240 FIRST YEAR COURSE IN GENERAL SCIENCE 



Nile in Egypt, which formerly occurred annually, has been 

 a real benefit to the land. It has deposited the fertile soil 

 upon which depends the support of millions of people. The 

 great dam at Assouan now holds back the water, so that 

 it may be distributed by irrigating canals. 



259. Changes in the Continents. It is said that the 

 sediment deposited in one year by the Mississippi would 

 make a pyramid half a mile square and seven hundred feet 

 high a small mountain, in fact. This material must have 

 been removed from the land through which the river and its 

 tributaries flowed. What the Mississippi is doing, other 

 rivers are doing to a lesser degree. That means that the 

 mountains have been lowered and the hillsides have lost some 

 of the slowly made soil perhaps only the fraction of an inch; 

 but the final result must be the lowering and leveling of 

 the continents. By the work of rivers, mountains become 

 rounded and low, valleys are filled, and the borders of conti- 

 nents are extended. (LABORATORY MANUAL, Exercise 

 XXII.) 



260. The Usefulness of Rivers. Rivers are invaluable 

 to pioneers in opening up a new country. They furnish an 

 easy means of traveling and of carrying supplies, and often 

 penetrate forests otherwise impassable. After settlement is 

 completed, they continue to furnish the cheapest means of 

 carrying freight. 



The shorter, swift-flowing rivers, which are not navigable 

 because of falls and rapids, are useful for the power they 

 furnish to run machinery. The great rivers often have 

 rapids in their upper courses, where they are known as young 

 rivers, because of their velocity, great erosive power, and un- 

 even bed. The mature streams, characterized by gentle flow, 

 greater volume, and absence of rapids, transport fine rock 

 waste which the younger part of the stream has brought in. 

 They carry upon their surface millions of dollars of freight 

 from the interior to the seaports. 



