Lawson.] 



The Upper Kern Basin. 



863 



had been, as is now generally recognized, reduced to a region of 

 very moderate relief, and on its western flanks was in the condi- 

 tion of a peneplain. The more eastern portion was, however, 

 still a distinct, though low, mountainous country draining across 

 the peneplain to the sea, and was higher to the south than to the 

 north. This being the case we cannot refer the flat tops of 

 the Summit Upland, such as are exemplified in the summits of 

 Mt. Whitney, Table Mountain, Sheep Mountain and Cirque 

 Peak, to the residuals of a peneplain. If these flat tops of the 

 summit region be remnants of a peneplain, then they must in 

 in later Tertiary time have stood at base level. And if this 

 were so it is difficult to see how they should have escaped sub- 

 mergence in the Miocene sea which overflowed the western flanks 

 of the range and left there a thick deposit of sediments. Again, 

 the gradual mergence of these flat tops into the western slope of 

 the summit region by smoothly flowing curves militates against 

 the notion that such summits were ever at base level. For if 

 such a base leveled plain were elevated it would be sharply dis- 

 sected, aud we should have some trace left of the contrast 

 between the original surface of the peneplain and the more 

 abrupt slopes due to dissection. The smoothly flowing curves by 

 which the flat summits descend to lower levels may indicate 

 advanced maturity of geomorphic evolution, but they lend no 

 support to the view that such summits were ever in the perfectly 

 senile condition implied in a reference of them to residuals of a 

 peneplain. 



This hypothesis being rejected, it is sought to explain the 

 flat tops as due to differential degradation under the control of 

 structure, the particular structure involved being the contact 

 plane between the granite and its roof of less resistant rocks. 

 Such a structural feature certainly existed over the region, and 

 must have influenced the course of erosion in no small degree. 

 The only question is whether the contact of the granite and its 

 roof was sufficiently near the present surface to have given 

 character to that surface. Analogy of northern California and 

 the fact that portions of the roof of the granite still remain, even 

 in the region under consideration, indicate that the contact was 

 not far above the present surface of the mountain summits, and 



