BLACK HILLS, an isolated group of mountains in the U.S.A. covering an area of about 6,000 sq.m. in the adjoining corners of South Dakota and Wyoming. They rise on an average some 2,000f t. above their base, the highest peak, Harney, having an altitude above the sea of 7, 216f t. They are drained and in large part enclosed by the North (or Belle Fourche) and South forks of the Cheyenne river, and are surrounded by semi-arid, alkaline plains lying 3,00o to 3,5ooft. above the sea. The mass has an elliptical shape, its long axis, which extends nearly north-north west to south-south-east, being about I2om. and its shorter axis about 4om. long. The hills are formed by a short, broad fold, which is flat or nearly so on its summit. From this fold the strati fied 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. 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. Low grade quartz mines were later found, Which furnished a large portion of the country's gold supply (about $100,000,00o from 1875 to 1901) . There is still a good yield from deep mines (over 500,00o fine ounces in 1933 with a current value of about $15,200,000). Silver production from 1879 to 1901 was about $4,154,000, and nearly $43,000 for Deposits of copper, tin, iron, tungsten, and a variety of other minerals have also been discovered. In contrast to the surround ing plains the climate is sub-humid, 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.