ORIGIN OF SOILS 65 



The chemical classification of igneous rocks depends on the 

 relative quantities of silica and bases present. Since silica is the 

 acid portion of minerals, rocks containing 65 to 75 per cent, of 

 silica are termed acid rocks, those containing 55 to 65 per cent, 

 are called intermediate, and those carrying 40 to 55 per cent, are 

 called basic. Crystalline acid rocks contain quartz, while basic 

 rocks do not contain enough silica for free quartz to crystallize 

 out. 



The granite group comprises rocks rich in silica and alkalies, 

 containing 65 to 75 per cent, silica and 5 to 8 per cent, of alkalies, 

 of which y^ to 2 /z consists of potash. They are, as a rule, much 

 richer in potash than other igneous rocks, and form correspond- 

 ingly better soils. 



Granite, the crystalline rock of this group, is very abundant, 

 and soils derived from it are quite common. Granite soils are 

 usually clay containing particles of quartz and mica, and they are 

 often fertile, being especially rich in potash. Rhyolite, which 

 is a porphyritic rock of this group, is extensively distributed 

 in the western part of the United States. 



The syenite group of rocks resembles the granite group, except 

 that the rocks contain less silica (55 to 65 per cent.) and more 

 bases to correspond. Like the granites, the syenites are rich in 

 potash. 



The diorite group contains about the same amount of silica as 

 the syenites, but less alkalies and more lime and magnesia. 



The basalt group contains the basic rocks (40 to 55 per cent, 

 silica). These rocks contain small amounts of alkalies, and are 

 rich in iron, lime, and magnesia. 



The soils of the Piedmont plateau, in the eastern part of the 

 United States, are derived mostly from igneous or metamorphic 

 rocks, and. consist of sands and clays containing quartz and mica. 

 This area extends from New York City to near the middle of 

 Alabama. The soils of the eastern Appalachian Mountain region 

 are also of similar origin. An extensive area in Washington, 

 Oregon, and Montana is covered with soils derived from the 

 weathering of basalt and other igneous rocks. 



