2 GEOLOGY OE THE BLACK HILLS. 



and Wyoming, and it will be seen by referring to the map accompanying 

 this report, that by far the greater area of the Hills is in Dakota, and that 

 but a comparatively small portion of the northern and middle parts is in 

 Wyoming. The entire Bear Lodge range is in Wyoming. 



The area of the Hills proper is about 5,000 square miles — not far from 

 that of the State of Connecticut. 



The whole area of the main section of the Hills determined to be gold 

 bearing — the French Creek, Spring Creek, Rapid Creek, Deadwood Creek, 

 Terry Peak, &c, districts — is in Dakota, while the small and unimportant 

 gold district of the Bear Lodge range lies in Wyoming. 



The Black Hills are embraced between the two forks of the Cheyenne 

 River, the Belle Fourche and South Fork, which, rising near Pumpkin 

 Buttes, in eastern Wyoming, spread out, the former northeasterly and the 

 latter southeasterly, to enfold the Black Hills, and then approaching each 

 other join about 60 miles east of the Hills in latitude 44° 25', forming the 

 Big Cheyenne, which then flows northeast and empties into the Missouri 

 River some 15 miles above New Fort Sully. Surrounded on all sides by the 

 comparatively unbroken sea of the Great Plains, they are entirely separated 

 from the main chain of the Rocky Mountains on the west, to which system, 

 however, in character and structure they belong, and rise an island of 

 rough and rugged mountains complete within themselves. 



The elevation of the Hills above the surrounding country — an aver- 

 age of 2,000 to 3,000 feet — and their heavy growth of timber cause them to 

 act as condensers upon the water-bearing clouds, and the precipitation of 

 moisture upon them is much greater than upon the neighboring region of 

 the Plains. Though they give rise in themselves to no great river or impor- 

 tant system of drainage, the enfolding branches of the Cheyenne become 

 augmented to fully double their previous volume by the few running streams 

 that drain from their heights and valleys. While the region of the Hills 

 proper is well watered, containing innumerable springs and brooks of clear 

 and cold water, all the important streams, save about five, sink and disap- 

 pear in their beds before they reach the regions of the Plains, and though 

 they occasionally rise and sink again, they carry no running water to the 

 Cheyenne. 



