Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note xviii. § 112.

Transportation of Stones, &c.

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.

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.

[176] Voyages aux Alpes, tom. i. § 308.

352. Again, in the narrow vale or glen which separates the Great from the Little Saleve, the strata are all calcareous, but a great number of loose blocks of granite and primary schistus are scattered over the surface. A block of the former, near the lower end of the valley, is about the size of 1200 cubic feet. Two other large blocks of the same kind of stone rest on a base of horizontal limestone, elevated two or three feet above the rest of the surface. This elevation arises no doubt from the protection which the stones have afforded to the calcareous beds on which they lie, so that these beds do not wear away so fast as those which are fully exposed to the weather. But it is surely to take a very limited view of the operations on the surface, to suppose, with Saussure, that the parts of the calcareous rock under these stones has suffered no waste whatsoever, so that the stones remain now in the identical spot where they were placed by the great debacle which brought them down from the high Alps.[177] For my part, I have no doubt that the Arve, which is still at no great distance, when it ran on a higher level, and in a line different from the present, aided by the glaciers and superior elevation of the mountains, was an engine sufficiently powerful for effecting the transportation of these stones.

[177] Ibid. § 227.

353. These phenomena are not peculiar to the Alps, but prevail, in a greater or less degree, in the vicinity of all primary or granite mountains. In the island of Arran, a fragment of the same kind with that which constitutes the upper part of Goatfield, is found on the sea shore, at least three miles from the nearest granite rock, and with a bay of the sea intervening. Its dimensions are not far from those of the pierre de gouté. In some former state of the granitic mountains in that island, the declivity from the top of Goatfield may have been very uniform, and more rapid than it is at present.

354. Besides glaciers and torrents, which have no doubt been the principal instruments in producing these changes, other causes may have occasionally operated. Large stones, when once detached, and resting on an inclined plane, from the effects of waste and decomposition, may advance horizontally, at the same time that they descend perpendicularly, and this will happen though they be not urged by any torrent, or any thing but their own weight; for the surface of the ground, as it wastes, remains higher under the stone, and for a little way round it, than at a greater distance, on account of the protection which it receives from the stone, as in the instances at Saleve, just mentioned. The stone itself also becomes rounded at the bottom; and thus the surface in contact with the ground is diminished in extent, and the two surfaces rendered convex towards one another. It must therefore happen, that the support, continually weakening, will at length give way, and the stone incline or roll toward the lower side, and may even roll considerably, if its centre of gravity has been high above its point of support, and if its surface has had much convexity: Thus the horizontal may very far exceed the perpendicular motion; and, in the course of ages, the stone may travel to a great distance. A stone, however, which travels in this manner, must diminish as it proceeds, and must have been much greater in the beginning than it is at present.

355. This kind of motion may be aided by particular circumstances. When a stone rests on an inclined plane, so as to be in a state not very remote from equilibrium, if a part be taken away from the upper side, the equilibrium will be lost, and the stone will thereby be put in motion. That stones which lie on other stones, may, by wearing, be brought very near an equilibrium, is proved by what are called rocking-stones, or in Cornwall Logan stones, which have sometimes been mistaken for works of art; but are certainly nothing else than stones, which have been subjected to the universal law of wasting and decay, in such peculiar circumstances, as nearly to bring about an equilibrium of that stable kind, which, when slightly disturbed, re-establishes itself.[178] The Logan stone at the Land's End, is a mass of granite, weighing more than sixty tons, resting on a rock of granite, of considerable height, and close on the sea shore. The two stones touch but in a small spot, their surfaces being considerably convex towards one another. The uppermost is so nearly in an equilibrium, that it can be made to vibrate by the strength of a man, though to overset it entirely would require a vast force. This arises from the centre of gravity of the stone being somewhat lower than the centre of curvature of that part of it on which it has a tendency to roll; the consequence of which is, that any motion impressed on the stone, forces its centre of gravity to rise, (though not very considerably,) by which means it returns whenever the force is removed, and vibrates backward and forward, till it is reduced to rest. Were it required to remove the stone from its place, it might be most easily done, by cutting off a part from one side, or blowing it away by gunpowder; the stone would then lose its balance, would tumble from its pedestal, and might roll to a considerable distance. Now, what art is here supposed to perform, nature herself in time will probably effect. If the waste on one side of this great mass shall exceed that on the opposite in more than a certain proportion, and it is not likely that that proportion will be always maintained, the equilibrium of the Logan stone will be subverted, never to return. Thus we perceive how motion may be produced by the combined action of the decomposition and gravitation of large masses of rock.

