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PECULIARITIES OF THE GRAVELS. 
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Among the occurrences which arrest the attention of the investigator in 
the hydraulic mining region there are two which seem very noteworthy. 
One is, the fact that in some localities the gravel is almost entirely made 
up of quartz boulders and pebbles; the other, that some of the boulders are 
of such enormous and quite exceptional size. Each of these peculiarities 
demands a few words of comment. 
The gravels, of course, consist essentially of the hardest portions of the 
rock masses, which have been eroded away during the time of their aceumu- 
lation. A large part of the metamorphic crystalline schists of the auriferous 
belt are of a decidedly indestructible character. Hence the gravel deposits, 
as a general rule, do contain a very considerable proportion of this kind of 
rock, as has been mentioned so frequently in the preceding pages; but 
there are districts where the bed-rock is chiefly made up of finely laminated 
slates, which are sometimes very soft, or are easily rendered so by exposure 
to air and moisture. Such softer rocks are not unfrequently traversed by 
large quartz veins; and, in some cases, the slates adjacent to the veins seem 
to have undergone some chemical change, rendering them peculiarly liable 
to disintegration. It follows, therefore, that in the wearing away by water 
of a region where the bed-rock is of this character, it would be the natural 
result that the softer material should become almost entirely pulverized, and 
the resulting mud be carried off to a considerable distance, leaving behind 
only the indestructible portion, or the quartz. We know that, at the present 
time, the masses of quartz enclosed in the slates are sometimes of great width, 
occasionally exceeding a hundred feet. The breaking up of such masses 
would be much assisted by the occasional softer streaks of slaty rock which 
they are liable to contain, and of these only faint traces might remain after 
the abrasion, removal, and redeposition of the quartzose material had been 
effected. 
There may have been localities, however, where the silicious deposit spread 
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itself extensively over the entire surface, so that large masses were accumu- 
lated, quite free from any admixture of other rock. Something of this kind 
may be seen at the present time in process of formation at the well-known 
locality of Steamboat Springs, in Nevada. The breaking up and washing 
away of this enormous deposit might naturally give rise to an accumulation, 
somewhere along the line of direction in which it was carried, of quartz- 
ose material, quite free from any admixture of slaty or other kinds of 
metamorphic rock. 
