THE EROSION OF THE BED-ROCK DONE BY WATER. 



295 



". ^L 



all these con- 



tlie whole of the mass of the upper portion of the chain was somewhat more 

 elevated than it now is cannot, of course, be denied, since erosive agencies 

 have been continually bringing down materials from higher and depositing 

 them in lower regions. That volcanic agencies also have played an important 

 part during the gravel epoch, and especially towards its close, is also perfectly 

 clear, for the innumerable sections presented in the hydraulic mining region 

 show the presence of eruptive materials almost everywhere, while an inspec- 

 tion of the various maps accompanying this volume would bear witness to 

 the same fact. We may, however, with propriety discuss the bearing and 

 importance of the volcanic deposits, as connected with the other occurrences 

 of the gravel epoch, without its being necessary to enter upon any investi- 

 gation of the nature of volcanic phenomena in general, — a subject, as is well 

 known, of the greatest obscurity and difficulty. The effect of a heavy cover- 

 ing of lava over the gravels, acting as a mechanical protection of that which 

 is beneath; the blocking up of old channels by masses of eruptive material 

 borne down from above, either directly in a melted condition, or indirectly 

 through the aid of water ; the impossibility of getting at the gravel, except 

 by means of drifting, where it has been too deeply covered, 

 ditions are of extreme simplicity, and without theoretical difficulty. 



Again, it is perfectly clear that the shaping of the surface of the bed-rock 

 and all the erosion which has taken place since the beginning of the gravel 

 epoch have been exclusively the work of water. Of course it is not intended 

 by this to exclude the idea that aqueous causes have acted independently of 

 all others. On the contrary, the descending currents have taken advantage 

 of every favorable condition to do their work more quietly and more thor- 

 oughly. Chemical decomposition of the bed-rock, under the influence of 

 volcanic agencies ; heavy grades, giving the necessary velocity to the down- 

 moving waters ; and, above all, copious precipitation, producing streams of 

 large volume, — these are among the favorable conditions which would insure 

 a rapid performance of the erosive work. It can be set down, however, as 

 established beyond any possibility of doubt, that ice had nothing to do with 

 «uiy part of the erosion of the gravel period. It was not until the whole 

 mass of gravels and other dctrital deposits had been accumulated, and the 

 volcanic strata piled over these, often to a thickness of several hundred feet, 

 in short, not until the whole of Tertiary time had passed, that the higher 

 portions of the Sierra Nevada were occupied by glaciers. More than this, 

 it is clearly established that the present river cafionshad been 6ut down into 



