CHAP. VI. MOULD OVER THE CHALK. 307 



worm-castings, is washed down, and accumu- 

 lates in the lower parts, notwithstanding the 

 percolation of an unknown quantity into the 

 underlying chalk ; a supply of fresh earthy 

 matter being afforded by the dissolution of 

 the chalk through atmospheric and other 

 agencies. 



