RIVER EROSION 97 



amount, though, of course, some soluble materials are withdrawn 

 from the rocks, over which the waters flow. In limestones this 

 may be considerable, especially if the water be charged with 

 organic acids from a swamp or peat-bog. 



The mechanical work of a river is much greater than the chemi- 

 cal, and is dependent upon the velocity of the current, varying 

 directly as the square of that velocity. The velocity of a stream 

 is the rather complex resultant of several factors, the chief of which 

 is gravity ; the steeper the slope of the bed, the swifter the flow of 

 the water. A second factor is the volume of water, the velocity 

 varying as the cube root of the volume. That is to say, if one of 

 two streams which flow down the same slope has eight times as 

 much water as the other, it will flow twice as fast. Other factors 

 enter into the result, but slope of bed and volume of water are 

 much the most important. 



Pure water can do little to abrade hard rocks, though it can 

 wash away sand, gravel, and other loose materials. As in the case 

 of the wind, the stream merely supplies the power ; the implement 

 with which the cutting is performed is the sand, pebbles, and other 

 hard particles, which the water sets in motion. These abrade the 

 rocks against which they are cast, just as the wind-driven sand 

 does, but more effectively, because of the ceaseless activity of the 

 stream, and because many rocks are rendered softer and more 

 yielding by being wet. The cutting materials are themselves 

 abraded and worn finer and finer by continued friction against the 

 rocks and against one another. In the case of complex minerals 

 this abrasion is accompanied by more or less chemical decompo- 

 sition, as has been shown experimentally by rotating crystals of 

 felspar in a drum half filled with water. When the felspar was 

 ground down to mud, the water showed the presence of potash 

 and soda in solution. 



A river which is subject to sudden fluctuations of volume, being 

 now a rushing torrent and again almost dry, is a much more effi- 

 cient agent, both of erosion and of transportation, than is one 

 which carries nearly the same quantity of water at all times, or 

 which fluctuates only slowly. 



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