1871.] 



GEIKIE TERTIARY VOLCANIC ROCKS, 



301 



runs across the dolerite beds with a trend from N. 10" W. Descending 

 the upper part of the cliif, the vein continues downward ; but the por- 

 phyry 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 por- 

 phyry. The pitchstone suggests the idea of intense liquidity, as it 

 seems to have thrust itseK 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 duU 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 

 Fig. 8. Pitchatone filling a cavity vein just described a second vein 



in Dolerite, Eigg. 



of pitchstone is seen traversing the 

 compact close-grained dolerite or 

 anamesite of the cliffs. At the up- 

 per part of the cliff a detached 

 portion (which, however, is pro- 

 bably 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 Cuninngharn found liquid bitumen in cavitie? of this voek. 

 VOL. XXVII. — rARX I. Y 



