We may here observe, this violent change of subterranean matter from the interior to the exterior of the earth not only lowered the crust of the earth in one direction and raised it in another, but had the effect of flexing and frequently inverting the strata which had been formed in the vicinity of the deeper basins, and within the influence of the contraction, caused by the spasmodic action of the internal volcanic heat; that is, the strata deposited on the sharp axes of deep synclinal', or basins, would be further deepened by the contraction or subsidence of those basins; while the sur face would rather contract, also, than expand under the same influence, and thus the angle of the strata become greater, and, in many cases, even inverted. But this we do not assume to be the only cause of the inverted strata, so prevalent in the East; though we think it the chief or normal cause.
The sedimentary strata would naturally conform to the surface over which they were stratified; and as the granite surface is, and apparently was, generally uneven, the metamorphic or gneissic strata first deposited assumed the corrugations and angles of the original foundation. These appear to have been in long lines running northeast and southwest, wide and broad in the west and centre of the great basin, and narrow, steep, and deep towards the east. On those the Appalachian chains were folded, growing narrower and steeper as the basins subsided, with the general crust movement towards the east; and, as the crusts of the strata came closer together, by the general or lateral contraction.
It is very evident that our sedimentary strata were not laid horizontally when originally deposited, but conformed, as we have stated, to the general uneven surface of a granitic and volcanic period; yet many of our geolo gists assume such to be the general law, and they then assume a natural impossibility to account for the flexures of the strata in the phenomena of earthquakes and internal throes or pressure to uplift the mountain-axis. But those folds and deep basins must have been formed naturally and gradually as the current subsided in certain directions, and as the surface of the earth generally contracted by the condensation of its materials. When we consider how much the tire of a wagon-wheel contracts on cool ing, we can form some idea of what the contraction of the crust of the earth must be. This contraction first tends to eject the fluid matter of the interior in volcanic violence. But when the exterior crust has expended its heat and ceased its condensation, the continued contraction of the interior crust, or condensation of the liquid mass forming the great bulk of the earth, the exterior strata must yield in folds, depressing or rising as the interior condenses. If this effect was common to all parts of the earth's crust, and evenly distributed over the surface, the undulations would not be so great. But such is not the effect, since the weakest points are always folded first, and the deepest basins are always made deeper by the same cause, on the same principle, that it is easier to bend a weak sapling already partially bent, than to bend a tree that is strong and straight.
Prof. Rogers, in his Geology of Pennsylvania, assumes the great Appa lachian basin to have been elevated bodily from the ancient sea, to account for its drainage and its coal formations. But a moment's thought is suffi cient to dispel an idea so impractical and at variance with the laws of Nature, particularly when aft elevation and depression of half a continent —whose crust must have been over twenty miles thick—is required to account for the formation of each coal-seam. When we know how weak and comparatively thin the crust of the earth must have been at that early day, when volcanic action was in full vigor in the East, and perhaps, too, in the West, since it has not, even to this day, entirely subsided, we can realize how impossible a thing it would have been to lift this incalculable mass. How much more readily would the pent-up gases have found a vent through the volcanic channels which had served as an escape so long! We do not intend to extend this chapter by simply refuting absurd theories while we are trying to establish a new hypothesis from the existing facts, but will now proceed to offer additional evidence and give additional facts to establish this wonderful creation as the result of plain and common processes of Nature.
We will not reiterate the facts already given, but again state the hypothesis briefly. The strata now reposing in the bed of the ancient Appalachian Sea were derived chiefly from the volcanic mountains which bounded its coasts on the east and northeast; and the processes effecting the same, as before stated, were natural and simple.
To the period of the Auroral or Great Limestone, there appears to have been but little interruption to the volcanic agencies and the process of change, or metamorphism of matter, from the fluid lava to the sedimentary strata resulting therefrom. But at this period there must have been a long calm, since the Great Limestone could not have been created in the manner and from the material of the subordinate strata. The volcanoes were comparatively quiet, having vented, perhaps, the fluids and vapors which had formerly existed in a state of tension in the bowels of the earth; and this assumption is sustained by the fact, that this state of quiet or rest from violent volcanic action was general, since the limestones were the cotempo rary formation of this period in nearly all parts of the earth.
Its formation was on a grand scale in the ancient Appalachian Sea. Its area was, of course, coextensive with the waters in which it was formed, while its maximum thickness was a mile! When we consider the boundaries and extent of this great sea,—whose landmarks are so well preserved,— we can form some idea of the magnitude of the limestone formations of this era.