Illustrations of the Huttonian Theory of the Earth

by John Playfair

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.