A glacier is a moving stream of ice formed in the transverse valleys and furrowed gorges of alpine chains. They are of great depth and indefinite length; and as they proceed slowly but progressively in their courses they carry along with them all the loose and prehensible mountain debris with which they come in contact. On their surface they bear every falling splinter, small and great, from the overhanging rocks. The sides and bottom of the ravines through which they pass are stripped, polished, and striated. The avalanche breaks upon them with its accumulated load; and every mountain rill, upon the melting of the snows in summer, deposits over their flanks the materials with which they are charged. Immense masses of matter are, in these various ways, collected and transported from the higher into the lower valleys: these at the outgoing of the glacier generally assume a ridge shaped form, and are termed moraines. The underlying blocks are all rounded and grooved: those borne on the surface are sharp and angular, until they are swept away by the torrents into the rivers, where they are in turn subjected to their smoothing operations. There can be no doubt, therefore, either as to the disintegrating or transporting power of this mighty agent. When I stood upon the Mer de Glace I saw before me, in one gorge of the mountain, a continuous stretch of icy machinery fourteen miles in length by two to three in breadth, and several hundred feet in depth. The whole was in motion; and, whether we adopt as the principle of translation the mechanical pressure of Agassiz, or the hydraulic law of Forbes, the instrument of an incalculable carriage-power was there. And yet, upon the first glance, it shrank into a span, or appeared but as a small lake, as we viewed the glacier pouring down that deep gorge of Mont Blanc; a sheer depth of dark perpendicular rocks rising on its edges many thousand feet in height; several of the sharp-pointed Aiguilles, the Grandes Mullets, and above all, the Peak de Dru, unrivaled in symmetrical grandeur, penetrating still higher into the clear sky above. How many such glaciers are dispersed through that vast alpine chain; and how immense, upon any rule of calculation, have been the earthy and rocky materials which they have borne downward in the lapse of time!
Familiarized to such gigantic operations among his native Alps, M. Agassiz came to the conclusion that not only the bowlder drift of Switzerland, but nearly all the superficial accumulations of northern Europe, were to be ascribed to glacial action. In the straths and glens of Scotland he fancied a moraine in every talus of a mountain, and in every bar of a river. He saw the polishing of glaciers in the pass of Killiecrankie, on the sides of Ben Nevis, and the steep promontories of Morven. The parallel roads of Glenroy originated in the same cause. From the Mediterranean to the Arctic zone a polar climate universally prevailed, and the whole was covered with a mantle of ice; vast fields of ice, too, depending from the mountains penetrated into the adjacent valleys; the plains in succession were invaded, and erratic blocks were scattered in every direction; when at last, upon a change of temperature consequent upon other changes in the planetary relations of the earth, all these erosive influences were for a time increased, and the glacial power attained its maximum. Not only the upper and transverse furrows in the Alps but all the lower and great longitudinal valleys of the Cantons were the seats of glaciers during this period. Along the passes of the Rhine, the Rhone, the Drance, the Doire, the Arve, and the Isêre, the irrepressible tide of ice maintained its course, leaving portions of the drift at different elevations, and dropping bowlders on the intermediate hills and on the more distant and loftier barriers of the Jura. Sir R. Murchison opposes all these speculations of Agassiz and others. The elevation of the alpine chain, of which there is abundant evidence in comparatively recent times, he regards as cotemporaneous with the translation of the bowlder-drift, and considers that during the sub-aqueous condition of northern Europe, the Alps and the Jura were from two to three thousand feet below their present altitude. He finds that the famous blocks of Monthey opposite Bex are composed exclusively of the granite of Mont Blanc—that they have been transported on ice-rafts through the gorge of St. Maurice to their present locality—and reasons with justice that had they formed part of a moraine the debris of all the intervening rocks, along the valley through which the glacier passed, must have been associated with them. None of the glaciers of the Alps, he thinks, could have been of the extent implied in the transport through their agency of the Jura blocks, nor have ever the upper longitudinal and flanking valleys around Mont Blanc been filled with general ice-streams. The materials, likewise, of true glacier moraines he conceives can be readily distinguished, on the one hand, from the more ancient alluvia, and, on the other, from tumultuous accumulations of gravel bowlders and far-transported erratic blocks. And, looking at the various causes which have affected the surface, Sir Roderick concludes, that all the chief difficulties of the bowlder-clay formation are removed, when it is admitted that frequent and vast changes of the land and waters have taken place since the distribution of large erratics—that a great northern glacial continent has subsided—that the bottom of the sea over Britain and the adjacent continent has been raised into dry land, while the Alps and Jura, formerly at lower levels, have been considerably and irregularly elevated.