[178] I do not presume so far as to say, that all rocking-stones are produced by natural means: I have not sufficient information to justify that assertion; but the great size of that at the Land's End, its elevated position, and the approaches toward something of the same kind which are to be seen in other parts of that shore, prove that it is no work of art. They who ascribe it to the Druids, do not consider the rapidity with which the Cornish granite wastes, nor think how improbable it is, that the conditions necessary to a rocking-stone, whether produced by nature or art, should have remained the same for sixteen or seventeen hundred years.

356. Besides the gradual waste to which stones exposed to the atmosphere are necessarily subject, those of a great size appear to be liable to splitting, and dividing into large portions, no doubt from their weight. This may be observed in almost all stones that happen to be in such circumstances as we are now considering; and from this cause the subversion of their balance may be more sudden, and of greater amount, than could be expected from their gradual decay.

Thus, if to the wasting of a stone at the bottom, we add the accidents that may befal it in the wasting of its sides, we see at least the physical possibility of detached stones being put in motion, merely by their own weight. It is indeed remarkable, that some of the largest of these stones rest on very narrow bases. Those at the foot of Saleve touch the ground only in a few points: The Boulder stone of Borrowdale is supported on a narrow ridge like the keel of a ship, and is prevented from tumbling by a stone or two, that serve as a kind of shores to prop it up. Very unexpected accidents sometimes happen to disturb the rest of such fragments of rock as have once migrated from their own place. Saussure mentions a great mass of lapis ollaris[179] that lies detached on the side of a declivity in the valley of Urseren, in the canton of Uri. The people use this stone as a quarry, and are working it away on the upper side, in consequence of which it will probably be soon overset, and will roll to the bottom of the valley.

[179] Voyages aux Alpes, tom. iv. § 1851.

357. In many instances it cannot be doubted, that stones of the kind here referred to are the remains of masses or veins of whinstone or granite, now worn away, and that they have travelled but a very short way, or perhaps not at all, from their original place. Many of the large blocks of whinstone which we find in this country, sometimes single, and sometimes scattered in considerable abundance over a particular spot, are certainly to be referred to this cause. But the most remarkable examples of this sort are the stones found at the Cape of Good Hope, on the hill called Paarlberg, which takes its name from a chain of large round stones, like the pearls of a necklace, that passes over the summit. Two of these, placed near the highest point, are called the Pearl and the Diamond, and were mentioned several years ago in the Philosophical Transactions.[180] From a more recent account, these stones appear to be a species of granite, though the hill on which they lie is composed of sandstone strata.[181] The Pearl is a naked rock, that rises to the height of 400 feet above the summit of the hill; the Diamond is higher, but its base is less, and it is more inaccessible.

[180] Vol. lxviii. p. 102.

[181] Barrow's Travels into Southern Africa, p. 60.

From the above stones forming a regular chain, as well as from the immense size of the two largest, it is impossible to suppose that they have been moved; and it is infinitely more probable, that they are parts of a granite vein, which runs across the sandstone strata, and of which some parts have resisted the action of the weather, while the rest have yielded to it. The whole geological history of this part of Africa seems highly interesting, since, as far as can be collected from the accounts of the ingenious traveller just mentioned, it consists of horizontal beds of sandstone or limestone, resting immediately on granite, or on primary schistus. Loose blocks of granite are seen in great abundance at the foot of the Table Mountain, and along the sea shore.

358. The system which accounts for such phenomena as have been considered in this and some of the preceding notes, by the operation of a great deluge, or debacle, as it is called, has been already mentioned. In Dr Hutton's theory, nothing whatever is ascribed to such accidental and unknown causes; and, though their existence is not absolutely denied, their effects, whatever they may have been, are alleged to be entirely obliterated, so that they can be referred to no other class but that of mere possibilities. A minute discussion, however, of the question, Whether there are, on the surface of the earth, any effects that require the interposition of an extraordinary cause, would lead into a longer digression than is suited to this place. I shall briefly state what appear to be the principal objections to all such explanations of the phenomena of geology.

