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AILSA CRAIG , an island rock at the mouth of the Firth of Clyde , ro m. W. of Girvan, Ayrshire, Scotland . It is of conoidai form, with an irregular elliptic base, and rises abruptly to a height of 1114 ft. The only side from which. the rock can be ascended is the east ; the other sides being for the most part perpendicular, and generally presenting lofty columnar forms, though not so regular as those of Staffa . This island is composed of micro-granite with riebeckite, of great interest on account of the rare occurrence of this type in Britain. It is comparatively fine-grained' and of a greyish colour. Its essential constituents are felspar, quartz and riebeckitea soda amphibole. The last of these minerals occurs in small irregular patches between the idiomorphic felspars which Dr J. J. H. Teall has found to be a soda orthoclase. The rock is allied to paisanite described by C. A. Osann and has been termed ailsite by Professor M. F. Heddle. It forms part of an intrusive mass which, on the south and west cliffs of the island, has a columnar arrangement and is traversed by dykes of dolerite, most of which run in a north-west direction. The age of this mass is uncertain, as its relations to other rocks are not visible in the island. As riebeckitegranophyre has been found in Skye it may be of Tertiary age. The rock is a favourite material for curling-stones, about three-fourths (according to estimate) of those in use in the countries where the game obtains being made of it. On this account curling-stones are popularly known as " Ailsas " or " Ailsa Craigs." A columnar cave exists towards the northern side of the island, and on the eastern are the remains of a tower, with several vaulted rooms. Two springs occur and some scanty grass affords subsistence to rabbits, and, on the higher levels, to goats. The precipitous parts are frequented by large flocks of solan geese and other sea birds. The lighthouse on the southern side shows a flashing light visible for 13 m. In 1831 the twelfth earl of Cassillis became first marquis of Ailsa, takingthe title from the Craig, which was his property. When John Keats was in Girvan during his Scottish tour in 1818 he apostrophized the rock in a fine sonnet.
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