The Conterminous United States Mineral Assessment Program; background information to accompany folio of geologic, geochemical, remote sensing, and mineral resources maps of the Butte 1 degree x 2 degrees Quadrangle, Montana - Elliott, James E.; Trautwein, C. M.; Wallace, C.; Lee, G.; Rowan, L.; Hanna, W. (Circular, 1993)