ORIGIN OF THE SOIL 13 



Jrom a distance by running water or other means are called 

 drift or transported soils. 



Weathering. The agencies that have served to break up the 

 bed rock into mantle rock and the mantle rock into soil are 

 grouped under the general title of weathering agencies. Two 

 phases of weathering may be distinguished, namely, weather- 

 ing by decomposition and weathering by disintegration. The 

 first is largely a chemical process in which some or all of 



FIG. 1. An outcrop of limestone showing weathering 



the minerals in the rock are resolved into their simpler com- 

 pounds, resulting in very fine particles ; the second is largely 

 a mechanical or grinding process and the resulting particles 

 are usually larger. 



Weathering by decomposition. Air and water are the chief 

 elements that are effective in decomposition. The oxygen in 

 the air readily enters into combination with other elements in 

 the rocks and soon tears down the surface layers exposed to 

 it, forming new compounds. Results of this process are seen 

 in the rusting of iron and the dulling of nearly all surfaces 

 exposed to the air for any length of time. In the quarry we 



