitude 16.0 millimeters, latitude 16.5 millimeters, semidiameter 

7.4 millimeters; left valve, altitude 17.0 millimeters, latitude -17.3 millimeters, semidiameter 

8.6 millimeters. 

Type locality: Station 8833, about 7 miles 

up the Rio Grande from Laredo, at Knob 

Bluffs, a quarter of a mile above pump of Santa 

Barbara farm, Webb County, Tex. 

Gallocard1:a astartoides is a remarkable species,. 

uniting the den.tition and sinal characters of the 

Veneridae with the external outline and sur-· 

face sculpture of certain of the Astartidae. The 

occurrence· oJ so primitive a type of a highly 

specialized group in the early Tertiary 'beds is 

of unusual interest. 

Gallocardia astartoides is the dominant species. 

at the type locality and common at a number 

of other localities in the Cook Mountain forma-· 

tion. 

Stations in Cook Mountain fo_rmation. 

?8850 (c). 500 yards southwest of Espejo ranch, 8 miles: 

south of Laredo, Webb County. 

8833 (a). About 7 miles up the Rio Grande from Laredo, 

in Knob Bluffs, a quarter of a mile above pump of Santa 

Barbara farm, Webb County. · 

8768 (c). 1! miles -north of and a trifle east of the third 

gate, 7 miles southeast of Velenzuela ranch house, Webb 

County. · 

8770 (c). 39.9 miles north of Rio Grande City, Starr 

County, on river road to Zapata, just north of Lopeiio post 

office, Zapata County. 

Family TELLINIDAE. 

Genus TELLINA (Linnaeus) Lamarck. , 

Tellina sp. A. 

Plate XXXII, figures 8, 9. 

Shell large, not very thin, much compressed, 

transversely ovate-trigonal in outline, nearly 

equilateral; umbones flat, inconspicuous except 

by reason of their position at the apex of an 

angle of not far from 120°; anterior and posterior dorsal margins nearly similar, the posterior declining, perhaps, a little more steeply; 

lateral extremities quite sharply rounded; base 

line symmetrically arcuate; · an inconspicuous 

rostral fold developed near the posterior 

dorsal margin, defined rather by the abrupt 

change in the direction of the incrementals than 

by its elevation; surface sculpture incremental 

in character, least feeble posteriorly and toward 

the basal margin, becoming abruptly stronger 

l?ehind the rostral fold and indicating by the 

undulation a second very obscure fold; characters of ligament and hinge not known; adductor scars commonly quite prominent;. pallial 

sinus obscure but apparently very deep, produced almost to the anterior adductor, not 

confluent below. 
114 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

The species is decidedly larger than any other 

found in the Eocene of Texas. A latitude of 

40 millimeters is not uncommon and one 

cast, presumably identical, reaches an altitude 

of 40 with a probable latitude of 60 millimeters 

or more. Unfortunately, only casts, some of 

them with a few fragments of shell adhering, 

are preserved. 

Stations in Cook Mountain formation. 

8771 (p). 4 miles southeast of Laredo, Webb County. 

6404 (p). 1 mile west of Canada Verde ranch, Webb 

County. 

6436 (c). · La Perla ranch below Laredo, Webb County. 

?8837 (r). 3~ miles above San Ygnacio, Zapata County. 

?8770 (p). Lopeno post office, Zapata County. 

Family PHOLADIDAE. 

. Genus MARTESIA Leach. 

Martesia? Iaredoensis Gardner, n. sp. 

Plate XXXII, figure 10. 

Shell very thin, transversely elongated, tapering posteriorly, strongly inflated; anterior third 

of shell cut off by a deep but narrow groove, 

dropped from the umbones and extending 

obliquely backward; umbones low, conforming 

to the tubular outline of the shell, prosogyrate; 

anterior extr0mity rounding smoothly into the 

upcurved base; posterior dorsal and ventral 

margins slowly converging, the narrow lateral 

extremity vertically truncate; shell usually 

decorticated but so thin that the sculpture is 

reflected on the cast; a c~:mcentric sculpture, 

incremental in character and more or less irregular, developed over the entire shell hut stronger 

upon the anterior third; radials approximately 

equal in strength to the concentric rugae and, 

like therri, somewhat irregular, developed only 

in front of the groove; charact.ers of the hinge 

and of the interior not known. 

Dimensions: Altitude, 9.0 millimeters; iatitude, 20.0 ± millimeters. 

Type locality: Station 6434, a quarter of a 

mile south of Espejo ranch, south of Laredo, 

in Webb County, Tex. 

Martesia texana Harris is similar to this 

species in outline and general dimensions, but 

the umbones of the Cherokee County species 

are not so near to the anterior extremity, the 

groove cuts off decidedly more than the anterior third, and there is an apparent absence 

of any radial sculpture. No trace of either 

M." texana or M. laredoensis has been found 

except at the type localities. 

Station in Cook Mountainformation. 

6434. A quarter of a mile south of Espejo ranch, south 

of Laredo, Webb County. 

Family TEREDINIDAE. 

Genus TEREDO Linnaeus. 

Teredo maverickensis Gardner, n. sp. 

Tubes of moderate' dimensions, somewhat 

irregular in growth habit though tending to follow the grain of the wood; closely and quite 

sharply rugose; characters of valves not known. 

Dimensions: Diameter, 6.0 millimeters; thickness of shell, 0.6 millimeters. 

Type locality: Station 1/277, Rio Grande, 

lower end of Maverick County, about 40 feet 

below Midway-Wilcox contact . 

This teredo, which packs a large fragment of 

fossilized wood, is the first that has been reported from the Midway of the Gulf. It differs {rom 

the later Eocene teredos in the less regular 

growth habits and particularly in the very 

decided, close, sharp wrinkling of the surface. 

The value of these differences is increased by 

the difference in the habitat: It is interesting 

to find a record· of the unfortunate appetite 

of this animal in the Midway of Texas. 

Stp,tion in Midway formation. 

1/277. Rio Grande, lower end of Maverick County, 

about 40 feet below Midway-Wilcox contact. 

GASTROPODA. 

Family CERITHllDAE. 

Genus CERITHIUM Bruguiere. 

Cerithium? sp. A. 

Plate XXIX, figures 1, 2. 

Internal casts of Oerithium-like forms occur 

in considerable numbers in southwestern Texas 

and serve as excellent horizon markers of the 

Midway. These casts indicate a rudely biconic 

shell with the maximum diameter in front of the 

median horizontal. The whorls of the conch 

were probably about 6 or 7 in number, rudely 

trapezoidal in outline and increasing quite 

rapidly in diameter. The body was smoothly 

rounded in the adults, though probably rather 

acutely angulated in the young. Some individuals show a suggestion of an irregular 

Oerithium-like ribbing. The aperture was ap- . 

parently between one-third and one-half as 

long as the entire shell and was obliquely 

lenticular in outline. A specific name · is 

reserved in the hope that the shell may come 
NEW SPECIES OF MOLLUSCA FROM SOUTHWESTER~ TEXAS. 115 

to light later. In the meantime these easily 

recognizable casts may be of service . in indicating the Midway age of the formation in which 

they are carried. 

Dimensions of imperfect figured specimen: 

Altitude, 42.0 millimeters; maximum diameter, 

21.0 millimeters. 

Stations in Midway formation. 

