'. 



THE BED-ROCK SURFACE AND THE CHANNELS. 



337 



/ 



very good idea of the bed-rock surface under the gravel, such as it has been 

 fashioned by the passage over it of immense quantities of fresh water bear- 

 ing detritus with it. 



Nowhere does this surface present those markings which are essentially 

 characteristic of ice-action. The rock is often furrowed with more or less 

 deep and persistent longitudinal depressions ; but these have nothing indica- 

 tive of ice-work about them. There are none of those fine stria?, parallel 

 for a considerable distance, and quite straight, such as we find to have been 

 produced where a mass of rock has been very slowly moved under the pres- 

 sure of a great body of overlying ice. It is true that to distinguish with 

 certainty between the work of ice and that of running water requires some 

 previous opportunities for studying both kinds of phenomena, under present 

 glaciers as well as in the beds of running streams, aided by at least a modi- 

 cum of natural ability for geological observation. Where, as seems to have 

 been frequently the case, both these advantages have been lacking, it is no 

 wonder that all kinds of mistakes have been made. 



Besides the character of the bed-rock surface as an indication of the work 

 of water rather than of ice, we have, as bearing testimony to the same 

 fact, the form and distribution of the gravel itself. This never resembles 

 morainic debris, or, at least, only in a few exceptional cases, and then only 

 to a very limited extent. It is almost always more or less rounded and 

 water-worn, and usually very considerably so. It is never deposited in the 

 form of moraines, or in accumulations having any other character than that 



of materials laid in their present position by the action of water. 



Again, the character of the fossils contained in the gravel deposits, espe- 

 cially that of the plants so abundantly distributed through the series, fur- 

 nishes corroborative evidence that the climate lias never been one of a 

 glacial character. Everything, on the contrary, indicates a warmer epoch 

 than that now prevailing in the same region. 



It being, then, as the writer conceives, beyond doubt that all that we see 

 as the work of eroding agencies in the Sierra has been effected by rain and 

 rivers, we have now to examine, with some care, what the precise char- 

 acter of this erosion has been. 



The circumstances which have contributed to bring about the present 

 relief of the surface in any region of country form, in almost all cases, a very 

 complicated series of operations. It is with difficulty that we can know 

 anything absolutely certain in regard to any of them. It is only those 



