Investigations of the distribution of uranium and other trace and minor elements in a major welded ash flow tuff (the Gillespie Tuff) and in several rhyolite flow units (all Oligocene) included petrographic examination to detect characteristic textures produced during the process of crystallization, and comparison of the chemical composition of rocks of identical original composition, but crystallized by different processes. In all units investigated, unaltered vitrophyres were used to indicate initial content of uranium and elements.

The major process of crystallization in these rocks involved growth of spherulitic and large crystal units in a supercooled magma, beginning at temperatures 200°C below the liquids and continuing at lower degrees of supercooling. This is not strictly devitrification because it probably occurs above the glass-transition temperature. Several distinctly different textures probably result from different crystallization conditions. Vapor-phase crystallization is rare in the Gillespie Tuff. "Granophyric" texture is common in both lavas and the Gillespie Tuff. In other areas, rocks displaying this texture may have lost some uranium, suggesting that volcanic rocks in Hidalgo County may have released considerable amounts during crystallization.

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