Petrography, northern Davis Mountains, Trans-Pecos, Texas
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The northern Davis Mountains are composed of Tertiary volcanic rocks of the group. This group, resting unconformably on southward dipping Cretaceous strata, contains a basal conglomerate and 1,500-2,000 feet of alternating beds of tuff and lava. Streams have cut steep-walled canyons in the outer margins of the mountains, and differential erosion between the tuffs and lavas has produced a stair-step profile. Landslides occur along the front as Toreva-blocks. The volcanic units of the McCutcheon group in the northern Davis Mountains are divided into six major mappable units consisting of three dissimilar flows, each underlain by tuff. The lavas are all rhyolites, the lowermost characterized by aegirite; the middle, by riebeckite and chlorite (?); and the upper, by riebeckite. The tuff units are rhyolitic and are classified as lithic, vitric, and crystal tuffs. The indices of refraction of natural glasses or fused igneous rocks vary inversely with the silica content. A standard silica-index of refraction curve is drawn for rocks from the alkalic province of the nearby Terlingua-Solitario district. Indices of refraction of artificial glasses of the Davis fountain rocks are referred to this curve to determine their approximate silica percentages. The igneous rocks of the Davis Mountains are similar in composition to igneous rocks in the Barrilla Mountains, the Big Bend, and the Buck Hill Quadrangle. The similarity of the rocks, indicated by the presence of soda-rich minerals, suggests that they were probably derived from the same parent magma.