EAGLE PASS AND 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 Uvalde, 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 journey 
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 Iiypothenuse, 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. (PI. 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 
Grulf and Atlantic States, comprising a western prolongation of the 
same up the Rio Grande, included in the angle formed by the con- 
/ergence of the Balcones escarpment line and the eastern front of the 
fcrdilleran region in Coahuila. 
The Texas side of the region is a vast plain, slightly inclined to the 
;outheast, sloping from an elevation of 1,082 feet at Johnstone, Val- 
verde County, to about 600 feet opposite Santo Tomas, or at the rate 
>f about 3 feet to a mile. The streams have cut their beds in this plain 
,nd now lie considerably below its former level. The larger of these 
tream valleys are accompanied by series of terraces. The structure 
i the Texas portion of the plain throughout the greater part of its 
xtent is that of a gently southeasterly dipping monocline. In the 
i Hill and Vaughan, Eighteenth Ann. Rept. U. S. Geol. Survey, Part II, 1898, p. 202. 
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