60 THE SOIL. 



ences of the atmospliere, it becomes so 'ooft tliat it 

 inay be easily crushed between the thumb and finger. 



Other actions, of a less simple kind, exert an in- 

 fluence on the stubbornness of rocks, and cause them 

 to be resolved into soils.* Of course, the composi- 

 tion of the soil nmst be similar to that of the rock 

 from which it was formed ; and consequently, if we 

 know the chemical character of the rock, we can tell 

 whether the soil formed from it can be brought under 

 profitable cultivation. Thus felspar, on being pul- 

 verized, yields potash ; talcose slate yields magnesia ; 

 marls yield lime, etc. 



The soil formed entirely from rock, contains, of 

 course, no organic matter. Still, it is capable of 

 bearing plants of a certain class, and when these die, 

 they are deposited in the soil, and thus form its or- 

 ganic portions, rendering it capable of supporting 

 those plants which furnish food for animals. Thou- 

 sands of years must have been occupied in prepa- 

 ring the earth for habitation by man. 



As the earthy part of the soil is usually the lar- 

 gest, we will consider it first. 



As we have stated that this portion is formed 

 from rocks, we will examine their character, with a 

 view to showing the different qualities of soils. 



As a general rale, it may be stated that all rocks 



* In verj' many instances the crevices and seams of rocks are 

 permeated by roots, wMch, by decaying and thus iaducing the 

 growth of other roote, cause these crevices to become filled with 

 organic matter. This, by the absorption of moisture, may expand 

 with sufficient power to burst the rock. 



