

CONDITIONS FAVOEING HYDRAULIC MINING. 



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quite to the base of the mountains, it is not difficult to understand that 

 when the streams had that greatly increased power which they would 

 acquire in consequence of the higher grade, there would be a proportion- 

 ately larger amount of detritus swept entirely away into the valley below. 



Hence, we conclude that a remarkable combination of favoring circum- 

 stances has given rise to those peculiar conditions which make hydraulic 

 mining a success in that part of the western slope of the Sierra which lies 

 between the most southern tributaries of the American and the most north- 

 ern of the Yuba Eiver. The bed-rock itself has been mineralized, so that 

 it is richly supplied with veins of quartz bearing gold. Orographic forces, 

 acting through the geological ages, resulted in the formation of a mountain 



, 



range of great extent, with a long, gently descending slope on the west side. 

 Certain other conditions favored the rapid erosion of these mineralized rocks, 

 raised in the way indicated ; and the consequence was, that the abraded 

 material was carried down the slope with just sufficient velocity, on the 

 whole, not to spread itself out over too wide an area, and so as to have only 

 its finer particles swept entirely into the Great Valley. In the course of this 

 operation a large part of the gold which the abraded material originally con- 

 tained found its way to the bottom of the detrital mass, where it was to lie 

 until the activity and ingenuity of man should find the means of getting 

 hold of it with profit. The bed-rock itself seems admirably adapted, in its 

 lithological and stratigraphical peculiarities, to help in the operation ; for its 

 slaty lamina) are turned up in an almost vertical position, as if it was intended 

 that, as they wear away irregularly, they should form natural " riffles " to 

 arrest the gold in its downward progress. But this is not all. The aurifer- 

 ous gravel might have been spread so uniformly and so deeply over the 

 surface of the country as to render gold at the bottom practically inac- 

 cessible. If the metallic particles all rested on the bed-rock, and were 

 uniformly disseminated over its surface, and this again covered with a large 

 mass of barren detritus, the chances for profitable mining would have been 

 much less than they now are. If, however, in such a case the precious 

 metal, instead of being uniformly spread over the bed-rock surface, were more 

 or less concentrated in channels — or gutters, as they are called in Australia, 

 then shaft-sinking and drifting might be profitably carried on, but the 

 conditions for successful hydraulic mining would not exist. For the appli- 

 cation of that method, the material to be operated on must be raised to a 

 considerable elevation above the adjacent lines of drainage, because other- 







