



- 



■ 



REVIEW AND GENERAL DISCUSSION. 



505 



as elsewhere during the gravel period, that from time to time they shifted their beds, and there- 

 fore, that if the actual system of channels could be fully traced, it would prove a complex network 

 here as elsewhere, and finally, that all the volcanic matter which afterwards buried them so 

 deeply (every particle of which is fragmentary) was brought here in one way or another by the aid 

 of water, from the high Sierra far up towards the summit of the range. On the contrary, I can 

 see no other theory that is capable of accounting for the facts throughout this region, any more 

 than elsewhere, where I went last summer. 



It is worthy of special note that wherever the ancient gravel rests on limestone bed-rock, there 

 it is arranged in most peculiar and utter confusion. No such scenes of disorder can be found in 

 the structure of gravel banks anywhere else, as at these localities. This fact may perhaps be 

 partially explained by the extremely irregular erosion of the limestone previous to the deposition 

 of the gravel upon it. But I think also that in many instances it is largely due to subsequent 

 erosion and land-slips since the volcanic period, and to the slow excavation by the percolating 

 waters of subterranean caverns in the limestone, whose roofs have occasionally broken in, and 

 allowed the overlying mass to settle down. Another interesting phenomenon is the occasional 

 finding of such subterranean caverns of considerable size, perfectly filled with the finest of silt, 

 which has found its way in through cracks and crannies too small to admit the passage of pebbles 

 or even of the coarser sand. 



My estimates of the gravel in the vicinity of Michigan Bar would give for the total amount 

 already washed away in the first hill visited, 775,000 cubic yards, with some three or four million 

 yards remaining yet to wash ; in the second hill 200,000 washed away, with a couple of millions 

 yet remaining ; and in the third hill between two and three hundred thousand washed away ; 

 while the total original aggregate amount of gravel in this vicinity, in the hills to the south of the 

 Cosumnes River, I fix at from twelve to fifteen million cubic yards. These figures are simply 

 round numbers, of course, and being only rapid estimates made chiefly by the eye, they must be 

 taken only for what they are worth. On the north side of the river there is comparatively little 



gravel in the hills. 



My examination of the gravel in the vicinity of Folsom was only cursory, and I made no at- 

 tempt to estimate its amount. Bat its quantity is immense (including the volcanic gravels), and a 

 more careful investigation of it could hardly fail to develop many facts of interest .which I failed 

 to learn. 



With reference to the region between Michigan Bar, Fiddletown, Volcano, the Mokelumne 

 River and the Valley, I can add little on any special points to what is contained in the preceding 



notes; 



I may notice, however, the existence at Butte City, at the southwestern foot of the Jackson 

 Butte, of what seems to be a curious gravel-basin, whose outlet is not known, and which is proba- 

 bly worthy of further study if time and opportunity should offer. And I am tempted to refer once 

 more to the intense resemblance which the Jackson Butte bears to a peak of metamorphic rock 

 when seen from a little distance northwest of it. 



I now proceed to the consideration of some more general questions. 



Section II. — General Observations on the Occurrence of the Gravel. 



The history of the Sierra Nevada, even if fully known, would be neither a simple nor an easy 

 thing to write. With reference to its earlier history I shall venture to say little here, my notes 

 relating chiefly to the later period, when the mountains were already formed, and after the aurif- 

 erous gravel had begun to accumulate upon their slopes. A few words, however, may not be out 

 of place respecting some of those structural features of the range which date back to the earliest 

 period of its history. 