359. The general structure of valleys among mountains, is highly unfavourable to the notion that they were produced by any single great torrent, which swept over the surface of the earth. In some instances, valleys diverge, as it were from a centre, in all directions. In others, they originate from a ridge, and proceed with equal depth and extent on both sides of it, plainly indicating, that the force which produced them was nothing, or evanescent at the summit of that ridge, and increased on both sides, as the distance from the ridge increased. The working of water collected from the rains and the snows, and seeking its way from a higher to a lower level, is the only cause we know of, which is subject to this law.

360. Again, if we consider a valley as a space, which perhaps with many windings and irregularities, has been hollowed out of the solid rock, it is plain, that no force of water, suddenly applied, could loosen and remove the great mass of stone which has actually disappeared. The greatest column of water that could be brought to act against such a mass, whatever be the velocity we ascribe to it, could not break asunder and displace beds of rock many leagues in length, and in continuity with the rock on either side of them. The slow working of water, on the other hand, or the powers that we see every day in action, are quite sufficient for this effect, if time only is allowed them.

361. Some valleys are so particularly constructed, as to carry with them a still stronger refutation of the existence of a debacle. These are the longitudinal valleys, which have the openings by which the water is discharged, not at one extremity, but at the broadside Such is that on the east side of Mont Blanc, deeply excavated on the confines of the granite and schistus rock, and extending parallel to the beds of the latter, from the Col de la Segne to the Col de Ferret; its opening is nearly in the middle, from which the Dora issues, and takes its course through a great valley, nearly at right angles to the chain of the Alps, and to the valley just mentioned. From the structure of these valleys, Saussure has argued very justly against Buffon's hypothesis, concerning the formation of valleys by currents at the bottom of the sea.[182] It affords indeed a complete refutation of that hypothesis: and it affords one no less complete of the system which Saussure himself seems on some occasions so much inclined to support. For if it be said, that this valley was cut out by the current of a debacle, that current must either have run in the direction of the valley of Ferret, or in that of the Dora, which issues from it. If it had the direction of the first, it could not cut out the second; and if it had the direction of the second, it could not cut out the first. Besides, the force which excavated this valley must have been nothing at the two extreme points, viz. at the Col de la Segne and the Col de Ferret, and must have increased with the distance from each. It can have been produced, therefore, only by the running of two streams in opposite directions, on a surface that was but slightly uneven, these streams at meeting taking a new direction, nearly at right angles to the former. A clearer proof could hardly be required than is afforded in this case, that what is now a deep valley was formerly solid rock, which the running of the waters has gradually worn away; and that the waters, when they began to run, were on a level as high, at least, as the tops of those mountains by which the valley is bounded toward the lower side.

[182] Voyage aux Alpes, tom. ii. § 920.

362. Longitudinal valleys, with the water bursting out transversely from their sides, like the preceding, are by no means confined to mountains of the first order. We have a very good example, though on a small scale, of a valley of this sort, within a few miles of Edinburgh. The Pentland Hills form a double ridge, separated by a small longitudinal valley, that runs from N. E. to S. W., the water of which issues from an opening almost in the middle, and directed towards the south. This, therefore, is not the work of any great torrent, which overwhelmed the country; for no one direction, which it is possible to assign to such a torrent, will afford an explanation, both of the valley and its outlet.[183]

[183] In Scotland there is one valley, of a kind that I believe is extremely rare in any part of the world, in accounting for which, the hypothesis of a torrent or debacle might, if any where, be employed to advantage. This is the valley which extends across the island, from Inverness to Fort William, or from sea to sea, being open at both ends, and very little elevated in the middle. It is nearly straight, and of a very uniform breadth, except that towards each end it widens considerably. The bottom, reckoning transversely, is flat, without any gradual slope from the sides towards the middle. From the sides the mountains rise immediately, and form two continued ridges of great height, like ramparts or embankments on each side of a large fossé. A great part of the bottom of this singular valley is occupied by lakes, namely, Loch Ness, Loch Oich, and Loch Lochy. Its length is about sixty-two miles, and the point of partition from which the waters run different ways, viz. north-east to the German Ocean, and south-west to the Atlantic, is between Loch Oich and Loch Lochy; and, by the estimation of the eye, I should hardly think that it is elevated more than ten or fifteen feet above the surface of either lake. The country on both sides is rugged and mountainous, and the streams which descend from thence into the valley, either fall directly into the lakes, or turn off almost at right angles when they enter the valley. Though the bottom of this valley, therefore, is every where alluvial, with the exception, perhaps, of a few rocks which appear at the surface, it is certainly not excavated by the rivers which now flow in it. The direction of the valley, it is to be observed, is the same with that of the vertical strata which compose the mountain on either side.

