66 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 



have become gradually familiar, as many years of workings have revealed 

 all the facts necessary to enable them to understand the mode of occurrence 

 of those deposits, in regard to whose real nature there was for a number of 

 years the greatest uncertainty. The names given to the gravels in question 

 have varied from time to time as more became known about them. At first, 

 they were called simply " high gravels," as being at a higher level than those 

 which had previously been ordinarily worked; then they were known as 

 " deep gravels," because usually occurring in heavier deposits than the river 

 gravels. As they are exclusively worked by the hydraulic process, they are 

 often called "hydraulic gravels."* It is now necessary to explain, with some 

 detail, the manner in which these high gravels occur, in order that the reader 

 may understand the meaning of the terms used in describing the various 

 localities where these are worked and the circumstances in which they are 

 found as preparatory to a general discussion of the facts observed. 



There are many localities in the gold region where the high gravels were 

 sufficiently well exposed as to be easily recognized previous to any mining 

 upon them ; hence the earliest scientific observers noticed their existence, 

 and commented upon their geological relations. As early as 1849, Mr. P. T. 

 Tyson passed through the mining districts between the Mokelumne and the 

 South Yuba rivers, and noticed the occurrence of gravels and volcanic rocks 

 in many places ; the former he repeatedly mentions under the name of con- 

 glomerates, and he supposed that they were the remains of a formation once 

 continuous over the whole of the lower portion of the Sierra slope. He con- 

 siders this formation to be not older than the Eocene, and accounts for the 

 elevated position by the upheaval of the range since their deposition.! 



From 1852 on, the nature of the high gravel deposits began to be a matter 

 of o-reat interest to the miners, and much was written and published in the 



CD 



* Instead of saying "to work by the hydraulic method," the much less cumbersome — if inelegant — 



phrase " to hydraulick" is in general use. 



f " We have evidence in the existence of sedimentary rocks near the Mokelemy River, that they have 

 been elevated 2,000 feet at least since their formation, which is certainly not anterior to the Eoccne 



period It was during this long-continued diluvian era that denudations were most rapidly 



effected. It was then that the large valleys, before noticed, south of the American Biver, were mainly 

 formed, by the removal of perhaps one half the area of the conglomerates, sandstone, etc., which once cov- 

 ered the entire surface of the flanks of the mountain (at least between the Cosumes and the Calaveras) to 

 an extent of not less than twenty miles eastward from the valley. And, more than this, it scoured out 

 innumerable ravines among the slates and other soft rocks beneath them, which were thus again exposed 

 to the light of day." — P. T. Tyson, Geology and Industrial Resources of California. Baltimore, 1851. 

 pp. 23 and 26. 



