« AnteriorContinuar »
of the moistened materials that lay between the surface and the great subterranean laboratory from which the heat and the explosive forces generated by it proceeded.
Let us suppose the waters of Lake Superior to be drained, and its bed scooped down through the whole series of stratified and crystallized rocks that lie beneath it, till a region were reached at a depth perhaps of fifteen or twenty miles, where, let it be assumed, a vast magazine is treasured up of volcanic materials. Let us then suppose the chasm to be filled by successive layers, each hundreds or thousands of feet in thickness, of silex, alumine, lime, potash, soda, iron, magnesia, and a proportionate share of the other elements that entered into the composition of the strata, in minute primitive particles. Let us suppose the waters of the lake then to be readmitted to its bed, and gradually to descend through it till they reached the magazine of volcanic matter, and generated an expansive force by which the superincumbent mass should be pushed upwards; it is manifest that that portion of the upper layer, at the points where the impulse from below was the greatest, would be the first that would be raised above the general level and mixed with the waters of the lake; and that if it were silex, it would, on being subjected to the proper agencies, form quartz rock or sandstone; if alumine, with an intermixture of silex, it would form marl, or some species of schist; and if lime, limestone. The effect of the impelling force from beneath, however, especially if large volumes of gas were driven upwards, would soon be to open a passage to the surface by a vein or chasm, through which a current of the moistened or liquid matter would be driven up into the waters of the lake, and diffused over its bottom; and if that process were continued, a portion of each layer in the series would be raised to the surface and spread in a stratum over the bed of the lake —before a stream of melted lava would mount through the passage and pass into the waters, or shoot into the atmosphere. But such a stupendous enginery acting, with slight intervals, at innumerable points throughout the circuit of the globe, would have been amply adequate to throw the whole materials of the strata on the surface in the lapse of fifteen, sixteen, or eighteen hundred years. Such a period would, indeed, seem excessive rather than too short for such a work. Such powerful agents, acting at points not more numerous than those at which igneous rocks and lavas have been driven to the surface, would undoubtedly have been sufficient for that effect.
We have thus the most ample evidence of the existence at that period of the requisite materials and agents in the proper conditions for the accomplishment of that work. We shall now proceed to show that this view of the origin of the strata is corroborated and verified by a variety of considerations.
It is confirmed by the great number of the points at which igneous rocks and lavas have been forced up to the surface. The number of volcanoes that burned during the formation of the secondary and tertiary strata was not only far greater than at present, but in the ratio probably of hundreds to one. Their traces are seen on a vast scale in many regions where uo eruptions have taken place for centuries. Thus they are very numerous in Central and Southern France, sixty cones being distinguishable in the single province of Auvergne. They exist in great numbers in Germany, Hungary, Transylvania, and Styria. In Northern, Central, and Southern Italy they are very frequent; in Sicily, also, Sardinia, and the neighboring islands. Traces of them are seen in Spain and Portugal. Their relics exist on a great scale in the islands of the Grecian Archipelago, and throughout Asia Minor. They are seen also in Syria, Southern Arabia, Persia, Northern and Eastern Asia, and the islands of the Chinese seas. Most of the islands of the Atlantic, and—except those of coral—nearly the whole of the vast crowd that stud the Indian and Pacific oceans, have been the seat of volcanoes; and craters that no longer burn are found in great numbers along the whole line of the mountains that skirt the Pacific coast, from the Arctic ocean to Cape Horn. If the number still active in different parts of the globe is, as is supposed, from one hundred and seventy-five to two hundred, the whole series that have burned at successive periods must undoubtedly amount to many thousands.
But the number of points at which igneous rocks— granite, porphyry, basalt, and trap—have been forced up to the surface, is immensely greater. Some, or all of them, are found in almost every considerable district of the globe. Though there are no traces in the British islands of modern volcanoes, granite, porphyry, greenstone, hypersthene, basalt, and trap form the crust, or lie immediately beneath the soil in England, Scotland, Ireland and the Hebrides and Orkneys, in thousands of places, indicating that there has been at least an equal number of passages from the molten abyss beneath, through which first the materials that lay above it, and then a portion of its own contents, have been driven up to the surface. In this country no lavas occur throughout the wide space betwixt the Mississippi and New Brunswick, or the great lakes and the Atlantic; yet granite, porphyry, hypersthene, hornblende, greenstone, serpentine, basalt, and trap, rise to the surface, or tower into the atmosphere in myriads and perhaps hundreds of thousands of places, so distinct from each other as to show that the passages through which they generally made their way to the surface were separate from each other. They are equally numerous also on other portions of the globe. Half as many channels of ejection from below, and probably a much smaller number, would have been adequate for the transfer to the surface, in a very few centuries, of a sufficient mass of materials for the formation of the strata. That such a vast number of openings have been formed from the interior, through which immense volumes of matter have been thrown up and incorporated in the crust of the globe, demonstrates, at least, the possibility and probability that it was through them or others of a like nature that the silex, alumine, lime, soda, iron, potash, and other elements of which the strata are built, were forced up into the oceans and seas from which they were depo- \sited.
It is corroborated by the deposition of the great elements—silex, alumine, and lime—of which the strata consist, in separate layers, instead of a promiscuous mixture;—silex constituting sandstone chiefly; lime forming limestone and chalk; and alumine, potash, and soda, which are conspicuous ingredients of felspar, entering, in a large measure, into the composition of shales, clays, and marls. That the ingredients of the strata are treasured up in masses separately from each other in the depths of the earth, is demonstrated, as we have already shown, by their being often separately ejected and embodied in the igneous rocks. Their distribution into separate strata is explicable, therefore, on the supposition that they were drawn from such depositories, and not on any other theory of their origin. Had they been formed,