Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/443

 owing their dark hue to the abundant included hair-like aggregations of a ferruginous silicate ; 2nd, those formed of a coloured glass in which the colouring-matter is impalpably diffused ; and, 3rd, those formed of a coloured glass where the hue is further intensified by the abundance of included " hairs."

3. Pitchstone and Porphyry Coulees of the Scur.

That feature of the island of Eigg which renders it so remarkable and conspicuous an object on the west coast is the long ridge of the Scur. Rising gently from the valley which crosses the island from Laig Bay to the Harbour, the basaltic plateau ascends south- westwards in a succession of terraces, until along its upper part it forms a long crest, from 900 to 1000 feet above the sea, to which it descends on the other or south-west side, first by a sharp slope, and then by a range of noble precipices. Along the watershed of this crest runs, in a graceful double curve, the abrupt ridge of the Scur, terminating on the north-west at the edge of the great sea-cliff (975 feet), and ending off on the south-east in that strange well- known mountain-wall (1272 feet high) which rises in a sheer cliff nearly 300 feet above the basalt-plateau on the one side and more than 400 feet on the other. The total length of the Scur ridge is two miles and a quarter, its greatest breadth 1520, its least breadth 350 feet. Its surface is very irregular, rising into minor hills and sinking into rock-basins, of which nine are small tarns, besides still smaller pools, while six others, also filled with water, lie partly on the ridge and partly on the basaltic plateau. No one, indeed, who looks on the Scur from below, and notes how evenly it rests upon the basalt-plateau, would be prepared for so rugged a landscape as that which meets his eye everywhere along the top of the ridge. Two minor arms project from the east side of the ridge ; one of these forms the rounded isle called Beinn Tighe (968 feet), the other the hill of A chor Bheinn.

Singular as the Scur of Eigg is, regarded merely as one of the landmarks of the Hebrides, its geological history is not less peculiar. The natural impression which arises in the mind when this mountain comes into view for the first time is, that the huge wall is part of a great dyke or intrusive mass which has been thrust through the older rocks *, It was not until after some time that the influence of this first impression passed off my own mind, and the true structure of the mass became apparent. The ridge of the Scur, though formed of one great mass of rock

the relations of the pitchstone-porphyry of the Scur and the trap-rocks with which it is connected, it can, after a most careful examination around the whole mass, be confidently asserted that it exists as a great vein which has been erupted through the other Plutonic rocks — thus agreeing in age with all the other pitchstones of the island." Macculloch leaves us to infer that he regarded the rock of the Scur to be regularly interstratified with the highest beds of the dolerite series (' Western Isles,' i. p. 522). Hugh Miller speaks of the Scur of Eigg as " resting on the remains of a prostrate forest." — Cruise of the Betsy, p. 32.
 * Hay Cunningham, in the paper before quoted, remarks: — " In regard to

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