19. But with regard to this series of Organic _Creations_, if, for the sake of brevity, we may call them so; we may naturally ask, in what manner, by what agencies, at what intervals, they succeeded each other on the earth? Now, do the researches of geologists give us any information on these points, which may be brought to bear upon our present speculations? If we ask these questions, we receive, from different classes of geologists, different answers. A little while ago, most geologists held, probably the greater number still hold, that the transitions from one of these periods of organic life to another, were accompanied generally by seasons of violent disruption and mutation of the surface of the earth, exceeding anything which has taken place since the surface assumed its present general form; in the same proportion as the changes of its organic population go beyond any such changes which we can discern to be at present in operation. And there were found to be changes of other kinds, which seemed to show that these epochs of organic transition had also been epochs of mechanical violence, upon a vast and wonderful scale. It appeared that, at some of these epochs at least, the strata previously deposited, as if in comparative tranquillity, had been broken, thrust up from below, or drawn or cast downwards; so that strata which must at first have been nearly level, were thrown into positions highly inclined, fractured, set on edge, contorted, even inverted. Over the broken edges of these strata, thus disturbed and fractured, were found vast accumulations of the fragments which such rude treatment might naturally produce; these fragmentary ruins being spread in beds comparatively level, over the bristling edges of the subjacent rocks, as if deposited in the fluid which had overwhelmed the previous structure; and with few or no traces of life appearing in this mass of ruins; while, in the strata which lay over them, and which appeared to have been the result of quieter times, new forms of organic life made their appearance in vast abundance. Such is, for example, the relation of the coal strata in a great part of England; broken into innumerable basins, ridges, valleys, strips, and shreds, lying in all positions; and then filled into a sort of level, by the conglomerate of the magnesian limestone, and the superincumbent red sandstone and oolites. In other cases it appeared as if there were the means of tracing, in these dislocations, the agency of igneous stony matter, which had been injected from below, so as to form mountain-chains, or the cores of such; and in which the period of the convulsion could be traced, by the strata to which the disturbance extended; _those_ strata being supposed to have been deposited before the eruption, which were thrust upwards by it into highly-inclined positions; while those strata which, though near to these scenes of mechanical violence, were still comparatively horizontal, as they had