ORIGIN OF SOII^S 63 



pied its position. The soils derived from limestone deposits in 

 Kentucky and in Texas are sedentary, and so are the soils 

 derived from granite and other igneous rocks which are common 

 in the Piedmont plateau of the Atlantic states. Old sedentary 

 soils, from whatever kind of rock derived, are as a rule clays 

 colored by iron. The various mineral constituents are often in 

 an advanced stage of decay, the more soluble constituents having 

 been largely washed out. 



A section of the soil may not show a gradual change to the 

 underlying rock, but the change may be abrupt and sudden. 

 Such a soil may be formed when the soil particles are brought 

 from other localities, and deposited, in which case the soil is 

 termed a transported soil. 



Colluvial and Cumulose Soil. A colluvial soil is one which 

 has been removed, to some extent, from the original position, so 

 as to mingle with other rocks and layers, as when a soil is 

 washed or moved down hillsides or sloping land. Such soils 

 commonly "creep" or have a slow annual movement. Colluvial 

 soil particles have been partly moved by water, but have not 

 been laid down under water as have alluvial soils. A cumulose 

 soil has been formed by the accumulation of vegetable matter, 

 such as occurs in swamps. Peat and muck are cumulose. 



Soils from Igneous Rocks. Igneous rocks are formed by the 

 cooling of molten matter which has been spread out upon the 

 surface of the earth or injected between layers of other rocks. 

 Metamorphic rocks were laid down by water or other agencies, 

 but were afterwards subjected to such intense heat and pressure 

 as to crystallize minerals in them. 



The physical and chemical character of the rock and of the 

 soil which may be derived from it, depends upon its chemical 

 composition, and the rapidity with which the igneous rock solidi- 

 fies. If the molten mass cools off rapidly, so that it solidifies in 

 a comparatively short time, minerals do not have time to crys- 

 tallize, and a hard, homogenous, glassy mass is produced (glassy 

 rock). If the molten material remains liquid for a long time 

 and cools slowly, the rock produced is a mixture of definite 

 minerals which can be easily distinguished (crystalline rock). 



