BLACK HILLS. A small group of moun tains known as the Black Hills rises several thousand feet above the plains in western South Dakota and eastern Wyoming. Having abun dant rainfall it constitutes through its vegeta tion and streams an oasis in the semi-arid region. The hills are carved from a dome shaped uplift of the earth's crust and consist largely of rocks that are older than those form ing the surface of the Great Plains. In these older rocks are extensive mineral deposits, which give the region great importance in the produc tion of precious metals. The length of the Black Hills is about 100 miles and the greatest width about 50 miles. The hills rise abruptly from the plains to an elevation of about 4,000 feet. Of its total area of 6,000 square miles 1,900 square miles are now a United States for est reserve. The salient features are an en circling °hogback') ridge, constituting the outer rim of the hills, the Red Valley, which extends completely around the uplift, then a limestone plateau with inf acing escarpment, and finally a central area of high ridges culminating in the precipitous crags of Harney Peak at an altitude of 7,216 feet. Two branches of Cheyenne River nearly surround the hills and receive from them many tributaries of running water, notably Spearfish, Rapid, Redwatev and Beaver creeks.
The rocks exposed comprise sandstones and shales of Cretaceous Age, red shales of Per mian and possibly Triassiac Age, limestones and sandstones of Carboniferous, Ordovician and late Cambrian Age with a central nucleus of Algonkian granite and schist. In the latter are veins of gold ore, which have been extensively mined for many years. The largest mine is the Homestake, at Lead, which produces from $4,000,000 to $5,000,000 in gold a year. Other mines yield smaller amounts of gold and con siderable lead, silver, lithia minerals, mica and wolf ramite are also produced. In the northern hills are many bodies of igneous rocks and on the northwest slope is Devil's Tower. The largest cities in the Black Hills are Lead, largely sustained by the Homestake and other mines, and Deadwood, on the site of part of the great free gold placers, the cause of the memorable stampede to the Black Hills, 1876-78.
N. H. DArcroN, Geologist, United States Geological Survey.