Chap. V. AND DENUDATION. 243 



in the ground. The living roots of plants, 

 moreover, as Sachs and others have shown, 

 quickly corrode and leave their impressions 

 on polished slabs of marble, dolomite and 

 phosphate of lime. They will attack even 

 basalt and sandstone.* But we are not hero 

 concerned with agencies which are wholly 

 independent of the action of worms. 



The combination of any acid with a base 

 is much facilitated by agitation, as fresh 

 surfaces are thus continually brought into 

 contact. This will be thoroughly effected 

 with the particles of stone and earth in the 

 intestines of worms, during the digestive pro- 

 cess ; and it should be remembered that the 

 entire mass of the mould over every field, 

 passes, in the course of a few years, through 

 their alimentary canals. Moreover as the old 

 burrows slowly collapse, and as fresh castings 

 are continually brought to the surface, the 

 whole superficial layer of mould slowly re- 

 volves or circulates ; and the friction of the 

 particles one with another will rub off the 

 finest films of disintegrated matter as soon as 



• See, for references on this subject, S. W. Johnson, " How 

 CJrops Feed," 1870, p. 326. 



