It is the same with the formations themselves in which the organic things are imbedded. The course of creation progresses, but always under such breaks and renewals as clearly to manifest, that the same power which watches over the organic, is operative also in respect of the inorganic structures of the earth. Various are the genera and species of once animate forms, imbedded in the different strata beneath our feet; but equally various are the strata themselves; as a new race arises, so are there new forms of rocks produced along with them. And when we compare the two extremes of the fossiliferous strata, the silurian and the tertiary, or any of the intermediate—the old red sandstone and the oolites, the carboniferous and the chalk—we find that the rocks are just as various in quality, structure, and appearance, as are the animals which existed and perished during their respective epochs. The lines of demarkation are distinct. They may sometimes run into each other, so as to leave it doubtful where the one series ends and the other begins; but so it is with the organic remains themselves, a few of an antecedent epoch living into and invading the province of another, when the limit is reached, and the family altogether disappears. The same law holds in the great mineral masses of the earth’s crust. Rocks are of different families, even as plants and animals are; and over the entire surface of the globe, they display in their various suites such changes and diversities as demonstrate an interfering hand and a new creative energy. Indeed, there is, it may be avowed, a much greater diversity of type in the mineral groups themselves than in their organisms, the living genera and species of one formation differing, often, less from each other than do the rocky matrices in which their remains are imbedded. II. But, in estimating _the time_ that elapsed during the formation of the various sedimentary strata, are geologists warranted in assuming such principles of calculation as have been adopted?—There are two aspects under which the subject may be approached—the one, as respects the formative process of rocks—the other, the probable duration of life in the different epochs, or rather, as connected with the formations which indicate the epochs.