The Christmas and Rosillos Mountains area in southern Brewster County covers 145 square miles and in it are exposed Cretaceous sedimentary formations, some extrusive bodies, and many hypabyssal intrusions of probable early Tertiary age. The upper Devils River limestone, which is equivalent, at least in part, to the Georgetown formation of central Texas, is the oldest Cretaceous formation. The Tornillo clay, probably of Navarro age, is the youngest Cretaceous formation. The igneous rocks occur on 18 laccoliths, many dikes, sills, plugs or necks, and lava flows. They are composed of porphyritic granite, sodic and non-sodic rhyolite and trachyte, basalt, and gabbro. The Christmas Mountains are an elliptical dome of Devils River limestone probably underlain by a gabbro laccolith which uplifted the limestone about 4,000 feet. The sedimentary formations formerly overlying the Devils River limestone at the crest of the dome have been removed by erosion and form dip slopes around the flanks of the dome. Several intrusive and extrusive masses of rock are exposed at the crest. The dome has been considerably faulted and terminates abruptly at the northwest with a down-dropped block in which about 1,000 feet of volcanic agglomerate is exposed. Traces of quicksilver occur in the mountains. The Rosillos Mountains are a laccolith that has intruded Aguja sandstone. Erosion has removed the sandstone from the top and small outcrops occur around the margin. In an escarpment around the northern end, an excellent cross section of the laccolith overlying its Aguja sandstone floor is exposed. The main mass is porphyritic olivine-augite-hornblende granite. A fault has depressed the northeast portion of the laccolith, and canyons have been eroded along faults around the southern end