SOIL FORMATION, COMPOSITION, ETC. 81 



which the soils are derived, the method of rock decay, 

 and the conditions under which it has existed since its 

 formation. The soils that have remained where they were 

 originally formed are of two kinds, residual and cumulose, 

 the latter being the result of the accumulations of plant 

 residues. The residual soil is the result of rock decay, 

 and represents the portions of the products that remain 

 on the parent rock. This consists, in a large measure, of 

 the elements most insoluble, which, however, represent but 

 a small part of the original rock. There is very little car- 

 bonate of lime in residual soils; even those derived from 

 limestone rock are often deficient in calcium carbonate, 

 the soil itself being simply the remains of the impurities 

 in the original limestone rock. The carbonate of lime has 

 been converted into soluble calcium bicarbonate, when it 

 has come in contact with water containing carbon dioxide, 

 and then it is leached out of the rock. Soils derived from 

 granite or gneiss are generally of a clayey nature. Soils 

 from marine formations are often sandy. Both classes 

 grade into clay loam or sandy loam as the case may be. 



The transported soils are formed from the products of 

 rock decay mixed with a certain amount of organic matter. 

 These materials have been transported from the place where 

 they were formed by such agencies as water, ice, wind, 

 and gravity. Their composition will vary to a consid- 

 erable degree. 



" Soils that have been transported by water are classified 

 as marine, alluvial, and lacustrine. Marine soil is formed 

 by the deposits in the ocean beds, which are subsequently 

 elevated. Alluvial soils are formed by the deposition of 

 material along the shores of streams, and are very variable 

 in composition. Lacustrine soils are formed in the beds 

 of lakes or ponds which are subsequently drained. 



The wind-borne, or aeolian, soils are rather extensively 

 represented by the loess soils of the central parts of the 

 United States. This wind -deposited soil covers to varying 



