The Terlingua-Solitario region in southern Brewster and Presidio counties, Texas, is arid and mountainous with many hypabyssal intrusions. Lava flows border it on the west. The sedimentary rocks, excepting some of Paleozoic age in the center of the Solitario, the largest laccolithic dome, are Cretaceous and have been deformed by laccolithic intrusions and faults, many of the rift type. The Solitario contains many small rhyolite dikes and sills and is encircled by a rim sill. Volcanic agglomerate in a central cauldronlike area records an extrusion. The eastern part of the area is in a block depressed several thousand feet and contains many dikes, sills, and smaller laccoliths.

The igneous rocks include an analcite-bearing series and intermediate, trachytic, and rhyolitic types, most of which are soda-rich. The analcite-bearing types range from melanocratic gabbroid to syenitic types. Analcite is primary, deuteric, and hydrothermal and was formed through a rather long period. The complete assemblage is alkaline and resembles the suites from Spanish Peaks, Colorado, and other alkaline subprovinces along the front of the Rocky Mountains.

The analcite-bearing rocks are most abundant in the sunken block and evidently were derived from several differentiating submagmas. The other rocks, especially the rhyolites, are associated with higher anticlinal structures. A tectonic control of differentiation is postulated.

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