By much the larger part of the mass of the Scuir consists of vitreous material. 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 Palæozoic 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 Scuir differs considerably in petrographical character from other pitchstones of the island which occur in dykes and veins. 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.[254] A chemical analysis of the rock by Mr. Barker North,[255] gave the following composition:-- Silica 65·81 Alumina 14·01 Ferric oxide 4·43 Lime 2·01 Magnesia 0·89 Soda 4·15 Potash 6·08 Loss in ignition 2·70 ------ 100·08 [Footnote 254: _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.] [Footnote 255: _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 379.] The grey devitrified bands, which occur as a subordinate part of the mass of the Scuir ridge, are usually somewhat decomposed. Where a fresh fracture is obtained, the material shows a fine-grained, sometimes almost flinty, grey felsitic base, containing clear granules of quartz, and facets of glassy felspar. In some places the rock is strongly porphyritic. Examined under the microscope it presents a more thoroughly devitrified groundmass, with the minutest depolarizing microlites, large porphyritic crystals of plagioclase and sanidine, grains of augite, and sometimes exceedingly abundant particles of magnetite.[256] [Footnote 256: The microscopic structure of the identical pitchstone of Hysgeir is given on p. 247.] [Illustration: Fig. 282.--Section at the base of the Scuir of Eigg (east end).]