Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/103

 Rh of a small spiral univalve, (Paludina viridis), fixed to the outside of this tubular case of a larva of the genus Phryganea. See Lyell's Principles of Geology, 3d edit. vol. iv. p. 100. It is difficult to conceive how strata like these, extended over large tracts of country, and laid one above another, with beds of marl and clay between them, should have contained the coverings of such multitudes of aquatic animals, by any other process than that of gradual accumulation during a long series of years.

In the case of deposites formed in estuaries, the admixture and alternation of the remains of fluviatile and lacustrine shells with marine Exuviæ, indicate conditions analogous to those under which we observe the inhabitants both of the sea and rivers existing together in brackish water near the Deltas of the Nile, and other great rivers. Thus, we find a stratum of oyster shells, that indicate the presence either of salt or of brackish water, interposed between limestone strata filled with freshwater shells among the Purbeck formations; so also in the sands and clays of the Wealden formation of Tilgate forest, we have freshwater and lacustrine shells intermixed with remains of large terrestrial reptiles, e. g. Megalosaurus, Iguanodon, and Hylæosaurus; with these we find also the bones of the marine reptiles Plesiosaurus, and from this admixture we infer that the former were drifted from the land into an estuary which the Plesiosaurus also having entered from the sea, left its bones in this common receptacle of the animal and mineral exuviæ of some not far distant lands.

Another condition of organic remains is that of which a well-known example occurs in the oolitic slate of Stonesfield, near Oxford. At this place a single bed of calcareous