V. WW II AN.] 
NEOCENE AND PLEISTOCENE DEPOSITS. 
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NEOCENE. 
In the area that is the subject of this paper the summits and high 
divides are the remnants of a plain the general features of which have 
been previously described. This plain was once entirely covered with 
a surface coating of gravel to which Hill has given the name "Uvalde 
formation." These deposits form the top of the bluff along the Rio 
Grande from Del Rio to beyond Santo Tomas, occupying a position 150 
to 200 feet above the river and extending eastward far beyond the 
limits of the area discussed. 
PLEISTOCENE. 
Below the level of the Uvalde formation several terraces have been 
developed along the principal stream. The uppermost of this series 
of terraces usually occurs about 120 feet below the Uvalde formation, 
Palafox 
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..MILE 
Fig. 7.— Section of bluffs and terraces at Palafox. 
at the foot of the bluff forming the walls of the Rio Grande Valle}^. 
This terrace is generally several miles wide, and is composed of 
fine gray calcareous silt. Beneath it are several narrower silt ter- 
races. Terraces occur on all of the other streams, but they will not 
be described here. 
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Fig. 8. — Generalized diagram of Rio Grande terraces. 
ECONOMIC GEOLOGY. 
EAGLE PASS COAL FIELD. 
EXTENT. 
As stated in the introduction, this coal field lies partly in Texas and 
partly in Mexico. It extends from Sabinas, Mexico, northeastward to 
Eagle Pass, crossing the Rio Grande some 5 or miles above the latter 
town. It continues for more than 8 miles to the north of this town. 
In the vicinity of Eagle Puss the coal horizon dips at a rather steep 
