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REVIEW AND GENERAL DISCUSSION. 



521 



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 canons that a gulch, 

 — like the little Golden Gate Canon 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 

 inHit 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 decree of slope upon which gravel will accumulate and rest varies under different circum- 

 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 debris 

 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 lias 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 lias 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 largo 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 canons in the mountains, or whether the two localities 





