This report is one of a series of reports that present, chiefly with maps at a scale of 1:250,000 and 1:500,000, various aspects of the geology, geochemistry, geophysics, and mineral resources of the Dillon 1 °X2° quadrangle, s uthwestem Montana and east-central Idaho. These studies were made largely under the Conterminous United States Mineral Assessment Program (CUSMAP), the primary purpose of which is to determine the mineral resource potential of selected 1 °X2° quadrangles by means of a multidisciplinary approach. CUSMAP is intended to provide information on mineral resources to assist Federal, State, and local governments in formulating minerals policy and land-use policy and to produce sound scientific data that may be of value to private industry and the general public in mineral exploration and development.