175

The Basement sands, although varying in thickness, being a bed nearly 100 feet thick at Colorado River in the southeast corner of Burnet County and a comparatively thin band of limestone conglomerate where the contact crosses the north line of the county, average about 100 to 120 feet in thickness throughout their extent.

From the Burnet-Lampasas county line northward across Lampasas and Mills counties the conglomerate increases very gradually in thickness, from a thin band to a bed nearly 100 feet thick, surmounted by 100 feet of sand. Along the base of the Callahan Divide the sand varies in thickness from 60 to 160 feet. At Buffalo Gap it is 140 feet thick. On the west side of the valley of Bitter Creek in Nolan County, where it rests on the Permian, it is 120 feet thick, with a 10-foot band of conglomerate at the base. In Horse Mountain, nearly due south of Bitter Creek, it is 160 feet thick. In Church Mountain section, Runnels County, it is 90 feet thick. About 10 miles west of Horse Mountain there are only 50 feet. West of Weatherford the Basement sands are 115 feet thick. Continuing northward across the State into the region where they represent the united Trinity and Paluxy sands, the formation becomes at least 300 feet thick, and is probably over 400 feet thick in Indian Territory.

On account of the irregular surface of the Paleozoic rocks upon which the Basement sands rest and the varied erosion of the many streams that pass across it with the dip of the rocks, the contact lines along the western border mark a course so tortuous that it is difficult to locate it by description. It is traced in detail on the map (Pl. LXVI) and is shown on the general map accompanying the First Annual Report of the Geological Survey of Texas, on which all the divisions of the Cretaceous traced out in the field by the writer and his assistants were mapped.

Wherever the Trinity sands are exposed they rest unconformably upon Paleozoic rocks. Above they come in contact in complete conformity with harder rocks. Where the superimposed strata are harder than the sands the contact of the sands and their overlying bed is clearly marked, and can be readily and accurately located. Such is not always the case at the base of the sands, however. In the valleys of the larger streams crossing the Trinity, and toward the southeast, and where erosion is very rapid, the contact of the sands with the Paleozoic rocks is easily discerned; but where these conditions are not present the sands spread out over the edge of the irregular base-level in attenuated sheets and remnant areas, obscuring actual contacts.

The lower contact of the Basement sands of the Trinity division begins at the Colorado River, where it crosses the Travis-Burnet county