CnAi>. Yl. MOULD OVER THE CHALK. 297 



whole it appears that the crowns and fur- 

 rows on land formerly ploughed, but now 

 covered with grass, tend slowly to disappear 

 when the surface is inclined ; and this is pro- 

 bably in large part due to the action of 

 worms ; but that the crowns and furrows last 

 for a very long time when the surface is 

 nearly level. 



Formation and amount of mould over the 

 Chalk Formation. — Worm-castings are often 

 ejected in extraordinary numbers on steep, 

 grass-covered slopes, where the Chalk comes 

 close to the surface, as my son William 

 observed near Winchester and elsewhere. If 

 such castings are largely washed away during 

 heavy rains, it is difficult to understand at 

 first how any mould can still remain on our 

 Downs, as there does not appear any evident 

 means for supplying the loss. There is, more- 

 ovei, another cause of loss, namely in the per- 

 colation of the finer particles of earth into the 

 fissures in the chalk and into the chalk itself. 

 These considerations led me to doubt for a time 

 whether I had not exaggerated the amount 

 of fine earth which flows or rolls down grass- 

 covered slopes under the form of castings ; and 



