Formation of Mould. 507 



fine particles of earth mixed with decayed vegetable matter, those substances 

 which had been spread on the surface fifteen years before. 



The appearance in all the above cases was, as if (in the language of the 

 farmers, who are acquainted with these facts) the fragments had worked 

 themselves down. It is, however, scarcely possible that cinders or pebbles, and 

 still less powdered quick-lime, could sink through compact earth and a layer 

 of matted roots of vegetables, to a depth of some inches ; nor is it at all probable 

 that the decay of the grass, although adding to the surface some of the consti- 

 tuent parts of the mould, should separate in so short a time the fine from the 

 coarse earth, and accumulate the former on those objects, which so lately had 

 been on the surface. I may add, that I have repeatedly observed fragments 

 of pottery and bones buried beneath the turf, in fields near towns, (on which 

 such substances are often thrown with manure) ; and as these fields did not 

 appear to have been ploughed, the circumstance often surprised me. On the 

 contrary, I have noticed in gardens lately dug, that the rain, by washing 

 away the finer particles, leaves stones and other hard bodies accumulated on 

 the surface. 



The explanation of these facts, which occurred to Mr. Wedgwood, although 

 it may appear trivial at first, I have not the least doubt is the correct one, 

 namely, that the whole operation is due to the digestive process of the com- 

 mon earth-worm. On carefully examining between the blades of grass in the 

 fields above described, I found scarcely a space of two inches square without 

 a little heap of the cylindrical castings of worms. It is well known, that worms, 

 in their excavations, swallow earthy matter, and that, having separated the 

 portion which serves for their nutriment, they eject at the mouth of their 

 burrows the remainder in little, intestine-shaped heaps. These partly retain 

 their form until the rain and thaws of winter, as I have observed, spread the 

 matter uniformly over the surface. The worm is unable to swallow coarse 

 particles, and as it would naturally avoid pure or caustic lime, the finer earth 

 lying beneath the cinders, burnt marl, or lime, would be removed, by a slow 

 process, to the surface. This supposition is not imaginary ; for in the field 

 in which cinders had been spread out only half a year before, I actually saw 

 the castings of the worms heaped on the smaller fragments. Nor, I repeat, 

 is the agency so trivial as at first it might be thought : the great number of 

 earth-worms, as every one must be aware who has ever dug in a grass field, 

 making up for the insignificant quantity of the work which each performs. 

 On the idea of the superficial mould having been thus prepared, the advan- 

 tage of old pasture land, which it is well known farmers in England are par- 

 ticularly averse to break up, is explained ; for the length of time required 



VOL. V. SECOND SERIES. 3 U 



