{224b} 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. {225a} ‘Archives de Zoolog. expér.’ tom. iii. 1874, p. 409. {225b} ‘Nouvelles Archives du Muséum,’ tom. viii. 1872, pp. 95, 131. {226} Morren, in speaking of the earth in the alimentary canals of worms, says, “præsepè cum lapillis commixtam vidi:” ‘De Lumbrici terrestris Hist. Nat.’ &c., 1829, p. 16. {227} Perrier, ‘Archives de Zoolog. expér.’ tom. iii. 1874, p. 419. {228a} Morren, ‘De Lumbrici terrestris Hist. Nat.’ &c., p. 16. {228b} ‘Archives de Zoolog. expér.’ tom. iii. 1874, p. 418. {234} This conclusion reminds me of the vast amount of extremely fine chalky mud which is found within the lagoons of many atolls, where the sea is tranquil and waves cannot triturate the blocks of coral. This mud must, as I believe (‘The Structure and Distribution of Coral-Reefs,’ 2nd edit. 1874, p. 19), be attributed to the innumerable annelids and other animals which burrow into the dead coral, and to the fishes, Holothurians, &c., which browse on the living corals. {236} Anniversary Address: ‘The Quarterly Journal of the Geological Soc.’ May 1880, p. 59. {244} Mr. James Wallace has pointed out that it is necessary to take into consideration the possibility of burrows being made at right angles to the surface instead of vertically down, in which case the lateral displacement of the soil would be increased. {259} ‘Elements of Geology,’ 1865, p. 20. {265} ‘Leçons de Géologie pratique, 1845; cinquième Leçon. All Élie de Beaumont’s arguments are admirably controverted by Prof. A. Geikie in his essay in Transact. Geolog. Soc. of Glasgow, vol. iii. p. 153, 1868. {266} ‘Illustrations of the Huttonian Theory of the Earth,’ p. 107.