Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/441

 runs across the dolerite beds with a trend from N. 10° W. Descending the upper part of the cliff, the vein continues downward ; but the porphyry is replaced by pitchstone, which descends to the beach. A detached portion of the former rock is involved in the latter ; but it is hardly possible to decide here which is the newer mass, or if they are not of contemporaneous origin*.

The portion of the vein filled with pitchstone runs with a much less even course and regular thickness than the part filled with porphyry. The pitchstone suggests the idea of intense liquidity, as it seems to have thrust itself into every minute crevice of the crack in the dolerites up which it rose. Its path is somewhat tortuous ; and its width varies from a few inches to 2 feet or more. Sometimes its walls are upright and parallel like those of a dyke ; but in a little distance this feature disappears, and the pitchstone bends now to one side, now to the other, along the irregularly jointed surface of the dolerites. At the margin of the vein it is deep jet-black, and as glassy and lustrous as bottle-glass, but much more brittle ; away from the edge it assumes the ordinary dull resinous lustre of pitchstone, while in the middle, where the vein is broadest, the rock takes a porphyritic texture. Examined microscopically, the outer or jet- black obsidian-like part of the vein is a perfect glass, of a pale brown colour, this hue being equally diffused as a tint through the mass, and not arising from the abundance of coloured crystals. I have not been able, even with a magnifying power which renders an object one five-thousandth of an inch in diameter clearly visible, to resolve this coloured matrix into component crystallites, grains, hairs, or any other structure. It is traversed by minute cracks, which, along with elongated cavities, run in one general direction parallel with the sides of the vein. On either side of these cracks the colour has in some cases been bleached out of the rock for a short distance ; in others the colour is intensified into a deep brown. The cavities vary from one two-thousandth of an inch or less up to one fiftieth of an inch or more, sometimes empty, sometimes filled with a brown colouring-matter.

About four yards west from the vein just described a second vein of pitchstone is seen traversing the compact close-grained dolerite or anamesite of the cliffs. At the upper part of the cliff a detached portion (which, however, is probably only a part of the vein) fills up a cavity like a pot-hole, about a foot broad (fig. 8). It is traversed by numerous divisional planes, which run parallel with the sides of the cavity, and converge towards its bottom in such a way as to look


 * Hay Cunningham found liquid bitumen in cavities of this rock.

VOL. XXVII. — PART I. Y

Fig. 8. Pitchstone filling a cavity in Dolerite, Eigg.