Volcanic rocks of the Davis and Barrilla Mountains constitute the largest contiguous segment of the mid-Cenozoic volcanic field of Trans-Pecos Texas. Salic volcanic rocks form a silica-oversaturated series ranging in composition from trachyte to peralkalic rhyolite. Mafic lavas are subordinate in volume to the salic, silica-oversaturated series.
Stratigraphic relations and K-Ar age determinations allow a reconstruction of the evolution of the area. Major volcanic activity occurred in a period of about 3 m.y. in early Oligocene time (38 to 35 m.y. B.P.), during which several large lava shields were constructed and major ash-flow sheets emplaced. A late episode of mafic lava extrusion and intrusion of phonolite bodies remains undated, but may have occurred later than 31.5 m.y. ago.
The silica-oversaturated volcanic rocks are the eruptive products of a group of highly evolved magmas emplaced into high crustal levels during early Oligocene time. In rapid succession, plutons vented to the surface, forming as many as five different eruptive centers, each of which was active for a period not exceeding 1 m.y. The eruptive centers are marked by concentrations of salic lava flows, dike swarms, intrusive masses, and at least one caldera.