A detailed study of the Rock Creek basalt flow reveals significant relationships between chemical composition of the plagioclase feldspar and texture. The An content of the groundmass plagioclase varies inversely with size and directly with the amount of associated glass. The rate of cooling, after extrusion, determined the composition of the groundmass plagioclase. The plagioclase phenocrysts, averaging 20 percent more calcic than the corresponding groundmass plagioclase, are intratelluric.

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