!208 ORIGIN OF SOILS. 



But this mode of waste of tlie existin<T solid rocks is not the one hj^ wliich soils are made : 

 these originate almost exclusively from mechanical action by abrasion, and from at- 

 mospheric influences, b}' which particles are separated from the rock and from each other. 

 This atmospheric action, however, is promoted by certain chemical changes among the 

 elements of tlie rock. Iron, in a state of protoxide, absorbs another equivalent of oxygen 

 from the atmospliere, and is converted into the peroxide, and such a change would be 

 one step towards disintegration. So almost any change whatever in the constitution of 

 the elements of a rock, though it is only a mechaniral product, will be followed by a se- 

 paration of its parts. All changes affecting the composition of a rock are promoted or 

 aided by frost. Water is absorbed more or less by rocks during the frosts of winter, and 

 the superficial portions gradually crumble and become detached. The exposed surface is 

 thus greatly increased, and hence the chemical changes are proportionally promoted. 



The nature of tlie rock itself may or may not favor disintegration. Rocks wiiose ele- 

 ments contain an alkali, or alkaline earth, undergo changes by wliich they are directly 

 converted into soils. Some granites and greenstones are of this description. Aluminous 

 rocks, soft slates and shales, are eminently disposed to disintegration : they break down 

 by moisture, without freezing. The presence of sulphuret of iron in these, or in any other 

 rocks, promotes those changes by which they become soils, especially when the iron is in 

 the state of a protosulphiuet. Other rocks, the pure sandstones and limestones, are acted 

 upon more slowly. 



Another condition which promotes the formation of soils, is the alternation of hard and 

 soft layers ; the latter are destroyed, leaving those which rest upon them to fall by their 

 own weight. 



Rocks exposed on the tops of mountains decay rapidly : the intensity of the frost, and 

 the length of time during which they are exposed to it; the suddenness of the changes of 

 temperature to wliich they are subjected ; and the dampness of the air during the summer, 

 when watery vapours condense upon their summits and sides, are circumstances that favor 

 the destruction of rocks in these places. 



With these causes in continual operation, the solid strata are broken down into soil. 

 No matter how hard the rock mtiy be : some change takes place ; some impression is made 

 upon it, and some matter is separated from it, v/hich goes to increase the amount of debris 

 covering the surface of the earth. 



If these, however, were the only causes in operation ; if there were no other movements 

 than those of the simple separation of the particles of rock from each other, the soil would 

 be very different from what we now find it : it would be less in quantity, or thinner, 

 over the whole earth, and its general characters would be somewhat different. Each rock 

 would then be covered by its own debris, and the soil would partake exclusively of the 

 character of the rock from which it is derived. But soil or debris, when formed, is not 

 suffered to remain in situ ; and this leads us to the consideration of those causes by which 

 it is and has l)eeu distributed. 



