Giants Causeway, a series of columnar basaltic rocks in the county Antrim, on the N. E. coast of Ireland, between Bengore Head and Port Rush. For 8 m. along the coast, from Bengore to Fairhead, the land abuts upon the sea in cliffs of basalt, many of which are made up in great part of rude vertical columns which alternate with layers of amorphous beds of the same class of rock. Ranges of these piled upon each other sometimes reach the height of 400 and at Fairhead even 550 ft. As seen from the sea in front, the uniformity of the arrangement of vertical columns and horizontal beds suggests rude resemblances to architectural forms. At the base of the cliffs is a talus of ruins that have fallen from the structures above and slope down to the water. But though the name of Giants' Causewav is often applied to all this coast range, it is properly applicable to but a small portion of it, a locality quite unpretending in its extent or in the grandeur of its features. It is a platform of basalt, composed of closely arranged columns, ranging from 15 to 36 ft. in height. This platform extends from a steep cliff down into the sea, till it is lost below low-water mark.
* length exposed at low water is differently given, but probably is less than 6OO ft. It is divided across its breadth into three portions, which are called the Little, the Middle, and the Large or Grand Causeway; the first being that on the east. These are separated from each other by dikes of amorphous basalt. The Great Causeway, which is the principal object of interest, is only from 20 to 30 ft. wide, though detached outliers of the same columnar structure standing on the shore near by might be added to increase the width. They no doubt connect with the same group below the surface. The columns are for the most part hexagonal prisms; but they are found also of five, seven, eight, and nine sides, and in one instance at least of three sides. They are all jointed into short irregular lengths from a few inches to a few feet each, the articulations being perfectly fitted by a convex end entering the concavity of the adjoining piece, so that the blocks form a true column. There is no uniformity in the arrangement of the convexities and concavities, but generally the upper part of this section is concave. The diameter is variable, but ranges generally from 15 to 28 in.
The columns fit together with the utmost precision, the corresponding faces of adjacent prisms being always equal, and so continuing from the top of the platform till the lines of separation are lost beneath the ground. It is said that water even cannot penetrate between adjoining columns. This portion is about 100 yards in length, extending from high-water mark to within 20 ft. of the cliff. The other portions are more uneven, and the columns in them are not uniformly vertical, but slope outward along the sides. The name causeway is given to the group from the circumstance of the columns terminating at a nearly uniform height, and thus presenting a tolerably smooth area gently inclining to the water. The columns of basalt do not retain their articulated character throughout the cliffs. At Fairhead they rise in single pieces, and, as measured by the officers of the ordnance trigonometrical survey of Ireland, some are found to stand 317 ft. in height, with sides occasionally of 5 ft. in breadth. These are flat at their extremities.
The formation is intersected by narrow dikes of columnar basalt, in which the prisms are piled horizontally, ranging across the line of the dikes. (See Basalt.) In this region it appears to have been protruded after the period of the deposition of the lias and chalk, the strata of these formations being penetrated by its dikes and overlaid by its horizontal beds.