Spherical geodes possessing a botryoidal outer surface, and ranging from a few centimeters to nearly a meter in diameter have developed in the Oligocene Cambria Felsite, a rhyolitic tuff and/or flow unit. The enclosing country rock has been altered locally to montmorillonite, clinoptilolite, and quartz adjacent to the sharp outer margin of the geodes. Peripheral portions of the geodes consist of an intimate, fibrous, subparallel to radiating intergrowth of potassium-rich feldspar (Or42±02 to Or63±03) of high structural state, sodic plagioclase and cryptocrystalline quartz. The geodes become more siliceous near their centers; they characteristically possess an internal cavity lined with chalcedony or rock crystal quartz. Sets of fractures, or microfaults, typically intersect at the central cavities.
Sill-like masses of Miocene diabase have invaded the Cambria Felsite extensively in the vicinity of the geode locality. Hydrothermal fluids charged with alkalis and silica are postulated to have been mobilized by this magmatic intrusion from pore solutions initially present in the country rock. The cavities, produced by the penecontemporaneous microfaulting, evidently acted both as nucleation centers and as sources of the metasomatizing fluids. In moving outward, the hot aqueous solutions promoted both the spherical growth of feldspathic + silicic material and the late, lower temperature, purely siliceous cavity linings. Judging from the lack of zeolites (except in the clayey country rock rims), the compositions and structural states of the coexisting feldspars and the estimated overburden of approximately 1000 m at the time of diabase intrusion, physical conditions for the early stage botryoidal growth of the geodes were: Ptotal = 100–300 bars; T = 600–650+ °C, assuming chemical equilibrium. Temperatures could have been substantially lower if the feldspars crystallized metastably.