Lawson.I 



The Upper Kern Basin . 



313 



evolution. Such senility implies an approximation by erosional 

 reduction, to base level. But under the hypothesis of differen- 

 tial degradation, if applied to the Summit Upland, no such 

 approximation to base level is implied, and is unnecessary to 

 produce the effects observed. That differential degradation docs 

 actually effect the results here ascribed to it is finely demon- 

 strated on the northern flanks of the Caucasus, on the route from 

 Kislowodsk to Elbrus. Here the stripping of soft Cretaceous 

 clays, down to a thick stratum of hard, Jurassic limestone, gives 

 rise to gently sloping plateaux of great expanse which are high 

 above the streams of the region. Farther down the slope a 

 similar stripping of the soft Tertiary sediments, from a hard 

 stratum of the upper Cretaceous upon which they repose, results 

 in similarly expansive, gently sloping plateaux far above base 

 level. The conditions are not the same as in the case we are 

 here concerned with, but the principle involved is identical. 



In applying this hypothesis of differential degradation to 

 the Summit Upland, then, the favoring considerations are: 

 ( 1 ) The adecpiacy of differential degradation to produce the 

 results observed at altitudes far above base level, provided (a) 

 that the structural plane involved was near and but little above 

 the present surface of the upland; (&) that the plane was uneven 

 in the same sense that the present surface is; and (c) that the 

 rocks above the contact on the roof of the granite were much 

 more easily eroded than the underlying granite. (2) That these 

 three provisions are in general realized. 



The alternative hypothesis is that the fiat summits are the 

 remnant of a region of senile geomorphy, representing an 

 approximation to base level by processes of erosional reduction, 

 independent of structural control. The character of the surface 

 of the Summit Upland as described, and as shown in the photo- 

 graphic illustrations, seems to the writer to favor the former 

 hypothesis rather than the latter. 



A remarkable feature of the upland surfaces is the prevailing- 

 absence of water courses or incisions of any kind due to stream 

 cutting. This indicates a peculiar status of the erosional 

 process. The surface is everywhere encumbered by blocks of 

 granite, formed by the intersection of joints and dislodged by 



