GEOLOGICAL CLASSIFICATION OF SOILS 45 



In many cases muck and peat are underlaid at varying 

 depths by a soft impure calcium carbonate, called bog-lime. 1 

 Such a deposit may come from the shells of certain of the 

 Mollusca, which have inhabited the basin, or from aquatic 

 plants, such as mosses, algae and species of Chara. In car- 

 bonated water these plants become incrusted with calcium 

 carbonate, possibly because of their ability to absorb carbon 

 dioxide, 2 thus precipitating the carbonate. In most cases 

 this carbonate accumulation is due to a combination of the 

 two agencies. Such material, because of its richness in cal- 

 cium, is valuable as a soil amendment, and often, where it is 

 found pure enough in quality and in sufficiently large quan- 

 tities, it is handled commercially. 



27. Colluvial soils. — This soil is formed in regions of 

 precipitous topography, and is made up of fragments of rocks 

 detached from the heights above and carried down the slopes 

 by gravity. Talus slopes, cliff debris, and other heterogeneous 

 rock detritus are examples of colluvial soil. Avalanches are 

 made up largely of such material. 



As the physical forces of weathering are most active in the 

 formation of these soils the amount of solution and oxidation 

 is usually small. The upper part of the accumulation ex- 

 hibits physical action to the greatest extent, the particles being 

 angular, coarse, and comparatively fresh; farther down the 

 slope the material may merge by degrees into ordinary soil. 3 

 Such soils are usually shallow and stony, and approach the 

 original rock in color unless large amounts of organic matter 

 have accumulated. Colluvial soils are not of great importance 



1 Bog-lime is often spoken of as marl. Marl, as used by the geologist, 

 refers to a calcareous clay of variable composition. Bog-lime, when it 

 contains numerous shells, is often termed shell-marl. See Stewart, C. F., 

 The Definition of Marl; Econ. Geol., Vol. 4, No. 5, pp. 485-489, 1909. 



2 CaH 2 (C0 3 ) 2 ?± C0 2 + H 2 G + CaC'O,. 



'Colluvial soils generally merge so gradually into alluvial fans that 

 the line of separation is difficult to establish. When the area of col- 

 luvial material is small, as it usually is, it is best included in the fan 

 soils. 



