226 AGRICULTURE 



When masses of rock are heated by the sun they expand; 

 sudden cooling at night or from change of weather causes 

 so rapid a contraction that they are rent asunder. Water 

 freezing in crevices also constantly breaks masses of rock 

 into smaller pieces. 



Rocks undergo decomposition largely through changes 

 due to the action of chemicals carried in water. The min- 

 eral substances in the rocks are dissolved, and new prod- 

 ucts formed. 



A plowing contest. 



Surface and subsoil. The terms soil and subsoil are 

 often used to distinguish the top portion from the soil that 

 lies underneath. The line between the dark humus-colored 

 part and the lighter soil below- is sometimes spoken of as the 

 division between soil and subsoil. Another distinction made 

 is to call all that lies below the depth of tillage subsoil. No 

 such dividing line can be sharply drawn, however, as much 

 organic matter is found below the humus line, and the depth 

 of tillage does not mark a natural division in layers of the 



