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In the midway regions of the secondary strata, are the earliest remains yet discovered of Cetacea.* In the tertiary formations, we find both Birds, Cetacea, and terrestrial Mammalia, some referable to existing genera, and all to existing orders. See Pl. 1, fig. 73—101.

Thus it appears, that the more perfect forms of animals become gradually more abundant, as we advance from the older into the newer series of depositions: whilst the more simple orders, though often changed in genus and species, and sometimes losing whole families which are replaced by new ones, have pervaded the entire range of fossiliferous formations.

The most prolific source of organic remains has been the accumulation of the shelly coverings of animals which occupied the bottom of the sea during a long series of consecutive generations. A large proportion of the entire substance of many strata is composed of myriads of these shells reduced to a comminuted state by the long continued movements of water. In other strata, the presence of countless multitudes of unbroken corallines, and of fragile shells, having their most delicate spines, still attached and undisturbed, shows that the animals which formed them, lived and died upon or near the spot where these remains are found.

Strata thus loaded with the exuviæ of innumerable generations of organic beings, afford strong proof of the lapse of long periods of time, wherein the animals from which they have been derived, lived and multiplied and died, at the bottom of seas which once occupied the site of our present continents and islands. Repeated changes in species, both of animals and vegetables, in succeeding members of diffe

* There is, in the Oxford Museum, an ulna from the great Oolite of Enstone near Woodstock, Oxen, which was examined by Cuvier, and pronounced to be cetaceous; and also a portion of a very large rib, apparently of a whale, from the same locality.

rent formations, give farther evidence not only of the lapse of time, but also of important changes in the physical condition and climate of the ancient earth.

Besides these more obvious remains of Testacea and of larger animals, minute examination discloses occasionally prodigious accumulations of microscopic shells that surprise us no less by their abundance than their extreme minuteness; the mode in which they are sometimes crowded together, may be estimated from the fact that Soldani collected from less than an ounce and a half of stone found in the hills of Casciana, in Tuscany, 10,454 microscopic chambered shells. The rest of the stone was composed of fragments of shells, of minute spines of Echini, and of a sparry calcareous matter.

Of several species of these shells, four or five hundred weigh but a single grain; of one species he calculates that a thousand individuals would scarcely weigh one grain. (Saggio Orittografico, 1780, pag. 103, Tab. III. fig. 22. H. 1.) He farther states that some idea of their diminutive size may be formed from the circumstance that immense numbers of them pass through a paper in which holes have been pricked with a needle of the smallest size.

Our mental, like our visual faculties, begin rapidly to fail us when we attempt to comprehend the infinity of littleness towards which we are thus conducted, on approaching the smaller extremes of creation,

Similar accumulations of microscopic shells have been observed also in various sedimentary deposites of fresh-water formation. A striking example of this kind is found in the abundant diffusion of the remains of a microscopic crustaceous animal of the genus Cypris. Animals of this genus are enclosed within two flat valves, like those of a bivalve shell, and now inhabit the waters of lakes and marshes. Certain clay beds of the Wealden formation below the chalk, are so abundantly charged with microscopic shells of the Cypris Faba, that the surfaces of many laminæ into

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which this clay is easily divided, are often entirely covered with them as with small seeds. The same shells occur also in the Hastings sand and sandstone, in the Sussex marble, and in the Purbeck limestone, all of which were deposited during the same geological epoch in an ancient lake or estuary, wherein strata of this formation have been accumulated to the thickness of nearly 1000 feet. (See Dr. Fitton's Geol. sketch of Hastings, 1833, p. 68.)

We have similar evidence of the long duration of time, in another series of Lacustrine formations, more recent than the chalk, viz. in the great fresh-water deposites of the tertiary period in central France; here the district of Auvergne presents an area of twenty miles in width, and eighty miles in length, within which strata of gravel, sand, clay, and limestone have been accumulated by the operations of freshwater, to the thickness of at least seven hundred feet. Mr. Lyell, in his Principles of Geology, 3d edit. vol. iv. p. 98, states that the foliated character of many of the marly beds of this formation is due to the presence of countless myriads of similar exuviæ of the Cypris which give rise to divisions in the marl as thin as paper. Taking this fact in conjunction with the habit of these animals to moult and change their skin annually, together with their shell, he justly observes that a more convincing proof of the tranquillity of the waters, and of the slow and gradual process by which the lake was filled up with fine mud cannot be desired.

Another proof of the length of time that must have elapsed during the deposition of these tertiary fresh-water formations in Auvergne, is afforded near Cleremont by the occurrence of beds of limestone, several feet in thickness, almost wholly made up of the fossil Indusiæ, or Caddis-like coverings, resembling the cases that enclose the larvæ of our common May-fly.

Mr. Lyell states that a single individual of these Indusiæ is often surrounded by no less than a hundred minute shells

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 impossible 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 fresh-water shells among the Purbeck formations; so also in the sands and clays of the Wealden formation of Tilgate forest, we have fresh-water 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 land.†

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

*See Madden's Travels in Egypt, vol. ii. p. 171-175.

+ For the detailed history of the organic remains of the Wealden formation, see Mr. Mantell's highly instructive and accurate volumes on the geo. logy of Sussex,

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

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