26 THE ORIGIN OF SOILS [chap. 



carbonate of lime from the surface soil, which generally 

 shows an acid reaction and a tendency towards the 

 accumulation of humus or peat. The method of forma- 

 tion of these deposits is still a matter of dispute. The 

 theory at present most generally accepted is that the 

 surface soil loses all its soluble material by leach- 

 ing. Then as the rain-water continues to percolate, 

 the oxides of iron, which are colloids (see p. 39), go 

 into pseudo-solution as "sols" and wash down into 

 the subsoil. During the dry season, however, the 

 soil dries out to a certain depth, at which the " sol " 

 solution begins to evaporate and at a certain concentra- 

 tion deposits solid hydrated oxide of iron. The thin 

 layer thus formed constitutes a nucleus on which further 

 oxide of iron deposits in succeeding seasons until the 

 iron band is formed. Carbonate of lime in the surface 

 soil prevents the action because it is always going into 

 solution, and as long as soluble salts are present the oxides 

 of iron will not assume the " sol " condition. This argu- 

 ment is strengthened by the fact that a definite band 

 or pan is usually associated with a comparatively 

 dry summer climate, such as would permit of the com- 

 plete drying out of the soil down to a certain depth. 



It has been supposed that the oxides of iron in the 

 surface soil are reduced by the presence of the organic 

 matter and carbon dioxide to the state of ferrous 

 carbonate, which reoxidises at the evaporating surface 

 in the summer months. However, the pan always 

 contains alumina as well as iron, and the oxide of 

 alumina will not undergo reduction to a soluble 

 compound, though it will form " sols " like the oxide 

 of iron. Again, ferrous compounds are not to be found 

 in soils, the test generally relied upon as indicating 

 their presence being deceptive. Other factors may, 

 however, be at work, particularly the action of certain 



