The Van Horn-Sierra Blanca region lies athwart part of the boundary between two contrasting geologic provinces. The contrast between these two provinces, splendidly displayed by Holocene landforms in the present map area, has a history that dates back to at least Late Precambrian time.

Tertiary rocks, including sandstone, conglomerate, shale, pyroclastics, tuff, and lava, are preserved in Big Bend National Park and in a much larger area to the west and northwest. Some of the rocks have distinctive characteristics that enable recognition by their lithology.

Geologic map that depicts the surface geology of El Paso County and parts of Hudspeth. Jeff Davis, and Culberson Counties, Texas. The 12-page booklet indicates geologic formations, abbreviations, and ages. Published in honor of E.

The Davis Mountains are an erosional remnant of a volcanic field that probably covered 5 to 10 times their present areal extent of approximately 2,000 square miles.

Study of Permian, Cretaceous, and Cenozoic rocks in the 30 Kent quad- rangle is important to an understanding of the geology of Trans-Pecos Texas. Most of the Cretaceoua rock units of central Texas lose their lithostratigraphic identity when traced into west Texas.

About 10 miles south of Van Horn, Texas, the Van Horn Mountains rise abruptly above an intermontane plain and extend southward to the Sierra Vieja. The area lies primarily in the southwestern part of Culberson County but extends into Hudspeth and Jeff Davis counties.

At the northern end of the Coahuila platform, the thinned Comanchean sequence consists of the newly named Sixshooter group of carbonate formations underlain by the newly named Yearwood formation.

The Barrilla Mountains, in the northeastern part of the Davis Mountains of Trans-Pecos Texas, are composed of Tertiary volcanic materials. Five tuffs and five lava flows, 1500 feet thick occurring throughout the mountains, persist in thickness and lithologic characteristics.

This preliminary report describes the Medley district in Jeff Davis County, Texas, where kaolin deposits have been found and where rutile has recently been discovered. Baker (2) has already briefly described the Medley Ranch kaolin deposit, but so far nothing has been published on the rutile.