78 CHEMISTRY OF FARM PRACTICE 



when sudden changes of temperature occur, there is a 

 tendency to break up the rock. The surface of the rock 

 is subjected to more rapid changes, due to outside influence 

 of heat and cold, and this influence tends to form flakes, 

 cracks, and crevices, even on the outer surface of the same 

 mineral. Water is retained in the cracks and crevices, and 

 exerts its solvent influences. Then also, when water cools, 

 it first contracts, becoming densest at 4 Centigrade, then 

 expands till the temperature falls to zero. These changes 

 of volume tend to break up rocks. When the water freezes, 

 there is a sudden expansion by which such enormous pres- 

 sure may be exerted as to shatter the strongest rocks. 



After ^oit is formed in the crevices of rocks, plants 

 grow, and the roots of these plants have a solvent effect 

 on the rock, due to the excretion of sap. Later, when 

 more soil has formed, trees may grow between the rock 

 masses, exerting a powerful force tending to separate the 

 masses. In addition to this action of plants and trees 

 in separating masses of rock, minute plant organisms, such 

 as lichens and mosses, grow on rock surfaces and form soil 

 which is either washed off or deepens until it furnishes a 

 home for higher orders of plants, which in turn are followed 

 by trees. These growing trees and plants get their plant 

 food from deep down in the soil, and drop their leaves on 

 the surface of the ground. The decay of these leaves 

 impregnates the soil moisture with carbon dioxide, in 

 which condition it has a much greater solvent effect on 

 the mineral portion of the soil underneath. The soil layer 

 is thus continuously deepened through additional deposits 

 above and the continued solution of the rock beneath. 



Glaciers have exerted in past ages a powerful influence 

 on soil formation. These vast ice fields crept slowly south- 

 ward, grinding the rocks beneath them to a condition of 

 great fineness. Some of the richest soils of the world are 

 of glacial origin. 



Rivers emptying into the ocean, on account of the 



