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ORIGIN OF SOILS. 
But this mode of waste of the existing solid rocks is not the one by which soils are made : 
these originate almost exclusively from mechanical action by abrasion, and from at¬ 
mospheric influences, hy 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 the rock. Iron, in a state of protoxide, absorbs another equivalent of oxygen 
from the atmosphere, 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 mechanical 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 the rock itself may or may not favor disintegration. Rocks whose ele¬ 
ments contain an alkali, or alkaline earth, undergo changes by which 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 protosulphuret. 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 which 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 may be : some change takes place ; some impression is made 
upon it, and some matter is separated from it, which 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 been distributed. 
