CHAPTER III 



THE PLACING OF SOIL MATERIAL 



I. RESIDUAL, GRAVITY-LAID AND WATER-LAID DEPOSITS 



THE mineral part of soils is derived from rocks through the work 

 of the geological forces given. Only a small part of the disinte- 

 grated and decomposed rock material produces soil where first 

 formed. By far the larger portion is moved from the place of its 

 origin a few feet, or it may he thousands of miles. The mate- 

 rial remaining in place produces sedentary soils. 



I. SEDENTARY FORMATIONS 



Sedentary Formations are those in which the greater part of 

 the material was formed in place, as when rocks weather into 

 debris fitted for the formation of soils, or when large amounts of 

 organic matter accumulate through the growth and partial decay 

 of mosses, grasses, sedges, and other plants. This class of forma- 

 tions is divided into residual and cumulose soils. 



1. Residual Soils. A residual soil is one formed in situ 

 through the decomposition and disintegration of rocks and the 

 action of organic agencies. It varies in composition with the rocks 

 from which it is derived, and we have in general those materials (a) 

 from igneous rocks, as granites, syenites, diorites, diabases and 

 others; (b) from aqueous rocks, such as sandstones, limestones, 

 shales; and (c) from metamorphic rocks, as gneisses, schists, mar- 

 bles, and slates. By subsequent changes quite different soils result 

 even from the same kind of rocks. The impression often prevails that 

 most soils are residual. This, however, is not the case. Not over two 

 per cent of the soils surveyed in the I'liitcd States by the Bureau of 

 Soils' are derived from igneous and metamorphic rocks, and not over 

 five per cent from sedimentary rocks such as sandstones and shales. 



2. Cumulose Soils. Cumulose soils are formed by the accumu- 

 lation of organic matter in undraincd areas to such an' extent that 

 it forms a very large portion of the soil. These are divided into 

 swamps and marshes, tf irn.m ftx are fresh water formations, while 

 the marches are formed in brackish or salt water areas. The organic 

 matter of those cumulose deposits is derived chiellv from mosses, 

 sedges, and grasses, but almost any form of vegetation may add to 

 the deposit. In north temperate and subarctic regions sphagnum 



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