AILSA CRAIG, a remarkable island-rock at the mouth of the Firth of Clyde, off the coast of Ayrshire, Scotland. It is of a conoidal form, with an irregular elliptic base, and rises abruptly from the sea to the height of 1139 feet. 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 rock is a greenstone or syenite, with a basis of grayish compact felspar traversed by numerous trap veins. A columnar cave exists towards the north side, and on the eastern are the remains of a tower, with several vaulted rooms. Two springs occur on the island, and some scanty grass affords subsistence to numerous rabbits. The precipitous parts of the rock are frequented by large flocks of solan geese and other aquatic wild fowl. It is situated in 55° 15' N. lat., 5° 7' W. long.