336 RESUME AND THEORETICAL DISCUSSION. 
has been in the way of excavating valleys during the different geological 
ages. And, furthermore, we have here the action of the sea entirely ex- 
cluded: all the erosion which has been done in the region in question 
has been the result of the combined action of rain and rivers. There can be 
no mistake about this; neither can there be as to the absence of ice during 
the entire period of the accumulation of the gravels, and of the wearing 
away of the channels which the detrital materials now fill. 
That the ocean has not had anything to do with the erosion of the bed- 
rock or the accumulation of the gravels will at the present time be admitted 
by all. The labors of the California Geological Survey have established the 
main facts so clearly, that the days of the crude theories advocated before 
that work was begun* may now be said with truth to have entirely gone 
by. A few words, however, in this connection, in regard to the proofs of 
the absence of ice agencies during the whole of the gravel period may prop- 
erly here find a place; for at the present time there seems to be no theory, 
however absurd, which does not find favor, provided we have the word 
“ olacial’’ connected with it. The entire subject of the glacial epoch in the 
Sierra Nevada, and in the Cordilleras in general, has been discussed by 
the present writer in another work, to which reference has already been 
made.t It is therefore unnecessary here to enlarge on this subject, although 
it will be proper to give concisely the reasons why it appears to be un- 
questionably the fact that the glacial epoch in California did not occur 
until long after the accumulation of the gravels had ceased, and the topog- 
raphy of the country had assumed its present form, down to almost its 
minutest details. 
In the first place, the reader will bear in mind that the operations of 
the hydraulic miner are constantly uncovering large areas of bed-rock all 
through the gravel region, so that we have far better opportunities for see- 
ing the character of the surface under the detrital masses than we can 
usually have in drift-covered regions. For instance, a large part of the 
surface of New England is overlain by gravel deposits, but it is only here 
and there that we can see the surface of the underlying bed-rock well 
exposed. It is true that the slates of the bed-rock series in the auriferous 
belt of the Sierra are liable to decompose rapidly when uncovered, and this 
circumstance often renders the opportunities for inspection somewhat less 
satisfactory than they would otherwise be; but, on the whole, we have a 
* See ante, pp. 66-72. + See Climatic Changes, ete., Chap. 2. 