3180 (p). Bluff on Frio River, half a mile below Myrick;'s (Evans's) apiary, Uvalde County. . 

6576 (c). Land of Indio Cattle Co., It miles below 

White Bluff, about 4! miles southwest of Jacal ranch 

house, Maverick County. 

8799 (p). Texas side of the Rio Grande 1 mile below 

the Blesse ranch house, Maverick County. 

CEPHALOPODA. 

Family CLYDONAUTILIDAE Hyatt. 

Genus ENCLIMATOCERAS Hyatt. 

Enclimatoceras vaughani Gardner, n. sp. 

Plate XXXIII. 

Shell large, rather compressed toward the 

apex, more broadly rounded ventrally toward 

the aperture, obliquely flattened laterally; 

whorls numerous, increasing but slowly in 

latitude; the altitude of the earlier whorls a 

little more than th~ee times their average width 

from suture to suture, of the later whorls a little 

less; final whorl of a half grown specimen 

rudely reniform in outline, somewhat auriculate laterally, concave, the diameter of the 

whorl approximately double the altitude; 

umbilici quite str-ongly depressed, -their peripheries obscurely carinate; ventral saddle very 

broad and nearly horizontal; lateral lobes 

broad a.nd very shallow; lateral saddles relatively narrow and moderately deep; siphuncle 

dorsad, migrating slowly toward the center 

with increasing age; surface not known. 

Dimensions: Maximulll: diameter of shell, 

168.0 ± millimeters; diameter of shell at 

right angles to maximt~;m diameter, 140.0 ± 

millimeters; thickness 100.0 millimeters. Adolescent individual: Maximum diameter of shell, 

93.0 ± millimeters; diameter of shell at right 

angles to maximum diameter, 64. 0 ± millimeters; maximum thickness, 73.0 ± millimeters. A larger but imperfect individual 

attains a maximum diameter of 220.0 millimeters. 

Type locality: Station 3178, three-fourths 

mile northwest of Myrick's (Evans's) apiary, 

Frio River, Uvalde County. 

Enclimatoceras vaughani is the Texas ana-. 

logue of Enclimatoceras ulrichi (White), of the 

Midway of Alabama, a form very similar in 

~eneral dimensions and outline. The adult 

E. vaughani are, as a rule, more compressed 

than the adult E. ulrichi, but the young of the 

two species show similar ranges of variation. 

The umbilici also seem to be a little more 

depressed in the Texas species. The most 

obvious and constant difference, however, is 

in the outline of the lobes and saddles .. The 

sutures in E. ulrichi are much more flexu,ous 

than those of E. vaughani. In E. vaughani th~ 

ventral saddle is nearly horizontal and the 

lat.eral lobes exceedingly shallow: In E. ulrichi the ventral saddle is perceptibly arcuate 

and the lateral lobes are strongly incurved. 

The lateral saddles are well developed in both 

species, though they are deeper in the Texas 

form. 

I have the honor of dedicating this species 

to Dr.1 T. Wayland Vaughan. 

Stations in Midway formation. 

3178 l(p). Three-fourths mile northwest of Myrick's 

(Evans's) apiary, north of Frio River, Uvalde County. 

3179 (p). Ha~f a mile northwest of Myrick's (Evans's) 

lower apiary,· south side ,of Frio River, Uvalde County. 

Frio River half a mile above Bob Evans's (formerly 

Myrick's) apiary, Uvalde County. 

/ . 

PLATES XXIX-XXXIII. 

I 

I 

. ' 

) \ 117 

U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 131 PLATE XXIX 

1 

5 

3 4 

6 

EOCENE MOLLUSCA FROM SOUTHWESTERN TEXAS. 

1, 2. Cerithium? sp. A. 1, Dorsal view, X 1; 2, apertural view, X 1. 

3. Nucula sp. Exterior of right valve, X 1. 

2 

7 

4, 5. Cucullaea ( Macrodonta subsp. ?) texana Gardner, n. subsp. 4, Front view, X 1; 5, exterior of left valve, X 1. 

6, 7. 08trca alabamien8i8 subsp. georgiana Conrad. 6, Exterior of left valve, X 1; 7, interior of left valve, X 1. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 131 PLATE XXX 

EOCENE MOLLUSCA FROM SOUTHWESTERN TEXAS. 

Ostrea alabamiensis subsp. georgiana Conrad. Exterior of right valve, X 1. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 131 PLATE XXXI 

1 

2 

3 4 

EOCENE MOLLUSCA FROM SOUTHWESTERN TEXAS. 

1-4. Ostrea tasex Gardner, n. sp. 1, Exterior of left valve, X 1; 2, interior of left valve, X 1; 3, exterior of right valve, X 1; 4, interior 

of right valve, X 1. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 131 PLATE XXXII 

3 

1 9 

8 

2 

4 6 

5 7 11 

EOCENE MOLLUSCA FROM SOUTHWESTERN TEXAS. 

1, 2. Venericardia alticostata subsp. hesperia Gardner, n. subsp. 1, Front view, X 1; 2, exterior of right valve, X 1. 

3. Venericardia alticostata subsp. whitei Gardner, n. subsp. Exterior of right valve, X 1. 

10 

4-7. Callocardia astartoides Gardner, n. sp. 4, Exterior of right valve, X 2; 5, interior of right valve, X 2; 6, exteriorof left valve, X 2; 

7, interior of left valve, X 2. 

8, 9. Tellina sp. A. 8, Internal cast of left valve, X 1; 9, internal cast of left valve, X 1. 

10. lorfartesia laredoensis Gardner, n. sp. External cast of right valve, X 2. 

11. Teredo maverickensis Gardner, n. sp. Longitudinal views and natural cross sections of tubes in fossil wood, X 1. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 131 PLATE XXXIII 

EOCENE MOLLUSCA FROM SOUTHWESTERN TEXAS. 

1-3. Enclimatoceras vaughani Gardner, n. sp. 1, Lateral view X 1; 2, cross section of adolescent individual, X 1; 3, apertural view of adolescent 

individual. X 1. 

PRELIMINARY REPORT ON FOSSIL VERTEBRATES OF THE SAN PEDRO 

VALLEY, ARIZONA, WITH DESCRIPTIOKS OF NEW SPECIES 

OF RODENTIA AND LAGOMORPHA. 

By JAMES w. GIDLEY. 

INTRODUCTION. 

On the initiative of the United States Geological Survey and with th~ cooperation of the 

United States National Museum, I was privile ed during February, March, and . part of 

April, 1921, to collect fossil vertebrates in the 

San Pedro Valley, Ariz., where remains had previously been located by Kirk Bryan, of the 

Geological Survey. The results of this expedition are here published in part. 

The special problem involved was the determination of the age of the sedimentary deposits, 

which up to that time had been termed PleistoGene. Two localities, a,pparently at slightly 

different . horizons, yielded material representing a fauna sufficiently varied and distinctive 

to establish their age as Pliocene. 

