254 GEOLOGY OF THE BLACK HILLS. 



ble to prospect them, there being not a drop of water in the vicinity at the 

 time of my visit (July 3) ; but in the spring this stream probably flows out 

 into the plains, and these dry arroyos and placers can then be worked. 



The available water-supply furnished by Spring Creek for working 

 purposes varies with the season of the year, being probably greatest in 

 April, May, June, and July, the rainy months, and least in October, 

 November, and December, when there is little precipitation of moisture. 



In turning Spring Creek from its channel a ditch 2£ feet wide and 2- 

 feet deep, with a grade of 2 inches in 30 feet, was entirely filled, besides 

 about 30 miner's inches which leaked through the dam ; and the available 

 quantity of water at that time (July 25) I estimated at not less than 350 

 miner's inches. During the summer months Spring Creek will afford from 

 300 to 800 miner's inches of water for mining purposes, the quantity vary- 

 ing with the season and the yearly rainfall. I had no instruments to accu- 

 rately measure the grade of the valley of this stream, but by an aneroid 

 barometer I found that the descent of the main stream from Newton's Fork 

 to where the creek enters the limestone is about 1,400 feet in a distance 

 of nineteen miles, or an average grade of 74 feet to the mile. The descent, 

 however, is less than this for the more open and broad portions of the val- 

 ley, probably 50 or 60 feet to the mile, and greater in the narrow, rocky 

 canons. The gold of this district occurs in the placers in coarse, flat scales, 

 thick and heavy, intermixed with very little fine dust, and often shows 

 brown oxide of iron adhering in the cavities of the larger pieces. 



The assays made by Mr. Ricketts, of the School of Mines, show that this 

 gold is 22| carats, or 946 fine, equal to the average of the best Australian 

 placer gold, and exceeding that from California. It is found associated 

 with small water-worn red garnet crystals, nodules of hematite iron ore, 

 magnetic iron sand, and crystals of iron pyrites, but no other minerals were 

 detected. 



The average proportion of gold in the native gold of California, as derived from 

 assays of several hundred millions of dollars' worth, is eight hundred and eighty 

 thousandths, while the range is mostly between 870 and 890. (Prof. J. C. Booth, of 

 United States mint, in a letter to the author, of May, 1867.) The range of the metal 

 of Australia is mostly between 909 and 9G0, with an average of 925. (Dana System 

 Mineralogy, fifth edition, 18G8.) 



