244 



GEOLOGY OF THE BLACK HILLS. 



over a bowlder, dug- from beneath it a blue gravelly clay, a shovelful of 

 which gave on washing six flat scales of gold about a tenth of an inch in 

 diameter, or five cents to the pan. Putting all my force of miners at work 

 in this place, several of the bowlders were rolled to one side, affording a 

 passage for the stream and decreasing the depth of water above the riffle. 

 On examination the pay gravel was found to be a deposit about 35 feet in 

 width, crossing the creek and extending into the banks on either side, 

 resulting from the decomposition of a stratum of soft clay-slate in the bed- 

 rock, which had caught and retained the gold swept over it by the stream, 

 The deposit consisted of — 



1. Loose gravel of slate and quartz, with small bowlders, containing 

 traces of gold; thickness, about 6 inches. 



2. " Pay streak," a compact gravel, full of small red garnet crystals, 

 rich in coarse gold; from 3 to 6 inches in depth. 



3. Plastic blue clay mixed with rotten slate bed-rock, yielding gold in 

 paying quantities, but not as rich as the garnet-gravel; thickness, 1 to 2 

 feet. 



4. Lamellar clay-slate, soft and decomposed, forming bed-rock. 



The " Mammoth" quartz ledge crossed the bed of the creek just below 

 this rich deposit, and subsequently it was found that the line of bowlders 

 forming the riffle had lodged in the soft slate above the upper side of the 

 ledge, as shown in the accompanying sketch. 



cmaL hard, 

 Quartzite. 



Fig. 28. Longitudinal section of the bed of Spring Creek at the point of original discovery. 



A. Water of stream. 



B. Riffle of large bowlders. 



C. Loose gravel and bowlders, poor in gold. 



D. Pay streak heavy red garnet-gravel ; auriferous. 



