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 soils 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 limestone 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 silica, and known as 
flint 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 
little 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 finer 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 soilin 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, 
