WATER, AND ITS USES 



107 



leveling down continents goes on ceaselessly. Wherever the 

 bedrock of the earth's crust appears at the surface without 

 any covering of soil, it may be supposed that erosion by the 

 agencies of wind and water has occurred, and that the dis- 

 integrated rock particles resulting from weathering have 

 been carried elsewhere. 



Sr^sirKw^W 



FIG. 39. Principal irrigation projects in western United States. 



SUMMARY 



The navigable streams of the United States figure largely in the 

 history of its exploration and settlement. 



Water power is a resource fundamental to the nation's industrial 

 life. Agricultural enterprise on the largest scale is possible only in 

 broad and fertile valleys where the slopes are gentle, and the soil is deep. 



