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Show 136 AMOUNT OF EARTH CHAP. III. and a layer of the marl fragments could be traced at a depth, carefully measured, of 12 inches in some parts, and of 14 inches in other parts. This difference in depth depended on the layer being horizontal, whilst the surface consisted of ridges and furrows from the field having been ploughed. The tenant assured me that it had never been turned up to a greater depth than from 6 to 8 inches; and as the fragments formed an unbroken horizontal layer from 12 to 14 inches beneath the surface, these must have been buried by the worms whilst the land was in pasture before it was ploughed, for otherwise they would have been indiscriminately scattered by the plough throughout the whole thickness of the soil. Four-and-a-half years afterwards I had three holes dug in this field, in which potatoes had been lately planted, and the layer of marl-fragments was now found 13 inches beneath the bottoms of the furrows, and therefore probably 15 inches 1809, that is twenty-eight years before the first examination of the field by my friend. 'l'he error, as far as the figure 80 is concerned, was corrected in an article by me, in the ' Gardeners' Chronicle,' 1844, p. 218. CHAP. III. BROUGHT UP BY WORMS. 137 beneath the general level of the field. It should, however, be observed that the thickness of the blackish sandy soil, whi.ch had been thrown up by the worms above the marlfragments in the course of 32! years, would have measured less than 15 inches, if the field had alwa~s remained as pasture, for the soil would in this case have been much more compact. The fragments of marl almost rested on an undisturbed sub-stratum of white sand with quartz pebbles ; and as this would be little attractive to worms, the mould would hereafter be very slowly increased by their action. We will now give some cases of the action of worms,' on land differing widely from the dry sandy or the swampy pastures just described. The chalk formation extends all round n1y house in Kent; and its surface, from having been exposed during an immense period to the dissolving action of rain-water, is extremely irregular, being abruptly festooned and penetrated by many deep welllike cavities.* During the dissolution of the * These pits or pipes are still in process of formation. During the last forty years I have seen or heard of five cases, in which a |
