Part I. The Geological Structure During the progress of the geological survey of the Island of Arran Mr. Gunn mapped the network of igneous rocks that occurs about halfway between Brodick Bay on the east and Machrie Bay on the west, and lying to the south of the String Road which crosses the island from Brodick to Shiskine and Blackwaterfoot (Sheet 21 of the 1-inch Geological Survey Map). These rocks form the mass of high ground culminating in Ard Bheinn (1676 feet), A'Chruach (1679 feet), and Beinn Bhreac (1649 feet), whence the headwaters of the Glen Cloy Burn and Benlister Burn drain to the east and the Black Water and Machrie Water to the west. The area in which these igneous rocks are exposed has obviously been a focus of great volcanic activity. It may be compared with some of the larger and more complex vents of Palæozoic and Tertiary age in Scotland, but it is exceptionally gigantic in its proportions. It is oval in shape, and covers an area of about 7 to 8 square miles. Its greater axis, which trends east-north-east and west-south-west, is 3½ miles long, and its shorter axis is about 3 miles in length. It is surrounded by various formations, ranging in age from the Lower Old Red Sandstone to the Trias. On the north side, conglomerates, sandstones, and mud-stones of Lower Old Red Sandstone age, dipping steeply southeastward, strike obliquely against its northern margin. On the east side, the Lower