Illustrations of the Huttonian Theory of the Earth

by John Playfair

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.