The Sierra Nevada batholith is an amalgam of many plutons of varied compositions that intruded the continental crust during subduction. The once-present volcanic rocks at the surface have, for the most part, been eroded away during uplift and exposure of the pluton. Plutonic igneous rocks are classified on the basis of their mineral content. As silica content decreases, the ratio of plagioclase to K-spar increases, and the quartz diminishes in abundance. Geologic mapping in the Sierra Nevada batholith requires defining rock units and naming them.
Plutons are mappable plutonic rock bodies and are often given names.
A map unit can be given a formal name such as the El Capitan Granite or an informal name such as the granodiorite of Kuna Crest. Note that the names include a place name or landmark that represents the type locality, and a rock name which represent the predominant rock type in the unit. However, there can be gradational compositional changes within plutons. As a consequence, not all the rocks exposed in the El Capitan Granite are necessarily granite as defined above. For example, the Tuolumne Intrusive Suite contains the granodiorite of Kuna Crest on the margin, and concentrically inward contains the Half Dome Granodiorite, the Cathedral Peak Granodiorite, and the Johnson Granite Porphyry.