io8 



GENERAL SCIENCE 



Continued erosion of hillsides and valley slopes is a menace to the 

 wealth of the nation. There is need for intelligent cultivation of the 

 soil on the steeper slopes to conserve its productiveness. 



Irrigation has brought under cultivation large tracts of land highly 

 productive when well watered. Many of these irrigation projects are 

 government enterprises not only in this country but in Egypt, India, 

 and elsewhere. 



Hills very generally represent relatively hard formations of the 

 earth's crust which have resisted erosion. In general, too, the slopes of 

 the older valleys are less abrupt, and the streams through them are 

 wider, more shallow, and slower in current. 



FIG. 40. A type of dredge used in removing sand bars and silt deposits in 

 the Mississippi River. 



A "divide" is the crest at the upper edge where two slopes meet, 

 and where the waters from rainfall and snowfall drain off in different 

 directions. A river system consists of a main stream and all its tribu- 

 taries. A river basin is the land area whose run-off contributes water 

 to a river system. 



Exercises 



1. Name sections of this country where water power is abundant and cheap. 

 Why is it not so in other well-watered regions? 