Here, then, we have a valley, not cut out by the working of any streams which now appear; and we may therefore make trial of the hypothesis of a debacle. This, however, will afford us no assistance; because, if we suppose what is now hollow to have been once occupied by the same kind of rock which is on either side, no force of torrents can have suddenly loosened and removed from its place a body of such vast magnitude. A greater column of water, than one having for its base a transverse section of the valley, could not act against it, and this would have to overcome the cohesion and inertia of a column of rock of the same section, and of the length of sixty-two miles. It is not hazarding much to affirm, that no velocity which could be communicated to water, not even that which it could acquire by falling from an infinite height, could give to it a force in any degree adequate to this great effect.

The explanation of this valley, which appears to me the most probable, is the following. It will be shown hereafter, that there is good reason to suppose, that, in most parts of our island, the relative level of the sea and land has been in past ages considerably higher than it is at present. In such circumstances, this valley may have been under the surface of the sea, the highest part of it being scarcely 100 feet above that level at present. It may have been a kind of sound, therefore, or strait, which connected the German Sea with the Atlantic; and the strong currents, which, on account of the different times of high water in these two seas, must have run alternately up and down this strait, may have produced that flatness of the bottom, and straightness of the sides, and that widening at the extremities, which are mentioned above. In this way, too, some difficulties are removed relative to Loch Ness, which is so deep as hardly to be consistent with the indefinite length of the period of waste that must be ascribed to the mountains on each side of it. Its depth is said, where greatest, not to be less than 180 fathoms. According to this hypothesis, it may, at no very distant period, have been a part of the bottom of the sea.

363. They who maintain the existence of the debacle, will no doubt allege, that though these valleys were not cut out by means of it, yet others may. But it must be recollected, that if some of the greatest and deepest valleys on the face of the earth, such as that just mentioned, on the east side of Mont Blanc, are thus shown to be the work of the daily wasting of the surface, what other inequalities can be great enough to require the interposition of a more powerful cause? If a dignus vindice nodus does not exist here, in what part of the natural history of the earth is it likely to be found?

364. The large masses of rock so often met with at a distance from their original place, are one of the arguments used for the debacle. It has, however, been shown, that, supposing a form of the earth's surface considerably different from the present, especially, supposing the absence of the valleys which the rivers have gradually cut out, the transportation of such stones is not impossible, even by such powers as nature employs at present. Now, without the supposition that the surface was more continuous, and that its present inequalities did not exist, no force of torrents, whatever their velocity and magnitude may have been, could have produced this transportation. No force of water could raise a stone like the pierre de goutté from the bottom of a valley, to the top of a steep hail. Indeed, if we suppose a great fragment of rock to be hurried along on a horizontal or an inclined plane, by the force of water, the moment it comes to a deep valley, and has to rise up over an ascent of a certain steepness, it will remain at rest; the water itself will lose its velocity, and the heavy bodies which it carried with it will proceed no farther. Thus, therefore, we have the following dilemma. If the surface is not supposed to have had a certain degree of uniformity in past times, a debacle is insufficient for the transportation of stones: If it is supposed to have had that uniformity, a debacle is unnecessary.

365. Another fact, which has been supposed favourable to the opinion of the action of great torrents at some former period, is, that in countries like that round Edinburgh, where whinstone hills rise up from among secondary strata, a remarkable uniformity is observed in the direction of their abrupt faces. Thus, in the country just mentioned, the steep faces generally front the west, whiles in the opposite direction, the slope is gentle, and the hills decline gradually into the plain. Hence it is supposed, that a torrent, sweeping from west to east, has carried off the strata from the west side of these hills, but, being obstructed by the whinstone rock, has left the strata on the east side in their natural place.

