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With regard to our own country, the principal deposits subsequent to the carboniferous rocks fringe these rocks both on the western and eastern sides. They pass in some cases by regular transition from the coal-measures, occasionally containing similar fossil vegetable and animal remains, and rarely indicating the lapse of any long period corresponding to a considerable break in the continuity of the successive
Overlying the coarse sandy beds which rest upon the coal-measures, there is next met with a limestone, (the magnesian limestone,) which differs considerably from most of the ordinary limestones in its general appearance, and which, in the possession of a variable proportion of carbonate of magnesia, mixed with its carbonate of lime, seems to have required either a totally different condition of deposition from any with which we are acquainted, or a subsequent change only seen at present in recent volcanic districts, and then on a small scale. This limestone, however, contains fossils, and among them a few corals and shells; and there are also found in it fragments and sometimes complete skeletons of fishes, which seem to have been tolerably abundant in the seas, since whole beds are charged with animal bitumen, probably derived from their decomposition, and in these beds the skeletons and other indications of the fishes are more than usually plentiful. But the most remarkable phenomenon of the magnesian limestone and contemporaneous strata is the presence in them of distinct reptilian remains, at least five species of which have already been made out.
Over various parts of the continent of Europe, as
in our own island, the coal-measures are more or less covered up with beds, consisting of this coarse grit overlaid by clay and limestone, the clay being often remarkably bituminous, and the limestone generally exhibiting the chemical peculiarity already alluded to, containing a certain proportion of carbonate of magnesia, mixed with the carbonate of lime. In the east of Europe, however, and more especially in Russia, this series is exhibited in its greatest completeness. It is there found occupying a hollow or troughlike depression in the carboniferous strata, and is said to extend for a distance of nearly seven hundred miles from north to south, and for four hundred miles between the Ural chain and the river Volga, in the ancient kingdom of Permia, now included within the vast compass of the Russian empire. In this tract conglomerates and grit-stones, with magnesian and other limestones, make up the series, and contain fossils identical with those common in Durham and the neighbourhood of Bristol. Over the whole of Europe, therefore, similar causes seem to have acted in producing this series of magnesian strata at the close of the carboniferous period; and a dreary waste of sandy unproductive beds seems to mark the disappearance of land clothed with vegetation, and the gradual deepening of the sea, which at first received rolled and pounded fragments of rock, carried out to a distance in the form of sand, until afterwards, the land diminishing and disappearing, even this small supply ceased, and scarcely any deposit or any fragment of organic existence was retained, in consequence of the absence of material in which it could be buried and preserved.
The nature of the remains of fishes found in the magnesian limestone rocks indicates also a diminution in the size of the prevailing species, perhaps arising from the gradual diminution and increasing distance of the land, and the deepening of the sea in the district where such remains occur.
The reptilian fishes remain indeed, but they also become small: those of the shark tribe are few, and exhibit some peculiarities of structure, but are comparatively unimportant; and the rest were chiefly the bad and slow swimmers, or bottom fish, living on offal and on the invertebrated groups.
But a time of much greater change was approaching a time of disturbance, which should shake to their foundations all the solid and massive rocks that had been then deposited; and of subterranean movements, which in their course should break asunder the hardest and the strongest among these rocks; crushing and grinding into small fragments whole strata that had become compact and closely consolidated, and crumpling into complicated folds the toughest and most unyielding beds, as if they had been layers of some soft material carelessly squeezed in the grasp of a powerful hand.
It is indeed impossible for words to express the complication of disturbance, or the amount of confusion, that has been produced in some districts by forces acting on the solid crust of the globe, between the close of what we have called the first epoch, and the commencement of the second; and yet all this was done with a certain degree of order, and doubtless occupied a long period of time. Volcanic eruptions have taken place in some districts, and their effect is seen in tor
rents of ancient lava, heaps of erupted ashes, and rocks chemically changed by the intrusion of heated vapours charged with gases. In others, enormous cracks extending for many hundred yards, or even for miles together, may be traced in the more brittle rocks; and the rocks themselves have been burnt as in a furnace by the boiling and bubbling mass of molten lava which has been poured from beneath into such wide fissures. Sometimes extensive tracts, where the rocks are thinner and tougher, have exhibited these cracks in systems of hundreds in number parallel to one another; while here and there the intense fiery action from beneath has thrown up the surface into blisters and domes, which are often fractured at the top, and thus reveal the history of their elevation. Still more frequently, also, the irresistible subterranean force has snapped asunder the strata, as a violent blow would pierce through a few folds of paper, and one side of the broken bed has been lifted high in the air, or has sunk into a deep hollow beneath. And if, as happened occasionally, the force was not sufficiently energetic to break up in this way the whole group of overlying matter, it might yet effect a no less striking result, raising up the strata upon a line or on a point, and producing a saddle-shaped or a dome-like elevation, according to the circumstances of the case.
All these effects, and all of them on the grandest scale, were produced in some way or other upon many of the old rocks towards the close of the first epoch of creation; and every Geologist, familiar with the structure of our own island, could readily point to abundant examples of each particular disturbance above alluded to. Every coal-field is so split asunder and broken into
small fragments by what are called 'faults,' (cracks and consequent disturbances of the strata,) that they alone might be appealed to as sufficient proof; and, indeed, the very appearance of the smaller coal-fields of the middle of England, lifted as they are far above the great expanse of the new red sandstone, is due solely to these under-ground movements, which have borne to the surface portions of the carboniferous and lower strata, that would otherwise have been hidden.
It is not unlikely that much of the general contour of the high ground of England and many parts of northern Europe was originally marked out during the restless disturbances of this interval of violence. The districts occupied by the mountain limestone and the older rocks, at least, have probably in later times been disturbed only by movements affecting the general level of large tracts; and there cannot be a question as to the intensity and continuance of the forces acting beneath the surface at that time having been then much greater than any that have since affected that portion of the earth's crust exposed for investigation in our own island.
These remarks apply chiefly to the physical geography and geology of England, but they also describe with very little modification a large proportion of all those tracts in which the carboniferous and older rocks appear. Exceptions, it is true, are not wanting; and a very interesting one is met with in Russia, where various rocks of this first epoch stretch over a vast extent of country, and seem to have been little disturbed, except by exceedingly slow movements of elevation, since they were originally deposited. We shall find hereafter, that, on the one hand, similar