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Cumbraes, The, two islands forming part of the county of Bute, Scotland, lying in the Firth of Clyde, between the southern shores of Bute and the coast of Ayrshire. Great Cumbrae Island, about 1½ m. W.S.W. of Largs, is 3¾ m. long and 2 m. broad, and has a circumference of 10 m. and an area of 3200 acres or 5 sq. m. Its highest point is 417 ft. above the sea. There is some fishing and a little farming, but the mainstay of the inhabitants is the custom of the visitors who crowd every summer to Millport, which is reached by railway steamer from Largs. This town (pop. 1901, 1663) is well situated at the head of a fine bay and has a climate that is both warm and bracing. Its chief public buildings include the cathedral, erected in Gothic style on rising ground behind the town, the college connected with it, the garrison, a picturesque seat belonging to the marquess of Bute, who owns the island, the town hall, a public hall, library and reading room, the Lady Margaret fever hospital, and a marine biological station. The cathedral, originally the collegiate church, was founded in 1849 by the earl of Glasgow and opened in 1851. In 1876 it was constituted the cathedral of Argyll and the Isles. Millport enjoys exceptional facilities for boating and bathing, and there is also a good golf-course. Pop. (1901) 1754, of whom 1028 were females, and 59 spoke both English and Gaelic. Little Cumbrae Island lies to the south, separated by the Tan, a strait half a mile wide. It is 1¾ m. long, barely 1 m. broad, and has an area of almost a square mile. Its highest point is 409 ft. above sea-level. On the bold cliffs of the west coast stands a lighthouse. Robert II. is said to have built a castle on the island which was demolished by Cromwell’s soldiers in 1653.

The strata met with in the Great and Little Cumbrae belong to the Upper Old Red Sandstone and Carboniferous systems. The former, consisting of false-bedded sandstones and conglomerates, are confined to the larger island. The Carboniferous rocks of the Cumbrae belong to the lower part of the Calciferous Sandstone series with the accompanying volcanic zone. In the larger island these sediments, comprising sandstones, red, purple and mottled clays with occasional bands of nodular limestone or cornstone, occupy a considerable area on the north side of Millport Bay. In the Little Cumbrae they appear on the east side, where they underlie and are interbedded with the lavas. The interesting geological feature of these islands is the development of Lower Carboniferous volcanic rocks. They cover nearly the whole of the Little Cumbrae, where they give rise to marked terraced features and are arranged in a gentle synclinal fold. The flows are often scoriaceous at the top and sometimes display columnar structure, as in the crags at the lighthouse. Those rocks examined microscopically consist of basalts which are often porphyritic. In Great Cumbrae the intrusive rocks mark four periods of eruption, three of which may be of Carboniferous age. The oldest, consisting of trachytes, occur as sheets and dikes trending generally E.N.E., and are confined chiefly to the Upper Old Red Sandstone. They seem to be of older date than the Carboniferous lavas of Little Cumbrae and south Bute. Next come dikes of olivine basalt of the type of the Lion’s Haunch on Arthur’s Seat, which, though possessing the same general trend as the trachytes, are seen to cut them. The members of the third group comprise dikes of dolerite or basalt with or without olivine, which have a general east and west trend, and as they intersect the two previous groups they must be of later date. They probably belong to the east and west quartz dolerite dikes which are now referred to late Carboniferous time. Lastly there are representatives of the basalt dikes of Tertiary age with a north-west trend.