EROSION. 299 



The whole area of the gold field in the Black Hills was, at the time of 

 tlie upheaval of the range, covered by the Potsdam and subsequent forma- 

 tions. It is probable that the Potsdam conglomerate, formed by the pri- 

 mary erosion of the metamorphic rocks and their inclosed auriferous quart/, 

 ledges, contains considerable quantities of gold, and, by the disintegration 

 and denudation of this conglomerate since the elevation of the Hills, the 

 gold which it contained has been set free and concentrated anew in the 

 placer gravels. This may in part account for the richness of some of the 

 older and more elevated gravel deposits along the valleys of the present 

 streams. 



The fourth erosion in the Black Hills has taken place in comparatively 

 recent times — since the Glacial period — and is still in progress, modifying 

 the effects of the previous denudation, and often removing and redepositing 

 the gravel beds resulting from them. 



This last erosion is solely the work of time, frost, rainfall, and river 

 action, as they occur at the present time, all working extremely slowly, but 

 gradually wearing down the rocks, deepening the valleys and canons of the 

 streams, and, by working over the older auriferous gravels, increasing the 

 richness of the bars and flats along the present beds of the streams. But 

 tlie amount of gold swept into the valle}"s by the present forces and rain- 

 fall must be inappreciable, compared with that resulting from the enormous 

 tearing down of metamorphic rocks and quartz ledges by the previous ero- 

 sions. The placer miners have a saying that "Gold does not go down 

 stream at the present day," else we would look for it in beaver dams and 

 other similar places. Though it is true that fine gold will sluice through a 

 canon over bare bed-rock, and be deposited in the bars below the mouth, 

 it being only a question of time how long a particle of gold is making the 

 journey, no matter how many times it may be caught on bed-rock, only 

 to be liberated by the wearing away of the rock or by freshets. Some of the 

 richest placers found in the Hills the past season were thus situated below 

 the mouths of the canons of the streams, where the valleys widened out, 

 allowing permanent gravel deposits to form, retaining the gold. 



In conclusion, in reviewing the gold placers of the Black Hills, it 

 should be noted that at best the gold field has been but partly prospected, 



