BLACK HILLS, an isolated group of mountains, covering an area of about 6000 sq. m. in the adjoining corners of South Dakota and Wyoming, U.S.A. They rise on an average some 2000 ft. above their base, the highest peak, Harney, having an altitude above the sea of 7216 ft. They are drained and in large part enclosed by the North (or Belle Fourche) and South forks of the Cheyenne river (at whose junction a fur-trading post was established about 1830); and are surrounded by semi-arid, alkaline plains lying 3000 to 3500 ft. above the sea. The mass has an elliptical shape, its long axis, which extends nearly N.N.W.-S.S.E., being about 120 m. and its shorter axis about 40 m. long. The hills are formed by a short, broad, anticlinal fold, which is flat or nearly so on its summit. From this fold the stratified beds have in large part been removed, the more recent having been almost entirely eroded from the elevated mass. The edges of these are now found encircling the mountains and forming a series of fairly continuous rims of hogbacks. The carboniferous and older stratified beds still cover the west half of the hills, while from the east half they have been removed, exposing the granite. Scientific exploration began in 1849, and systematic geological investigation about 1875. Rich gold placers had already been discovered, and in 1875 the Sioux Indians within whose territory the hills had until then been included, were removed, and the lands were open to white settlers. Subsequently low-grade quartz mines were found and developed, and have furnished a notable part of the gold supply of the country (about $100,000,000 from 1875 to 1901). The output is to-day relatively small in comparison with that of many other fields, but there are one or two permanent gold mines of great value working low-grade ore. The silver product from 1879 to 1901 was about $4,154,000. Deposits of copper, tin, iron and tungsten have been discovered, and a variety of other mineral products (graphite, mica, spodumene, coal, petroleum, &c.). In sharp contrast to the surrounding plains the climate is subhumid, especially in the higher Harney region. There is an abundance of fertile soil and magnificent grazing land. A third of the total area is covered with forests of pine and other trees, which have for the most part been made a forest-reserve by the national government. Jagged crags, sudden abysses, magnificent canyons, forests with open parks, undulating hills, mountain prairies, freaks of weathering and erosion, and the enclosing lines of the successive hog-backs afford scenery of remarkable variety and wild beauty. There are several interesting limestone caverns, and Sylvan Lake, in the high mountain district, is an important resort.
See the publications of the United States Geological Survey (especially Professional Paper No. 26, Economic Resources of the Northern Black Hills, 1904), and of the South Dakota School of Mines (Bulletin No. 4, containing a history and bibliography of Black Hills investigations); also R. L. Dodge, The Black Hills: A Minute Description . . . (New York, 1876).