Abstract

A siliceous hornfels in the Wallowa mountains of northeastern Oregon contains small quartz porphyroblasts. In thin section the porphyroblasts show several stages of development from initial forms which are barely distinguishable from the groundmass on account of the abundance of minute inclusions, to later forms which are gradually cleared of inclusions. Some of the porphyroblasts have euhedral boundaries yet consist of several individuals of varying optical orientation. Idiomorphic pyrite is contained in some of the quartz porphyroblasts. The petrographic evidence seems to indicate a facility of crystallization in environment which might not be expected to favor idiomorphism and may indicate an initial step in the recrystallization of the rock.

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