Illustrations of the Huttonian Theory of the Earth

by John Playfair

343. Nature supplies the means of tracing with considerable certainty the migration of fossil bodies on the surface of the earth, as only the more indurated stones, and those most strongly characterized, can endure the accidents that must befal them in travelling to a distance from their native place.

It is a fact very generally observed, that where the valleys among primitive mountains open into huge plains, the gravel of those plains consists, of stones, evidently derived from the mountains. The nearer that any spot is to the mountains, the larger are the gravel stones, and the less rounded is their figure; and, as the distance increases, this gravel, which often forms a stratum nearly level, is covered with a thicker bed of earth or vegetable soil. This progression has particularly been observed in the valleys of Piémont and the plains of Lombardy, where a bed of gravel forms the basis of the soil, from the foot of the Alps to the shores of the Hadriatic.[170] We may collect from Guettard, that a similar gradation is found in the gravel and earth which cover the great plain of Poland, from Mount Krapack to the Baltic.[171] The reason of this gradation is evident; the farther the stones have travelled, and the more rubbing they have endured, the smaller they grow, the more regular is the figure they assume, and the greater the quantity of that finer detritus which constitutes the soil. The washing of the rains and rivers is here obvious; and each of the three quantities just mentioned, if not directly proportional to the distance which the stones have migrated from their native place, may be said, in the language of geometry, to be at least proportional to a certain function of that distance.

[170] Voyages aux Alpes, tom. iii. § 1315.

[171] Mém. Acad. des Sciences, 1762, p. 234, 293, &c.

344. The immense quantity of cailloux roulés, or rounded gravel, collected in the immediate vicinity of mountainous tracts, has led some geologists to suppose the existence of ancient currents, which descended from the mountains, in a quantity, and with a momentum, of which there is no example in the present state of the world. Thus Saussure imagines, that the hill of Supergue, near Turin, which is formed of gravel, can only be explained by supposing such currents as are just mentioned, or what he terms a debacle, to have taken place at some former period.[172] If, however, we ascribe to the mountains a magnitude and elevation vastly greater than that which they now possess; if we regard the valleys between them as cut out by the rivers and torrents from an immense rampart of solid rock, neither materials sufficiently great, nor agents sufficiently powerful, will appear to be wanting, for collecting bodies of gravel and other loose materials, equal to any that are found on the surface of the earth. The necessity of introducing a debacle, or any other unknown agent, to account for the transportation of fossils, seems to arise from underrating the effects of action long continued, and not limited by such short periods as circumscribe the works, and even the observations, of men.

[172] Voyages aux Alpes, tom. iii. § 1303.

345. The supply of gravel and cailloux roulés, for the plains extended at the feet of primitive mountains, is doubtless in many cases much increased by the pudding-stone, interposed between the secondary and the primary strata. The beds of pudding-stone contain gravel already formed on the shores of continents, that ceased to exist before the present were produced; and the cement of this gravel, yielding easily to the weather, allows the stones included in it to be washed down by the torrents, and scattered over the plains. I know not if the hill of Supergue above mentioned, is not in reality a mass of the pudding-stone which forms the border of the Alps, and of which the materials have suffered no transportation since the time of their last consolidation. This at least is certain, that Saussure, notwithstanding his accuracy, has sometimes confounded the loose gravel on the surface with that which is consolidated into rock; an inaccuracy which is to be charged, as I have elsewhere observed, rather against his system than himself.

346. The loose stones found on the sides of hills, and the bottoms of valleys, when traced back to their original place, point out with demonstrative evidence the great changes which have happened since the commencement of their journey; and in particular serve to show, that many valleys which now deeply intersect the surface, had not begun to be cut out when these stones were first detached from their native rocks. We know, for instance, that stones under the influence of such forces as we are now considering, cannot have first descended from one ridge, and then ascended on the side of an opposite ridge. But the granite of Mont Blanc has been found, as mentioned above, on the sides of Jura, and even on the side of it farthest from the Alps. Now, in the present state of the earth's surface, between the central chain of the Alps, from which these pieces of granite must have come, and the ridge of Mont Jura, besides many smaller valleys, there is the great valley of the Rhone, from the bottom of which, to the place where they now lie, is a height of not less than 3000 feet. Stones could not, by any force that we know of, be made to ascend over this height. We must therefore suppose, that when they travelled from Mont Blanc to Jura, this deep valley did not exist, but that such an uniform declivity, as water can run on with rapidity, extended from the one summit to the other. This supposition accords well with what has been already said concerning the recent formation of the Leman Lake, and of the present valley of the Rhone.

347. We can derive, in a matter of this sort, but little and from calculation; yet we may discover by it, whether our hypothesis transgresses materially against the laws of probability, and is inconsistent with physical principles already established. The horizontal distance from Mont Jura to the granite mountains, at the head of the Arve, may be accounted fifty geographic miles. Though we suppose Mont Blanc, and the rest of those mountains, to have been originally much higher than they are at present, the ridge of Jura must have been so likewise; and though probably not by an equal quantity, yet it is the fairest way to suppose the difference of their height to have been nearly the same in former ages that it is at present, and it may therefore be taken at 10,000 feet. The declivity of a plane from the top of Mont Jura to the top of Mont Blanc, would therefore be about one mile and three quarters in fifty, or one foot in thirty; an inclination much greater than is necessary for water to run on, even with extreme rapidity, and more than sufficient to enable a river or a torrent to carry with it stones or fragments of rock, almost to any distance.

Saussure, in relating the fact that pieces of granite are found among the high passes near the summits of Mont Jura, alleges, that they are only found in spots from which the central chain of the Alps may be seen. But it should seem that this coincidence is accidental, because, from whatever cause the transportation of these blocks has proceeded, the form of the mountains, especially of Mont Jura, must be too much changed to admit of the supposition, that the places of it from which Mont Blanc is now visible, are the same from which that mountain was visible when these stones were transported hither. It may be, however, that the passes which now exist in Mont Jura are the remains of valleys or beds of torrents, which once flowed westward from the Alps; and it is natural, that the fragments from the latter mountains should be found in the neighbourhood of those ancient water-tracks.