320 VOLCANIC CORES AND LAVAS OF SKYE. 



the north end of the island; it is sometimes columnar, and of a 

 pale-grey colour resembling the fine-grained quartz porphyries of Skye 

 and Raasay. One or two other masses of felstones occur in the 

 island. The Scur of Eigg is an elevated ridge two and a quarter miles 

 long, and rising three hundred to four hundred feet above the basalt 

 plateau. It consists of pitchstone and felstone, the former dark- 

 coloured and columnar. The porphyry is grey and interbedded in 

 the pitchstone. Under the microscope the base of this rock is glassy 

 or granular with crystals of orthoclase which are sometimes a quarter 

 of an inch long. The eruption of the pitchstone is considered to 

 have been long subsequent to the eruption of the basalt, and the Scur 

 of Eigg was formerly a river valley in which gravels and drift-wood 

 were buried under the products of successive volcanic eruptions. 

 Subsequent denudation has reduced Eigg to much the same condition 

 as the fragments of the older basaltic plateau of the Auvergne. 1 



Skye. A volcano which was probable larger than any of the 

 others was situate in the south of Skye. The intrusive igneous rocks 

 burst through strata of Cambrian, Liassic, and Oolitic age, and often 

 transmute the lias into white granular and compact limestone at the 

 junction. The great central granite mass of the Red Mountains, 

 remarkable for their smooth contours, reaches a height of 2670 feet in 

 the pyramidal peak of Ben Glamaig. The granite gradually changes 

 at the circumference into felsites, and is pierced by contemporary 

 veins of the same rock. As in the associated volcanoes, this granite 

 is the core of the older or acidic cone. The later eruption formed the 

 wild hills of gabbro, jagged in outline, which are known as the 

 Cuchullin Hills and Ben Blabheinn, both more than 3000 feet high. 

 On passing outward from the central cores of gabbro, with its large 

 metallic bronzy crystals like hypersthene embedded in the grey labra- 

 dorite, the rock passes insensibly, first into dolorite, and then into the 

 liner-grained basalt. Patches remain of the old agglomerates and 

 deposits of volcanic ash, mostly mingled with felstone lavas. They 

 are well seen in the hill on the south of Ben Dearg ; and in the 

 island of Scalpa they lie on Primary and Secondary strata. The 

 basalt was remarkably fluid, and extended from the Cuchullin Hills 

 in every direction. Much of it may be now covered by the sea, but 

 large fragments stretch to the north-west and north, and form the penin- 

 sulas of Skye known as Trotternish, Vaternish, Duirinish, and Mingi- 

 nish. In the island of Kaasay an outlier of basalt forms the hill of 

 Dun Can. This volcano of Skye may perhaps have had its principal 

 crater in Loch Coruisk, which is a deep and desolated amphitheatre in 

 the gabbro. 



Professor Judd has suggested that the remote island of St. Kilda 

 is an extinct volcano now represented by cores. M'Culloch describes 

 the eastern part as of granite, and the west as of gabbro, with the 

 hills surrounded by basalt. 2 



1 Arch. Geikie, Q. J. G. S., vol. xxvii. p. 285. 

 8 Judd, Q. J. G. S., vol. xxx. p. 255. 



