The very numerous Tertiary dykes of the Clyde islands (Arran, Bute, and the Cumbraes), and of the adjacent mainland and peninsulas, have been little studied either from the geological or petrographical point of view; but they are of great interest, not only in themselves, but as providing a link between the dykes of the better known areas of the North of England, and of Skye and Mull. The material is not yet gathered on which could be based a complete account of the series. The present paper is designed to present a full description of a hitherto unrecognized type of Tertiary dyke, typically exposed in the Great Cumbrae, and to indicate its relationships to other types already described from Mull and the North of England. Furthermore, a few other dykes from the Clyde area, all of basaltic composition, will be briefly described.