CHAP. V. AND DENUDATION. 241 



64 per cent. ; but with these latter cases we 

 are not here concerned. The carbon in the 

 soil tends gradually to oxidise and to dis- 

 appear, except where water accumulates and 

 the climate is cool ; * so that in the oldest 

 pasture-land there is no great excess of 

 organic matter, notwithstanding the con- 

 tinued decay of the roots and the underground 

 stems of plants, and the occasional addition 

 of manure. The disappearance of the organic 

 matter from mould is probably much aided 

 by its being brought again and again to the 

 surface in the castings of worms. 



Worms, on the other hand, add largely to 

 the organic matter in the soil by the astonish- 

 ing number of half-decayed leaves which 

 they draw into their burrows to a depth of 2 

 or 3 inches. They do this chiefly for obtain- 

 ing food, but partly for closing the mouths 

 of their burrows and for lining the upper 

 part. The leaves which they consume are 

 moistened, torn into small shreds, partially 

 digested, and intimately commingled with 



* I have given some facts on the climate necessary or favour* 

 able for the formation of peat, in my * Journal of Researches,' 

 1845, p. 287. 