The elevation of this stupendous chain of rocks, not by one but by a succession of upheavals and depressions ere they assumed their present position and grouping, is a point generally admitted, and not difficult to demonstrate. The Alps, for example, are folded all round with successive belts or zones of sedimentary matter, marking, as so many milestones at different points of altitude, the measure of increment attained during the intervals of their deposition. These belts contain each their own peculiar class of fossils which determine their relative ages. In succession, the several suites or families of rocks rest upon the inverted outcrop or inclined edges of the older groups. Thus the history of organic life upon the globe, the incoming of new races and the extinction of old ones, as contained in these deposits, becomes a scale of measurement of the elevations, disruptions, and ever-varying conditions of the inorganic crust, while in the inverted, dislocated state of the crust itself, we mark the several throes by which it was lifted above the waters. Not one of the fossiliferous beds enveloping the granitic and crystalline nucleus of the chain of the Alps but has been shifted out of its original horizontal position, and the shift of the subjacent having always preceded the deposition of the overlying formation, it follows that, in addition to the intumescence of the chain, there must have been a series of oscillatory and elevatory movements before attaining its final altitude. But after the consolidation of the whole rocky strata, and while the waters were still many thousands of feet in depth, the superficial accumulations were being deposited—the bowlder drift, and erratic blocks, either by icebergs or other causes, were floated into position—and it was not until every one of these traveled stones, fresh even now as when torn from the living rock, were quietly settled down into the bottom of the sea, that Mont Blanc had displayed a moiety of its massive outline, or towered to one-half of its present colossal grandeur. The elevation of Ben-Mac-Dhui dates from the era of the old red sandstone formation. Mont Blanc was invaded on all sides by a sea that received the latest of the tertiary deposits. Both were submerged during the cataclysm which produced the bowlder clay; but as no increment to its bulk was derived from this cause, Ben-Mac-Dhui falls geologically to be reckoned a completed, and therefore a far older, mountain than Mont Blanc, which had not attained its full altitude and bulk until the expiration of the Pleiocene age!
Such are the mighty agencies contemplated by the geologist in the various later changes which have affected the surface of our globe. The rill, the river, the torrent, the glacier, the earthquake, the volcano, are still in operation, but only as faint images of the enormous powers which in the more ancient times have been at work. That the earth has been repeatedly encroached upon by the waters every principle of his science goes to establish; but out of every convulsion he sees a better and more stable condition of things to have emerged. If the bowlder drift and the cold plastic clay formation point to a continuance of sunless, lifeless seasons, he forgets not, as the products of the period, that two-thirds of the soil of Great Britain and of the grain-bearing lands of the continent, have been derived from these accumulations—the industrial monuments of their invasion in every quarter of the world.
CHAPTER III.
THICKNESS OF THE EARTH’S CRUST—CENTRAL HEAT.
The question arises, since upon geological grounds it is demonstrable that the crust of the earth has been repeatedly upheaved and broken, have we reason to conclude that similar states of paroxysm and convulsion may not again return? This brings us to the consideration of two very interesting problems, namely,—The Thickness of the Earth’s Crust—And the Doctrine Of Central Heat. Have we any means of determining either of these points? The doctrine of the igneous origin of granite and other rocks proceeds upon the assumption of a vast reservoir of heat existing somewhere within the interior; and the question to be solved is—What is the thickness of the solid crust beneath which the molten rocks have their origin? and what the cause of their fusion?
