The curtain again rises. A last day had at length come to the period of the middle formation; and in an ocean roughened by waves, and agitated by currents, like the ocean which flowed over the conglomerate base of the system, we find new races of existences. We may mark the clumsy bulk of the _Holoptychius_ conspicuous in the group; the shark family have their representatives as before; a new variety of the _Pterichthys_ spreads out its spear-like wings at every alarm, like its predecessors of the lower formation; shoals of fish of a type more common, but still unnamed and undescribed, sport amid the eddies; and we may see attached to the rocks below substances of uncouth form and doubtful structure, with which the oryctologist has still to acquaint himself. The depositions of this upper ocean are of a mixed character: the beds are less uniform and continuous than at a greater depth. In some places they consist exclusively of sandstone, in others of conglomerate; and yet the sandstone and conglomerate seem, from their frequent occurrence on the same platform, to have been formed simultaneously. The transporting and depositing agents must have become more partial in their action than during the earlier period. They had their foci of strength and their circumferences of comparative weakness; and while the heavier pebbles which composed the conglomerate were in the course of being deposited in the foci, the lighter sand which composes the sandstone was settling in those outer skirts by which the foci were surrounded. At this stage, too, there are unequivocal marks, in the northern localities, of extensive denudation. The older strata are cut away in some places to a considerable depth, and newer strata of the same formation deposited unconformably over them. There must have been partial upheavings and depressions, corresponding with the partial character of the depositions; and, as a necessary consequence, frequent shiftings of currents. The ocean, too, seems to have lessened its general depth, and the bottom to have lain more exposed to the influence of the waves. And hence one cause, added to the porous nature of the matrix, and the diffused oxide, of the detached, and, if I may so express myself, churchyard character of its organisms.