The Hood Spring quadrangle, in the central part of Brewster County, Texas, contains a segment of the southeast rim of the Marathon basin.

Preparation of a desk-side reference manual of Cretaceous foraminifera was undertaken as a project of the Bureau of Economic Geology, where I was a staff member during the summers of 1945 to 1947 and during July of 1951.

The 15-minute Agua Fria quadrangle in southwestern Brewster County, Texas, is arid, sparsely vegetated, and includes diverse topographic features that result chiefly from complex structure and variation in rock resistance to erosion.

The Tascotal Mesa quadrangle is in southeastern Presidio and southwestern Brewster counties, Trans Pecos Texas. Bandera Mesa and Tascotal Mesa are reached most easily from Marfa, 50 miles north, by the Marfa-Lajitas road which traverses the quadrangle from north to south.

The fossils described in this paper substantiate the Permian age of the Rustler formation. The Rustler fauna is largely molluscan and has a Whitehorse aspect.

Rocks of Cretaceous age crop out along the western margins of the deeper playa basins and along the southern and southeastern margins of the Llano Estacado.