SOIL-FOBMING PROCESSES 19 



providing better circulation of air and water. This 

 brings about a deeper and more effective action of the 

 other physical agencies of weathering. Earthworms 

 produce similar effects. Their holes provide channels 

 for ready drainage, and large quantities of soil are brought 

 to the surface yearly by them. Darwin estimates that 

 this amounts to as much as one or two inches in a 

 decade. 



16, Oxidation and carbonation. — The physical and 

 chemical forces do not act alone, but, as a general thing, 

 combine in their effects. Thus one set of factors aids 

 and accelerates the other. Scarcely has the disintegration 

 of a rock begun, then, before its decomposition is also 

 apparent. Of the chemical forces oxidation is usually, 

 especially near the surface of the earth, the jBrst to be 

 noticed. It is perceptibly manifested in rocks carrying 

 iron, and consists in such a change that the added oxygen 

 may be accommodated. Sulfides readily succumb and 

 become oxides, while these same oxides are prone to take 

 up oxygen to their fullest extent. This oxidation is dis- 

 closed by a discoloration of the rock, which is first streaked 

 and stained with iron oxide but at last changed to a 

 uniform ochre. The change may be exemplified by the 

 following reactions : — 



2FeS2 + 7O2 + 4H2O = 2FeO + 4H2SO4 

 4 PeO + O2 = 2 FesOs (red) 



While not all the minerals contain iron, enough of them 

 do to impart a fatal weakness to most rocks. The ferrous 

 oxide (FeO), being soluble, is washed out and the rock is 

 creviced and crumbled. A way is now open for more 

 energetic physical and chemical decay. 

 With the oxidizing action there is also the influence of 



