[69-A] See Mammat's Geological Facts, &c. p. 90. and plate. [69-B] Conybeare's Report to Brit. Assoc. 1842, p. 381. [70-A] Prestwich, Geol. Trans. second series, vol. v. pp. 452. 473. [75-A] Section given by Dr. Christie, Edin. New Phil. Journ. No. xxiii., called by mistake the Cave of Mardolce, by the late M. Hoffmann. See account by Mr. S. P. Pratt, F. G. S. Proceedings of Geol. Soc. No. 32. 1833. [78-A] I was directed by M. Deshayes to this spot, which I visited in June, 1833. [78-B] See Trans. of Geol. Soc., second series, vol. v. plate v. CHAPTER VII. ALLUVIUM. Alluvium described--Due to complicated causes--Of various ages, as shown in Auvergne--How distinguished from rocks in situ--River-terraces--Parallel roads of Glen Roy--Various theories respecting their origin. Between the superficial covering of vegetable mould and the subjacent rock there usually intervenes in every district a deposit of loose gravel, sand, and mud, to which the name of alluvium has been applied. The term is derived from _alluvio_, an inundation, or _alluo_, to wash, because the pebbles and sand commonly resemble those of a river's bed or the mud and gravel spread over low lands by a flood. A partial covering of such alluvium is found alike in all climates, from the equatorial to the polar regions; but in the higher latitudes of Europe and North America it assumes a distinct character, being very frequently devoid of stratification, and containing huge fragments of rock, some angular and others rounded, which have been transported to great distances from their parent mountains. When it presents itself in this form, it has been called "diluvium," "drift," or the "boulder formation;" and its probable connexion with the agency of floating ice and glaciers will be treated of more particularly in the eleventh and twelfth chapters. [Illustration: Fig. 99. Lavas of Auvergne resting on alluviums of different ages.]