25. ISLE OF STAFFA; FINGAL'S CAVE.-Many of the Hebrides, or Western Isles of Scotland, are almost wholly composed of trap-rocks. Of these Staffa* is the most celebrated, on account of a deep chasm or recess situated in a magnificent group of vertical columnar basalt, and which has been produced by the incessant action of the surges on the base of the cliff. This natural cavern is of singular beauty, and is known to the English tourist by the name of Fingal's Cave; but it is called by the islanders Naimh-bim, or the Cave of Music, from the murmuring echoes occasioned by the billows, which in rough weather dash with violence and a loud noise into the chasm.

The Isle of Staffa is a complete mass of columnar basalt; it is about two miles in circumference, and forms a table-land of an irregular surface, being surrounded on every side by steep cliffs, about seventy feet high, which are composed of clusters of angular columns, possessing from three to six or seven sides. It is intersected by one deep gorge, which divides the higher and more celebrated columnar portion from the other division of the island. At the highest tides, the columns which form the south-western cliffs appear to terminate abruptly in the water; but the retiring tide exposes a causeway of broken columns at their base. The greatest elevation of the island is about 120 feet, and its surface is covered with soil of considerable depth, clothed with herbage.t

Fingal's Cave, first made known to the public in 1772, by Sir Joseph Banks, is on the south-east corner of the island, and presents a magnificent chasm 42 feet wide, and 227 in length. The roof, which is 100 feet high at the entrance, gradually diminishes to 50, and is composed of the projecting extremities of basaltic columns; the sides are formed of perpendicular pillars; and the base consists of a causeway paved with the truncated ends of similar columns.

*Staffa, a Norse term, signifying a staff or column † Dr. Macculloch

The vaulted arch presents a singularly rich and varied effect; in some places it is composed of the ends of portions of basaltic pillars, resembling a tessellated marble pavement; in others, of the rough surface of the naked rock; while in many, stalactites mingle with the pillars in the recesses, and add, by the contrast of their colours, to the pictorial effect, which is still further heightened by the ever-varying reflected light thrown from the surface of the water that fills the bottom of the cave.

The depth of the water is nine feet, and a boat can therefore reach the extremity of the cave in tolerably calm

LIGN. 206.--FINGAL'S CAVE; VIEWED FROM WITHIN

weather; but, when the boisterous gales of that northern clime drive into the cavern, the agitated waves, dashing and breaking against the rocky sides, and their roar echoed with increased power from the roof, present to the eye and ear such a scene of grandeur as bids defiance to any description. The short columns composing the natural causeway before mentioned continue within the cave on each side, and form

a broken and irregular path, which allows a skilful and fearless climber to reach the extremity on the eastern side on foot: but it is a task of danger at all times, and impossible at high tide, or in rough weather. It would be useless, observes Dr. Macculloch, to attempt a description of the picturesque effect of a scene which the pencil itself is inadequate to portray. Even if this cave were destitute of that order and symmetry, and that richness arising from the multiplicity of its parts, combined with its vast dimensions and simple style, which it possesses, still the prolonged length, the twilight gloom half concealing the playful and varying effects of reflected light, the echo of the measured surge as it rises and falls, the transparent green of the water, and the profound and fairy solitude of the whole scene, could not fail strongly to impress a mind gifted with any sense of beauty in art or in nature.*

The basalt of which the columns are composed is of a dark greenish-black hue; a thin layer of silicious cement occurs between the joints or articulations, which is called mortar by the islanders, and strengthens their persuasion that this wonderful cave is the work of art. Another cave,

but of inferior dimensions, lies at a short distance; and many others of less note are seen in various parts of the cliffs, into which the sea breaks with a noise resembling that of distant heavy ordnance.

26. STRATA ALTERED BY CONTACT WITH BASALT.-In Ireland a magnificent range of basaltic pillars extends along the northern coast of Antrim. It consists of an irregular group of hundreds of thousands of pentagonal, jointed, basaltic columns, varying from one to five feet in thickness, and from twenty to two hundred feet in height. The structure of these masses I have already described; their prevailing colour is a dark greenish-grey. Along the shore, a vast area is covered by the truncated ends of upright

*Macculloch's Western Isles.

columns, the upper parts of which have been swept away by the action of the waves. The surface, therefore, presents the appearance of a pavement composed of enormous angular blocks of stone; whence has originated the popular name of the Giants' Causeway. In the cliffs, a natural cavern has been excavated by the inroads of the waves, about sixty feet high, and of great picturesque effect; the entrance is nearly thirty feet in width, and the walls are formed of dark basalt.

But the great interest of this spot, in a geological point of view, is the altered structure observable in the sedimentary rocks wherever they are in contact with the basalt. The Chalk, in this part of Ireland, constitutes a line of cliffs traversed by trap, which occurs in vertical dykes, and in extensive beds, and has a columnar structure.

LIGN. 207.-TRAP-DYKES TRAVERSING CHALK: IN THE ISLE OF RATHLIN.*

1, 1. Chalk.

2, 2, 2. Chalk changed into granular marble by contact with the
Trap-dykes.

3. A narrow Trap-dyke or vein (1 foot thick) traversing altered

Chalk.

4. Trap-dyke, 35 feet wide.

5. Trap-dyke, 20 feet wide.

The chalk-strata have a total thickness of about 270 feet, and rest on a green sandstone, called mullattoe, which is the equivalent of the Upper Greensand of the south-east of

66

* See the Memoir by Conybeare and Buckland, Geol. Trans. vol. iii. p. 196, &c. The student may also with advantage consult Portlock's Report on Londonderry," &c. for an account of the Chalk and Basalts of the Antrim coast; also Mr. E. Hall's paper in the Edinb. New. Phil. Journ. new series, vol. v. p. 53.

England (vol. i. p. 301); it contains flint-nodules, ammonites, belemites, echinites, terebratulæ, and other usual fossils of the cretaceous formation.

In the Isle of Rathlin, nearly vertical dykes of basalt are seen intersecting the chalk (as in this sketch, Lign. 207), which at the line of contact, and to an extent of several feet from the wall of the dyke, is completely metamorphosed. Those portions of the chalk which have been exposed to the extreme influence of the trap are now a dark-brown crystalline rock, the crystals running in flakes, like those of coarse crystalline limestone; in the next state the rock is of a saccharoid structure; then fine-grained and arenaceous; a compact variety with a porcellaneous aspect, and of a bluishgrey colour, succeeds; this gradually becomes of a yellowish-white, and passes insensibly into unaltered chalk. The flints in the hardened chalk are either of a yellowish or deepred colour, and the chalk itself is highly phosphorescent. The fossils are much indurated, but retain their usual appearance.

To the south of Fairhead, in the county of Antrim, syenite traverses mica-schist and chalk, and fragments of the latter are impacted in the erupted mass, being changed into granular marble. The geological structure of that part of Ireland consists of-1. The underlying rock, Micaschist; 2. Coal-shale; 3. Triassic strata; 4. Chalk.‡

In this place it is necessary to remind the reader of the examples of intruded basaltic rocks which have been noticed in the former part of this lecture, when treating of the paleozoic formations; viz. the trap of Dudley (p. 700 and 811), of the Malvern and Abberley Hills (p. 815 and 818), &c.; the toad-stones of Derbyshire (p. 698), and the

* Dr. Berger on the Geological Features of the North-east of Ireland, Geol. Trans. 1st ser. vol. iii. p. 172.

+ The beautiful statuary marble of Carrara is jurassic limestone, metamorphosed by the influence of contiguous igneous rocks.

Mr. Griffiths, Trans. Geol. Soc. 2nd ser. vol. v. p. 179, &c.