28 THE ORIGIN OF SOILS [chap. 



Marcasite readily oxidises in moist air to ferrous sul- 

 phate and sulphuric acid : and many clay soils contain 

 basic sulphates, soluble in dilute acids but not in water, 

 that have arisen in this way. Selenite and the soluble 

 sulphates present in well waters, especially in clay soils, 

 are probably secondary products arising from the oxida- 

 tion of marcasite. In a finely divided condition iron 

 pyrites forms the colouring matter of many dark green 

 or olive rocks and clays. 



Soil and Subsoil. 



Although the transition from soil to subsoil is 

 gradual, the distinction between the two is, as a rule, 

 easy to be made; the change begins an inch or so 

 below the usual limit of cultivation on arable soils, on 

 pastures at the depth to which the mass of the roots 

 penetrate. The most obvious difference between the 

 two lies in the comparative richness of the staple in 

 decaying vegetable matter or humus, which indeed 

 would be entirely confined to the surface layers were 

 it not for the decay of the deeper roots and the work of 

 worms. To the humus is also due the difference in 

 colour ; not only does the colour deepen towards black 

 as the proportion of humus increases, but by it the 

 sands and clay are to a greater or less extent bleached 

 through the removal of the iron oxides which colour 

 them, hence the inorganic material is lighter and duller 

 in colour in the soil than in the subsoil. In stiff clays 

 the subsoil often shows signs of imperfect oxidation at 

 comparatively slight depths. On an old pasture on the 

 Gault Clay a trench was dug, the top 3 inches were black 

 or nearly so and gradually changed to a stiff brown 

 loam which extended to a depth of 9 or 10 inches, 

 becoming lighter and more distinctively yellow as 

 the admixture of humus diminished ; below this depth 



