Maps showing geochemical analyses of the Ishi, Mill Creek, Polk Springs, and Butt Mountain further planning areas, Tehama and Plumas counties, California

The Ishi, Mill Creek, Polk Springs, and Butt Mountain Further Planning Areas occupy part of the region between Chico and Mount Lassen (fig. 1).  All of these areas are characterized by thick sequences of Tertiary and Quaternary pyroclastic and lava flows, predominantly of the Pliocene Tuscan Formation, overlying Paleozoic and (or) Mesozoic metavolcanic and metasedimentary rocks and the Cretaceous Chico Formation, a fossiliferous sandstone.  The Tuscan Formation contains several small altered areas that may represent areas of hydrothermal alteration.  The major rock types and altered areas are shown on the map.  For more detailed geology, refer to Peterson and others (1982).