Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/444

 very different from those around it, in reality consists of two distinct varieties of rock, pitchstone and felstone-porphyry, arranged in distinct and, in a general sense, horizontal beds. Looked at from the east side (P1. XIV. fig. 1) this feature is not clearly marked ; for the great cliff seems then to consist of one homogeneous mass, except a marked columnar band running obliquely along the base of the precipice. If, however, the side is viewed from the south, the bedded character of its component rocks becomes a conspicuous feature. Along the noble cliffs on that side the two varieties of rock are strongly distinguished by their contrasting colour and mode of weathering, the sombre-hued pitchstone standing up in a huge precipice striped with columns, and barred horizontally with bands of the pale-grey porphyry, which seems sunk into the face of the cliff. At the south-east end of the ridge the beds are very distinct. Further west of the precipices to the south of the Loch a Bhealaich, the dark pitchstone which forms the main mass is divided by two long parallel intercalations of grey porphyry, and two other short lenticular seams of the same material (see PI. XIV. figs. 2 & 3). It is clear from these features, which are not seen by most travellers, who pass Eigg merely in a steamer, that the Scur is in no sense of the word a dyke.

But although the Scur is thus a bedded mass, the bedding is far different from the regularity and parallelism of that which obtains among the interbedded basalt-rocks below. Even where no intervening porphyry occurs, the pitchstone can be recognized as made up of many beds, each marked by the different angle at which its columns lie. And when the porphyry does occur and forms so striking a division in the pitchstone, its beds die out rapidly, appearing now on one horizon, now on another, along the face of the cliffs, and thickening and thinning abruptly in short distances along the line of the same bed. Perhaps the best place for examining these features is at the Bhealaich, the only gully practicable for ascent or descent, at the south-eastern face of the ridge.

By much the larger part of the mass of the Scur consists of pitchstone. As a rule this rock is columnar, the columns being much slimmer and shorter than those of the basalt-rocks. They rise sometimes vertically, and often obliquely, or project even horizontally from the face of the cliff. They are seldom quite straight, but have a wavy outline ; and when grouped in knolls here and there along the top of the ridge, they remind one of gigantic bunches of some of the palaeozoic corals, such as Lithostrotion. In other cases they slope out from a common centre, and show an arrangement not very unlike that of a Highland peat- stack.

The pitchstone of the Scur differs considerably in petrographical character from any other of the pitchstones of the island, and indeed from any other pitchstone which I have yet met with in Scotland. Its base is of a velvet-black colour, and is so much less vitreous in aspect than ordinary pitchstone as to have been described by Jameson and later writers as intermediate between pitchstone and basalt*.


 * ' Mineralogy of the Scottish Isles,' vol. ii. p. 47. See also Macculloch, ' Western Isles,' vol. i. p. 521, and Hay Cunningham, ' Mem. Wern. Soc' vol. viii. p. 155.