304 GEOLOGY OF THE BLACK HILLS. 



series of observatioiiH (uot so regular as might be wished) gave a mean temperature of 

 02° and a daily variation of 14°. A few similar observations at Camp Crook, on liai)id 

 Creek, in the latter part of July, resulted in a mean temperature of G4°, with a daily 

 variation of 20°. On the 24th of July, on liapid Creek, a se\ere hailstorm sent the 

 mercury from 84° to 62°, 22° in half an hour. Camp Cook is at least a thousand feet 

 lower in altitude than Camp Harney.* 



The past winter of 1875-76 has been unusually mild all through the 

 Northern and Western States, and reports from the miners in the Black 

 Hills speak in glowing terms of the delightful weather they have experienced; 

 very little snow having fallen before February, the grass being green at 

 the roots, and the stock, when not overworked, keeping fat and in good 

 condition. The winter of 1874-75 was correspondingly extremely severe ; 

 yet the Sioux City party of miners, who built the stockade and cabins on 

 French Creek in December and January, rej)ort that the cold was not 

 intense, and that their cattle kept fat on the grass in the vicinity, where the 

 altitude is 5,600 feet above the sea. 



The Black Hills, with their copious rain-fall, rise like a high wooded 

 island from an ocean of grass-covered and treeless plains, watered by occa- 

 sional and scanty rains. The rain-fall on the plains, far from yielding a 

 constant and uniform supply of moisture, unfortunately is very uncertain 

 and irregular, both in quantity and occurrence, and sometimes falls so far 

 below the average that the grass almost perishes from the drought. 



This arid character of the plains is not peculiar to the region immedi- 

 ately about the Black Hills, but is equally true in a general way for this 

 whole belt of elevated plains, nearly five hundred miles in width, extending 

 along the eastern slope of the Rocky Mountains from Mexico to the British 

 Possessions. And the reason for the treeless character of this imiuense 

 tract of country is to be sought in its location with respect to the adjacent 

 ranges of the Rocky Mountains and to the oceans on both side of the con- 

 tinent. 



From the north the winds, coming from the colder regions in the British 

 Possessions, bring with them scarcel}' any moisture ; while from the south, 

 the direction of the prevailing winds and the character of the country is 

 such lliat any supplies of rain from that direction are cut ofi". The winds 



* The Black Hills, by Ricliartl Irving Dodge, Lieuteuant-Coloncl, United States Army. New York : 

 Jauies Miller, publisher, G47 Broadway, 1876. 



