OKlGm AND r'or.^rAtioN^ oi;" soils. 123 



The first step towards tlie formation of a soil must have 

 been the pulverization of tlie rock. This has been accom- 

 plished by a variety of agencies acting through long pe- 

 riods of time. The causes which could produce such re- 

 sults are indeed stupendous when contrasted with the 

 narrow experience of a single human life, but are really 

 trifling compared Avith the magnitude of the earth itself, 

 for the soil forms upon the surface of our globe, whose di- 

 ameter is nearly 8,000 miles, a thin coating of dust, meas- 

 ured in its greatest accumulations not by miles, nor 

 scarcely by rods, but by feet. 



The conversion of rocks to soils has been performed, 

 1st, by Changes of Temperature ; 2d, by 3Ioving Water 

 or Ice J 3d, by the Chemical Action of Water and Air j 

 4th, by the Influence of Vegetable and An imal life. 



1. — Changes of Tempekature. 



The continued cooling of tlie globe after it had become 

 enveloped in a solid rock-crust must have been accom- 

 panied by a contraction of its volume. One effect of this 

 shrinkage would have been a subsidence of portions of 

 the crust, and a wrinkling of other portions, thus produc- 

 ing on the one hand sea- basins and valleys, and on the 

 other mountain ranges. Another effect would have been 

 the cracking of the crust itself as the result of its own 

 contraction. 



The pressure caused by contraction or by mere weight 

 of superincumbent matter doubtless led to the production 

 of the laminated structure of slaty rocks, which may be 

 readily imitated in wax and clay by aid of an hydraulic 

 press. Basaltic and trap rocks in cooling from fusion often 

 acquire a tendency to separate into vertical columns, 

 somewhat as moist starch splits into five or six-sided frag- 

 ments, when dried. These columns are again transversely 

 jointed. The Giant's Causeway of Ireland is an illustra- 

 tion. These fractures and joints are, perhaps, tlie first oc- 

 casion of the breaking down of the rocks. The fact that 



