184 



THE SOIL: INORGANIC MATTER 



(a) Granite. — (Fig. 39). This is one of the most common 

 igneous rocks and many soils are derived from it. It is 

 usually composed of quartz, feldspar, mica, and hornblende. 



Fig. 39. — Soil-forming rock: Granite. United States Geological Survey. 



The action of water, heat, and cold serve to break it down 

 into small particles. Aided by the chemical action of water 

 and carbon dioxide the feldspar changes in part to kaolinite 



