14 THE PRINCIPLES OF SOIL MANAGEMENT 



are usually arranged in alternating layers of variable 

 thickness and extent, and have given rise to important 

 areas of soil. 



^Eolian rocks are relatively insignificant, and are 

 generally of a sandy or clayey character. 



Metamorphic rocks are correlated with all the other 

 types of rock, and have resulted from pronounced 

 alterations in other rocks. Their individual properties 

 are therefore similar to the rock from which they were 

 formed. Often their resistance to decay is increased 

 by the process, as in quartzite and slate. 



IV. CHEMICAL AND PHYSICAL AGENCIES OF 

 ROCK-DECAY 



There are five chief agencies of rock-decay. They 

 are, (a) the atmosphere, (b) heat and cold, (c) water, 

 (d) ice, and (c) plants and animals. The operations of 

 each of these agencies are of two sorts: (1) chemical; 

 (2) mechanical. The products of these two types of 

 force are distinctly different in their relaiton to the 

 plant. The chemical action of the various agencies 

 results in a changed composition of the minerals. 

 It results in the breaking down of the mineral com- 

 pounds, with the possible removal of the elements, as 

 when feldspar is changed to kaolinite. Here the base 

 potash, soda or lime is replaced by the elements of 

 water, and may be carried entirely away. The hydrated 

 residue loses some of its silica, and kaolinite is the result. 

 In other cases the change may be effected by the addi- 

 tion of material, as when pyrite is oxidized by the 



