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Show 298 DENUDATION OF TilE LAND. CIIAP. VI. I sought for additional information. In some places, the castings on Chalk Downs con ist largely of calcareous matter, and here the supply is of course unlimited. But in other places, for instance on a part of Teg Down near Winchester, the castings were all black and did not effervesce with acids. The mould over the chalk was here only from 3 to 4 inches in thickness. So again on the plain near Stonehenge, the mould, apparently free from calcareous matter, averaged rather less than 3l inches in thickness. Why worms 2 . should penetrate and bring up chalk In some places and not in others I do not know. In many districts where the land is nearly level, a bed sev-eral feet in thickness of red clay full of unworn flints overlies the Upper Chalk. This overlying matter, the surface of which has been converted into mould, consists of the undissolved residue from the chalk. It mav be well here to recall the case of the ol fragments of chalk buried beneath worm-castings on one of my fields, the angles of which were so completely roundeJ in the course of 29 years that the fragments now resembled water-worn pebbles. This must CHAP. VT. MOULD OVER TilE CHALK. 299 have been effected by the carbonic ac1d in the rain and in the ground, by the humusacids, and by the corroding power of living root·. Why a thick mass of residue has not been left on the Chalk, wherever the land is nearly level, may perhaps be account~d for by the percolation of the fine particles into the fissures, which are often present in tbe chalk and are either open or are filled up with impure chalk, or into the solid chalk itself. That such percolation occurs can hardly be doubted. My son collected some powdered and fragmentary chalk beneath the turf near Win chester ; the former was found by Colonel Parsons, R.E., to contain 10 per cent., and the fragments 8 per cent. of earthy matter. On the flanks of the escarpment near Abinger in Surrey, some chalk close beneath a layer of flints, 2 inches in thickness and covered by 8 inches of mould, yielded a residue of 3·7 per cent. of earthy matter. On t11e other band the Upper Chalk properly contains, as I was infonned by the late David Forbes who had made many analyses, only from 1 to 2 per cent. of earthy matter; and two samples from pits near my house con- |