172

Province, are many fragmentary disconnected patches of the Basement sands resting upon the Paleozoic rocks, notably in San Saba, Brown, Coleman, Callahan, Taylor, and Jack counties and in the Chickasaw Nation.

It having been shown that the Basement sands of the western border, which have the aspects of a mappable formation, are largely the attenuated interior margins of all the formations of the Trinity division, they will now be further described. They first reach northwest through Burnet, Lampasas, and Mills counties to Brown County, Texas, along the eastern slopes of the Colorado Valley. From Brown County the outcrop deflects west along the south side of the narrow Callahan Divide for many miles to the Llano Estacado, and thence back again on the north side. From northern Brown County on the north side of the divide, after a long indentation of 50 miles down the Leon Valley, the belt turns northeast through Comanche, Eastland, and Erath counties to the Brazos, and thence in zigzag lines through Hood, Parker, Wise, Montague, and Cooke counties in Texas northward to Healdton, Chickasaw Nation, and thence east to the Arkansas line at the crossing of Little River.

Roughly measured, and not including the minute meanderings, the length of the narrow belt of outcrop along the western border is estimated as follows: From the Colorado to the Callahan Divide in northern Brown County, 100 miles; west along the Callahan Divide, following the thirty-second parallel to the Colorado River and return, 300 miles; the indentation of the Leon Valley from May, Brown County, southeast to Hazledell, Comanche County, 35 miles, and north to Desdimonia, Eastland County, 35 miles; the main border from Desdimonia north of east to the Brazos, 45 miles; the zigzag meandering down and up the indenting valleys of the Brazos and Trinity to Decatur, 100 miles; from Decatur to Healdton, Indian Territory, 90 miles; from Healdton east to the Arkansas line, 175 miles; from the Arkansas line east to near Murfreesboro, Arkansas, where the sands finally disappear, 30 miles; a total of 910 miles.

The width of the belt of outcrop varies with the topography. It is governed by the degree of preservation of the rocks of the overlying slopes and scarps composed of beds of Glen Rose and Edwards limestones. The latter extends for the most part to within from one-fourth of a mile to 2 miles of the western crest of the Grand Prairie, forming low escarpments that overlook the slopes composed of the Glen Rose formation which lead down to the Basement sands. The belt of sands is usually upon a lower-lying plain extending from the base of the Glen Rose slopes to the outcrop of harder rocks of the Paleozoic border, which may vary in width according to the degree of erosion of the overlying beds.