162 



GEOLOGY. 



rapids or falls would promptly recede, and with the recession, the 

 acceleration of velocity resulting from the uplift would be felt farther 

 and farther up-stream, and ultimately to its source. The rejuvenated 

 streams would cut new valleys in the bottoms of their old ones (Figs. 

 152 and 153). The new valleys would begin where the increase in 

 velocity was first felt, and they would be lengthened by head erosion 

 just as valleys of the first cycle were lengthened. 



When the head of the new part of a valley of a rejuvenated stream 



Fig. 153. — Diagram to illustrate in ground plan an ideal case of rejuvenation as the 



result of uplift. 



recedes past the mouth of a tributary adjusted^ to the gradient of 

 the main stream before rejuvenation, the velocity of the tributary is 



1 This sort of adjustment may be called topographic adjustment. A tributary is 

 in topographic adjustment when its gradient is harmonious with that of its main. 



