6 W. M. DAVIS 



by persistent upheaval, are enabled to deepen their valleys as 

 rapidly as the region is upheaved. In this stage (if the valley sides 

 are still kept essentially graded from stream bank to ridge crest), 

 the ridges will be lowered as fast as the valleys are deepened, and 

 both the absolute altitude of the crests, which is then at its maxi- 

 mum, and their local rehef will remain constant; and the ridge 

 altitudes will still be as nearly alike as the streams are uniformly 

 spaced. (The deductive analysis which leads to this conclusion 

 is not presented. In its absence, it is difficult to understand how 

 the larger streams can be impelled to more rapid downward erosion 

 than before by the continued aiid uniform upheaval of the mountain 

 region, without being thrown out of their graded condition and 

 therefore incising rock-walled gorges in the bottom of their previ- 

 ously graded valleys; and in that case the control of ridge-crest 

 height by valley depth must be imperfect. Perhaps a postulate of 

 non-uniform upheaval would lessen this difficulty.) 



When upheaval at last ceases, it is supposed that for a time the 

 valleys will still continue to be degraded by downward erosion. 

 In this stage the ridges, in so far as their side slopes are graded 

 with respect to the valley bottoms, will for a time retain sharp 

 crests, and their local reUef will remain unchanged although their 

 absolute altitude now begins to decrease. (It is difficult to accept 

 this deduction. If the streams and the ridge slopes were essentially 

 graded during the latest phases of upheaval, the valleys could not 

 be deepened significantly until the detrital load that had to be swept 

 along them by the streams was decreased; and such decrease 

 would not ensue until the ridge slopes were reduced to a gentler 

 declivity than before, and such a reduction of declivity would 

 entail a reduction of ridge-crest relief over valley bottom.) At a 

 still later stage, the land mass remaining stationary, the streams 

 will further diminish their fall and become more perfectly graded. 

 The valley floors will then be opened and widened, the graded slopes 

 of the ridges will be worn to a gentler declivity, and the ridge crests 

 will be rounded. The ridges will then decrease in local relief as well 

 as in absolute altitude. Finally downward erosion ceases, the 

 valleys are still further broadened, the rounded ridges are worn« 

 down to broad swells, and the swells are eventually almost obHter- 



