I. Eagle Pass and the Eocene Coal Fields of the Middle Rio Grande Region of Texas.
Definition of the Area.
This part of this report is based upon a reconnaissance, made in 1895, of the Rio Grande region from Del Rio, in Valverde County, to Santo Tomas, in Webb County, thence to Uvaldc, in Uvalde County; and upon a second reconnaissance, made in 1898, from Cline, Uvalde County, to Eagle Pass, thence up the Rio Grande to Upson, across to Paloma, back to Eagle Pass, and from the last-mentioned place to Carrizo Springs, thence to the Rio Grande at the Webb-Maverick county line. After returning to Carrizo Springs the return journe}r to Uvalde County was made along the Nueces River. Besides the data accumulated on these reconnaissance trips some data obtained while studying the geology of the Brackett and Uvalde quadrangles have been utilized.
The area is approximately a right-angled triangle, the apex being at Santo Tomas; the bypothenuse, the Rio Grande; the base, a line from Del Rio to Uvalde, and the third side a line from Uvalde to Santo Tomas. It embraces portions of Kinney, Valverde, Maverick, Webb, Zavalla, and Uvalde counties. (PL I.)
General Physical Features.
These coal fields lie within the geographic province of the Texas region denominated the Rio Grande Plain. 1 This plain is a local modi fication of the southwestern attenuation of the coastal plain of the Gulf and Atlantic States, comprising a western prolongation of the same up the Rio Grande, included in the angle formed by the con vergence of the Balcones escarpment line and the eastern front of the Cordilleran region in Coahuila.
The Texas side of the region is a vast plain, slightly inclined to the southeast, sloping from an elevation of 1,082 feet at Johnstone, Val verde County, to about 600 feet opposite Santo Tomas, or at the rate of about 3 feet to a mile. The streams have cut their beds in this plain and now lie considerably below its former level. The larger of these stream valleys are accompanied by series of terraces. The structure of the Texas portion of the plain throughout the greater part of its extent is that of a gently southeasterly dipping monocline. In the
i Hill and Vaughan, Eighteenth Ann. Rept, U. S. Geol. Survey, Part 11, 1898, p. 202.