RIVERS AND THEIR WORK 



241 



Knowing the character of the towns and cities upon a 

 river, one can tell which is the youthful and which the 

 mature part. Manufacturing places on the borders of the 

 young working river are followed lower down by centers of 

 commerce and trade on the mature navigable stretch. 



261. The Absence of Rivers. A land without rivers is 

 usually a desert. The principal reason for the absence of 



FIG. 128. SPRUCE TREE CANYON, MESA VERDE, COLORADO 



1. What are the signs of surface drainage into the valley? 2. How 

 has weathering changed the side of the valley? 3. What is the cause of 

 the horizonal caverns? One of them contains ruins of a village deserted 

 long before the Spaniards discovered them 400 years ago. 



rivers is that the winds blowing from the great source of 

 vapor, the ocean, lose their moisture before reaching the 

 interior of the continent. If the warm vapor-laden winds 

 encounter a range of mountains, the air cools as it rises, and 

 the vapor condenses. Rain falls on the slopes of the moun- 

 tain nearest the ocean. Here there are forests, mountain 

 streams, and peaks whose melting snow keeps the supply 



