1871.] GEIKIE — TEETIAEY VOLCANIC ROCKS. 303 



owing their dark hue to the abundant inehided hair-like aggrega- 

 tions of a ferruginous sUicate ; 2nd, those formed of a coloured 

 glass in which the colouring-matter is impalpahly 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 vaUey 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-clitf 

 (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 hiU 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 moun- 

 tain comes into view for the first time is, that the huge wall is part 

 of a great dj^ke 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 struc- 

 ture of the mass became apparent. 



The ridge of the Scur, though formed of one great mass of rock 



* Hay Cunningham, in the paper before quoted, remarks : — " In regard to 

 the relations of the pitchstone-porphyry of the Sciir and the trap-rocts 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 pitch- 

 stones of the island." Maeculloch leaves us to infer that he regarded tlie 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 forpst." — Cruise of the Betsy, p. 32. 



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