Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note xvii. § 105.

Remains of Decomposed Rocks.

334. The plain of Crau was the Campus Lapideus of the ancients; and, as mythology always seeks to connect itself with the extraordinary facts in natural history, it was said to be the spot where Hercules, fighting with the sons of Neptune, and being in want of weapons, was supplied from heaven by a shower of stones: hence it was called Campus Herculeus.

This plain is on the east side of the Rhone, between Salon and Aries: it is of a triangular form, about twenty square leagues in extent, and is covered almost entirely with quartzy gravel. This immense collection of gravel has been supposed by some to have been brought down by the Durance from the Alps of Dauphine; by others it has been ascribed to the Rhone; and by many to the sea, as being a work too great for any river. The explanation mentioned above, [§ 105], namely, that the loose gravel on the plain arises from the decomposition of a great stratum of pudding-stone, which is the basis of the whole, is the opinion of Saussure, and is founded on his own observations.[167]

[167] See Voyages aux Alpes, tom. iii. § 1592 et 1597. See also on this subject a Memoir by Lamanon, Journal de Physique, tom. xxii. p. 477; and another by M. De Servieres, ibid. p. 270.

335. The theories that have been contrived for explaining the phenomena of the plain of Crau, afford an instance of the necessity of generalizing our observations before we can explain a particular appearance: in other words, they prove the truth of Lord Bacon's maxim, That the explanation of a phenomenon should not be sought for from the study of that phenomenon alone, but from the comparison of it with others. One of the theories of this plain is, that the breccia, which is the base of it, is formed from the consolidation of the loose gravel of the plain, by water percolating through it, and carrying some cementing substance along with it, or some lapidific juice, as it is called. And indeed, whether the gravel is formed from the breccia, or the breccia from the gravel, is a question which probably could never be resolved by the mere examination of the plain itself. But the question is very soon decided, when we compare what is observed here with other appearances in the natural history of the earth's surface, and consider how much more frequent the decomposition of solids is, than their reconsolidation, in any place above the level of the sea.

336. The argument for the decomposition of stony substances which is afforded by the state of this singular plain, may be confirmed by the appearances observed in many extensive tracts of land all over the world, and especially in some parts of Great Britain. The road to Exeter from Taunton Dean, between the latter and Honiton, passes over a large heath or down, considerably elevated above the plain of Taunton. The rock which is the base of this heath, as far as can be discovered, is limestone, and over the surface of it large flints, in the form of gravel, are very thickly spread. There is no higher ground in the neighbourhood from which this gravel can be supposed to have come, nor any stream that can have carried it, so that no explanation of it remains, but that it is formed of the flints contained in beds of limestone, which are now worn away. The flints on the heath are precisely of the kind found in limestone; many of them are not much worn, and cannot have travelled far from the rock in which they were originally contained. It seems certain, therefore, that they are the debris of limestone strata, now entirely decomposed, that once lay above the strata which at present form the base of this elevated plain, and probably covered them to a considerable height. This explanation carries the greater probability with it, that any other way of accounting for the fact in question, as the travelling of the gravel from higher grounds, or the immersion of the surface under the sea, will imply changes in the face of the country, incomparably greater than are here supposed. Our hypothesis seems to give the minimum of all the kinds of change that can possibly account for the phenomenon.

337. The same remarks may be made on the high plain of Blackdown, which the road passes over in going from Exeter to the westward. The flints there are disseminated over the surface as thickly as in the other instance, and can be explained only on the same supposition.

Again, in the interior of England, beginning from about Worcester and Birmingham, and proceeding north-east through Warwickshire, Leicestershire, Nottinghamshire, as far as the south of Yorkshire, a particular species of highly indurated gravel, formed of granulated quartz, is found every where in great abundance. This same gravel extends to the west and north-west, as far as Ashburn in Derbyshire, and perhaps still farther to the north. The quantity of it about Birmingham is very remarkable, as well as in many other places; and the phenomenon is the more surprising, that no rock of the same sort is seen in its native place. It is such gravel as might be expected in a mountainous country, in Scotland, for instance, or in Switzerland, but not at all in the fertile and secondary plains of England.

This enigma is explained, however, when it is observed, that the basis of the whole tract just described is a red sandstone, often containing in it a hard quartzy gravel, perfectly similar to that which has just been mentioned. From the dissolution of beds of this sandstone, which formerly covered the present, there can be no doubt that this gravel is derived. But, as the gravel is in general thinly dispersed through the sandstone, and abounds only in some of its layers, it should therefore seem, that a vast body of strata must have been worn away and decomposed, before such quantities of gravel as now exist in the soil could have been let loose.