The locality first visited is a small area of 

badlands about 3 miles east of the Curtis ranch ' which is on the State road about 14 miles 

northwest of Tombstone and an equal distance 

southeast of Benson by road. The second 

locality is west of the State road about .2 miles 

south of Benson. These two localities are 

about 12 miles apart and on opposite sides of 

the San Pedro Valley. The first locality 

yielded by far the greater bulk of material, 

but the species represented in the collection 

from the second are more numerous. Stratigraphically there seems to be no difference in 

the level of the beds of the two localities and ' structurally they are very similar, yet, so far 

as I am able to determine, they contain. no 

·species in common. Because of this fact as ' 

well as the general character and assemblage 

of the forms represented, I am led to consider 

them as of slightly different age, the beds at 

the Benson locality being apparently the older. 

Most of the mammal material is fragmentary, 

but several good specimens were procured, 

including enough material for the restoration 

of the dermal armor and skeleton of a Glyptotherium and a possible skeletal restoration of 

a mastodon, both from th~ Curtis locality. 

The material from the Benson ·locality .here 

described was all collected at one spot, a fossil 

bone quarry opened up and worl~ed by Kirk 

Bryan and me. Immediately associated material from this quarry includes fragmentary specimens of Hipparion, Pliauchena?, Platygonus, 

and Pliohippus or Hippidium?, together with a 

new species of h?x turtle and eight new species 

·of birds, not all of which are satisfactorily 

definable from the scanty material obtained. 

The greater part of the specimens included 

in these collections have not yet been prepared, 

but those belonging to the orders Rodentia 

and Lagonl.orpha have been cleaned and 

studied, and the new species are here described. 

Those from the Curtis locality were all found · 

during the excavation of one of the mastodon 

skeletons. Immediately associated with them 

were remains of a llama, a canid, a mustilid, 

a small land turtle, an edentate, and two 

species of small birds. 

A tentative list of the orders, families, 

gener!1, and species, so far as they have been 

determined, represented in each locality · is 

given on page 120. 

119 
120 SHORTER CONTRJBUTIONS TO GENERAL GEOLOGY, 1922. 

Preliminary list of fossil vertebrates from the San Pedro Valley, Ariz. 

Number of species. Number of species. 

Benson. Curtis. Benson. Curtis. 

--- ---

MAMMALIA. MAMMALIA-COntinued. 

Pro boscidia: Lagomorpha: 

Elephantidae: Leporidae: 

Of. Dibelodon, probably n. sp. ___ 2 L epus, 3 sp., possibly new ....... 2 1 

Cf.Gompotherium, probably n. sp. 1 Sylvilagus or Brachylagus sp ...... 1 ···-·-· Perissodactyla: · Edentata: 

Equiidae: Glyptodontidae: 

Equus, undet ................ _ ... ................... 2 Glyptotherium, n. sp ............. .. ........... 1 

Pliohippus, undet .............. 1 ................ ------ Hipparion, undet. .............. 1 ................. Total Mammalia .......... 23 22 

Artiodactyla: ------ Camelidae: REPTILIA. 

Lama, n. sp ..................... 1 

Of. Procamelus, undet ........... 1 1 Testudines: 

Of. Pliauchenia, undet .......... 1 1 Testudinidae: 

Tayassuidae: Testudo, 2 sp. undet . ........... 1 1 

Platygonus cf. P. vetus, or n. sp .. 1 ................. Kinosternodae: 

Cervidae: . Kinosternon, new sp ........... 1 / 1 

Odocoileus, n. sp ................ ................. 1 Batrachia: 

MerJtcodus near M. necatus ...... . 1 1 Salientia: 

Of. erycodus, n. sp ............. 1 Ranidae: 

Carnivora: Rana or Bufo, not yet deterCanids, 3 n. sp ..................... 1 2 Inined .................... 1 ............ Felid?, n. sp ........................ 1 ------ Mustilid, sp ......................... 1 1 Total Reptilia ........ 3 2 

Rodentia: ------ Sciuridae: AVES. 

Citellus cochisei, n. sp ............ ................. 1 

Citellus bensoni, n. sp ............ 1 Galliformes: 

Geomyidae: . Odontophoridae: 

Geomys parvidens, n. sp .......... 1 Undeterinined ... , ............. .................... 1 

Geomys minor, n. sp ............ 1 ............. eolinus sp. undet .. ----.-. 0 0 0. 0 0 1 .. ........... Cratogeomys bensoni, n. sp ...... 1 .................. Fainily undet ....................... 1 .. .............. 

Heteromyidae: Anseriformes: 

JJipodomys minor, n. sp .......... 1 ............. Anatidae: 

Muridae: Dendrocygna, n . sp ............... 1 - --...... - Subfainily Cricetinae: Brant a, n. sp .................... 1 - ........... - Peromyscus brachygnathus, Charadriiformes: " 

n. sp .................... 1 .. .............. Scolopacidae: 

P eromyscus minimus, n. sp .. 1 ................ Micropalama, n. sp .............. 1 - .......... - Peromyscus sp .............. 1 .................. Family undet., medium-sized form .. 1 .. ............... 

Eligmodontia arizonae, n. sp. 1 ................. Columbiformes: 

Onychomys pedroensis, n. sp. 1 Fainily undet. A pigeon somewhat 

Onychomys bensoni, n. sp ... 1 smaller than the domestic dove .... ................ 1 

Sigmodon curtisi, n. sp ...... 1 Passeriformes: I 

Sigmodonm~nor, n. sp .. _ ... '1 Fringillidae: 

Sigmodonmedius, n. sp .. _ . . 1 ................... Small sp., undet ............ . ... 1 ------- &ubfainily Neotomeniae: Other passerines represented by 

Neotomafossilis, n. sp ....... 1 .............. scanty material.. ....... •..... . .... 1 1 

Subfamily Microteinae: ------ Neojiber sp ....... 0 •• 0. 0 ••• 0 ................... 1 Total Aves ..... 0 •••••• _ ••••••••• 8 3 I 

The reptilian remains, including at least GEOLOGIC HORIZON AND CLIMATIC CONDITIONS 

two species of turtles, probably new, are INDICATED. 

being described by C. W. Gilmore, and the This preliminary study reveals littie positive 

bird bones are being studied and will be des- evidence regarding the exact geologic horizon 

cribe.d by Alexander Wetmore, of the Biolog- of the beds in San Pedro Valley, beyond what 

ical Survey, Department of Agriculture. The is stated' above. They seem to be clearly 

tentative lists of Reptilia and Aves given Pliocene? and the peculiar mingling of modern 

above are furnished by these gentlemen. and more ancient forms seems to point rather 
FOSSIL VERTEBRATES OF THE SAN PEDRO VALLEY, ARIZONA. 121 

definitely to a late stage of Pliocene time. 

Unfortunately, the material of these collections represents for the most part new species 

that can not be correlated with known faunas 

of other localities where the age of the b~ds 

has been established. However, the species 

are sufficiently numerous and varied to constitute a standard for comparison;' and future 

field work in this region will almost certainly 

bring to light material that will' defini~ely and 

-correctly determine the proper. sequence of 

these deposits. 

The facts that there are here two or three 

species of true horse (Eq_uus) and that the 

rodents are all referable to modern genera, 

some of which are found living in this general 

region, may suggest Pleistocene age. But 

to offset this evidence of a seemingly more 

modern fauna, the mastodons and glyptodonts are of distinctly Pliocene types, and one 

<>f the canids, though modern in skull and 

dental modifications, has the type of humerus 

belonging to the Miocene and Pliocene canids, 

in that the entepicondylar foramen is prominently present. This feature and the pres-

-ence of Hipparion, Pliohippus, and Merycodus, 

together with the fact that the 15 or more 

species of rodents include no living species, 

-constitute rather conclusive evidence of a 

faunal assemblage that is older than Pleistocene. 

