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The present bulletin completes the work of preliminary exploration of the mountain region of Trans-Pecos Texas begun twenty years since by the University of Texas Mineral Survey and completed by the University of Texas Bureau of Economic Geology. The region treated in the present paper is bounded on the north by the line of the Texas & Pacific Railway, on the west by the Malone Mountains, on the southwest by the Rio Grande, and on the east practically by the line 104° 30' west longitude. The field work was done at the rate of about thirty-five square miles per day. Much of the mapping was schematic and thicknesses of rock sections were estimated. The structure proved to be complicated and only the main structural features were determined. The main new results of the present work are four : 1. The determination of the probable Permian age of the gypsum beds of the Malone Mountains. 2. The classification in a general way of the Comanchean sediments, a work very greatly aided by the fossil determinations of Professor W. M. Winton. 3. The discovery of complicated structure in the Quitman and Eagle mountains, with overturning and overthrusting of strata. 4. The assignment of the epoch of this intense deformation to Cretaceous time later than Taylor and before the final disappearance of Cretaceous dinosaurs.