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POPULAR SCIENCE MONTHLY. 



water, there begin the attacks of the forces operating to destroy them. 

 Hand in hand there go on growth and destruction. The two may 

 keep an even pace; either may obtain the mastery. In the one case, 

 lack of considerable elevation and flatness result; in the other, great 

 altitudes may be attained. The rivers may cut their valleys down- 

 ward as fast as the land rises, or the down-cutting may be relatively 

 slower. In any case, after a given land mass has attained its greatest 

 height above the sea, the larger rivers soon cut their channels down 

 as far as river cutting is possible- — namely, to within a few feet of 



• 



-■■*• ". -«a»#i- ■ '" ■■■»;»'■;■ ;*f. ■**■'■ ^ftt'^F'. 



■■ ■■•■ '■ . & ■i'^mw^ WimbMFyti. 



1 Miff 



Fig. 8. — Horizontal Rocks, Grand Canon of the Colorado. 



sea level. With relatively rapid elevation, soft rocks, and large 

 rivers, the resultant valley takes the form of a canon, examples of 

 which are found along the courses of the Colorado and the Yellow- 

 stone Rivers (see Fig. 8).* Valleys of this nature soon lose their 

 steep sides by the action of weathering and all that this implies, and 

 pass into a more open state, like that shown in Fig. 9. 



These views have been selected in order that a comparison of this 

 type of mountain structure may be made with that shown in Fig. 



* The bottom of the canon at this point is between four and five thousand feet below 

 the flat surfaces in the foreground — a sheer descent of nearly a mile. 