This general assemblage of species, including 

:ptoboscidians, camels, horses, glyptodonts, 

and an extinct species of the ocellated turkey 

:related to a group now living only in Central 

.America and southern Mexico, suggests that 

the climate at the time the beds were laid 

down was rather warm and moist, probably 

. :subtropical or even tropical. 

The presence of a true llama, the glyptodont, and a rodent belonging to a genus now 

Jiving only in South America seems to indicate 

an intermingling of forms of South American 

·origin, and the presence in these Pliocene 

deposits of a rodent and camel of definite 

South American living types suggests the 

derivation of their South American relatives 

from North America and favors the suggestion 

of an exchange of faunas· between the two 

.continents which took place somewhere 'near 

this· epoch of geologic time. 

DESCRIPTIONS OF SPECIES. 

Order RODENTIA. 

The order Rodentia is represented in the San 

Pedro Valley by 17 species, 11 of which occur 

at the Benson locality and 6 at the Curtis locality. Of these species, 15 are here described as 

new, and 2 are not certainly determined on the 

scanty material in hand. However, though 

all the species represented seem to be new, each 

is readily referable to one of ten living genera, 

distributed among four families-the Sciuridae, 

Geomyidae, Heteromyidae, and Muridae. The 

species belonging to the Muridae represent three . 

subfamilies-the Cricetinae, N eotomeniae, and 

Microteinae. 

Family SCIURIDAE. 

The family Sciuridae is represented by two· 

new species, both belonging to the genus 

Oitellus. 

Citellus cochisei Gidley, n. sp. 

Plate XXXIV, figures 1, 2. 

Type.-Portion of a right maxillary containing all the cheek teeth (catalog No. 10490, U.S. 

N.at. Mus.). 

Paratype.-Portion of a left lower jaw (catalog No. 10491). 

Locality.-Both are from the Curtis locality, 

in sec. 25, T. 18 S., R. 21 E., and were found in 

exhuming a mastodon skeleton. 

Description.-Length of upper cheek-tooth 

series 10.5 millimeters. Molars relatively wide, 

m1 and m2 one-third wider than long; lophs 

and valleys simple and narrow; both posterior 

transverse lophs of the molars completely 

united with protocone, forming a short, narrow 

valley opening outward and extending inward 

not more than one-half the width of the tooth 

crown, as in Oynomys. The anterior transverse lophs are more depressed and except in 

m3 are much longer than the other lophs, extending inward and upward to disppear in the 

anterior wall of the protocone. P 4 differs 

from the anterior two molars only in being less 

wide and in having_ the anterior loph relatively 

and actually more extended anteriorly. 

In the greater width of tooth crowns, the less 

extent of their median external reentrant valleys, and the relative shortness of the heel of 

the last molar, this species suggests Oynomys, 

' i 
122 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922'. 

but it differs from species of that genus in the 

much more brachyodont tooth crowns, he 

greater relative depth of the median external 

reentrant valleys, and the incipiency, amounting to almost total absence, of the posterior reentrant valleys so prominent in species of 

Gynomys. The first of these features might be 

considered as a character of degree only, indicating · less advanced specialization, but the 

other two I consider characters that denote 

relationship with the living species of Gitellus 

rather than with those of the genus Gynomys. 

The lower jaw, as indicated by specimen No. 

· 10491, which carries the incisor and the anterior two cheek teeth, is relatively short and 

deep, the incisor narrow and pointed, and the 

cheek teeth relatively wide to a degree cor-

. responding with those of the upper series. 

This species compares in size with G. evermani, 

but it differs from all the living forms in one or 

more of the characters enumerated above. 

Citellus bensoni Gidley, n. sp. 

Plate XXXIV, figures 3, _4. 

Type.-A first or second upper molar of the 

right side and a last upper molar of the left 

side (catalog No. 10531, U.S. Nat. Mus.). 

Paratype.-The cheek-tooth series of a left 

lower jaw (catalog No. 10532, U.S. Nat. Mus.). 

Locality.-Both from the Benson locality. 

Description:-L~ngth of cheek-tooth series 

(estimated . from measurements of the paratype), 10.3 millimeters; upper molar of about 

the same proportion as the corresponding one 

of G. cochisei at the base but much narrower 

at the summit, owing to the greater slope of the 

inner wall of the protocone; the three transverse lophs about equal in length, the posterior 

·one being broken up into two distinct but 

slightly joined cuspules, the inner one of which 

is a rounded cone entirely disconnected from 

the protocone. 

An upper m3 which I associate with the type 

specimen, like that of G. cochisei, has arelatively small heel portion . as compared with 

living species, and in addition there is a welldefined isolated cone-shaped cuspule near the 

center of the posterior basin. 

This species seems to approach 0. beecheyi 

much more closely than any other living species but differs from it in a few apparently 

important particulars. In G. beecheyi the 

crowns of the upper molars are relatively narrower than in the fossil species, being nearly 

as long as wide; the lingual wall of the protocone is less sloping, and the exterior or buccal 

face of this cusp is much more sloping. Other 

differences noted in the living species are the 

generally less broken up condition of the posterior transverse lophs; the somewhat shallower transverse valleys, the posterior one of 

which usually has a small cuspule at its external 

entrance; and the relatively broader and less. 

completely inclosed posterior valley of the last 

upper molar. 

Beyond the fact that the last lower molar,. 

like the upper, is relatively smaller, and the 

valleys and cusps are somewhat more sharply 

defined than in G. beecheyi, there is little to 

distinguish between the lower cheek teeth of 

that species and G. be~soni. 

Family GEOMYIDAE. 

Three new species . referable to two living 

genera of geomids ~re represented in the collection. 

Geomys parvidens Gidley, n. sp. 

Plate XXXIV, figures 5, 6. 

·Type.-The anterior portion of a skull carrying the incisors and all the cheek teeth except 

the first and last of the right side (catalog No. 

10492, U. S. Nat. Mus.). 

Paratype.-Portion of the right lower jaw 

carrying all the teeth except m3 (catalog No. 

10493, U.S. Nat. Mus.). 

Locality.-Both from the Curtis locality. 

Description.-Length of upper cheek-tooth 

series 5.5 millimeters; width across first upper 

molars (measured. to outer borders) 6.7 millimeters; size small, about the same as that of 

Geomys texensis,· rostrum relatively short and 

heavy; incisors with two very unequal grooves, 

the smaller one close to the inner border of the 

crown, the larger one almost exactly bisectingits anterior face, making the outer of the three . 

ridges thus formed slightly but definitely largest; 

the cheek teeth are relatively small . and not 

greatly expanded laterally, m1 and m2 being 

almost evenly elliptical in outline but with the 

outer edges slightly compressed; p4 with anterior lobe decidedly smaller and less laterally 

expanded than the posterior one; m3 as wide 

as long, subtriangular, relatively small. 

