A 1,000-m section of volcanic rocks overlying Lower Cretaceous limestone in the Sierra del Gallego area 200 km south of El Paso can be divided into four lithologic sequences. About 44 to 38 m.y. ago, at least five separate crystal-poor sanidine-bearing outflow units of rhyolitic tuff covered parts of the area. The tuffaceous activity was closely followed by lava flows of fritted feldspar-bearing tholeiitic andesite. These flows were followed about 37-35 m.y. ago by a sequence of rhyolite flows, flow domes, and intrusions. A 6-m.y. period of quiescence was followed about 29 m.y. ago by massive outpourings of basalt with minor associated rhyolite tuff. Parts of the oldest sequence are probably correlative with stratigraphic units in the Pena Blanca area in New Mexico. The P zos limestone conglomerate at the deposit has a counterpart in the Sierra del Gallego area. Petrographic similarities, thickness relations, geodes, and K-Ar dating directly link the H member in the lower part of the Liebres Formation in the Sierra del Gallego area with the Nopal Formation at Pena Blanca. The Pozos, Nopal, Escuadra, Pena Blanca, and Mesa Formations are, as a group, lithologically and chronologically similar to the oldest sequence (the Liebres Formation) of the Sierra del Gallego area.

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