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Show 306 CONCLUSION. CnAr. VII. and the particles composing it are thus rubbed together. By these means fresh surfaces are continually exposed to the action of the carbonic acid in the soil, and of the humus-acids which appear to be still more efficient in the decomposition of rocks. The generation of the humus-acids is probably hastened during the digestion of the many half-decayed leaves which worms consume. Thus the particles of earth, forming the superficial mould, are su~jected to conditions eminently favourable for their decomposition and disintegration. Moreover, 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. Casting~ w ben dry often crumble into small pellets and these arc apt to roll down any sloping surface. ·where the land is quite level and is covered with herbage, and where the CITAP. VIT. CONCLUSION. 307 climate is hu1nid ·o that much dust cannot be blown away, it appears at first sight impo iblc that there shoulcl be any appreciable amount of sub-aerial denudation· but worm .. ' castings are blown, e pecia,lly whil t moist and viscid, in one uniform direction by tlHJ prevalent winds \vhich arc accompanied l)y rain. By these several means the superficial mould is prevented from accumulating to a great thickncs ; ancl a thick bed of nwuld checks in many ways the disintegration of the underlying rocks and fragments of rock. The removal of worm-castings by the above means leads to results which are far fron1 insignificant. It has been shown that a layer of earth, ·2 of an inch in thickness, 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', 2·4 cubic inches of earth which had been ejected by worms crossed, in the course X 2 |