A large complex of gabbroic rocks forms a series of related intrusions in the northern parts of the Stillwater Range and Clan Alpine Mountains of Churchill County and in the West Humboldt Range of Pershing County (Willden and Speed, 1974). It also may extend into the Trinity Range and Shawave Mountains in western Churchill County (Greene, Stewart, and others, 1991). The complex contains highly differentiated facies near the periphery of the body and more homogeneous gabbro in the interior. Layered rocks near the margins include picrite, olivine gabbro, hornblende gabbro, and anorthosite. The homogeneous rocks consist largely of feldspathic hornblende gabbro and analcite gabbro. The complex is interpreted to be part of a continental Jurassic volcanic arc that is the northern continuation of a Jurassic continental margin arc that extended from the Sonora Desert region in the south to northern California in the north (Dilek and Moores, 1995; Zientek, Sidder, and Zierenberg, 2004). Biotites from several places in the gabbro have been dated by K/Ar and range from 140 to 170 Ma.