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Show 300 DENUDA'l'ION OF rrrrE LAND. CrrAP. VI. tained 1·3 and 0·6 per cent. I mention these latter cases because, from the thickness of the overlying bed of red clay with flints, I had imagined that the underlying chalk might here be less pure than elsewhere. The cause of the residue accumulating more in some places than in others, may be attributed to a layer of argillaceous matter having been left at an early period on the chalk, and this would check the subsequent percolation of earthy matter into it. From the facts now given we may conclude that castings ejected on our Chalk Downs suffer some loss by the percolation of their finer matter into the chalk. But such impure superficial chalk, when dissolved, would leave a larger supply of earthy matter to be added to the mould than in the case of pure chalk. Besides the loss caused by percolation, some fine earth is certainly washed down the sloping grass-covered surfaces of our Downs. The washing-down process, however, will be checked in the course of time; for although I do not know how thin a layer of mould suffices to support worms, yet a limit must at last be reached; and then their cast- Crrar>. VT. MO OLD OVER TIIE CIIA.LK. 301 ings would cease to be ejected or would become scanty. The following cases show that a considerable quantity of fine earth is washed down. The thickness of the mould was measured at points 12 yards apart across a small valley in the Chalk ncar Winchester. The sides sloped gently at first; then became inclined at about 20°; then more gently to near the bottom, which transversely was almost level and about 50 yards across. In the bottom, the mean thickness of the mould from five measurements was 8·3 inches; whilst on the sides of the valley, where the inclination varied between 14° and 20°) its mean thickness was rather less than 3·5 inches. As the turf-covered bottom of the valley sloped at an angle of only between 2° and 3°, it is probable that most of the 8·3-inch layer of mould had been washed down from the flanks of the valley, and not from the upper part. But as a shepherd said that he had seen water flowing in this valley after the sudden thawing of snow, it .is possible that some earth may have been brought down from the upper part; or, on the other hand, that some may have been |
