40 ELEMENTARY AGRICULTITRAL CHEMISTRY 



*bout the erosion of the latter. In most cases the effects of 



•i 

 this form of erosion are masked and hidden by those of other 



denuding agents, but occasionally — e.g.^ at Brimham Rocks, in 



Yorkshire — the curious undercuttings produced by this action 



are very clearly shown. 



(h\ Chemically. — In many rocks are minerals capable of 

 taking up oxygen — e.g., ferrous carbonate. On exposure to air 

 oxidation occurs, and the mineral swells up, and often crumbles 

 to powder, thus loosening the other minerals in the rocks. The 

 oxidation is in many cases accompanied by a change in colour, 

 from green or grey to yellow or red. The carbon dioxide in the 

 air also acts corrosively on carbonates in the presence of water. 



3. Animals. — Burrowing animals — e.g., rabbits and moles 

 — admit air into soil or sand, and thus favour the changes which 

 air produces. The part played by the humbler creatures, 

 earthworms, is probably much more important. They bring 

 portions of the subsoil to the surface, they draw dead leaves 

 and other vegetable refuse into their burrows, and they pass 

 large quantities of the soil through their bodies and deposit it 

 on the surface at a rate which has been estimated, on the 

 average, to be about ten tons per acre per annum. 



Ants in some hot countries — e.g., Africa — perform much the 

 same work as earthworms, though perhaps even on a larger 

 scale. In many parts of South Africa the veld is thickly 

 studded with the hills of the white ant, usually about two feet 

 high and about two or three feet in diameter, though much larger 

 ones are often found. The ant hills are full of cavities and 

 chambers inhabited by the insects, and much vegetable matter 

 is stored in them. The material of the ant-hills consists of 

 the smaller parts of the surrounding soil, the particles being 

 cemented together and the whole made practically water-tight. 

 When the veld is ploughed and sown it is always noticed that 

 where ant-hills had formerly been, the crop is heavier than else- 

 where. The following analyses of ant-heap material and the 

 adjacent soil, taken by the writer near Christiana, in the 

 Transvaal, will show the richness of such substances : 



