Illustrations of the Huttonian Theory of the Earth

by John Playfair

348. Saussure observed in another part of the Alps, that where the Drance descends from the sides of Mont Velan and the Great St Bernard, to join the Rhone in the Valais, the valley it runs in lies between mountains of primary schistus, in which no granite appears, and yet that the bottom of this valley, toward its lower extremity, is for a considerable way covered with loose blocks of granite.[173] His familiar acquaintance with all the rocks of those mountains, led him immediately to suspect, that these stones came from the granite chain of Mont Blanc, which is westward of the Drance, and considerably higher than the intervening mountains. This conjecture was verified by the observations of one of his friends, who found the stones in question to agree exactly with a rock at the point of Ornes, the nearest part of the granite chain.

[173] Voyages aux Alpes, tom. ii. § 1022.

In the present state of the surface, however, the valley of Orsiere lies between the rocks of Ornex and the valley of the Drance, and would certainly have intercepted the granite blocks in their way from the one of these points to the other, if it had existed at the time when they were passing over that tract. The valley of Orsiere, therefore, was not formed, when the torrents, or the glaciers transported these fragments from their native place.

Mountainous countries, when carefully examined, afford so many facts similar to the preceding, that we should never have done were we to enumerate all the instances in which they occur. They lead to conclusions of great use, if we would compare the machinery which nature actually employs in the transportation of rocks, with the largest fragments of rock which appear to have been removed, at some former period, from their native place.

349. For the moving of large masses of rock, the most powerful engines without doubt which nature employs are the glaciers, those lakes or rivers of ice which are formed in the highest valleys of the Alps, and other mountains of the first order. These great masses are in perpetual motion, undermined by the influx of heat from the earth, and impelled down the declivities on which they rest by their own enormous weight, together with that of the innumerable fragments of rock with which they are loaded. These fragments they gradually transport to their utmost boundaries, where a formidable wall ascertains the magnitude, and attests the force, of the great engine by which it was erected. The immense quantity and size of the rocks thus transported, have been remarked with astonishment by every observer,[174] and explain sufficiently how fragments of rock may be put in motion, even where there is but little declivity, and where the actual surface of the ground is considerably uneven. In this manner, before the valleys were cut out in the form they now are, and when the mountains were still more elevated, huge fragments of rock may have been carried to a great distance; and it is not wonderful, if these same masses, greatly diminished in size, and reduced to gravel or sand, have reached the shores, or even the bottom, of the ocean.

[174] The stones collected on the Glacier de Miage, when Saussure visited it, were in such quantity as to conceal the ice entirely. Voyages aux Alpes, tom. ii. § 854.

350. Next in force to the glaciers, the torrents are the most powerful instruments employed in the transportation of stones. These, when they descend from the sides of mountains, and even where the declivity of their course is not very great, produce effects which nothing but direct experience could render credible. The fragments of rock which oppose the torrent, are rendered specifically lighter by the fluid in which they are immersed, and lose by that means at least a third part of their weight: they are, at the same time, impelled by a force proportional to the square of the velocity with which the water rushes against them, and proportional also to the quantity of gravel and stones which it has already put in motion. Perhaps, after taking all these circumstances into computation, in the midst of a scene perfectly quiet and undisturbed, a philosopher might remain in doubt as to the power of torrents to move the enormous bodies of rock which are seen in the bottom of the narrow valleys or deep glens of a mountainous country; but his incredulity, says an experienced traveller, will cease altogether, if he has been surprised by a storm in the midst of some Alpine region; if he has seen the number and impetuosity of the cataracts which rushed down the sides of the mountains, and beheld the ruin which accompanied them; and if, when the tempest was passed, he has viewed those meadows, which a few hours before were covered with verdure, now buried under heaps of stones, or overwhelmed by masses of liquid mud, and the sides of the mountains cut by deep ravines, where the track of the smallest rivulet was not before to be discovered.[175]

[175] See an account of a thunder storm near Bareges, in the Essai sur la Mineralogie des Pyrenées, p. 134.

It is but rarely, however, even on occasions like these, that such vast masses of rock can be seen actually in motion, as are often found on the surface, apparently removed to a great distance from their native place. The magnitude of these is so great, in many instances, that their transportation cannot be explained without supposing, that the surface was very different when these transportations took place from what it is at present; that the elevation of the mountains was greater, and the ground smoother and more uniform, at least in some directions. If these suppositions are admitted, and they are countenanced, as we have already seen, by almost every phenomenon in geology, the difficulties which present themselves here will not appear insurmountable.

351. One of the largest blocks of granite that we know of, is on the east side of the lake of Geneva, called Pierre de Gouté, about ten feet in height, with a horizontal section of fifteen by twenty.[176] Another block not far from it, and nearly of the same size, has some remains of schistus attached to it. These stones very much resemble those which have fallen from the Aiguilles, in the valley of Chamouni. The distance from their present situation to those Aiguilles is about thirty English miles, with many mountains and valleys at present interposed. By whatever means, therefore, these blocks were transported, their motion must have been over a surface of much more uniform declivity than the present. If the surface was without great inequalities, and its general declivity about one foot in thirty, as already computed, the glaciers, in the first place, and the torrents afterwards, may have served for the transportation even of these rocks.