Corresponding with these views it will be remembered that, after the deposition of the coal-measures, there succeeded a period of violent plutonic action, whereby the formation was dislocated, broken up into smaller sections or basins, and pierced by the igneous rocks. There would, consequently, during this season of paroxysm, be a vast destruction of animal and vegetable life. The indurated crust would everywhere undergo great trituration. Gravel, sand, and mud of every quality would be cast along the shores, or silted up in the deeper hollows. And then again would come a term of general repose, as the angry elements subsided, exhausted by their own violence. The scene was actually or nearly as described; and, in the aspect of the older denuded and uplifted rocks, as above represented, there are the most striking evidences of the agitations of the period. The exuberant flora of the carboniferous age, suffered prodigiously, or became utterly extinct. Conglomerates were formed which exhibit few traces of organic life. To these succeeded the vast areas of the fine-grained sandstones, and gypseous and saliferous marls, everywhere nearly horizontal, and still undisturbed on their ancient beds. The tribes of animals were abridged in numbers, changed or modified in structure, so as to suit the altered state of things. The rain-drop, ripple-mark, and foot-print are all witnesses to be adduced of the mighty change, as they are all proofs of the doings of Him who holds the waters in the hollow of his hand, makes the clouds to distill in showers, issues his command to the hurricane and the earthquake, and restores in renovated beauty the face of nature. The lowest bed of the formation is the magnesian limestone, which derives its name from the quantity of carbonate of magnesia distributed through the matter of the rock, amounting in some instances to as much as sixty per cent. It is likewise called the _dolomitic_ limestone, from M. Dolomieu, who first investigated its granular crystalline structure. This limestone is generally of a yellow color, glimmering luster, passing occasionally into blue and brick-red varieties, and exfoliates in thin plates, or breaks up in large botryoidal masses. In this form it occurs at the cliffs of Durham, where it assumes the grouping and arrangement of chain-shot; and, as the beds are distinctly stratified, the face of the rock has a very striking and pleasing effect. In the more southern counties, this formation exists generally in the form of a conglomerate, supposed to be derived from the debris of the older carboniferous limestone united by a dolomitic paste; thus illustrating the source and mode of the deposit, while in the organic remains there has been traced a regular gradation between the types of the older sub-carboniferous and the successive newer strata. Let us consider some of the more remarkable forms, tracings, and ingredients of the formation.