The rocks of the Huntington Lake area, 160 square miles of the western slope of the central Sierra Nevada of California, are dominantly granitic. Two square miles of the area are underlain by metamorphic rocks, chiefly quartzite and marble.
The granitic rocks range from quartz diorite to alaskite and form 10 large separate and sharply bounded intrusive plutons. Each pluton varies within itself, and a series of maps has been prepared illustrating the variables of rock type, quantity and relative proportions of ferromagnesian minerals, abundance of dark inclusions, and grain size. Gradational variations within some plutons are considerable; two plutons, for example, range from quartz diorite through granodiorite to quartz monzonite. In another pluton, content of ferromagnesian minerals varies from 2 to 19 per cent.
Plagioclase and its anorthite content, total dark minerals, and the proportion of hornblende to biotite in general increase together. These interrelations are irregular, however, and quantitative generalizations are not warranted.
The abundance of ferromagnesian minerals and of dark inclusions are closely parallel. The inclusions probably are xenolithic, and some and possibly most of the mafic minerals are products of assimilation of metamorphic rocks. Most of the granitic rocks formed from the upward intrusion of mobile materials.