ON THE EROSIONS OF THE EARTH'S SURFACE. 



General Remarks. 



The vast amount of denudation which the earth's surface has experienced is 

 shown by the following facts : — 



1. The great amount of boulders, gravel, sand, clay, and loam, that is spread 

 over the solid rocks. 



That these materials once constituted a part of the solid strata, it would seem, 

 cannot be doubted by any one who has observed natural operations at all. For 

 he must have seen the process of abrasion and comminution going on everywhere. 

 Let him go to the shores of any river, and he will see the work in progress. Where 

 the stream is rapid, the materials at its bottom and along its shores will be coarse 

 and not thoroughly rounded ; where less violent in its movement, well-worn pebbles 

 will be seen mingled with coarse sand; that is, such materials as that amount of 

 current would urge along. Fine sand, clay, and loam, will appear where the 

 stream is very slow ; because such a current can separate and sweep along only the 

 minute fragments of which such deposits are composed. But in all these cases the 

 fragments, if examined, will be found to be portions of the rocks over which the 

 stream passes ; and, moreover, we find in many places that the river, sometimes in 

 the form of ice, has power to break off and grind down portions of the rocks. 



Now these detrital materials are spread- over perhaps nine-tenths of the surface, 

 even in mountainous regions, save in some very precipitous and elevated parts. 

 Their thickness, also, often amounts to hundreds of feet. In short, the loose 

 materials spread over four-fifths of the surface, amount to a thick rock formation ; 

 and all accumulated by the slow processes of erosion now going on before our eyes. 

 How vast the period requisite to accomplish the work ! 



2. By the deep troughs worn out of the loose materials by rivers. After the 

 detritus has been deposited the stream sinks by wearing away a portion of the 

 mass. This process has sometimes gone on to a depth of 100 or 200 feet; and 

 though this is a small erosion in comparison with that already named, it deserves 

 notice in this connection. 



3. Nearly all the fossiliferous rocks are composed of materials abraded from 

 previously existing rocks, and subsequently consolidated. The former, it is well 

 known, are several miles thick in all the countries where they have been measured. 



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