io6 GENERAL SCIENCE 



In Physical Geography the relation of erosion to valley 

 formation is discussed at length, together with the wonder- 

 ful results accomplished by streams in making broad and 

 deep ways for themselves. When one looks upon channels 

 that have been worn down through solid rock hundreds of 



FIG. 38. The Roosevelt Dam on Salt River, Arizona, is 280 feet high. It 

 has a storage capacity sufficient to cover with water 1,367,000 acres one foot 

 deep. Below the dam, and out of sight in the picture, is a power-house 

 capable of developing 7,500 horsepower. 



feet in depth, and miles in length, the power of running water 

 to do work almost passes belief. And the wonder does not 

 grow less as one considers that this continuous flow of the 

 waters toward sea-level is made possible by their continuous 

 return as atmospheric moisture to elevated regions. As 

 result of solar heating and the vaporization of oceanic waters 

 and the precipitation of this moisture inland on the higher 

 levels, the flow of streams is continuous, and the work of 



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