FOKMATION OF SOILS. 237 



water aud the Incombustible ash come from the soil. In the case 

 of aquatics this general statement would not appear to hold, for 

 thejr obtain all their substance from the water in which they live ; 

 but, as will be seen later, this source is essentiallj' the same. 

 We have examined in the previous chapter one of the means 

 b}- which plants obtain their supplj' of water and ash materials, 

 and it will be best to consider now the source from which this 

 supply comes, before approaching the studj' of the combustible 

 substance of plants. 



SOILS. 



635. Formation of soils. Soils are produced bj' the disinte- 

 gration of rocks. This may be mechanical, as that caused by 

 crushing, attrition, and the action of frost; or it may be and 

 generally is associated with more or less chemical change. In 

 soils, some of the products of the decomposition of organic sub- 

 stances are usually intermingled with purely mineral matters 

 aggregated in various degrees of fineness. Soils exposed to 

 atmospheric influences constantly change both in their ph^ysical 

 properties and chemical composition, the changes being brought 

 about chiefly by the combined action of moisture, carbonic acid, 

 and oxygen. 



636. Water not only wears awaj- solid rocks by its mechanical 

 action, but after it has insinuated itself into the crevices of 

 rooks it accomplishes the work of disintegration far more rapidly 

 bj- its expansion during freezing.^ 



When rocks become loosened by running water, or by the 

 slow movement of glaciers, the crushing and grinding of the 

 pieces which come into contact are sufficient to pulverize 

 the hardest of the more common ones. 



Water, especiall3' when it holds carbonic acid in solution, is 

 a very important agent in ciianging the characters of rocks ; 

 sometimes it does this by dissolving out portions of the rocks, 

 sometimes by bringing about new combinations of their con- 

 stituents. Moreover, rain-water contains a minute quantity of 

 other matters besides carbonic acid, and these exert a powerful 

 eflec!t in disintegrating and dissolving certain rocks. 



637. The free oxygen of the atmosphere is also an efficient 

 agent in the changes by which rocks are broken down to form 



1 The amount of expansion ia usually given as approximately one fifteenth 

 of the volume. 