338. I have said, that a rock capable of affording such gravel as this, is not to be found in the tract of country just mentioned. This however, is not strictly true; for in Worcestershire, between Bromesgrove and Birmingham, about seven miles from the latter, a rock is found consisting of indurated strata, greatly elevated, and without doubt primitive, from the detritus of which such gravel as we are now speaking of might be produced. These strata seem to rise up from under the secondary, where they are intersected by the road; and, for as much as appears, are not of great thickness, so that they cannot have afforded the materials of this gravel directly, though they may have done so indirectly, or through the medium of the red sandstone; that is to say, a primary rock of which they are the remains, may have afforded materials for the gravel in the sandstone; and this sandstone may in its turn have afforded the materials of the present soil, and particularly the gravel contained in it.

339. Pudding-stones being very liable to decomposition, have probably, in most countries, afforded large proportion of the loose gravel now found in the soil The mountains, or at least hills, of this rock, which are found in many places, prove the great extent of such decomposition. Mount Rigi, for instance, on the side of the Lake of Lucerne, is entirely of pudding-stone, and is 742 toises in height, measured from the level of the lake. By the descriptions given of it, as well as of other hills of the same kind in Switzerland, we may, without due attention, be led to suppose that they are entirely formed of loose gravel. Even M. Saussure's description is chargeable with this fault, though, when attended to, it will be found to contain a sufficient proof, that this hill is composed of real pudding-stone.[168] The nature of the thing also, would be sufficient to convince us, that a hill, more than 4000 feet in height, could not consist of loose and unconsolidated materials.

If, then, we regard Mount Rigi as the remains of a body of pudding-stone strata, we must conclude, that these strata were originally more extensive, and the adjacent valleys and plains will serve, in some degree, to measure the quantity of them which time has destroyed.

[168] Voyages aux Alpes, tom. iv. § 1941.

340. If the theory of unstratified mountains, namely those of whinstone, porphyry, and granite, be admitted as laid down above, it will furnish a measure of the destruction which has taken place in the stratified rocks, and of the vast depredations which have been made upon them since they were raised up from the bottom of the sea. Like every other measure, however, of wasting, by a thing that is itself subject to waste, it can only give a minimum, or a limit which the quantity wasted must necessarily exceed.

The abrupt face of a whinstone rock must be understood as an evidence, that some body of strata which supported it when fluid, remained in contact with it, when it was become solid; and if this part of the mould in which the whinstone was cast, has disappeared, it must generally be ascribed to the operation of waste and decomposition. Such a face, for instance, as that which Salisbury Craig presents to the west, viz. a perpendicular wall of whinstone, about ninety feet high, raised on a body of sandstone strata of the height of about 300 feet, can have been produced only by having been abutted against some stratified rock, equally abrupt, and of the same elevation with itself. Of this rock no part remains.

The basaltic rock of Edinburgh Castle is nearly in the same state. Its perpendicular sides on the south, west, and north, are now disengaged from the strata by which they were once encompassed.

341. The granite mountains also, where they are quite unstratified, give rise to the same conclusion. Those central chains which we find in so many instances towering above the schistus which cover their sides, have probably been once completely enveloped by the latter; and, on this supposition, an estimate may sometimes be formed of the original height of such mountains. In these estimations, however, some uncertainty must arise, from our being unable to distinguish between the effects which are to be ascribed to the fracture and dislocation that took place when the compound body of stratified and unstratified rocks was raised up from the bottom of the sea, and the effects produced by the subsequent waste and decomposition at the surface. In this, as in many other instances, we are not always able to separate between the original inequalities of the surface, and those which wearing has produced.

342. It would be important to ascertain the rate at which the elevation of mountains decreases, and this is what we may perhaps expect to be accomplished, by the progress of geological science, and the multiplying of accurate observations. It has been supposed, that the Pyrenees diminish about ten inches in a century; but what confidence is to be put in this estimate, I am unable to determine.[169]

[169] Essai sur la Mineralogie des Pyrenées, p. 87.

A very unequivocal mark of the degradation of mountains is often to be met with in the heaps of loose stones found on their tops. These stones, it is obvious, cannot have come from any other place by natural means, and they are accordingly always sharp and angular, and have none of the characters of transported rocks. They are said sometimes to have been brought by men's hands; but this is highly improbable, their quantity is often so considerable, and the difficulty of transportation so great. Where any purpose was to be served by heaping them together, men have availed themselves of the stones that they found ready prepared on the summit, and have constructed from them cairns, which have served as signals, useful in their pastoral, and sometimes in their military occupations.