GIANTS' CAUSEWAY (deriving its name from a mythical legend that it was the com mencement of a road to be constructed by giants across the channel to Scotland), is a sort of pier or mole, of columnar basalt. projecting from the northern coast of Antrim, Ireland, into the North channel, about 15 m. from Coleraine. It is part of an extensive and overlying mass of basalt, from 300 to 500 ft. in thickness. which covers almost the whole county of Antrim and the eastern part of Londonderry, extending over an area of nearly 1200 sq. miles. The basalt occurs in several beds, interstratified with layers of ash. It covers secondary strata, converting the chalk into granular limestone, and the lies shale into Lydian stone, where it comes in contact with them. Several of the basaltic beds are more or less columnar, but three layers are remarkably so. The first appears at the bold promontory, of Fair bead;„ its columns are coarse and large, exceed ing 200 ft. in height. r The other two are seen together rising above the sea-level at Ben gore head. The lower one forms the causeway at the place where it is uncovered, as it again gradually dips under the sea. It is exposed for 300 yards, and exhibits an unequal
pavement, formed of the tops of polygonal columns, fitting so compactly that the blade of a knife can scarcely be inserted between them. The columns are chiefly hexagonal, though examples may. be found with 5, 7, 8, or 9 sides; and there is a single instance of a triangular prism. The diameter of the pillar is very variable, but the average size is from 15 to 20 inches. Each pillar is divided by joints of unequal length, the concave hollow at the end of one division fitting exactly into the convex projection of the other. Tlie rock is compact and homogeneous, and is somewhat sonorous when struck with a hammer.
The causeway is divided into the little, middle, and large causeways. The large causeway, which is formed by the lowest of the three columnar beds of basalt, is about 30 ft. wide, and runs more than 200 yards from its exposure on the cliff till it is covered by the sea. The little and middle causeways are formed from the second columnar stratum, and are less remarkable than the other.