302 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [Apr. 5, 



like a mass of rather viscid pitch which had been spilt at the top of 

 the cliff, and had slowly flowed into and fiUed 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 pre- 

 vious vein. A little below it the main portion of the vein descends the 

 lower part of the cliff, and crosses the beach, from IST. 25° W. to S. 

 25° E., with an average width of 1^ 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 ferru- 

 ginous silicate arranged singly and in oblong tufts. The individual 

 hairs of each tuft are not feathered, like those of the Corriegills pitch- 

 stone 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 Eudh 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 micro- 

 scope 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 sepa- 

 rated 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 depen- 

 dent 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 sub- 

 stance, 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 



