MECHANICAL ACTION OF WATER AND ICE 175 



3. MECHAKECAI. ACTION OF WATER AND OF ICE 



Aside from its solvent capacity, water acts as a powerful ero- 

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

 eroded materials. It is only its erosive power that need con- 

 cern lis here, though, as will be seen, 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, catching them up in the temporary rivulets, carries them 

 to the more permanent rills, to be spread out over the valley 

 bottoms, or perhaps, if the slopes be steep and the current ac- 

 cordingly 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 ex- 

 ceeds our comprehension, but it is estimated that at the rate 

 the Mississippi River is now doing its work, the entire Ameri- 

 can continent might be reduced to sea-level within a period of 

 four and one-half million years. The Appalachian Mountain 

 system, the uplifting of which began in early Cambrian times 

 and terminated at the close of the Carboniferous, 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 they bear, become themselves 

 converted into agents of erosion, filing away upon their rocky 

 beds, undermining their banks, and continually wearing away 

 the land by their ceaseless activity. The pot-holes in the bed 

 of a stream, formed by the swirl of sand and gravel in an 

 eddy, furnish on a small scale striking illustrations of this 

 cutting power, while the rocky canons of the Colorado of the 

 West, where thousands of feet of horizontal strata have been cut 

 through as with a file, show the same thing on a scale so gigantic 

 as to be at first scarce comprehensible.^ An item of no in- 

 significant importance to be considered here is the possibility, 

 indeed probability, of an incidental chemical decomposition 

 taking place during this abrasive action. Daubree showed'* 



^Captain C. E. Dutton has estimated (Tertiary History of the Grand 

 Canon of the Colorado) that from over an area of 13,000 to 15,000 square 

 miles drained by the Colorado Eiver, an average thickness of 10,000 feet of 

 strata have been removed. 



2 It will be remembered that this authority placed rock fragments in stone 

 and iron cylinders containing water and made to revolve horizontally at a 



