Chap. VI. MOULD OYER THE CHALK. 303 



tained 1*3 and 0*6 per cent. I mention these 

 latter cases because, from the thickness of the 

 overlying bed of red clay with flints, I had 

 imagined that the underlying chalk might 

 here be less pure than elsewhere. The cause 

 of the residue accumulating more in some 

 places than in others, may be attributed to a 

 layer of argillaceous matter having been left 

 at an early period on the chalk, and this 

 would check the subsequent percolation of 

 earthy matter into it. 



From the facts now given we may conclude 

 that castings ejected on our Chalk Downs suffer 

 some loss by the percolation of their finer 

 matter into the chalk. But such impure 

 superficial chalk, when dissolved, would leave 

 a larger supply of earthy matter to be 

 added to the mould than in the case of pure 

 chalk. Besides the loss caused by percola- 

 tion, some fine earth is certainly washed 

 down the sloping grass-covered surfaces ot 

 our Downs. The washing-down process, how- 

 ever, will be checked in the course of time ; 

 for although I do not know how thin a layer 

 of mould suffices to support worms, yet a limit 

 must at last be reached ; and then their cast- 



