AILSA CRAIG, an island rock at the mouth of the Firth of Clyde, 1 om. W. of Girvan, Ayrshire, Scotland. It is of conoidal form, with an irregular elliptic base, and rises abruptly to a height of I ,09 7f t. 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. The island is composed of micro-granite with riebeckite, of great interest on account of the rare occurrence of this type in Britain. It is fine-grained and of a greyish colour, and may be of Tertiary age. Pebbles of this rock were carried far south (e.g. to Welsh shores) by gla ciers of the Ice age. The rock is a favourite material for curling-stones. Hence curling-stones are popularly known as "Ailsas" or "Ailsa Craigs." The rock is also used for paving stones. A columnar cave exists towards the north side of the island, and on the east are remains of a tower. Two springs occur and some scanty grass affords subsistence to rabbits, and, on the higher levels, to goats. The precipices have large breeding colonies of gannets and other sea birds which spend the winter months at sea. A lighthouse on the south side flashes a light visible for 13 miles.