The increased thickness of the limestones in those regions, as in the Alps and Jura, where the clays are comparatively thin, arises from the calcareous matter having been derived from species of corals and other organic beings which live in clear water, far from land, to the growth of which the influx of mud would be unfavourable. Portions therefore of these clays and limestones have probably been formed contemporaneously to a greater extent than we can generally prove, for the distinctness of the species of organic beings would be caused by the difference of conditions between the more littoral and the more pelagic areas and the different depths and nature of the sea-bottom. Independently of those ascending and descending movements which have given rise to the superposition of the limestones and clays, and by which the position of land and sea are made in the course of ages to vary, the geologist has the difficult task of allowing for the contemporaneous thinning out in one direction and thickening in another, of the successive organic and inorganic deposits of the same era. [1] Quart. Journ., vol. xvi, p. 376. [2] Agassiz, Poissons Fossiles, vol. ii, tab. 28, 29. [3] Agassiz, Poissons Fossiles, vol. iii, tab. C, Fig. 1. [4] Bridgewater Treatise, p. 290. [5] Geol. Soc. Transactions, Second Series, vol. i, p. 49. [6] Conybeare and De la Beche, Geol. Trans., First Series, vol. v, p. 559; and Buckland, Bridgewater Treatise, p. 203. [7] See Darwin, Naturalist’s Voyage, p. 385. Murray. [8] Bridgewater Treatise, p. 115. [9] Geological Researches, p. 334. [10] Buckland, Bridgewater Treatise, p. 307. [11] A History of Fossil Insects, etc., 1846. London. [12] Tableau des Vég. Foss., 1849, p. 105. [13] Conybeare and Philips’s Outlines, etc., p. 166. [14] Geological Researches, p. 337. CHAPTER XXI. TRIAS, OR NEW RED SANDSTONE GROUP. Beds of Passage between the Lias and Trias, Rhætic Beds. — Triassic Mammifer. — Triple Division of the Trias. — Keuper, or Upper Trias of England. — Reptiles of the Upper Trias. — Foot-prints in the Bunter formation in England. — Dolomitic Conglomerate of Bristol. — Origin of Red Sandstone and Rock-salt. — Precipitation of Salt from inland Lakes and Lagoons. — Trias of Germany. — Keuper. — St. Cassian and Hallstadt Beds. — Peculiarity of their Fauna. — Muschelkalk and its Fossils. — Trias of the United States. — Fossil Foot-prints of Birds and Reptiles in the Valley of the Connecticut. — Triassic Mammifer of North Carolina. — Triassic Coal-field of Richmond, Virginia. — Low Grade of early Mammals favourable to the Theory of Progressive Development.