the surface deposit of casts is "22" per annum. Even in poor 

 soils a surface deposit of "08" per annum has been estimated. 

 As earth-worms go down several feet deep and come up 

 again, the mixing of the soil effected by them is often more 

 efficacious than that effected by cultivation. 



42. It has been observed that a stream of lava takes some- 

 times several years to cool. Even when cool it is incapable 

 of supporting ^higher vegetable life. Disintegration takes 

 |)laceiby hydration, oxidation and physical action. Nitrifica- 

 tion then proceeds with the help of Bacteria. Then lichens 

 and other minute vegetation are observed to appear. Gradu- 

 ally the quantity of soil on the hard surface of the rock 

 increases/'and the growth of vegetation becomes more vigor- 

 ous, mosses, ferns and grasses gradually taking the place 

 of lichens. When visible soil accumulates and fissures and 

 cracks appear on the rock, herbs and shrubs multiply and 

 by their root action further help to disintegrate the rock to 

 some depth. The formation of soils now goes on apace. 

 Lichens and bacteria are able to draw nourishment from the 

 most insoluble rocks, not only basalts, granites and schists, 

 but also quartz. Even quartz gets covered with lichens 

 when exposed long enough to air. Some lichens contain a 

 good deal of oxalic acid, and limestones which have been 

 long exposed, generate calcium oxalate by the action of 

 lichens. The action of higher vegetation on rocks is partly 

 mechanical and partly chemical. Roots get into the clefts of 

 rocks and tear them asunder. The chemical action is con- 

 cerned in the solution of some of the ingredients of the 

 rock. The solvent action of roots is due to the formation of 

 acids in them which act on particles of soil. All plants, large 

 or small, die each year wholly or partly and deposit their 

 dead organic matter on the rock. The falling leaves, seeds, 

 &c., when they accumulate in forests, marshes or bogs, pro- 

 duce a black or brown mass which is called Humus. By 

 decay of roots of plants Jalso, a similar substance is formed. 

 F 



