THE PAET PLAYED BY WORMS. 41 



over, the particles of the softer rocks suffer some amount 

 of mechanical trituration in the muscular gizzards of 

 worms, in which small stones serve as mill-stones. 



The finely levigated castings, when brought to the 

 surface in a moist condition, flow during rainy weather 

 down any moderate slope ; and the smaller particles are 

 washed far down even a gently inclined surface. Cast- 

 ings when dry often crumble into small pellets, and these 

 are apt to roll down any sloping surface. Where the 

 land is quite level and is covered with herbage, and where 

 the climate is humid so that much dust can not be blown 

 away, it appears at first sight impossible that there should 

 be any appreciable amount of subaerial denudation ; but 

 worm-castings are blown, especially while moist and vis- 

 cid, in one uniform direction by the prevalent winds 

 which are accompanied by rain. By these several means 

 the superficial mold is prevented from accumulating to a 

 great thickness ; and a thick bed of mold checks in many 

 ways the disintegration of the underlying rocks and frag- 

 ments of rock. 



The removal of worm-castings by the above means 

 leads to results which are far from insignificant. It has 

 been shown that a layer of earth, -2 of an inch in thick- 

 ness, is in many places annually brought to the surface 

 per acre ; and if a small part of this amount flows, or 

 rolls, or is washed, even for a short distance down every 

 inclined surface, or i^ repeatedly blown in one direction, 

 a great effect will be produced in the course of ages. It 

 was found by measurements and calculations that on a 

 surface with a mean inclination of 9° 26', 3 '4 cubic inches 

 of earth which had been ejected by worms crossed, in the, 

 course of a year, a horizontal line one yard in length ; so 

 that 340 cubic inches would cross a line a hundred yards 

 in length. This latter amount in a damp state would 



