The Geology of Bald Butte Ridge, Washington

Bald Butte Ridge is a stock of the Idaho batholith which protrudes through and above the late Tertiary basalts. Its rocks form a petrographic series which has crystallized in the following order: hornblende-biotite-tonalite, biotite-tonalite, aplite dikes, quartz-gabbro dikes, pegmatite dikes, and quartz dikes. Most of the rock in the area is biotite-tonalite. It contains schlieren of the hornblende-biotite-tonalite and is cut by all the dikes named above. The tonalites have a poorly developed gneissic structure which was produced by movement in the mass during the early stages of crystallization.