This mole or quay is entirely composed of basaltic co lumns : it is part of an immense bed, which here dips into the sea, and rises, as it is traced eastward, until it reaches a height of 200 feet above the level of the sea. These co lumns are arranged perpendicularly, and so accurately fitted into each other, that the point of a knife is not to be introduced between them, excepting where the seams have been opened by the action of the weather. This collection of columns extends from the base of the cliff into the sea about 725 feet, part of it at low water being still covered. It is divided into three parts, which arc denominated the great, the Middle, and the Little Causeway. These sepa rations are occasioned by two parallel dykes, which tra verse the columns in a northern direction; and to these perhaps the preservation of this mole is to be attributed ; for although they are excavated, and worn down on the surface, still they remain firm at the base, and afford an immoveable support to the columns. These are of all shapes, from the triangular prism to the figureS of nine sides. It is seldom that any among the multiplicity of forms which present themselves are very symmetrical, those of the pentagon and hexagon are most common ; and they sometimes, though rarely, occur perfectly equi lateral. In the highest part of the mole, the columns are from 25 to 30 feet in length, extremely straight, and well proportioned : to this place the name of the Loom has been given. The prisms are wonderfully sharp in the angles, and present the very curious phenomenon of arti culation throughout their whole extent. This articulation is not performed by a simple section of the column, but the joints are let into each other in the manner of the ball and socket, so that the angles of the under joint extend in the form of triangular projections, over those of the one above it. These projections or spurs, as they have been de nominated, are easily detached ; and in some places, par ticularly among the columns at the Organ a little east of the mole, where they are 45 feet in height, this mutilation renders the articulation particularly remarkable. The joints are from eight inches in length to a foot and a half, and sometimes even two feet; they are often longest towards the bottom. In diameter, the columns may average about 16 or 20 inches; they are wonderfully uniform in this re spect ; those of a triangular and square form are very rare, as well as those of nine sides.
The height of the cliff which overhangs this mole, is about 330 feet above the level of the sea, and varies from that to 400 feet, which is the elevation of Pleskin, one of the principal promontories towards the eastern extremity of this basaltic district. This portion of the coast is deep ly indented; each little bay is denominated a port, and dis tinguished by its particular name, as Port Nofer, Port na Spania, &c. and along the whole coast the basaltic forma tion is beautifully exposed to view in one of the most mag nificent facades perhaps in the world. In some of the pro montories, the ranges of columns placed over each other, and separated by amorphous trap, extend to the '.umber of four or five. This is particularly the case in the great headland which bounds the east side of Port na Spania.
At Port Pleskin, the visible ranges of columns are only two, but here they are magnificently displayed, and on a larger scale than in any other part of the causeway. The number of beds of trap are altogether about 16, partly very soft amygdaloid mixed with much zeolite, and partly irregular prismatic basalt. These are here and there in terspersed with beds of bright red ochre; on one of which, at an elevation of about 200 feet from the sea, the first bed of columnar basalt rests, measuring about 44 feet in thick ness. On this a bed of irregularly prismatic basalt lies, 54 feet thick ; and on it another colonnade, still more mag nificent tnan the first.
Pleskin is the highest elevation of this basaltic district ; from it the beds all dip to right and left, and that which we have just mentioned, as resting on a surface of red ochre, 200 feet above the level of the sea, on the west, sinks below its surface at the mole, which in fact is merely a portion of it, and on the cast it disappears in the middle of Port Moon. The view from the summit of Pleskin, is one of the most imposing that can be imagined ; the series of headlands, which are seen in perspective from this point, form one of the grandest pictures of coast scenery, that it is possible to conceive.
The substance of the columnar basalt is extremely com pact, of a dark iron grey colour, fine grained in the tex ture, and conchoidal in the fracture, with sharp edged an gular fragments. It is totally different from the substance of which Fairhead is composed ; and perhaps we could not point out where the distinction between basalt and green stone is better defined, than at Fairhead and the Giant's Causeway. The blocks or joints are extremely sonorous. Small pieces of calcedony, fine semi-opal, and even precious opal, have been found imbedded in it : it is occasionally cel lular, and in some places presents the very singular phe nomenon of containing fluid water ; a circumstance which may be observed in the columnar basalt that occurs in a quarry not far from the summit of Pleskin. This fact has beenurgedby Dr Richardson as an incontestible proof of impossibility mpossibility of basalt being of igneous origin ; but the theorists on that side of the question have no difficulty in accounting for it ; we may remark, however, with re gard to the value of the fact itself, that it cannot be of much consequence ; for if pieces of the stone be removed for a time from the quarry, water will no longer be found in them, having made its escape; hence, if the stone be sufficiently porous to admit of the escape of the fluid, it cannot be denied that water may also be admitted through the same channel.
The Causeway has one considerable advantage over its rival Staffa, being much more accessible ; it is distant about six miles from Colerain, between which and Belfast there is a regular mail-coach communication.
See the Rev. Mr Dubourdieu's Statistical Survey of An trim ; Dr Hamilton's Letters on the County of Antrim ; the Giant's Causeway, a Poem, by W. H. Drummond, D. D. ; the Rev. Richard Pocock's Account of the Giant's Cause way, in the Phil. Trans. 1747-8, vol. xlv. page 124 ; and Dr Richardson's Paper on the basaltic country in Ireland, in the Phil. Trans. 1808, vol. xcviii. p. 187. (s. 1‘; )