Besides being about equal in size to G. texensis, G. parvidens is almost identical with that 
FOSSIL VERTEBRATES OF THE SAN PEDRO VALLEY, ARIZONA. - 123 

species in the character of the incisors, but it 

differs from G. texensis and the other living 

sp~cies of the genus in general in the less lateral 

expansion of the cheek teeth, the shorter and 

deeper rostrum, and the less progressive character of p4 • 

The lower jaw referred to this species is 

relatively short, with prominent masseteric 

ridge and the submental foramina lying almost 

directly under the anterior end of the masseteric 

ridge. The anterior lobe of p4 is relatively 

small and subcircular, with the walls very little 

flattened. 

p4 with outer reentrant angle rather widely 

V-shaped, the inner one forming a distinct U; 

posterior wall of last molar in the type specimen 

with two distinct and two faint parallel longitudinal grooves forming three low but distinct 

closely grouped median longitudinal enamel 

ridges. , 

This species differs from all living species of 

geomids examined in having the area of the 

anterior porti'on of the masseteric ridge more 

depressed and in the position of the anterior 

submental foramen, which is nearly as in the 

two above-described species of Geomys. In 

Geornys minor Gidley, n. sp. living species this foramen is anterior to the 

Plate XXXIV, figures 7, 8. masset~ric ridge and nearly level with it. In 

-the fossil species it is lower and more posterior Type.-Portion of a right lower jaw lacking 

th I t 1 ( t I N 10494 U S N t in position, lying almost directly under the 

e as mo ar ca a og o. , . . a . anterior extremity of the masseteric ridge. 

Mus.)· There is also between the cheek teeth and the Paratype.~A right upper incisor (catalog anterior border of the coronoid process in 0. 

No. 10534, U.S. Nat. Mus.). b · b d' · 1 k d 1 · r;,nson~ a narrow u t 1st1nct y mar e ongi- Locality.-Both from the Benson locality. tudinal fossa. 

Description.-Length of cheek-tooth series This species is referred to Oratogeomys on the 

(estimated) 5.4 millimeters; the jaw is slightly ~vidence of an associated upper incisor of appro- - smaller than _ that of G. parvidens and differs priate size which has the single groove char_ac- frorn.it otherwi(Se in having the anterior masseteric area more depressed, the submental teristic of this genus. 

:foramen slightly farther back in position, the 

cheek t~eth relatively larger, and the anterior 

lobe of p4 more nearly circular in outline; no 

anterior fossa between cheek teeth and coronoid 

process. 

An associated upper incisor tooth is of appropriate size for the species It shows the characteristic grooving of .a true Geomys, and it is 

this fact which has determined the generic 

reference of G. minor. 

Cratogeornys bensoni Gidley, n. sp. 

Plate XXXIV, figures 9-11. 

Type.-A portion of a left lower j,aw carrying 

all teeth (incisors broken) (catalog No. 10495, 

U. S. Nat. Mus.). 

Paratype.-An upper incisor (catalog No. 

10496, U.S. Nat. Mus.). 

Locality.-Benson locality. The species is 

represented by four other pieces of the lower 

jaws from the same locality. . 

Description.-Lengt)l of cheek-tooth series 8 

millimeters (measured at summits of teeth). 

About the- size of the living species Geomys 

brevicepsJ· cheek teeth not greatly compressed; 

Dipodornys minor Gidley, n. sp. 

Plate XXXIV, figure 16 

Type.-A nearly complete right lower jaw 

containing the incisor and the first cheek tooth 

(catalog. No. 10499, U. S. Nat. Mus.)'. 

Locality.--Benson locality. 

Description.-Length of cheek-tooth row, 

measured from extreme borders of the alveoli, 

4.7 millimeters; size and general jaw characters 

about as in the living species Perodipus chapmani, differing only in its more _slender proportions and in the somewhat m'ore reduced 

condition of the coronoid process. The fourth 

lower premolar, the only one of the cheek-teeth 

series present, is a long crowned and anteriorly 

curved tooth which is double-lobed, the anterior lobe having an anterior reentrant angle as 

in P. chapmani, but this tooth differs from the 

tooth of that species in having the posterior 

lobe relatively wider. Also the alveolus of the 

last molar is only about one-half the width of 

that of the other molars, indicating that this 

tooth is relatively more reduced than in the 

living species~ 
124 SHORTER CONTRI~UTIONS TO GENERAL GEOLOGY, 1922. 

Family MURIDAE. 

The family Muridae is rather abundantly 

represented at both localities. The 12 determinable species are distributed among three· 

subfamilies, 10 belonging to the Cricetinae, 1 

. to the Microtinae, and 1 to the N eotomeniae. 

Genus PEROMYSCUS. 

Specimens found at the Benson locality are 

referable to three species of Peromyscu~. Two 

of these are new, although one is apparently 

closely related to the P. taylori group. The 

other new species is more distinctive. The 

third, represented by a single lower jaw lacking the cheek teeth, has not been determined. 

Peromyscus brachygnathus Gidley, n. sp. 

Plate XXXIV, figure 12. 

Type.-The greater portion of a right lower 

jaw c.arrying all the teeth (catalog No. 10501, 

U.S. Nat. Mus.). 

Locality. -Curtis locality. 

Description.-Length of cheek-tooth series, 

2.8 millimeters; about the size of P. tayloriJ· 

jaw relatively short anterior to cheek teeth; 

last molar very much reduced, as. much as in 

any living species of Onychomys. The teeth 

are too much worn to determine accurately 

their normal height in unworn condition, but 

they appear to be of the depressed type of P. 

taylori. 

The form and position of the anterior extension of the masseteric ridge, the general character of the coronoid region, and the relatively 

broadly expanded anterior lobe of the anterior 

cheek tooth seem to determine the generic reference of this species, but it differs from aU 

living species of the genus in the relatively 

short jaw and the greater reduction of the last 

lower cheek tooth, which seems to have nearly 

or quite lost its hinder lobe. 

Peromyscus minimus Gidley, n. sp. 

Plate XXXIV, figt.Ire 13. 

Type.-Portion of a left lower jaw carrying 

all the teeth (catalog No. 10500, U. S. Nat. 

Mus.). 

Locality.-Benson locality. 

Description.-Size very small, length of cheektooth series 2.6 millimeters; cheek-tooth cusps 

depressed, wit):! well-marked cingula at the entrance of the external valley, and with tendency to inclose the internal ones by an uprising of the inner enamel wall between the 

cusps; anterior lobe of the first cheek tooth, 

p4, relatively narrow but two-cusped at the 

summit; posterior lobe of last cheek tooth 

small, almost as much reduced as in Onychomys . 

This species seems more nearly related to 

P. taylori than to any other living species but 

differs from it in having a narrow divided anterior lobe of the first cheek tooth and a more 

reduced hinder molar. It further differs from 

that species and more nearly agrees with others 

of the genus in the wider angle at which the 

coronoid portio'n of the ascending ramus diverges from the alveolar portion of the jaw. 

P. minimus is distinguished from P. brachygnathus in that the jaw is somewhat more 

slender in proportions; the masseteric ridge is 

less advanced forward; the · cheek teeth are 

slightly narrower; the anterior lobe of the first 

cheek tooth is narrow and double cusped, while 

that of P. brachygnathus is wider and undivided, 

and the .last cheek tooth is somewhat less 

reduced. 

Peromyscus sp. 

Plate XXXIV, figure 14. 

