316 RESUME AND THEORETICAL DISCUSSION. 
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. 
There is no foundation in fact for the idea aavocated by Richthofen,* in 
his masterly essay on the relation of the distribution of the voleaniec rocks to 
the configuration of the surface of the globe, that, durmg 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 the 
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 Voleanic Rocks, p. 86. 
