A SUMMER RAMBLE AMONG THE HEBRIDES. 21 



them from the boat, was, that the inclosing mass was a pitch 

 cauldron, rather of the roughest and largest, and much be- 

 grimed by soot, that had cracked to the heat, and that the 

 fluid pitch was forcing its way outward through the rents. 

 The veins expand and contract, here diminishing to a strip 

 a few inches across, there widening into a comparatively broad 

 belt some two or three feet over ; and, as well described by 

 M'Culloch, we find the inclosed pitch-stone changing in co- 

 lour, and assuming a lighter or darker hue, as it nears the edge 

 or recedes from it. In the centre it is of a dull olive green, 

 passing gradually into blue, which in turn deepens into black ; 

 and it is exactly at the point of contact with the earthy amyg- 

 daloid that the black is most intense, and the fracture of the 

 stone glassiest and brightest I was lucky enough to detach 

 a specimen, which, though scarce four inches across, exhibits 

 the three colours characteristic of the vein, its bar of olive 

 green on the one side, of intense black on the other, and of 

 blue, like that of imperfectly fused bottle-glass, in the centre. 

 This curious rock, so nearly akin in composition and ap- 

 pearance to obsidian, a mineral which, in its dense form, 

 closely resembles the coarse dark-coloured glass of which com- 

 mon bottles are made, and which, in its lighter form, exists 

 as pumice, constitutes one of the links that connect the trap 

 with the unequivocally volcanic rocks. The one mineral may 

 be seen beside smoking crater, as in the Lipari Isles, passing 

 into pumice ; while the other may be converted into a sub- 

 stance almost identical with pumice by the chemist. " It is 

 stated by the Honourable George Knox of Dublin," says Mr 

 Robert Allan, in his valuable mineralogical work, " that the 

 pitchstone of Newry, on being exposed to a high tempera- 

 ture, loses its bitumen and water, and is converted into a light 

 substance in every respect resembling pumice. " But of pumice 

 in connection with the pitchstones of Eigg, more anon. 

 Leaving our boat to return to the Betsey at John Stewart's 



