12. But the mechanical structure of the strata, the result, as it seems, of aqueous sedimentary deposit, is not the only, nor the most important evidence, with regard to the length of time occupied by the formation of the rocky layers which now compose our mountains. As we have said, they contain shells, and other remains of creatures which live in the sea. These they contain, not in small numbers, scattered and detached, but in vast abundance, as they are found in those parts of the ocean which is most alive with them. There are the remains of oysters and other shell-fish in layers, as they live at present in the seas near our shores; of corals, in vast patches and beds, as they now occur in the waters of the Pacific; of shoals of fishes, of many different kinds, in immense abundance. Each of these beds of shells, of corals, and of fishes, must have required many years, perhaps many centuries, for the growth of the successive individuals and successive generations of which it consists: as long a time, perhaps, as the present inhabitants of the sea have lived therein: or many times longer, if there have been many such successive changes. And thus, while the present condition of the earth extends backwards to a period of vast but unknown antiquity; we have, offered to our notice, the evidence of a series of other periods, each of which, so far as we can judge, may have been as long or longer than that during which the dry land has had its present form. 13. But the most remarkable feature in the evidence is yet to come. We have spoken in general of the oysters, and corals, and fishes, which occur in the strata of our hills; as if they were creatures of the same kinds which we now designate by those names. But a more exact examination of these remains of organized beings, shows that this is not so. The tribes of animals which are found petrified in our rocks are almost all different, so far as our best natural historians can determine, from those which now live in our existing seas. They are different species; different genera. The creatures which we find thus embedded in our mountains, are not only dead as individuals, but extinct as species. They belonged, not only to a terrestrial period, but to an animal creation, which is now past away. The earth is, it seems, a domicile which has outlasted more than one race of tenants.