Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/440

 may be inferred from the fact that the pitchstone of the Scur, as will be shown in the sequel, is much younger than the rocks of the basaltic plateau. The pitchstone of most of the veins differs, indeed, microscopically from that of the Scur ; but the latter varies greatly even within itself, so that, though no evidence exists of any pitchstone vein having ever been connected with the rock of the Scur, we may, provisionally at least, class all the pitchstones together as the latest of the igneous rocks of Eigg.

Four separate veins of pitchstone have been noticed in Eigg. The best- known and most clearly exposed veins are two which traverse the dolerite beds at Rudh an Tangairt, near the famous Uamha Fhraing, or Frank's Cave, on the south side of the island. The eastern vein (fig. 7) consists, in its upper part, not of pitchstone, but of a pale compact quartziferous porphyry or felstone, like that of Scorr Scalleadh*. It is exceedingly hard, splintering under the hammer with a metallic sound. It weathers with a yellowish or reddish tint, which extends for an inch or two into the stone, and shows numerous cavities, resulting apparently from the decomposition of felspar crystals. Towards the margin of the vein it assumes a laminar texture, in plates which are in a general sense parallel to the walls of the vein. Examined in thin section with the microscope, this rock shows a curious confused mass of minute needle-like or hair- like bodies, with opaque partially decomposed grains of pyrites, or possibly titaniferous iron, and a still more decomposed brown mineral. The texture closely resembles that of some of the pitchstones. The vein has a thickness of about 2-1/2 feet, and

vol. ii. p. 45. The felspathic rock he terms " hornstone ;" it is called " chert " by Macculloch, and " porphyritic compact felspar " by Hay Cunningham. It differs in minute structure from evey other rock with which I am acquainted, and, as stated in the text, more nearly resembles pitchstone.
 * An engraving of this vein is given by Jameson in his work already cited,

Fig. 6. Basalt Veins traversing interbedded Dolerites, Kildonan, Eigg.

Fig. 7. Vein of Pitchstone traversing Dolerite, Rudh an Tangairt.