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. But 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. 
SEecTIon 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. 