A second and larger species, about equaling 

Peromyscus m. gambeli in size, is represented by a 

lower jaw (catalog No. 10502, U.S. Nat. Mus.) 

lacking all the cheek teeth. Length of cheektooth serie~ as estimated by measurement of 

the alveoli, 3.8 millimeters. This jaw, in its 

general form and in the position of the anterior extremity of the masseteric ridge, is 

sufficiently characterized to make certain its 

generic reference, but its specific features can 

not be determined until the cheek teeth are 

known. 

Genus ELIGMODONTIA. 

The genus Eligmodontia, the living species 

of which are confined to the South America 

continent, is represented by a single species in 

the fossil collection from Arizona. 

Eligmodontia arizonae Gidley, n. sp. 

Plate XXXIV, figure 15. 

Type.-Greater porti~n of a left lower jaw 

containing the complete dentition (catalog 

No. 10503, U.S. Nat. Mus.). 

Paratype.-Single lower jaws of two other 

individuals (catalog Nos. 10504, 10505). 

Locality.-All from Benson locality. 
FOSSIL VER,TEBRATES OF THE SAN PEDRO VALLEY, ARIZONA. 125 

Description.-Length of cheek-tooth series 

3.6 millimeters; about the size of. the living 

. species E. morgani, but may be distinguished 

from it by its relatively greater depth of jaws, · 

its decidedly shorter symphysis, and , the less 

reduced condition of the last molar. 

The anterior lobe of the first cheek tooth is 

distinctly notched by an anterior ~edian reentrant angle, as is usual in Peromyscus, whereas in the living species of Eligmodontia this 

lobe is usually evenly convex in front. The 

fossil species from Arizona, however, is readily 

distinguished from Peromyscus by the characters which ally it to Eligmodontia, namely, 

by the form and position of the masseteric 

ridge, which extends forward to the extrem.e 

anterior border of the cheek-tooth series; by 

the form and situation of the capsular process 

for the reception of the base of the incisor, 

which is placed high upon the ascending ramus 

and is marked by a decided sulcus between it 

and the coronoid; and by the form of the last 

cheek tooth, which in species of Eligmodontia 

is more reduced than in Peromyscus. 

Onychomys pedroensis Gidley, n. sp. 

Plate XXXV, figure 1. 

Type.-Portion of a left lower jaw carrying 

the first and last cheek teeth (catalog No. 

10506, U.S. Nat. Mus,). 

Paratypes.-Other lower jaw portions (catalog Nos. 10507, 1Q508, U.S. Nat. Mus.). 

Locality.-All from Curtis ·locality. 

Description.-Length of · the cheek teeth 

4.5 millimeters; size somewhat larger than 

0. leu. ruidosae, which it so~ewhat more 

nearly resembles than any other living species. 

It further differs from that species in its relatively greater depth of jaw, the greater degree 

to which the masseteric ridge is carried forward, and the somewhat less reduced condition of the last cheek tooth. 

Onychomys bensoni Gidley, n. sp. 

Plate XXXV, figure 3. 

Type.-Portion- of a right lower jaw containing complete dentition (catalog No. 10509, 

U., S. Nat. Mus.). 

Locality.-Benson locality. 

Description.-Length of cheek-tooth series 

3.9 millimeters; about the size of 0. torridus 

but with less reduced last molar, as in the 

larger sp~cies 0. leu. ruidosae. The hinder lol;>e 

91048°-23-9 

of the last molar is less reduced even than in 

the last-mentioned species. 0. bensoni differs 

from all the living species of the genus in 

having more widely open valleys and less conspicuous lophs in the mola_r teeth; a more 

depressed heel on the last lower molar, which 

is distinctly lower than the anterior or triconid 

portion; and apparently a · relatively larger 

and higher coronoid process. 

Sigmodc;m curtisi Gidley, n. sp. 

Plate XXXV, figure 2. 

Type.-The greater parts of both lower jaws 

carrying complete dentition (catalog No. 10510, 

U.S. Nat. Mus.). 

Paratype.-Portion of a . right lower jaw 

.with cheek teeth (catalog No. 10511, U. S. 

Nat. Mus.). 

Locality.-Both from Curtis locality. 

Description.-Length of cheek-tooth series 

7 ~illimeters; about th'e size of S. hispidus 

arizonae, which it 1Seems to . r-esemble more 

nearly, but it differs from that as well as the 

other living specieg in the more open valleys of 

the reentrant angles, the . more compressed 

lophs, the less hypsodont condition of the 

cheek teeth, the relatively deeper posterior 

inner reentrant valley of p4 and m1, and especially the form and proportions of the cusps of 

the last lower molar, which is relatively larger, 

with the hinder · cusp relatively broader and 

fuller than in the living species. A feature 

that is characteristic of this and the species 

described below· and is not observed in any of 

the living species is noted in the last molar, in 

which the great extension and flattening of 

the inner or lingual wall .of the posterior lobe 

forms a sharp right angle -rith the posterior . 

wall of the reentrant angle on that side. 

I take pleasure in naming this species for 

Mr. Milton Curtis, in recognition of his efficient 

assistance in the field. 

Sigmodon minor Gidley, n. sp. 

Plate XXXV, figures 4, 5. 

Type.-Portion of a left lower jaw carrying 

the complete dentition (catalog No. 10512, 

U. S. Nat. Mus.). 

Paratype.-Portion of a left maxillary carry- . 

ing the anterior two cheek teeth (catalog No. 

10513, .U. S. Nat. Mus.). 

Locality.-Curtis locality. Five other lower-

·jaw portions, most of them with complete 
126 SHOR.TER CONTRJBUTIONS '1"0 GENERAL GEOLOGY, 1922. 

dentition (catalog Nos. 10514 to 10518), are 

also referable to this species. 

Description.-Length of cheek-tooth series 

4.7 millimeters; smaller than S. medius and 

decidedly smaller than any of the living 

species examined. This species is distinguished from the living forms by the same 

characters observed in S. medius, except that 

the external reentrant valleys of the upper 

molars (as shown by specimen No. 10513) are 

of the normal modern type. S. minor differs 

from S. medius only in its smaller size, having 

a relatively ' narrower anterior lower cheek 

tooth, in which the anterior lobe is relatively 

smaller, with the adjacent reentrant valleys 

nearly equal in length. The reentrant valleys 

of the upper cheek teeth are oblique to the 

same degree and of about the same proportionate depth, but the molar crowns of both 

the upper and lower series are less hypsodont. 

Sigmodon medius Gidley, n. sp. 

Plate XXXV, figures 6, 7. 

Type.-Portion of a right lq.wer jaw containing the complete dentition and an associated fragment of a right maxillary carrying 

the anterior two cheek teeth, possibly of the 

same individual (catalog No. 10519, U. S. 

Nat. Mus.). 

Paratypes.-Four other lower-jaw portions 

(catalog Nos. 10520 to 10523, U.S. Nat. Mus.). 

Locality.-All from the Benson locality. 

Description.-Length of cheek-tooth series 5.5 

millimeters; somewhat smaller than the living 

species S. sanctae martae, with which it seems 

to compare more nearly than with other living 

forms. Its resemblances are noted in the form 

of the anterior lobe of the first cheek tooth, in 

which the anterior internal reentrant valley 

. considerably exceeds the opposing outer one in 

length; in the decided angulation of the posterior portion of the inner wall of the last molar; 

and in the relatively heavy symphysis and 

considerable depth of jaw. It differs from 

this and other living species in having the 

cheek-tooth crowns slightly less . hypsodont, , 

the reentrant valleys more open, and the lophs 

correspondingly narrower, while there is present 

in t4e last molar a small but distinct reentrant 

notch on its inner side opposite the posterior 

reentrant valley. 

