Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/442

 like a mass of rather viscid pitch which had been spilt at the top of the cliff, and had slowly flowed into and filled a cavity in the rocks. This detached portion of pitchstone has the same bright deep jet-black or bluish-black colour and obsidian-like texture noticed in the previous vein. A little below it the main portion of the vein descends the lower part of the cliff, and crosses the beach from 1ST. 25° W. to S. 25° E., with an average width of 1-1/2 to 2 feet. Its external parts, as in the previous example, are black and quite glassy ; the central portion possesses the common dull resinous lustre and dark-green colour characteristic of pitchstone. A thin section of the latter part of the vein, placed under the microscope, shows that the base of the rock is a nearly colourless homogeneous glass, through which are scattered abundant black or greenish hairs of some ferruginous silicate arranged singly and in oblong tufts. The individual hairs of each tuft are not feathered, like those of the Corriegills pitchstone of Arran. It is to the abundance of these particles that the dark colour of the rock is due.

A third, less distinctly traceable vein of pitchstone traverses the dolerites on the beach at the harbour. There is the same difference of texture in it as in the Rudh an Tangairt veins. The black brittle obsidian-like portion, when examined microscopically, shows a deep rich-brown homogeneous glass, with numerous small kernels, some of which are filled with an amber-coloured substance (bitumen?). Except for its much deeper colour and the presence of coloured kernels instead of much more minute elongated vesicles, the minute texture of this rock is analogous to that of the east vein. The dull dark-green portion is markedly porphyritic, and is mixed up, even in hand-specimens, with the more glassy variety. Under the microscope it shows considerable opacity, but on extremely thin edges and in certain less deeply coloured portions is found to consist of a thickly aggregated mass of minute black hairs, less distinctly separated than those of the Rudh an Tangairt vein, and imbedded in a glass mostly of a dark-green or black colour, but here and there colourless. The colouring-matter is therefore not entirely dependent in this rock upon the abundance of the hair-like particles. Large crystals of a beautifully striated felspar are scattered through the rock, also kernels filled with a brown or amber-coloured substance, as in the black part of the vein.

A fourth pitchstone occurs on the roadside, a little to the east of Laig Farm, and seems to be connected with the intrusive boss of quartziferous porphyry there. It differs considerably in external aspect from the other veins, being of a pale-green or greenish-grey colour, and thus resembling at first sight the pale slag of an iron- furnace. The base is minutely granular, and shows a few scattered felspar crystals. Under the microscope this rock appears as a pale-brown glass, through which are scattered abundant minute cavities, short dark bodies resembling the " hairs " already described, but less definitely formed, and crystals of an orthoclase felspar. Petrographically considered, the pitchstone veins of Eigg present us with three varieties : — 1st, those formed of a colourless glass and