I. An opinion has long prevailed among geologists of a certain school, that the crust of the earth is of very limited dimensions. A thin coating of primary crystalline rock is interposed betwixt the sedimentary strata above, and the intensely incandescent mass of which the interior is composed. The experiments of Fourier establish a formula of increasing temperature of the strata in a descending series, and from the rate of this increase, it is inferred, that about one hundred miles below the surface the entire nucleus is in a state of complete fusion. Some have even assumed the melting point to be less than thirty miles, when “the next contiguous matter is in a state of fusion, at a temperature probably higher than any that man can produce by artificial means, or any natural heat that can exist on the surface.”[11] Sir John Leslie attempted a demonstration of the ultimate resolution of the materials into light, as the only element capable of resisting the vast pressure of the outer crust; and, erroneously assuming the modulus of compressibility of air, water, the metals, and all known earthy substances to be invariable, however greatly the pressure may be increased, this ingenious philosopher came to the conclusion, that, instead of Tartarean darkness, the offspring of superstition, the inner chambers of the earth are filled with luminous ether, the most pure, concentrated, and resplendent. Darwin believes that much of the vast continent of South America is suspended over an inner sea of liquid fire, and says, that, “daily it is forced home on the mind of the geologist, that nothing, not even the wind that blows, is so unstable as the level of the crust of the earth.”
With the fires of Etna and Vesuvius raging on the one side, and the recent though extinct volcanoes of Auvergne and the Cantal seated so near on the other side, what security is there, amidst so many undoubted facts of the mobility of the land, that these vast piles of Alpine mountains may not again, through mere mechanical weight, break through the film of crust on which they rest, and sink into the abyss from which they so lately emerged! The doctrine of central heat, it may be replied, does not necessarily imply the universal fluidity of the central mass, an opinion supported by Lyell, Poisson, and other eminent philosophers; while there is reason to infer, as repeatedly stated, that there is no identity of scale and mechanism between volcanoes now active, and the igneous causes which gave birth to these and other stupendous mountain-chains.
But astronomy gives a different and more comfortable solution of the problem. The influence of the moon alone, it would appear, acting upon our planet, requires a thickness of crust of at least one thousand miles, to prevent the fabric of the globe from being severed into fragments. The earth, considered in connection with its own planetary system, has three distinct motions in space, a fact in science usually illustrated by the movements of the common spinning-top. A more striking illustration may be seen in the steam-vapor which has aided you onward, that living cloud of light and heat which towers and floats away in these beautifully curling wreaths. Like the trail of the comet, how gracefully it sweeps over the plains in its forward movement: then it turns to the right or left in the direction of the wind: and then, in a third convolution, every globule of the airy mass is twirling on an axis of its own. Equally buoyant is the earth, hung upon nothing, and cleaving the liquid firmament. It turns on its axis, causing the vicissitude of day and night; it moves through its orbit, making the circuit of the sun and the diversity of the seasons; and, in addition, there is an oscillatory motion like the unsteady zig-zag twistings of the carriage-train, occasioned by the excess of the equatorial over the polar diameter. This excess amounts to about a three-hundredth part. But, small as it is, it exerts an assignable influence over the cohesion or attraction of the solid framework. Now, by a nice mathematical demonstration, resting on the sun and moon’s attraction, Mr. Hopkins infers, as indicated by the phenomena of precession and nutation, that the minimum thickness of the earth’s crust cannot be less than one-fourth or one-fifth of the earth’s radius. The theorem is of too abstract a nature to be here introduced; but it appears from it that the observed amount of precession requires this degree of solid matter, which gives a clear depth of solid arch over either vacuum, resplendent light, or fiery fluid, of from eight hundred to a thousand miles. This may well allay the fears of the most timid as to the stability of the ground beneath his feet, whatever be the state of the interior, or under whatever modifications the materials therein may exist.