But, besides that no force which can ever be ascribed to a torrent could have removed at once bodies of strata 300 or 400 feet, nay even 800 or 1000 in thickness, which must have been the case if this were the true explanation of the fact, there is a circumstance which may perhaps enable us to explain these phenomena without the assistance of any extraordinary cause. The secondary strata in which the whinstone hills are found in this part of Scotland, are not horizontal, but rise or head towards the west, dipping towards the east. The side, therefore, of the whinstone hills which is precipitous, is the same with that towards which the strata rise. Now, from the manner in which these hills are supposed to have been elevated, the strata are likely to have been most broken and shattered towards that side, while, on the opposite, they had the support of the whinstone rock. They would become a prey, therefore, more easily to the common causes of erosion and waste on the upper side than on the lower. The streams that flowed from the higher grounds would wear them on the former most readily; and the action of these streams would be resisted by the superior hardness of the whinstone, just as the great torrent of the debacle is supposed to have been.

It should also be observed, that this fact of the uniform direction of the abrupt faces of mountains, is often too hastily generalized. In primitive countries, it is no farther observed than by the steep faces of the mountains being most frequently turned toward the central chain. In Scotland, as soon as you leave the flat country, and enter the Highlands, the scarps of the hills face indiscriminately all the points of the compass, and are directed as often to the east as to the west.

366. Where the strata are nearly horizontal, they afford the most distinct information concerning the direction and progress of the wasting of the land. The inclined position of the strata, which in all other cases must enter for so much into our estimate of the causes which have produced the present inequality of the earth's surface, disappears there entirely; and the whole of that inequality is to be ascribed to the operations at the surface, whether they have been sudden or gradual. A very important fact from a country of this sort, is related by Barrow, in his Travels into Southern Africa. The mountains about the Cape of Good Hope, and as far to the north as that ingenious traveller prosecuted his journey, are chiefly of horizontal strata of sandstone and limestone, exhibiting the appearance, on their abrupt sides, of regular layers of masonry, of towers, fortifications, &c. Now, among all these mountains, he observed, that the high or steep sides look constantly down the rivers, while the sloping or inclined sides have just the opposite direction. When, in travelling northward, he passed the line of partition, where the waters from running south take their direction to the north, he found, that the gradual slope, which had hitherto been turned to the north, was now turned to the south: The abrupt aspect of the mountains, in like manner, from facing the south, was directed to the north; so that, in both cases, the hills turned their backs on the line of greatest elevation.[184]

[184] Barrow's Travels into Southern Africa, p. 245.

It is evident, therefore, that the form of this land has been determined by the slow working of the streams. The causes which produced the effects here described, began their action from the line of greatest elevation, and extended it from thence on both sides, in opposite directions. This is the most precise character that can mark the alluvial operations, and distinguish them from the overwhelming power of a great debacle.

367. Lastly, if there were any where a hill, or any large mass composed of broken and shapeless stones, thrown together like rubbish, and neither worked into gravel nor disposed with any regularity, we must ascribe it to some other cause than the ordinary detritus and wasting of the land. This, however, has never yet occurred; and it seems best to wait till the phenomenon is observed, before we seek for the explanation of it.

368. These arguments appear to me conclusive against the necessity of supposing the action of sudden and irregular causes on the surface of the earth. In this, however, I am perhaps deceived: neither Pallas, nor Saussure, nor Dolomieu, nor any other author who has espoused the hypothesis of such causes, has explained his notions with any precision; on the contrary, they have all spoken with such reserve and mystery, as seemed to betray the weakness, but may have concealed the strength of their cause. I have therefore been combating an enemy, that was in some respects unknown; and I may have supposed him dislodged, only because I could not penetrate to his strongholds. The question, however, is likely soon to assume a more determinate form. A zealous friend of Dr Hutton's theory, has lately[185] declared his approbation of the hypothesis which has here been represented as so adverse to that theory; and, from his ability and vigour of research, it is likely to receive every improvement of which it is susceptible.

[185] Trans. Royal Society Edin. vol. v. p 68.