Читать книгу Cavern Researches - Rev. John MacEnery - Страница 6

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Before entering into further details of the proceedings, it is fit the reader should be presented with a brief outline of the geology of the environs of the cavern.

Devonshire is traversed through its centre by the great Granite chain which rises in Cornwall, dividing the county, like its spine, into two nearly equal parts, north and south.—The latter we have to describe at present.

This district, the tourist need not be told, is distinguished from the rest of England by those ridges and Tors, which diversify its surface, and impart to it much of its picturesque character.

Besides the granite, which skirts it on the North-west, (and on the side of which, it may be observed in passing, the Bovey Lignite reclines) the rocks are composed of two principal classes, viz., limestone (attended by its conglomerate above and shale below) and old red-sandstone (accompanied by its silicious group) on which the limestone reposes.

The Limestone ranges in an almost uninterrupted line from Petit-Tor to Devonport, embracing in its course among other towns, St. Mary-Church, Torquay, Totnes, and Ivy Bridge.—Until recently, it was always esteemed a member of the Transition series. Mr. De la Beche succeeded, by a comparison of its organic remains, in establishing its identity with the Carboniferous species. Another high authority, however, is disposed, for the sake of simplicity, to retain its ancient classification, considering it convenient to include within the Transition series all kinds of stratified rocks from the earliest slates, in which we find the first traces of animal and vegetable remains, to the termination of the great coal formation, “the animal remains in the more ancient position of this series, viz., the Grauwacke group, being nearly allied in general, and differing only in species, from those in its more recent portion, viz., the Carboniferous group.” (Bridg. T. p. 60.) It reposes on old red-sandstone, or, more strictly speaking, on beds of argillaceous shale interposed between it and the sandstones with which in some places it is seen to interstratify, and into which in others it seems to pass.[A]

From Meadfoot, where it reposes on red-sandstone, it proceeds to Upton, where it forms a zone round the shale; it thence stretches out in a tongue as far as Coffinswell. At this point it disappears under the Exeter Conglomerate. Emerging again at Kingskerswell, it holds on its march to Newton. Thence it may be traced through the vallies of Bradley and Ogwell, successively to Ipplepen, Dartington, and Totnes on its road to Plymouth, where it rejoins the branch which followed the line of the coast and from which it was separated by the intervention of schist.

Its laminæ are of various density, from a few inches to several feet. They attain great thickness in the quarries of Park Hill. In texture it passes from compact to Crystalline, especially in the vicinity of Trap. Its ordinary color is a pearly grey, occasionally assuming shades of deeper intensity.

The semi-crystalline variety of Petit-Tor broken into boulders of several tons weight, and composing a massive species of Conglomerate, supplies those curiously tinted marbles, which nearly rival the Italian, and are worked up at St. Mary-Church for like purposes of art.

The shale with which the limestone alternates, in its upper members takes a reddish hue, and a brownish grey in its lower, and, according to the authority of Mr. De la Beche whose views we follow in this sketch, is identical with the shale of Pembroke.

The Conglomerate which overlies it, is composed of fragments torn from the parent rock which it overlies. One of the most curious examples existing, is in the loose masses (of which we have spoken as furnishing ornamental marbles) at Petit-Tor, resting, in the form of conglomerate, on their native bed of limestone, and which are thought to have been detached by the action of water.

We have lastly to speak of the old red-sandstone, on which the limestone and shale rest. It is, according to Mr. Conybeare, a continuation of that silicious group which stretches south from the vale of Taunton into Devonshire,—advancing along the vale of Crediton to Exeter, and thence to the coast, in a southern and continuous direction by Chudleigh and the Teign, terminating at the S.W. angle of Torbay, between which and the Teign a branch runs in among the limestone,—another bends its course inland through Chelston and Cockington, occasionally disappearing beneath the Exeter conglomerate. Under the red silicious grit of Meadfoot occurs a grey fissile and micaceous sandstone. It traverses the point of land called Hope’s Nose, sustaining the limestone. This grit rock is traversed by veins of trap, like the red grit of Meadfoot in contact with the limestone of Kent’s Hole.

To the west of Babbicombe, trap pierces upwards through the argillaceous shale into the limestone, the adjacent beds of which are fractured and contorted, and portions of them inclosed in the mass of trap. The shale is elevated to the summit of the cliffs at Oddicombe, as well as the adjoining red-sandstone. The sandstone strata, between Livermead and Cockington, are confusedly huddled together,—the dip being at all angles. Such disturbance of the lower strata could not fail to be felt by the upper. Accordingly we find the limestones and shales included in the trap. Nowhere, except in the Alps, have I seen paralleled the contortions in the strata which Mr. De la Beche, at the time he was engaged in the examination of this district, pointed out to my notice, diagrams of which he has since given in his works. In the valley of Luterbrünnen and on the left flank of the Lake of Lucerne near Fluellen, may be seen sections, on a grander scale, which remind one of the bent strata of this district.

Such is the confusion prevailing through the whole that there is no where to be obtained a regular dip. In one quarry near the Torquay turnpike, the dip is at an angle of 35, at another little farther on the strata are vertical. On the new line through Stony Valley, the limestone projects from the sides of the ridges in bold crags, or bends over at the entrance, by the roadside, into natural arches.

The southern arm of Torbay exhibits evidence of convulsion no less decisive than that of the north. From Berryhead to Saltern Cove the limestone is blended together with the trap, and its calcareous quality so modified in consequence, that it almost refuses to effervesce with acids.

From this it is evident that the strata of this district have undergone two distinct revolutions—

1. From the action of water, in the abrasion of the rocks, the debris of which are accumulated in the form of conglomerate on the parent beds.

2. From the agency of another and more tremendous power, viz., of trap, in piercing through, bending, and ultimately enclosing solid masses of limestone and shale,—an event which Mr. De la Beche connects with the elevation, by the same cause, of the chalk and Oolitic formations in this country, and of the loftiest mountain range on the continent. This limestone may be justly called, from abounding with caverns, the Cave Limestone, as the Jura formation has been designated, for the same reason, Höhlenkalk.—There is a chain of caverns from Kent’s Hole extending through Dean and Buckfastleigh to Plymouth. It is strongly impregnated with iron, to which it is indebted for the variety of its tints. Carbonate of Lime and sulphate of Barytes are found in the veins.

For a fuller description of the formations of this district, which yields in interest to no other quarter of the globe, and from which Mr. De la Beche has drawn some of his most instructive illustrations, I must refer to his work, and others which treat expressly on the subject, where it will be seen that the geological construction of this region as justly merits a preeminent place in the annals of the structure of our globe, as does Kent’s Hole among the caverns in its bowels.

FOOTNOTES:

[A] This group has since been classified as a distinct formation, the Devonian, from its fullest development, being in this county. It has been identified by Sir R. Murchison with beds similar in geological, although not always in minerological character in Scotland and Russia. It is intermediate between the Silurian and carboniferous systems.—E. V.