The two upper ·cheek teeth show the same 

character of the more open valleys observed in 

the lower cheek · teeth. Another difference 

noted is the form of the external reentrant 

valleys of these teeth, which clearly distinguishes them from the corresponding teeth of 

living species. The external reentrant valleys, 

instead of being straight, open directly outward, while their inner portions bend backward, 

forming a decided angle in their posterior 

enamel walls. In the living species these valleys run obliquely forward and outward in a 

nearly straight or but slightly curved line. 

Two other lower jaws in the collection (Nos. 

10521 and 10522) differ slightly in minor characters from the type but are here referred to 

this species. These differences, which consist 

wholly in a somewhat smaller size and a less 

proportionate depth of jaw, correspond to 

differences observed between males an females 

of the living species. S. sanctae martae. I 

therefore assume that the type specimen and 

specimen No. 10520, which agrees with it in 

every particular, represent males, while the 

smaller jaws just mentioned represent females 

of the species. 

Neotoma fossilis Gidley, n. sp. 

. Plate XXXV, figures 8-10. 

Type.-Portion of a right maxillary carrying 

the anterior cheek tooth and the alveoli for 

middle cheek tooth (catalog No. 10524, U. S. 

Nat. Mus.). 

Paratypes.-Portion of a left lower jaw carrying the incisor and alveoli for the cheek teeth, 

and two right lower cheek teeth (catalog Nos. 

10525 and 10526, U. S. Nat. Mus.). 

Locality.-Benson locality. 

Description.~Length of anterior cheek tooth 

3.2 millimeters; width 2.2 m,illimeters; about 

the size of N. intermedia gilva. The following 

characters distinguished this from all living 

species: First upper cheek tooth relatively 

short and wide; no indication of an· anterior 

internal reentrant valley; posterior internal 

reentrant valley nearly as long as the the external one; posterior notch of anterior palatial 

opening opposite the anterior: root of the first 

cheek tooth. · 

The two lower teeth referred to this species 

also show distinctive characters. The anterior one (p4) is entirely devoid of the anterior 

reentrant valley so prominent in such a form as 

N. cinera, and both these teeth are like the· 

corresponding ones of this species in h~ ving the 
FOSSIL VERTEBRATES OF THE SAN PEDRO VALLEY, ARIZONA. 127 

posterior external reentrant valleys pitted in a 

way to form a nearly circular enamellJke in 

well-worn teeth. These lower teeth differ from 

those of all living species in having_ the main 

cross lophs directed somewhat obliquely inward and backward instead of squarely across 

the long axis of the tooth. I 

The lower-jaw specimen shows nothing peculiar except that the capsular bulb at the base of 

the incisor is somewhat less prominent than is 

usual in the living species. 

N eo fiber sp. 

Plate XXXV, figure 14. 

A nearly complete cheek tooth, apparently 

the left upper middle one; from the Curtis 

locality (catalog No. 10527, U. S. Nat. Mus.), 

seems to represent the genus Neofiber. It is 

slightly smaller than the corresponding tooth of 

Neofiber alleni, with which I compared it, but 

has all its characteristics. Except the slight 

variation in size, they differ only in that the 

base of the fossil tooth is closed by the formation of rootlets, whereas th~ specimens of the 

living species examined seemed to indicate 

g~owth from a persistent pulp. However, this 

difference may be, in part, at least, an age 

character, as in the closely related genus Ondatra, in which both conditions are found on 

examm1ng a number of· specimens of various 

ages. 

Order LAGOMORPHA. 

Four species of Lagomorpha are represented 

in the collection, three from th~ Benson locality 

and one from the Curtis locality. They seem 

not to be referable to any living species, but the 

material representing them is too fragmentary 

to warrant any attempt at diagnostic descriptions. 

Species No. 1. 

Plate XXXV, figure 15. 

Represented ·by a portion of a left lower jaw 

carrying p4, m11 m2 (broken), and the alveoli 

for the other two cheek teeth and the incisor; 

from the Benson locality (catalog No. 10530, 

. U. S. Nat. Mus.). This specimen represents a 

species about the -size of Lepus campestris, or 

slightly smaller, and it agrees with that species 

r~ther closely both in jaw and tooth characters. 

Species No. 2. 

Plate XXXV, figure 11. 

Represented by two upper incisors (broken), 

a first lower cheek 'tooth of the left side, and a 

median lower cheek tooth of the left side, from 

the Benson locality (catalog No. 10529, U. S. 

Nat. Mus.). 

These teeth represent a sm.all species about 

the size of Sylvilagus aud. arizonae or Brachylagus idahoensis. However, ·there are some 

peculiarities in one or another of all these teeth 

that make uncertain their definite reference to 

either of these living genera. The incisors are· 

of a type to fit in with either genus, although 

there is not quite the difference in relative size 

·of the two lobes formed by the anterior longitudinal groove. In the molar tooth the posterior 

lobe is about intermediate in relative size bebetween that of S. aud. arizonae and B. idahoensis, and both lobes are somewhat less laterally 

expanded and less sharply angulate than in 

either of these species. The anterior lower c4eek 

tooth, which is of the right side, is peculiar and 

differs from that of any living species that has 

come under my observation. It is of the same 

general form as that of S. aud. arizonae but is 

proportionately a little wider, being nearly as 

wide as long, and is void of any reentrant angles 

except the normal two outer ones. Moreover, 

the posterior one of these extends but little 

more than halfway across the tooth. In aU 

living forms examined this reentrant either 

extends entirely across to the inner wall of the 

tooth or -meets an opposing reentrant from the 

inner side. 

Species No.3 • . 

Plate XXXV, figure 13. 

In the collection from the Curtis locality 

are a portion of a right lower jaw carrying the 

anterior two cheek teeth (catalog No. 10528, 

U.S.Nat.Mus.) and a lower right anterior tooth 

of a· second individual, which represent a species 

about the size of Lepus californicus erennicus, 

or somewhat smaller. The fossil- spedes 

seems to differ from the latter, however, in the 

proportions of all the tooth lobes which are 

somewhat narrower and in having the anterior 

lobe of the first cheek and tooth set farther inward, so that the reentrant valley faces decidedly 

forward and outward, instead of almost directly 

outward, as in the living species. 
128 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1922. 

Cf. Lepus sp. 

Plate XXXV, :figure 12. 

A fourth species of Lagomorpha (catalog 

No. 10535, U. S. Nat. Mus.1 is represented in 

the material from the Benson locality by the 

two incisors of the right side in a fragment of 

the premaxillary. It differs from the others 

in having an unusually high inner ridge on the 

large incisor. In fact, the tooth is remarkable 

in that the plane of its upper face is very oblique 

to the perpendicular plane of' the skull, as 

in Ochotona, but the plane of its lower surfaces 

is nearly at -right angles to the perpendicular 

median plane of the skull, as in the true rabbits, 

and its summit has the characteristic wear of a 

short anterior chisel point and posterior ledge 

or shelf observed in all species of rabbits but not 

found in· the Pica group. The longitudinal 

enamel groove is filled with cement, but this 

feature has not even a generic significance. 

