Introduction.—The conditions described below were observed over a large part of South-West Texas; but the local details apply to an area of one or two hundred square miles in McMullen County, geologically surveyed by the writer in 1912. The centre of observations was a little mining ranch named Crowther, about 50 miles south of San Antonio and 80 miles from the Gulf of Mexico, near the inland limit of the Gulf coastal plain. The country for many miles around Crowther is practically untouched by man, though a somewhat primitive little local railroad now under reconstruction reaches to within 16 miles of it.