and sandy slate not six feet thick, contains an admixture of terrestrial animals and plants with shells that are decidedly marine; the bones of Didelphys, Megalosaurus, and Pterodactyle are so mixed with Ammonites, Nautili, and Belemnites, and many other species of marine shells, that there can be little doubt that this formation was deposited at the bottom of the sea not far distant from some ancient shore. We may account for the presence of remains of terrestrial animals in such a situation by supposing their carcasses to have been floated from land at no great distance from their place of submarine interment.
A similar explanation may be given of the mixture of the bones of large terrestrial mammalia with marine shells, in the Miocene Tertiary formations of Touraine, and in the Crag of Norfolk.
Cases of Animals destroyed suddenly.
The cases hitherto examined, are examples of the processes of slow and gradual accumulations in which are preserved the remains of marine, lacustrine, and terrestrial animals that perished during extended periods of time, by natural death. It remains to state that other causes seem to have operated occasionally, and at distant intervals, to produce a rapid accumulation of certain strata, accompanied by the sudden destruction, not only of testacea, but also of the higher classes of the then existing inhabitants of the seas. We have analogous instances of sudden destruction operating locally at the present time, in the case of fishes that perish from an excessive admixture of mud with the water of the sea, during extraordinary tempests: and also from the sudden imparting of heat, and noxious gases, to water in immediate contact with the site of submarine volcanoes. A sudden irruption of salt water into lakes or estuaries, previously occupied by fresh water, or the sudden occupation of a portion of the sea, by an immense body of
