174 THICKNESS OP THE MOULD Chap. m. 



which is usually from 4 or 5 to 10 and even 

 12 inches in thickness; and it is this mould 

 which passes over and over again through 

 their bodies and is brought to the surface. 

 But worms occasionally burrow into the sub- 

 soil to a much greater depth, and on such 

 occasions they bring up earth from this 

 greater depth ; and this process has gone on 

 Jbr countless ages. Therefore the superficial 

 layer of mould would ultimately attain, 

 though at a slower and slower rate, a thick- 

 ness equal to the depth to which worms 

 ever burrow, were there not other opposing 

 agencies at work which carry away to a 

 lower level some of the finest earth which is 

 continually being brought to the surface by 

 worms. How great a thickness vegetable 

 mould ever attains, I have not had good 

 opportunities for observing; but in the next 

 chapter, when we consider the burial of 

 ancient buildings, some facts will be given on 

 this head. In the two last chapters we 

 shall see that the soil is actually increased, 

 though only to a small degree, through the 

 agency of worms; but their chief work is 

 to sift the finer from the coarser particles, to 



