16 RIO GRANDE COAL FIELDS OF TEXAS. 
LOWER CRETACEOUS (COMANCHK SERIES) . 
At Del Rio the lowest rocks exposed belong to the Fort Worth lime- 
stone, but a short distance above Del Rio, before reaching the mouth 
of Devils River, exposures of the Edwards (Caprina) limestone are 
seen. The canyons of the Rio Grande and Devils rivers in this vicinity 
are cut through it. It is a whitish limestone and occurs in thick ledges. 
FORT WORTH LIMESTONE. 
Within the breaks 1 of the Rio Grande, on both sides of the niilroad 
where it crosses San Felipe Creek and in the valley of the creek to the 
north of the railroad, are good exposures of this limestone. Macro- 
scopically it is rather soft, chalky, and argillaceous, and possesses a 
minutely granular or subflocculent texture. Its color when freshly 
broken is white or whitish, with a yellowish tinge, due to the presence 
of ferruginous matter. It weathers to a grayish or }^ellowish color. 
The fossils which it contains are frequently ferruginous replacements. 
Microscopically the limestone is composed of minutely crystalline call 
cite and much floeculent (argillaceous) material. Foraminif era belong- 
ing to the Globigerina and Nodosaria types are present in very great 
numbers. 
Exposures. aggregating a thickness of about 40 feet were seen. In 
its upper part the limestone becomes very argillaceous and passes into 
a whitish clay which lies at the base of the Del Rio (Exogyra arietina) 
clay. 
The Valverde County court-house and jail are built of this limestone. 
Through it the San Felipe Springs burst forth, the water coming up 
along a system of joint planes. 
The following fossils were obtained near the San Felipe Springs 
(locality No. 269): 
Enallaster texana (Roemer) . 
Kingena wacoensis (Roemer) . 
Rhynconella ? sp. 
Ostrea (Alectryonia) sp. 
Lima, two species, probably undescribed. 
Neithea occidentalis Conrad. 
Inoceramus sp., small fragmentary specimens. 
Nemodon, two distinct species, undescribed. 
Cardium sp. 
Thracia sp. 
'Mr. \V J McGee defines break as follows: "The 'break' is the head of a small retrogressive 
ravine, a minor water course gradually eating its way back into the upland." Twelfth Ann. Rept. 
U. S. Geol. Survey, p. 434. 
What is here called "the breaks'' is a much-indented escarpment that constitutes the outer bound- 
ary of the Rio Grande Valley, in a restricted sense, and up to which the general level of the Rio 
Grande Plain extends. The indentation of the escarpment is due to the head- water erosion of numer- 
ous small streams which are cutting it away. 
