64 DISINTEGRATION". 



The avidity with which organic substances, whether 

 derived from dead plants or animals, absorb oxygen, 

 is still greater than that of iron. The breaking 

 down, decay, gradual solution, and disappearance of 

 these, is caused principally by air; for decomposi- 

 tion consists of a constant absorption of oxygen, 

 which forms with the elements of these organic 

 substances new, soluble, and volatile compounds, 

 which, where vegetation abounds, are in great part 

 absorbed by the roots and foliage ; otherwise they 

 are lost by evaporation. Air in motion, as in winds 

 or storms, may also, by its mechanical power, di- 

 rectly or indirectly promote the cracking or loos- 

 ening of the earth, as also the circulation of air in 

 the soil, by its pressure on the crown of a tree, thus 

 exercising a leverage on the roots." 



We have before stated that it was unsafe to judge 

 wholly of the nature of the soil by the rocks which 

 underlie it, on account of the numerous chemical 

 and mechanical agencies which are at work in their 

 disintegration, decomposition, and mixture. If the 

 surface of the earth was a vast level, the substratum 

 of rock would be a more correct index to the nature 

 of the soil ; but such a condition of things would 

 be decidedly unfavorable to fertility, as it would en- 

 tirely prevent the mixture of the separated particles 

 of the different rocks. " The agency of water to 

 produce this result is almost beyond conception. 

 In a violent storm, we can perhaps form an idea 



