ORIGIN AND FORMATION OF SOILS. 123 
‘The first step towards the formation of a soil must have 
been the pulverization of the 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 with 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 Moving Water 
or Ice ; 3d, by the Chemical Action of Water and Air ; 
4th, by the Influence of Vegetable and Animal Life. 
1.—Cuances oF TEMPERATURE, 
. The continued cooling of the globe after it had become 
enveloped ina 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, the first oc- 
easion of the breaking down of the rocks. The fact that: 
