From preface: This memoir is a general study of the region described, and not a detailed report. The field work upon which it is based formed a part of the work of mapping the areal geology of the Little Belt Mountains sheet of the Geological Survey. The topographic map, made in 1882 by the Northern Transcontinental Survey, is too small in scale and too general in character to warrant more detailed work; but as the facts obtained are of interest, and the region is one of economic importance, it has been considered worthy of a special treatment.