GEOLOGY AND CHEMISTRY OF SOILS. 



CHAPTER IV. 



ROCKS AND THEIR RELATIONS TO VEGETATION. 



Soil is formed by the decomposition and wearing down of 

 ihe rocks, which are mingled with variable quantities of ani- 

 mal and vegetable matters. All soils consist of compound 

 bodies, and these compounds are generally formed, by the 

 union of two or three simple substances. In order, therefore, 

 to understand the nature of soil, and its relations to vegeta- 

 tion, it will be necessary to give a general view of the simple 

 and compound bodies, which enter into the composition of 

 the rocks, the manner in which they are combined, and the 

 process by which the rocks are converted into soil. 



Sect. 1. Simple Bodies which enter into the Composition of 

 the Rocks. 



The number of simple bodies known to chemists, is fifty- 

 five. Of these, only fourteen enter into the composition of 

 rocks and soils ; hence, these fourteen bodies constitute 

 nearly the whole matter of the globe. The remaining sub- 

 stances are found either in too small quantities to affect the 

 general mass, or exist only in particular locations of limited 

 extent. 



The proportion in which these bodies exist in the rocks, 

 beginning with the most abundant, is nearly in the following 



