16

At Del Rio the lowest rocks exposed belong to the Fort Worth limestone, 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.

Within the breaks of the Rio Grande, on both sides of the railroad 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. Macroscopically 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 yellowish color. The fossils which it contains are frequently ferruginous replacements. Microscopically the limestone is composed of minutely crystalline calcite and much flocculent (argillaceous) Material. Foraminifera belonging 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):

Mr. W 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 boundary 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 numerous small streams which are cutting it away.