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.t 
From 1852 on, the nature of the high gravel deposits began to be a matter 
of great interest to the miners, and much was written and published in the 
* Instead of saying “to work by the hydraulic method,” the much less cumbersome — if inelegant — 
phrase “to hydraulick” is in general use. 
t “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 Eocene 
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 River, were mainly 
formed, by the removal of perhaps one half the area of the conglomerates, sandstone, ete., which once coy- 
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. 
