WEATHERING OF ROCKS 



gen, barium, manganese, and chromium) make up together only 

 0.35 per cent of all soil mineral matter. 



It is worthy of note that carbon, one of the most important 

 elements in the life of plants and animals and which plays such 

 an important role as a source of fuel and in the synthesis of hun- 

 dreds of compounds used by man, makes up only a very small 

 fraction of the surface of the earth, as well as of the whole litho- 

 sphere, hydrosphere and atmosphere. The constant circulation 

 of this element in nature is necessary to keep life from becoming 

 rapidly extinguished, and it is the soil microbes that bring about 

 certain important phases of the transformation of this element. 



Seventy-five per cent of all the sohd surface of the earth's 

 crust is composed of the two elements oxygen and siUcon, while 

 silica (Si02) as the compound and as combined in silicates makes 

 up 60 per cent of the crust. This silica thus comprises the major 

 part of the inorganic portion of the soil, which results from the 

 disintegration of the rocks, due not only to its being the most 

 abundant material in rocks but also as a result of its resistance to 

 solution by water or dilute acids excreted by plants or formed by 

 microorganisms in the soil. 



Weathering agencies remove certain rock constituents quite 

 rapidly, and others only in very small amounts even after long 

 periods of time. Table 3 brings out further the changes which 



TABLE 3 



Chemical Composition of a Rock and of a Residual Soil 



Formed from It (from Clarke) 



