290 GEOLOGY OF THE BLACK HILLS. 



j)liiir ill jippearauce is often seen on the surface of tliese clay-shales. On 

 testing-, it was found to be a basic sesquisulphate of iron, probably identical 

 with a yellow salt of similar composition used in medicine. 



In reviewing the gold deposits in the Black Hills, there are some 

 peculiarities in the occurrence which require to be .specially noticed. 



The gold contained in the trachytes of the northern part of the Hills 

 and the Bear Lodge range has been deposited in these rocks at the time of 

 the intrusion, which was probably coeval with the elevation of the range at 

 the close of the Cretaceous period. But the gold ledges in the schists and 

 slates are of Archaean age, and formed during the folding of the metamor- 

 phic rocks. 



There seems to have been no volcanic disturbance in the Black Hills 

 since the elevation, and the occurrence of basalt capping gravel deposits, so 

 common a feature in the California and Australian gold fields, is here 

 entirely wanting. No fossil plants or the bones of extinct animals have as 

 yet been found in the placers, whereby their age might be determined, but 

 from their position they have been deposited not later than the Tertiary, 

 and since the elevation of the Black Hills. 



Very few minerals have been found associated with the gold, except 

 garnets and magnetic iron sand. The occurrence of zircon, topaz, or plati- 

 num, so coumion in the gold-washings in other parts of the world, has not 

 been observed in the Black Hills. 



With the exception of a few of the gravel deposits in the foothills, 

 which may be of shore formation, no deep leads or old channels filled with 

 gravel were found, which could not be referred to the present streams and 

 system of drainage, assuming only a greater rain-fall than at present, as 

 numerous gravel beds occur in dry sags and gulches, where water rarely, if 

 ever, forms now a running stream. 



There are evidences of four distinct erosions having taken place in the 

 geological history of the Black Hills. The first at the close of the Archaean, 

 the second during the early Tertiary, the third in the Drift or Glacial 

 period, and the fourthj^ the result of the action of the present streams and 

 drainage in recent times. 



After the elevation and folding of the schists and slates, the formation 



