EXTENSION COURSE IN SOILS. 5 



or soil, the percentage composition of materials may be somewhat 

 changed by the difference in solubility of the compounds forming 

 the rock, and by other factors. Because of the wide variation of 

 rocks forming residual soils and the changes which may take place 

 during rock disintegration, these soUs are of many kinds. 



Granite rocks consist principally of the minerals feldspar, quartz, 

 hornblende, and mica. In the decomposition of granites carbon 

 dioxid, usually called carbonic acid, dissolved in soil water, combines 

 with the elements potassium, sodium, or calcium in the feldspar, 

 forming soluble carbonate compounds of these elements, while 

 insoluble alumina and silica, uniting with small quantities of water, 

 collect as clay. Quartz grains, on the other hand, are not appreciably 

 affected by carbon dioxid, and so collect as sand in the soil. In this 

 way there is formed from granites a mixture of clay, sand, and partly 

 decomposed particles of all the minerals found in the granite rocks. 



Soil is also formed from hmestone rocks by weathering and solu- 

 tion. Limestone consists principally of calcium and magnesium car- 

 bonates. These slightly soluble carbonates are made more soluble 

 through the action of carbonic acid in the water of the soil. A good 

 illustration of such solution is the so-called hard water from a lime- 

 stone well. When such water is boiled the carbon dioxid holding 

 the calcium carbonate in solution is driven off and the carbonate is 

 precipitated as a solid residue which often adheres to the containing 

 vessel, forming what is known as scale. In soil formation from lime- 

 stones, as the carbonates are dissolved and leach out, the impurities 

 in the limestone, chiefly fine clay and silt, are left to collect and form 

 a soil. Mixed with this fine residual clay and silt is usually found a 

 great deal of stony material consisting largely of sihca, and known as 

 flin t or chert. Soils formed from limestones are, therefore, largely 

 clay, containing more or less flint or chert. 



In the formation of soils from sandstone rocks the changes taking 

 place are largely physical, and the composition of the soils differs but 

 Httle from that of the rocks from which they are derived. The chief 

 process is the disintegration of the rock and the separation of the sand 

 grains through freezing and thawing and the action of water. Soils 

 formed in this way from sandstones are, of course, sandy in character, 

 though they may be somewhat fkier than the rock itself, since the 

 grains of sandstone not only separate one from another, but split 

 up into somewhat finer parts. 



The principal area of residual soil in the United States is south 

 of a line extending roughly from New York to Pittsburgh, thence fol- 

 lowing the Ohio River to the Mississippi River, up the Mississippi 

 and Missouri Rivers to the Dakotas, and from thence west to the 

 Puget Sound region in Washington, where it turns well southward. 

 From this area, however, should be excluded the coastal plains, 



