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LL 



74 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 



of detritus, for the most part auriferous, took place during the later Pliocene 

 epoch, and not as late as the drift or diluvial period, as is abundantly proved 

 by the character of the remains of plants and land animals which are im- 

 bedded in it. The deposition of this auriferous detritus was succeeded, 

 throughout the whole extent of the Sierra Nevada, by a tremendous outbreak 

 of volcanic energy, during which the auriferous gravel was covered by heavy 

 accumulations of volcanic sediments, ashes, pumice, and the like, finally 

 winding up by a general outpouring of lava, which naturally flowed from the 

 summits of the Sierra through the valleys, into the lake-like expansions, 

 filling them up and covering over the auriferous gravels, which were to 

 remain for ages, as it were, in a hidden treasure chamber, concealed under 

 hundreds of feet in thickness of an almost indestructible material." 



The above extract represents, very nearly, the general scope of the results 

 arrived at by the Geological Survey, during the first two or three years of 

 its existence, in reference to the origin of the high gravel deposits. No 

 detailed work had been done in the region where these occur, and the 

 extreme inaccuracy of all the existing maps would have rendered it im- 

 possible to coordinate any such observations if they had been made. There 

 still remained much to be done before a thorough understanding of the 

 phenomena in question could be arrived at, as will be evident to the reader 

 of the succeeding chapters of this volume. What has been accomplished 

 since the Geology of California, Vol. I, was published, in the way of a de- 

 tailed examination of the gravel region, will be set forth in the next section 

 of this chapter, and, after a statement of the facts observed, their theoretical 

 bearing will be discussed at some length. In the mean time we have become, 

 from what has been said in the preceding pages, sufficiently well acquainted 

 with the general character of the high gravel deposits to be able to under- 



- ■ 



stand the meaning of the terms employed by the miners in their work, 

 terms which we shall have to use frequently in giving an account of our 

 observations in the mining region. 



It being well understood that the high gravels belong to a system of an- 

 cient rivers, in the former beds of which detrital material has been deposited, 

 and which have since become in large part obliterated by accumulations of 

 lava, and also very extensively worn away by erosion, the character of the 

 miner's work will be in a great measure directed by these conditions. That 

 the portions of the gravel richest in gold should be found towards the bottom 

 of the deposit, on or near the bed-rock surface, and in the lowest portions of 





