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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,

at the foot of the bluff forming the walls of the Rio Grande Valley. This terrace is generally several miles wide, and is composed of fine gray calcareous silt. Beneath it are several narrower silt terraces. Terraces occur on all of the other streams, but they will not be described here.

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 6 miles above the latter town. It continues for more than S miles to the north of this town. In the vicinity of Eagle Pass the coal horizon dips at a rather steep