GREAT RIVERS OF THE UNITED STATES 359 



all its erosive helpers. The rill, the rivulet, the intermit- 

 tent torrent, the sand blast of the scouring wind, the pull 

 of gravity, the varied resistances of the rock layers, the 

 structure and composition of these layers have each and 

 all left their peculiar impress upon the resulting sculpture. 

 Standing beside this mighty chasm, one is impressed, as no- 

 where else, with the mighty power of erosive agents. 



THE COLORADO RIVER. 

 Flowing through a deep-cut, narrow valley. 



And yet here is seen only the beginning of the vast 

 work which these forces have before them. They have 

 built only the narrow trough of what must be developed 

 into a wide and gently sloping valley, and have hewn out 

 here and there a ravine in that great upland which in 

 time they must carve into the mature forms of a thoroughly 

 dissected country. If the region had not been so dry, the 



