Chap. V.
AND DENUDATION.
245
the powerful action of frost. The free access of air will likewise be excluded. From these several causes disintegration would be almost arrested, if the overlying mould were to increase much in thickness, owing to none or little being removed from the surface.[1] In my own immediate neighbourhood we have a curious proof how effectually a few feet of clay checks some change which goes on in flints, lying freely exposed; for the large ones which have lain for some time on the surface of ploughed fields cannot be used for building; they will not cleave properly and are said by the workmen to be rotten.[2] It is
- ↑ The preservative power of a layer of mould and turf is often shown by the perfect state of the glacial scratches on rocks when first uncovered. Mr. J. Geikie maintains, in his last very interesting work ('Prehistoric Europe,' 1881), that the more perfect scratches are probably due to the last access of cold and increase of ice, during the long-continued, intermittent glacial period.
- ↑ Many geologists have felt much surprise at the complete disappearance of flints over wide and nearly level areas, from which the chalk has been removed by subaerial denudation. But the surface of every flint is coated by an opaque modified layer, which will just yield to a steel point, whilst the freshly-fractured, translucent surface will not thus yield. The removal by atmospheric agencies of the outer modified surfaces of freely exposed flints, though no doubt excessively slow, together with the modification travelling inwards, will, as may be suspected, ultimately lead to their complete disintegration, notwithstanding that they appear to be so extremely durable.
