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more regular, more widely spread, and more characteristic beds of gravel than the Alpine rocks, whose range is, in every respect, more limited.
The whole subject of the distribution of gravel is, however, one abounding in difficulties which have as yet been only partially explained. Besides assuming the action of great waves acting for a very short time immediately after an earthquake shock, and propelling a mass of broken rock with irresistible power at a rapid rate for a short distance, some geologists have called in the aid of marine currents. The action of the waves on an ordinary coast-line is also itself sufficient to account for many even of the more striking phenomena. While these causes, analogous to those now in action, are thought by some geologists sufficient to explain the facts, others again have resorted to ice in some form as the only agent capable of solving the problem. One theory connected with ice is indeed only more improbable than it is bold and ingenious, its author and supporters assuming the whole of that part of the earth on which gravel is now found to have been once actually beneath a frozen surface, and to have been traversed by glaciers, such as those which in the Alps produce very similar and analogous appearances. There are certainly, however, no sufficient grounds for believing that true glaciers have ever covered Europe, since there is no evidence of the existence of mountain chains from which they could have proceeded.
Although these authors, misled by partial observation, have thus limited the action of ice to glaciers, or frozen streams descending from mountain sides and moving along on the plains, simply from the
action of gravitation, or by some supposed force connected with the alternate thawing and freezing of water, others have assumed that the ice may have acted in the form of icebergs. In other words, it has been supposed that glaciers, descending to the sea in cold climates, may have been broken off from time to time and floated away, conveyed by marine currents until they are either melted by the warmer waters of the ocean or stranded on some submarine mud-bank or shoal. In either case, and both are illustrated by recent examples, the load of broken rock which such masses of ice carry would form a bed of gravel, which, on subsequent elevation of the sea bottom, might become a portion of the general surface of a continent or island.
The breaking up of the surface, during or after the intense cold of an Arctic winter, or even of such cold as occurs annually in thickly-inhabited districts in Russia, is another means by which some of the phenomena of gravel when little removed from the parent rock have been explained. Geologists are indebted to Sir Roderic Murchison for this and many other ingenious suggestions concerning the origin of gravel; and there can be no question, that the careful examination of existing nature, so far as it is exposed to our view, is the most satisfactory as it is the safest and most reasonable mode of explaining the various appearances which are presented in the course of geological investigations.
Whatever the cause or causes may have been, the distribution of numerous blocks of stone, sometimes rounded, but more frequently angular, and of every size and shape, and the removal of these to various
distances from the parent rock, are facts distinctly made out. Such blocks, also, are not confined to northern Europe, but are met with both in North and South America, and in other parts of the world. It is, however, certain, that true gravel with rolled blocks of stone is not universally distributed; and the effects thus produced have been as partial as they were frequent, the result being often quite different. It thus happens that while in most cases common gravel, or transported and erratic blocks and boulders, have been deposited, we find elsewhere only great masses of mud and clay, mixed with stones, sand, or any other material, drifted into recesses, and left there by the iceberg or the retiring wave.
It is not likely that a great system of elevation can have acted during a long period, bursting asunder in some districts the hard and brittle rocks at the surface, and sending up granite in a soft and pasty or melted state; tearing asunder in others the tough superficial beds, and allowing the escape of gaseous vapours and currents of molten rock; while in others, again, wide tracts were slowly but permanently lifted above their former level, without a re-action having taken place after the force had ceased to act, causing a general or partial subsidence over some vast areas. Possibly, the more extreme and Arctic temperature which many things seem to indicate as characterising a late geological period, may have been connected with a more uniform expanse of land near the poles, the general level of that land being also somewhat higher than at present. After this partial elevation there may also have been a partial depression, especially, perhaps, in
north-western Europe; the climate there may have become ameliorated, while at the same time considerable tracts, which had long existed as dry land, were gradually covered up by water.
It was probably after this last depression, succeeding the period of deposition of the gravel, and itself accompanied by undulation of the surface admitting of many superficial deposits in certain districts, that the final separation of our own islands, and the destruction of many species of animals which had before been their chief inhabitants, took place. Possibly, also, the land may then have sunk some two or three hundred feet, or even more, below the present level, so that there remained only the higher grounds on which the smaller animals were enabled to live, while the larger ones died out. Another movement of elevation then occurred, once more bringing large tracts of land above the surface, especially along the north-western extremity of Europe and the neighbouring British Islands, and by this were formed the numerous raised beaches of the south and west coast of England. It was not till this late modification of the surface, that the courses of the rivers, the general contour of the land, the general relations of land and sea, the climate, and the general fauna and flora distinctly assumed their present character; nor was it, perhaps, till long after this time, that, on the introduction of man,* new changes and modifications took place, and new races were introduced, not, indeed, naturally indigenous, but otherwise well adapt
* There is, however, some evidence tending to prove that man was an inhabitant of our own island even so long ago as the cavern bear and hyæna.
ed to the soil and climate. Nor, indeed, is this all; for the same mighty influence has changed even the conditions of climate and the natural course of seasons by the removal of forests and the draining of marshes. In this way have been effected those final, and, in their way, mighty changes, which this closing chapter of the earth's history calls upon us to notice, although, from their recent production, they rather belong to recent natural history than to geology in the general acceptation of the term.
In Europe, and, above all, in England, where every corner of land is considered as waste if it is not employed directly by human agents and for human purposes, and where man is in everything paramount, these changes have now so far affected the surface of the land, as to render it difficult to pursue our investigations with regard to the true history of unfettered nature. We must go to distant countries and other climes, where nature is still free, to discover the great facts of general progress; it is there, if at all, that we shall find distinct traces of the progress of that well-adapted system, according to which all things, animate and inanimate, work together in harmony; and we must travel with the enterprising and the active, over plains and into forests hitherto untrodden by man, or, with the geologist, we must look far back into the ancient history of the earth, if we would know truly and fully what nature is, and how far the laws originally imposed on matter are real and have been perpetual.