SOIL FORMATIONS AND CLASSIFICATIONS 



55 



Alluvial soils are the common formation in river valleys and other 

 lowlands that receive deposits of material washed from the higher 

 lands. 



Soil materials. Soil materials consist of stones, gravel, sand, 

 silt, clay, and organic matter. The term day, as correctly used, 

 is applied to the material that gives to certain soils their sticky, 

 plastic property, including hydrated aluminum silicate and other 

 plastic substances, in part reduced probably to the molecular 

 state of division and without granular character, although most 

 so-called " clay " contains more or less undecomposed, or but 

 partially decomposed, mineral particles. Silt includes a grade 

 of particles that are smaller than sand, impalpable in fact, but 

 still granular as seen through the microscope, and not plastic 

 when free from clay. 



Soil types. Soil types are based largely upon the relative propor- 

 tion of these several soil materials, as may be noted by inspection 

 or determined by mechanical analysis. The following general 

 groups are recognized: 



TABLE 6. SOILS: GENERAL GROUPS 



