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Show 290 DENUDATION OF rriiE LAND. CnAP. VI. tained long ago, by Playfair, who, in 1802, wrote, "in the permanence of a coat of '' vco·etable mould on the surface of the earth, b "we have a demonstrative proof of the con- " tinued destruction of the rocks."* Ancient encampments and turnuli.-B. de Beaumont adduces the present state of many ancient encampments and tumuli and of old ploughed fields, as evidence that the sur~aco of the land undergoes hardly any degradatwn. But it does not appear that he ever exa1nined the thickness of the mould over different parts of such old remains. l-Ie relies chiefl! on indirect, but apparently trustworthy, evidence that the slopes of the old embankments are the same as they originally were; and it is obvious that he could know nothing about their original heights. In ICnole Park a mound had been thrown up behind the rifletargets, which appeared to have been formed of earth originally supported by square blocks of turf. The sides sloped, as nearly as I could estimate them, at an angle of 45° or 50° with the horizon, and tbey were covered, especially on the northern side, with long coarse grass, * 'Illustrations of the Huttonian Theory of tho Earth,' p. 107. CHAP. VI. ANCIENT MOUNDS. 2Hl beneath which many worm-castino-s wore found. These had flowed bodily do.:nwards, and others had roll d down as pellets. lienee it is certain that as long as a mound of this kind is tenanted by worms, its height will Le continually lowered. ~rhe fine earth which flows or rolls down tho sides of such a mound accumulates at its base in the form of a talus. A bed, even a very thin bed, of fine earth is eminently favourable for worms; so that a greater number of castings would tend to be ejected on a talus thus fonned than elsewhere. and these would be partially washed a way b; every heavy shower and be spread over the adjoining level ground. The final result would be the lowering of the whole monnd whilst the inclination of the sides would not' be greatly lessened. The same result would assuredly follow with ancient embankments and tumuli ; except where they had been formed of gravel or of nearly pure sancl, as such matter is unfavourable for worms. Many old fortifications and tumuli are believed to be at least 2000 years old ; and we should bear jn mind that in many places about one inch of mould is brought to the surface in 5 years or u 2 |