The volcanic rocks in the north and west parts of the Buck Hill Quadrangle are a southeastern extension of the Davis Mountains of trans-Pecos Texas. Field evidence suggests a division of the volcanics of this area into two regions: (1) west of Calamity Creek the volcanic series, consisting of flows of trachyte prophyry, andesite porphyry, basalt (five flows), and rhyolite porphyry separated by beds of ash, tuff, and breccia, has a combined thickness of 2,100 feet; (2) east of Calamity Creek the rock series is distinctly different: here porphyritic olivine-free diabasic basalt flows (?) and intervening beds of ash and fresh-water conglomerates predominate and are 552 feet thick.

A major stratigraphic break separates the volcanic rocks from the underlying Upper Cretaceous rocks, and everywhere a basal conglomerate marks this unconformity. No difference in the dips of the underlying Cretaceous rocks and the volcanics is noticeable in the field, therefore the break at the base of the volcanics is considered one of disconformity rather than of angular unconformity. The age of these volcanic rocks is not known, but from available data it seems probable that they are of Cenozoic age.

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