Nevada Bureau of Mines and Geology
Gneiss (Precambrian age) dominated by felsic minerals (orthoclase, KAlSi 3 O 8, plagioclase, NaAlSi 3 O 8CaAl 2 Si 2 O 8, and quartz, SiO 2) with less biotite, K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2); collected below the "Great Unconformity" (between the Precambrian and Paleozoic eras) at the base of Frenchman Mountain, south of Lake Mead Boulevard ...