The claims cover an erosional window through Miocene Plateau Basalts in which the oldest exposed bedrock consists of a series of metamorphosed Permian-Pennsylvanian Cache Creek Group pelitic and argillaceous sedimentary rocks. The metasediments have been hornfelsed by a complex multiphase intrusive body composed dominantly of porphyritic quartz diorite. The contact between the intrusive and the sediments commonly contains pyrite and is usually highly silicified. All quartz veins known in the area appear to be associated with silicified zones.