WEATHERING OP SOILS 35 



and animal life. From the calcareous muds thus 

 deposited our limestone rocks have originated. From 

 the sandstones, clays, and limestones thus produced, 

 our present soils have been mostly formed. 



The action of the tveather on soils is similar in kind to 

 that which has already taken place on a larger scale 

 on rocks. The expansion of water when freezing 

 tends to reduce a soil to fine powder. The oxygen of 

 the air takes a share in the disintegration if silicates 

 containing ferrous oxide are present. The most potent 

 chemical agent is, however, the carbonic acid present 

 in rain, and to a still larger extent in the water 

 permeating soils containing vegetable matter. This 

 solution of carbonic acid dissolves, and removes as 

 drainage water, the carbonates of calcium and mag- 

 nesium ; it also, especially when reinforced by the 

 presence of the carbonates just mentioned, attacks any 

 undecomposed siHcates, and removes in solution the 

 alkalies and some of the siHca which they contain. 

 When a soil has become the seat of vegetation the 

 chemical agents of decomposition gain in power ; the 

 carbonic acid in the soil is much increased, and is 

 assisted by the humic acids, and by the nitric acid, 

 which appear on the scene ; the solvent action of plant 

 roots must also be taken into account. 



Weathering action is thus destructive, and especially 

 tends to remove from the soil in drainage water the 

 lime, magnesia, and alkalies which it contains ; a sur- 

 face soil is thus generally poorer in lime, and fre- 

 quently in potash, than the subsoil beneath it. The 

 impoverishment of the soil is hindered by its absorp- 

 tive power (pp. 43, 45), but chiefly by the conservative 

 action of vegetation, The plant is continually collect- 



