AGENTS THAT DESTROY ROCKS 133 



lor just as we now behold them; but those who 

 have followed us thus far will have seen that such 

 a conclusion must necessarily be erroneous. The 

 remains of animals and vegetables imbedded in the 

 solid rock, thousands of feet beneath the surface, 

 must have led them to conclude that such rocks 

 were formed after the animals and vegetables con-" 

 tained in them had existed ; and, in short, that rocks 

 are produced by agents similar to those that are 

 now exerting their influence upon the surface of the 

 globe. But, in order to show how rocks are con- 

 tinually forming at the present time, we must first 

 point out how they are destroyed ; for it is by their 

 disintegration and decomposition that materials are 

 furnished for new formations. 



Slight observation teaches us that the surface 

 of the earth is everywhere undergoing important 

 changes. The hardest rocks are broken down and 

 decomposed, and a variety of causes are aiding in 

 this work. Although, to the casual observer, the solid 

 inorganic world seems to be steadfast and immo- 

 vable, yet the earth is constantly taking new states, 

 even beneath his own feet. The effects, it is true, 

 may be gradual or almost imperceptible, but they 

 are none the less certain or efficient. Every stream 

 or river which flows through our country bears along 

 a greater or less proportion of mineral substances ; 

 which, being carried into the sea, are again thrown 

 by the waves upon the beach, thus forming a sandy 

 soil like that of New- Jersey or on the eastern side 

 of Long Island. 



Rocks suffer disintegration from three different 

 causes, namely: 1. Mechanical Agents ; 2. Chemi- 

 cal Action ; 3. The Freezing and Thawing of Water. 



I. Mechanical Agents which Destroy Rocks. 

 Water, in its influence upon rocks, may be justly 

 considered as a mechanical agent, although, to a 

 certain extent, it is a chemical one also, as it has 

 M 



