206 GEOLOGY OF THE BLACK HILLS. 



its crevices as far below the surface as decomposition had penetrated the 

 substance of the rock. 



Very few large bowlders were seen in the gravel deposits, the pay 

 dirt, as well as the whole thickness of the bars, being mostly a slate-wash, 

 easily treated in sluices with a low grade and slow cuiTcnt of water, owing 

 to the lightness of the material operated upon. Th& gravel deposits are 

 flats along the course of the stream, filling tlie bottom of the valley, and 

 elevated bars on slate benches, 10 to 30 feet above the creek, often quite 

 extensive at the bends of the stream Several pits had been sunk to bed- 

 rock in the flats, reaching it at a depth of 12 to 15 feet, and in every case 

 striking on bed-rock pay dirt yielding from 3 to 10 cents to the pan The 

 elevated bars showed in places 10 to 12 feet of gravel, and the dnfts run in 

 on rim-rock exposed the bed-rock pitching in toward the center of the bar. 

 A number of these high bars were tested by my assistants, with a return 

 from the pay streak of from 3 to 15 cents to the pan. The gold was coarse, 

 flattened scales and rounded grains, stained a rusty- brown color by a thin 

 coating of oxide of iron, derived from the presence of iron pyrites in the 

 slates. 



This portion of Castle Creek, extending from the bend to its junction 

 with the north fork of Rapid Creek, a distance of about eight miles, is in a very 

 mountainous and broken region. The valley is deep, and in places quite 

 narrow, and surrounded by high, steep hills of clay-slates, occasionally 

 intersected by a stratum of ferruginous quartzite mixed with white quartz, 

 similar to the Mammoth ledge on Spring Creek. It is not necessary to dwell 

 particularly on the wealth of this district, as there is no doubt that the 

 gravel deposits on this part of Castle Creek will prove remunerative when 

 opened. They are very favorably situated for working. The Avater supply 

 is quite large, with a grade of at least 7G feet to the mile. The gravel is 

 easily mined and sluiced, and the gold coarse and readily saved, even 

 Avithout the use of quicksilver. Castle Creek at this point is a fine rapid 

 stream of water, averaging in June and July 10 to 12 feet in width, with 

 a depth of 12 inches, probably yielding 300 to 500 miner's inches of water. 



