CHAP. VII. CONCLUSION. 309 



and the particles composing it are thus 

 rubbed together. By these means fresh sur- 

 faces are continually exposed to the action of 

 the carbonic acid in the soil, and of the 

 humus-acids which appear to be still more 

 efficient in the decomposition of rocks. The 

 generation of the humus-acids is probably 

 hastened during the digestion of the many 

 half-decayed leaves which worms consume. 

 Thus the particles of earth, forming the 

 superficial mould, are subjected to conditions 

 eminently favourable for their decomposition 

 and disintegration. Moreover, the particles 

 of the softer rocks suffer some amount of 

 mechanical trituration in the muscular giz- 

 zards of worms, in which smail stones serve 

 as mill-stones. 



The finely levigated castings, when brought 

 to the surface in a moist condition, flow during 

 rainy weather down any moderate slope ; and 

 the smaller particles are washed far down 

 even a gently inclined surface. Castings 

 when dry often crumble into small pellets 

 and these are apt to roll down any sloping 

 surface. Where the land is quite level and 

 is covered with herbage, and where the 



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