316 



EESUME AND THEOEETICAL DISCUSSION. 









o 



between the two systems. It does not appear, however, that this tilting of 

 the land masses in the northern region produced any effect on the sys- 

 tems of drainage of the gravel region; at least, no important changes 

 can be traced at present which might be referred to an uplift of the kind 

 described. 



t 



There is no foundation in fact for the idea advocated by Richthofen * in 

 his masterly essay on the relation of the distribution of the volcanic rocks to 

 the configuration of the surface of the globe, that, during the period of vol- 

 canic activity in that region, the crest of the Sierra was raised at a higher rate 

 than its western foot, and that consequently those ancient rivers, which were 

 flowing parallel to that crest, have been gradually turned from their chan- 

 nels and made to flow in the present direction of the drainage, that is, down 

 the slope and at right angles to their former direction. At least, the most 

 that could be said in favor of this view would be, that in the extreme north- 

 ern portion of the mining region there have been disturbances since the 

 deposition of the Cretaceous which have, to some extent, modified th 

 courses of the channels of the ancient rivers; but in what direction, has not 

 yet been made out. The natural course of things would seem to be, that 

 elevating the region to the north would reverse the direction of the streams 

 previously flowing towards the lower area. Such a change in the course of 

 the ancient rivers as that supposed by Richthofen would not be likely to 

 have been brought about by the simple more rapid rising of the crest than 

 the base of the chain ; this, as it would seem, might more naturally have 

 only augmented the velocity of the streams previously flowing in a normal 

 course, — that is, down the slope and at right angles to the crest. If we 

 could suppose such a thing possible as a general southerly direction of the 

 ancient rivers before the volcanic epoch, then it may be conceived that a rise 

 of the chain, proceeding more rapidly at its southern extremity than at its 

 northern, might have turned the streams down towards the Great Valley, 

 one after the other, in the manner indicated by Richthofen. His views, 

 however, are in fact tantamount to an assumption that the range of the 

 Sierra was not elevated until after the old river channels had been in exist- 

 ence some time, and a large portion of the high gravels deposited ; in short, 

 it demands an entire reconstruction of the topography of the region during 

 later Tertiary times. But this idea is entirely unsupported by the investiga- 

 tions, of which the results are recorded in the present volume, as has already 



* Natural System of the Volcanic Kocks, p. 86. 



