Studies for Students. 



THE PRINCIPLES OF ROCK WEATHERING. 



( Concluded. ) 



(<:) MECHANICAL ACTION OF WATER AND OF ICE. 



Aside from its solvent capacity, water acts as a powerful 

 erosive agent, as well as an agent for the transportation of the 

 eroded materials. It is only its erosive power that need concern 

 us here, though as we shall see this is to a considerable extent 

 dependent upon its power of transportation. Every raindrop 

 beating down upon a surface already sorely tried by heat and 

 frost serves to detach the partially loosened granules, and, catch- 

 ing them up in the temporary rivulets, carries them to the more 

 permanent rills to be spread out over the valley bottoms, or per- 

 haps if the slopes be steep and the current accordingly strong 

 to the rivers and thence to the sea. The amount of detrital 

 matter thus mechanically removed from the hills and spread out 

 over valley and sea bottoms quite exceeds our comprehension, 

 but it is estimated that at the rate the Mississippi River is now 

 doing its work the entire American continent might be reduced 

 to sea level within a period of four and one half million years. 

 The Appalachian Mountain system has already, through this 

 cause, lost more material than the entire mass of that which now 

 remains. But the rivers, like the winds and glaciers, in virtue of 

 this load of sand they bear, become themselves converted into 

 agents of erosion, filing away upon their rocky beds, undermin- 

 ing their banks and continually wearing away the land by their 

 ceaseless activity. The pot-holes in the bed of a stream, formed 

 by the constant swirl of sand and gravel in an eddy, furnish on 

 a small scale striking illustrations of this cutting power, while 

 the rocky canyons of the Colorado of the west, where thousands 

 of feet of horizontal strata have been cut through as with a file. 



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