SOIL FORMATION, COMPOSITION, ETC. 



79 



checking of the current and the action of the salts in the 

 sea water, deposit the materials carried in suspension. 1 hese 

 materials gradually accumulate on shallow bottoms until 

 marsh lands are formed, and the building is continued 

 through the action of winds, waves, tides, and plants until 

 soil is gradually formed. There are many organisms in 

 the ocean, such as coral polyps and shell-fish, that build 

 up islands. The sea bottom is the seat of many soil- 

 forming activities, and vast areas of sea bottom have been 



-H 



FIG. 27. Residual soil, Piedmont region. (By permission F. G. 

 Tarbox, S. C. Exp. Station.) 



elevated to form some of our most fertile soils. Sandstones, 

 shales, and marls are formed at the bottom of the ocean. 



63. Composition of Soils. About ninety-eight per cent 

 of the earth's solid crust consists of the eight elements: 

 oxygen, silicon, aluminium, iron, calcium, potassium, sodium, 

 and magnesium, here arranged in the order of their abun- 

 dance. These elements make up the common minerals, 

 which, in turn, make up the common rocks. 



The difference in the chemical composition of different 

 soils is due to the differing compositions of the rocks from 



