We ought not, I think, to feel surprise that we have not hitherto succeeded in detecting the signs of such aerolites in older rocks, for, besides their rarity in our own days, those which fell into the sea (and it is with marine strata that geologists have usually to deal), being chiefly composed of native iron, would rapidly enter into new chemical combinations, the water and mud being charged with chloride of sodium and other salts. We find that anchors, cannon, and other cast-iron implements which have been buried for a few hundred years off our English coast have decomposed in part or entirely, turning the sand and gravel which enclosed them into a conglomerate, cemented together by oxide of iron. In like manner meteoric iron, although its rusting would be somewhat checked by the alloy of nickel, could scarcely ever fail to decompose in the course of thousands of years, becoming oxide, sulphuret or carbonate of iron, and its origin being then no longer distinguishable. The greater the antiquity of rocks,--the oftener they have been heated and cooled, permeated by gases or by the waters of the sea, the atmosphere or mineral springs,--the smaller must be the chance of meeting with a mass of native iron unaltered; but the preservation of the ancient meteorite of the Altai, and the presence of nickel in these curious bodies, renders the recognition of them in deposits of remote periods less hopeless than we might have anticipated. FOOTNOTES: [134-A] Geol. Trans. 2d series, vol. vi. p. 135. Mr. Smith of Jordanhill had arrived at similar conclusions as to climate from the shells of the Scotch Pleistocene deposits. [134-B] Proceedings of Geol. Soc. No. 63. p. 119. [135-A] Travels in N. America, vol. ii. p. 141. [135-B] Ibid. p. 99. chap. xix. [136-A] Bulletin Soc. Geol. de France, tom. iv. 2de ser. p. 1121. [138-A] See Travels in N. America, vol. i. chap. ii. [140-A] Agassiz, Etudes sur les Glaciers. [143-A] Archiac, Hist. des Progres, &c. vol. ii. p. 249. [143-B] See Elements of Geology, 2d ed. 1841. [144-A] Darwin's Journal, p. 283. [144-B] More recently Sir R. Murchison, having revisited the Alps, has declared his opinion that "the great granitic blocks of Mont Blanc were translated to the Jura when the intermediate country was under water."--Paper read to Geol. Soc. London, May 30, 1849. CHAPTER XIII. NEWER PLIOCENE STRATA AND CAVERN DEPOSITS.