Illustrations of the Huttonian Theory of the Earth

by John Playfair

I have dwelt longer on the description of these appearances than on any others of the same kind, because, from the great mass of secondary strata which here covers the primary, the circumstances are such as we cannot expect to see very often exemplified.

198. The Lakes of Cumberland are much visited by travellers; and it may be worth remarking, on that account, that, as the site of these lakes is a patch of primary country, bounded on all sides by secondary, so, in the rivers that run from the lakes, such junctions as we are now treating of may be expected to be found. Under Dun-Mallet, on the side toward Ulles Water, we observed a breccia, which was in horizontal layers, and seemed to lie on the primary schistus, so that the whole hill is perhaps a piece of more indurated breccia, or secondary rock, which has resisted the wearing and washing down of the rivers better than the rest.

199. After ascertaining the fact of the disturbance of the strata, and their removal from their original position, it is of consequence to inquire into the direction of the force by which these changes have been produced. Now, if the disturbed or elevated strata, were every where in planes, without bending or sinuosity, it might perhaps be hard to determine, whether that force had acted in the direction of gravity, or in the opposite. Either supposition would account for the appearances; and, as gravity is a known force, providing we can find some place fit to receive the matter impelled downward by it, its action would furnish the most probable solution of the difficulty.

It is on this principle that the Neptunian system proceeds, imagining, that certain great caverns or vacuities having been opened in the interior of the globe, a great part of the waters which formerly covered its surface, retired into them, and much of the solid rock also sunk down at the same time. In this way, one extremity of a stratum has been elevated, while the other has been depressed, and a certain inclination to the horizon has been given to the whole of it. Thus one cause serves two purposes; the vacuities in the interior of the earth account, both for the depression of the sea, and the elevation of the land; and the Neptunists, if the phenomena were all such as have been now stated, might boast of a felicity of explanation, not very usual in their system.

But this appearance of success vanishes, when the elevation and disturbance of the strata are more minutely examined, and are found to include waving and inflection, in a great variety of forms. It then becomes evident, that the beds of rock, at the time when they were disturbed from their horizontal position, had not their present hardness and rigidity, but were, in a certain degree at least, soft and flexible. Without these qualities, they could not have received, as they have often done, the curvature of a circle, not many feet, nay, not many inches, in diameter; nor could they have been bent into superficies, with their curvature in opposite directions, so that the same surface is in one part convex, and in another concave, on the same side, with a line of contrary flexure interposed. These are appearances, not reconcilable with the mere falling in, and breaking down of indurated rocks.

200. The inflections and wavings that we are here speaking of, though not peculiar to the primary strata, are found most frequently among them, and are perfectly familiar to every one who his travelled among mountains with any view to the study of geology. The following are a few instances of this phenomenon out of a great number which might be produced.

Saussure, in describing the route from Geneva to Chamouni, mentions many remarkable instances of the bending of the strata, and particularly where the small stream of Nant d'Arpenaz forms a cascade, by falling over the face of a perpendicular limestone rock. The strata of this rock are bent into circular arches, extremely regular, and with their concavity turned to the left. What deserves particularly to be remarked, is, that a mountain behind the cascade has its strata bent in a direction opposite to the former, or with their concavity to the right. There is no doubt that the strata of both rocks are the same, so that a vertical section of them would give a curve, in the figure of an S.[99] These circumstances are mentioned by Saussure, and from them we may infer this other property of these strata, that their section by a horizontal plane, must exhibit a system of straight lines, probably all parallel to one another.

[99] Voyages aux Alpes, vol i. § 472; also, Theory of the Earth, vol. ii. p. 30.

The same mineralogist describes the calcareous strata which compose the mountain Axenberg, on the side of the Lake of Lucerne, as having from top to bottom of the mountain the form of the letter S compressed, (ecrasée) with their curvature in some places very great. These inflections are repeated several times, and often in contrary directions; the layers are sometimes broken, where their curvature is greatest.[100]

[100] Voyages aux Alpes, tom. iv. § 1935.