bo 
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REVIEW AND GENERAL DISCUSSION. 5 
streams leave the mountains. Yet at every such point there is more or less of it, and the quantity 
is greatest near the largest streams. ‘This appears, therefore, to point very strongly towards the 
conclusion that the main streams issued from the mountains in the gravel period at very nearly 
the same points where they do now. 
The bed of fine volcanic breccia underlying a heavy bank of metamorphic gravel in the hills near 
Folsom is a fact of much interest, and possibly of much significance in some directions, which will 
' better appear when it shall be further investigated. 
It appears to have sometimes happened during the excavation of the modern cajions that a gulch, 
— like the little Golden Gate Cation at Damascus, — after being excavated to a certain depth, 
has had its bed filled by a heavy slide, and has then excavated a new channel alongside of it, 
leaving its old bed still buried in the hillside. 
It is certain that, unless it be in very rare instances, as, for example, that of the Jackson 
Butte, the belt of volcanic activity in this part of the country never extended to any considerable 
distance down the western slope of the Sierra. I saw no signs of volcanic action in any part of 
the country which I traversed in 1871. 
With reference to any arguments for a change of grade or other disturbance of the bed-rock on 
the western slope of the Sierra since the gravel period, based upon the assumption that any par- 
ticular amount of grade is necessary in order that gravel may accumulate, and the conclusion which 
might be drawn from them that the alternate prevalence at different times of denudation and 
accumulation has been determined by changes of grade in the mountains, I will only remark, in 
addition to what I have already said upon the subject of such supposed recent disturbances in the 
Sierra, that any such argument as this appears to me extremely ill-founded, because of the fact 
that the degree of slope upon which gravel will accumulate and rest varies under different cireum- 
stances between very wide limits indeed, and is dependent between these limits almost entirely 
upon the character and force of the streams which carry it. We all know that a torrent like the 
American in flood would move boulders of considerable size over grades of less than fifty feet to 
the mile, while a stream of 300 or 400 miner’s inches of water, when allowed to spread, as it will 
on leaving a flume, to a stream four or five feet in width, can only just roll the ordinary débris 
from a hydraulic claim down a slope as high as twelve degrees, which was the actual slope of one 
pile of tailings, which I noted under such circumstances. 
It has also been often supposed that the quantity of water in the Sierra during the gravel period 
must have been vastly greater than it is to-day in order to enable it to accumulate such immense 
quantities of gravel. I do not see any necessity for this supposition either. It may have been so, 
or it may not. But other things being equal, an increased quantity of water would produce an 
increased tendency to excavation. Whereas, given a moderate quantity of water, and given the 
alternations of drought and freshet, it is easy enough to see how accumulation might have gone 
on, even with no more water than is furnished by the present climate, and how, if it did go on 
under such circumstances, it would be productive of exactly that kind of structure which we 
see in the banks, and, with time enough, might accumulate even greater quantities of gravel than 
were accumulated. But there was certainly a far later period, namely, that of the glaciers, during 
which the quantity of water in the Sierra was greater. 
The low, terraced hills, which form a belt of greater or less width, skirting the foot-hills of the 
Sierra proper for a long distance southeasterly from Folsom along the eastern margin of the Sacra- 
mento and San Joaquin Valley, present some interesting facts and problems. The whole mass of 
these hills has undoubtedly been derived at some time from the slopes of the Sierra. Moreover, a 
very large portion of their volume has been gathered since the commencement of the volcanic era, 
as is proven by the very large extent to which volcanic materials enter into their structure. 
Furthermore, there are in the history of these hills two well-marked periods, —the period of accumu- 
lation, and the period of erosion. But whether the material of these hills continued to accumu- 
late until the close of the volcanic era, and then their erosion commenced simultaneously with the 
beginning of the excavation of the modern cafions in the mountains, or whether the two localities 
