172 THICKNESS OF THE MOULD Chap. Ill, 



the Chalk at Down, the soil being argillaceous, very poor, and 

 only just converted into pasture (so that it was for some years 

 unfavourable for worms), amounted to 0*83 inches in 10 years. 



In these cases (excepting the last) it may 

 be seen that the amount of earth brought 

 to the surface during 10 years is somewhat 

 greater than that calculated from the castings 

 which were actually weighed. This excess 

 may be partly accounted for by the loss which 

 the weighed castings had previously under- 

 gone through being washed by rain, by the 

 adhesion of particles to the blades of the sur- 

 rounding grass, and by their crumbling when 

 dry. Nor must we overlook other agencies 

 which in all ordinary cases add to the 

 amount of mould, and which would not be 

 included in the castings that were collected, 

 namely, the fine earth brought up to the 

 surface by burrowing larvse and insects, espe- 

 cially by ants. The earth brought up by moles 

 generally has a somewhat different appearance 

 from vegetable mould ; but after a time would 

 not be distinguishable from it. In dry coun- 

 tries, moreover the wind plays an important 

 part in carrying dust from one place to another, 

 and even in England it must add to the mould 



