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 number of genera and species ; and some of the forms are poor and dwarfed. It is a north-sea fauna ; and when the area was first separated from the main ocean, it was first freshened by influx of rivers, while it is now again becoming salter by evaporation. The result is the poverty and dwarfing of the forms. In the Black Sea there are misshapen or monstrous forms of Mollusca, stated by Forbes to be due to the freshened state of the water. Both of these cases, relating to what may be called continental seas, bear upon the subject in question, especially since the Rhaetic fauna of England is also comparatively poor in genera and species, when compared with the well-developed fauna of Lombardy and other parts of the south and east of Europe.

I now come to a difficult point. It is undoubtedly true in England that the Lower Lias follows the Rhaetic beds wherever they go ; and though there are symptoms of erosion between them at Penarth and at Curry Rivell, in Somerset*, yet the conformity is, on the whole, so complete, that wherever we meet with the base of the Lower Lias we look for the Rhaetic beds below, and as yet we have not been disappointed. The question then arises, how is it that the transition in these areas from the Rhaetic to the Liassic forms is so sudden ? It is hard to answer this question ; but it may perhaps be met by an analogous case. The estuarine and the lagoon beds of the Purbeck and Wealden series commenced in the Oolitic epoch, and ended in the Neocomian or Lower Cretaceous epoch ; and the change between the life of the Oolitic and Neocomian and Cretaceous deposits is as great as, and in some respects greater than, that between the Rhaetic and Liassic strata ; and though the Rhaetic beds were not deposited in fresh water, yet, like parts of the Purbeck series, I believe they were formed in shallow water under brackish semi- estuarine conditions which endured for a long period.

In conclusion, I may state that the same kind of reasoning, with differences, applies more or less to other red-coloured and to some calcareous strata of England/ including the Permian, Old Bed Sandstone, and even the red Cambrian formation. This I hope to treat of in a subsequent memoir. If this idea is true, and if this kind of work be carried out, it must have an important bearing on certain departments of palaeontology in a manner already partly indicated by Professor Huxley, and it may throw much fight on the distribution of the various forms of animal and vegetable fife in time and space. Without it, we must still in great part continue to regard the various formations very much as we might a pack of

in which the beds were being very tranquilly deposited " (" Abnormal Secondary Deposits," Quart. Journ. Geol. Soc. 1867, vol. xxiii. p. 470). See this memoir for a great deal of valuable information on these and other deposits. See also memoir by Dr. Wright " On the Avicula-contorta beds," Quart. Journ. Geol. Soc. 1860, vol. xvi., and others.

sufficient to produce it when the Lias-sea first came across a slowly sinking area.
 * This kind of erosion was evidently not accompanied by marked unconformity, the result of serious disturbance of the Rhaetic beds before the deposition of the ordinary Lias. Estuarine or tidal sea-currents would have been