190 



GEOLOGY AND CHEMISTRY OF THE SOIL. 



When the pyrites exists m slate rocks, containing much 

 alumina, magnesia and lime, the sulphuric acid combines 

 with these bases, by which nearly the whole rock is gradual- 

 ly converted into soil. Were this the only agent acting upon 

 the rocks, the character of the soil would be accurately 

 known, by examining the rock which underlays it ; but this is 

 rarely the case. 



3. The mechanical agency of water, aided by cold and 

 heat, and by its currents and waves, not only aids in break- 

 ing down the solid masses, but transports the pulverized ma- 

 terials in the form of detritus, and deposits them in lower 

 lands. Thus the substances of different rocks are mingled 

 together. Freezing water exerts an immense power in this 

 respect. The water penetrates every seam and crevice of 

 the rocks, and, by its expansive power in the act of freezing, 

 forces the parts asunder, and creates new fissures, which are 

 each year increased in number and width. Nor does this 

 influence cease after the rocks are fully converted into soils ; 

 each year the expansive force of water tends to pulverize, and 

 render the earth light and porous. 



The friction of running water wears off" the rocks, and re- 

 moves that which has become broken down by chemical ac- 

 tion. The particles being suspended are carried down by 

 the force of the stream, and deposited along the banks and at 

 the mouths of rivers. 



That the agency of water, at some ancient period, has ex- 

 erted a very great influence upon the rocks and soils appears 

 from the fact, that over the whole northern hemisphere, the 

 rocks and soils have nearly all been removed in a southerly 

 direction, and the materials of different formations variously 

 mingled together. This has been shown to have resulted 

 from the action of glaciers, by which the whole surface has 

 become scratched, and the sand, gravel and boulders rolled 

 up into hills, with ponds and vallies between. 



4. Decaying plants tend to convert the rocks into soils. 



