132 



GEOLOGY. 



less rock decay, and less vegetation to hold the products of decay. 

 The more resistant beds of rock therefore come into greater prominence, 

 especially on slopes, where they develop chffs (Figs. 109 and 110). 

 These general principles find abundant illustration in the plateaus of 

 the western part of the United States/ where the chffs are by no means 

 confined to the immediate valleys of the streams (Fig. 1, PL XII). 



Fig. 110. — A Butte. A characteristic feature of the arid plateau region of the West. 

 (Button, Mono. II, U. S. Geol. Surv.) 



EFFECTS OF UNEQUAL HARDNESS. 



In the preceding pages incidental reference has been made to the 

 results of inequahties of rock resistance. This topic will now be con- 

 sidered more fully. 



Rapids and falls. — Returning for a moment to the hypothetical 

 island with which our study of erosion began, let a horizontal layer of 

 hard rock be assumed to run through it (H, Fig. 111). As the rain 



1 Dutton. Tertiary History of the Grand Canyon District, Mono. II, U. S. Geolog- 

 ical Survey. 



