534 Mr R. ALLAN on a Suite of Volcanic Rocks 



globules imbedded in the black matrix being as hard, and almost 

 as vitreous, as the obsidian itself. They present, in fact, a mine- 

 ral resembling pearlstone, each of the spheroids being composed 

 of a double series of rings, of two distinct hues, enclosing a cen- 

 tral point, of a lighter colour ; but equally, as in the last specimen, 

 presenting a radiated structure. This is very similar to the ap- 

 pearances familiar to us in the productions of glass-furnaces. 



No. 11. presents a variety of colour not often met with 

 among the obsidians of these islands. It is of a dark liver-brown, 

 streaked with black, and contains imbedded masses, which are of 

 a white colour, and have a pumaceous texture. 



No. 12. may be denominated pitchstone-lava. It has some- 

 what of the pearlstone structure, and presents a stratified aspect 

 parallel to the faces of the bed. Its colour is dark ash-grey, mixed 

 with some brownish tinges. It is devoid of the brilh'ant vitre- 

 ous aspect of the obsidian, and is more nearly allied to pitch- 

 stone, than to any other volcanic product. 



No. 13. This specimen was taken from a rock at some dis- 

 tance from the rest. It broke in slaty fragments, like some por- 

 tions of the pitchstone from Arran. Its texture is vitreous 

 throughout, its colour brown and mottled, bearing traces of the 

 globular structure described in some of the preceding speci- 

 mens ; while on its surface there is an iridescent tarnish, pecu- 

 liar, in so far as I remarked, to this variety. 



The specimens from Nos. 14. to 23, form a suite of the 

 pumice rocks, all occurring within the space of a few yards, at a 

 quarry situated high up on the hill of Campo Bianco, the great 

 deposit of this mineral at the northern extremity of the island. 

 No. 14. is so light and spongy it floats on water like froth. 

 Nos. 1 8. and 1 9. present the most delicate glassy texture, while 

 No. 20. is the variety usually exported as an article of commerce. 

 Its structure is somewhat fibrous, and its filaments have a pecu- 

 liarly silky aspect. In No. 22. the pumice reassumes a charac- 