As this form of incis~r is suggested in some 

degree in certain species of Lepus, as L. 

.siamensis, I regard the fossil specimen as belonging to that genus. 
. I 

PLATES XX~IV-XXXV. 

I 

I 

129 

I , 

I • 

I 
PLATE XXXIV. 

[All figures about twice natural size.] 

Page. 

FIGURE 1. Citellus cochisei Gidley, n. sp. Type. Upper cheek teeth of right side, Grown view. No. 10490, 

U. B. Nat·. Mus ........................................................ ·..................... 121 

2. Citellus cochisei Gidley, n. sp. Portion of left lower jaw, tooth-crown view; 2a, side view. No. 

10491, U.S. Nat. Mus....................................................................... 121 

3. Citellus bensoni Gidley, n. sp. Type. Last left upper molar, crown view; 3a, 3b, right upper molar 

probably of same individual. No. 10531, U. S. Nat. Mus..................................... 122 

4. Citellus bensoni Gidley, n. sp. Lower cheek teeth in fragment of jaw of left side, crown view. No. 

10532, U. S. Nat. Mus...................................................................... 122 

5. Geomys parvidens Gidley, n. sp. Type. Anterior portion of skull, palate view; 5a, side view. 

No. 10492, U.S. Nat. Mus.................................................................. 122 

6. Geomys parvidens Gidley, n. sp. Portion of a right lower jaw, tooth-crown view; 6a, side view. No. 

10493, U.S. Nat. Mus...................................................................... 122 

7. Geomys minor Gidley, n. sp. Type. Portion of a right lower jaw, tooth-crown view; 7a, side view. 

No.10494, U.S. Nat. Mus ................. ~---············································· 123 

8. Geomys minor Gidley, n. sp. Upper incisor of left side, front view. No. 10534, U. S. Nat. Mus... 123 

9. Cratogeomys bensoni Gidley, n. sp. Type. Portion of a left lower jaw, tooth-crown view; 9a, side 

view. No. 10495, U. S. Nat. Mus ... . ·................ ~........ ............................. 123 

10. Cratogeomys bensoni Gidley, n. sp. Portion of a right lower jaw, tooth-crown view; lOa, side view. 

No. 10497, U.S. Nat. Mus................................................................. 123 

11. Cratogeomys bensoni Gidley, n. sp. Upper incisor of right side, front view. No. 10496, U. S. Nat. 

Mus ................................................. ,........ . ........................... 123 

12. Peromyscus brachygnathus Gidley, n. sp. Type. Portion of a right lower jaw, tooth-crown view; 

12a, side view. No. 10501, U. S. Nat. Mus ............ ~..................................... 124 

13. Peromyscus minimus Gidley, .n. sp. Type. Portion of a left lower jaw, tooth-crown view; 13a, 

side view. No. 10500, U.S. Nat. Mus ... :................................................... 124 

14. Peromyscus sp. Portion of a right lower jaw, side view. No. 10502, U.S. Nat. Mus ............. ·... 124 

15. Eligmodontia arizonae Gidley, n. sp. Type. Greate;r portion of a right lower jaw, crown view; 

15a, side view. No. 10503, U. S. Nat. Mus.................................................. 124 

16. Dipodomys minor Gidley, n. sp. Type. Greater portion of ~ right lower jaw, tooth-crown view; 

16a, side view. No. 10499, U.S. Nat. Mus ............................... ,.................. 123 

130 
U. B. GEOLOGICAL SURVEEY PROFESSIONAL PAPER 131 PLATE XXXIV 

3a 

2a • 3b 

4 

5 

7a 

9 

6a 7 

11 

9a 

10 

16 

12a 

lOa 

13 16a 

13a 14 

15a 15 

FOSSIL VERTEBRATES OF SAN PEDRO VALLEY, ARIZ. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 131 PLATE XXXV 

4 

1 

1a. Sa 4a 

2a 

7 

5 

6a 8a 

11a 

12 12a 

lOa 

13 13a 14 

14a 

l5a 

FOSSIL VERTEBRATES OF SAN PEDRO VALLEY, ARIZ. 
PLATE XXXV. 

[All figures about twice natural size.] 

Page. 

FIGURE 1. ' Onychomys pedroensis Gidley, n. sp. Type. Portion of a left lower jaw, tooth-crown view; la, side 

view. No.l0506, U.S. Nat. Mus............................................................ 125 · 

2. Sigmodon curtisi Gidley, n. sp. Type. Portion of a right lower jaw, tooth-crown view; 2a,, side 

view. No.10510, U.S. Nat. Mus ................................................... :........ 125 

3. Onychomys bensoni Gidley, n. sp. Type. Portion of a left lower jaw, tooth-crown view; 3a1 side 

view. No.10509, U.S. Nat. Mus............................................................ 125 

4. Sigmodon minor Gidley, n. sp. Type. Portion of a left lower jaw, tooth-crown view; 4a, side view. 

No.10512, U.S. Nat. Mus .......................... .'........................................ 125 

5. SigmoJon minor Gidley, n. sp. Two anterior cheek teeth in fragment of left maxillary, inner-side 

view. No. 10513, U.S. Nat. Mus............................................................. 125 

6. Sigmodon medius Gidley, n . sp. Type. Portion of a right lower jaw, tootl crown view; 6a, side 

view. No.l0519, U.S. Nat. Mus........................................ . .................... 126 

7. Sigmodon medius Gidley, n . sp. Anterior two upper cheek teeth of right si :~ e, in fragment of jaw, , 

crown view. No. 10519, U.S. Nat. Mus .. . ...................... : ............................ 126 

8. Neotomafossilis Gidley, n. sp. Portion of a left lower jaw viewed from above; 8a, side view. No. 

10525, U.S. Nat. Mus........................................................................ 126 

9. Neotomafossilis Gidley, n. sp. Type_. First upper cheek tooth of right side in fragment of maxillary, 

crown view. No. 10524, U. S. Nat. Mus .......................................... _ ... _....... 126 

10. Neotomafossilis Gidley, n. sp. Anterior two lower cheek teeth of right side, crown view; lOa, inner 

side view. No. 10526, U.S. Nat. Mus. : ..................... _ . ............................... · 126 

11. cf. Sylvilagus sp. Median incisors in fragment of premaxillary, palate view; lla, front view. No. 

10529, U.S. Nat. Mus ......... "················-·-········································--- -127 

12. Lepus sp. Two upper incisors of right side in fragment of premaxillary, palate view; 12a, front view. 

No. 10535, U.S. Nat. Mus.................................................................. 128 

13. Lepus sp. cf. L. californiC'lts. Anterior two cheek teeth of right side in fragment of lower jaw, crown 

view; 13a, outer side view. No. 10528, U. S. Nat. Mus_ ........... , ...................... _..... 127 

14. Neofiber sp. Portion of a left upper molar, oblique crown view; 14a, outer side view. No. 10527, 

U S. Nat. Mus ___ ._ ..... _ ... __ . _____ . _. _ ... _. _. _ ... _. _. _ .......... _ ...... __ .. ___ .......... _ 127 

15. Lepus sp. Portion of left lower jaw, tooth-crown view; 15a; side view. No. 10530, U.S. Nat. Mus._ 127 

131 

\