were not only instrumental in disintej^rating (hom, but played an 

 important part in the formation of the contour of the surface of 

 sections of the earth. 



Changes in Temperature 



causing contraction and expansion, tend to crack and dislodge rock. 

 Plant Roots 



which either grow into crevices or are blown or washed there, form 

 carbonic acid when they decay, which acts upon some rock formations, 

 disintegrating and dissolving them. 



Insects and Earth Worms 



play a part in making soil. They burrow through loams and soft rock, 

 admitting air and water, which further hastens disintegration. Micro- 

 organisms on the surface and in crevices of rock residues and nitrifying 

 organisms furnish nitrogen, which stimulate the growth of vegetation, 

 hastening disintegration. 



Briefly, rock disintegration is accomplished by the combined action 

 of water, heat and cold, air and other gases, vegetable growth, micro- 

 organisms, insects, earthworms, chemical elements and substances all 

 working in combination both mechanically and chemically. 



CLASSIFICATION OF SOILS 



SOILS are classified as sedentary and transported. 

 Sedentary soils are those which remain where disintegration took 

 place. There may be residual deposits, namely, gravels, sands, clays, 

 etc., or cumulose. This sub-class includes peat, muck and swampy 

 soils made so by the accumulation of organic matter, both by growth 

 and decomposition. 



Transported Soils 



are called colluvial, alluvial, aeolian and glacial deposits. These soils, 

 like all others, are composed of disintegrated rock and organic matter. 

 They are transported or shifted by winds, water, drifts, glaciers and 

 other forces. 



Soils are further classified according to their characteristics, as sand 

 and sandy soils, clays, silt and loam. 



Sand 



is disintegrated rock, the particles ranging in size between 0.5 mm. and 

 0.05 mm. in diameter. Sand is not cohesive nor does it retain moisture 



9 



