This report is the result of studies made in the field during the seasons of 1901, 1902, 1903, 1904, and 1905. It relates to an area of about 9,000 square miles, situated mainly in the north-central portion of Wyoming and extending northward into Montana. Its location and general surroundings are shown on PL II. It covers the greater portion of the Bighorn uplift, together with an adjoining area of the Great Plains on the east. It also includes a small part of the Bighorn Basin and the eastern end of the Bridger Range. The report describes the various rocks, their structure, history, and mineral resources, including underground water, coal, gypsum, and various other products. It also contains information as to surface waters available for irrigation.

Throughout the work assistance has been rendered by Mr. C. A. Fisher, who mapped portions of the area, measured numerous sections, collected fossils, and examined most of the crystalline rocks; and the author is indebted to Mr. Albert Johannsen for some of the petrographic descriptions.

Previous observers have given but little information regarding the geology of the Bighorn Mountains, though Dr. F. V. Hayden ascertained the general relations of the uplift in his exploration of the Northwest, and Mr. George H. Eldridge, who crossed the range near Bald Mountain and southwest of Buffalo during the summer of 1893, described 0 some of the broader features of the geologic succession and structure in a remarkably comprehensive manner, considering how small an area he had the opportunity to examine. The Bighorn region is very thinly settled, there being no permanent habitations among the mountains, and it is but little visited by others than hunters, prospectors, and herders. There have recently been established two summer resorts in the mountains, and each year a larger number of persons visit the region. Unfortunately the mineral prospects have proved disappointing to the prospectors, and there appears to be but little promise that the area will become important on account of its mineral resources. In the Bridger uplift promising prospects of gold and copper have recently been discovered, which may prove of value.

There are extensive, forests in the mountains, which are now included in a Government forest reserve, but their timber is not of great value. Much of the area below timber line contains an abundance of luxuriant grasses and other plants, which afford excellent pasturage for stock, and large herds of sheep and cattle are ranged in the region during the short summer season. Game is moderately abundant, and most of the streams contain large numbers of trout. The region is one of great interest geologically on account of its variety of sedimentary rocks, interesting structure, and remarkably instructive glacial features. The central area, with its high peaks, presents alpine scenery of notable character. Doubtless in the future the region will be extensively visited by tourists, hunters, and geologists.