

336 



RESUME AND THEORETICAL DISCUSSION. 



has been in the way of excavating valleys during the different geological 

 a^es. 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 

 " glacial " 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. 00 - 72. 



i" See Climatic Changes, etc., Chap. 2. 









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