450 PROF. J. W. JUDD AN"D G. A. J. COLE ON THE 



acccording to Klaproth, of 2-67 *. Mohl states that the hasalt-glass 

 of Sababurg has in its most vitreous condition a density of 2*68, 

 and in its less vitreous of 2*76 f . The density of the basalt-glass 

 of Bobenhausen is, according to the same author, 2-71 J, and that 

 of Ostheim 2*74 §•. Cohen || records the specific gravity of six 

 varieties from the Sandwich Islands, the average of which is 2*71. 



The basalt-glass of the Western Isles of Scotland appears to be 

 generally distinguished by its very high specific gravity. That of 

 the Beal, near Portree, is 2°72 ; that of Lamlash, 2-78 ; that of 

 Gribun, in Mull, 2*82 ; that of Screpidale, in Eaasay, 2-84 ; while 

 the basalt-glass of Some has a density of no less than 2*89. This 

 difference appears to be connected with peculiarities of chemical 

 composition to be hereafter noticed. 



Of somewhat analogous substances we may observe that the 

 tephrite-glass of Klein Priesen has a density, according to Boricky ^[, 

 of 2-65, the tephrite itself giving 2-696. The augite-andesite 

 glass of Beinn Shiant, Ardnamurchan, gives 2*62, and the probably 

 similar rock of Eskdale 2*7 **. 



4. Other Physical Properties. 



The colour of the freshly broken surfaces of the basalt- glass of 

 the Hebrides varies from velvet-black to pitch-black. The joint- 

 planes and all exposed surfaces, however, are covered with a 

 film of a grey or greenish-grey colour, occasionally passing into 

 brown. This surface-film, which may be due either to chemical 

 change or to molecular alteration of the exposed faces, appears to 

 be analogous to what is seen in the pitchstones of Ponza, which, 

 when freshly broken, are perfectly vitreous in appearance, but in a 

 few seconds become coated with a delicate film which impairs their 

 lustre ff. The beautiful iridescence seen on some surfaces of the 

 basalt-glass of the Beal, in Skye, is probably due to interference 

 produced by a thin film of the same kind. 



The lustre of the rock at the extreme edge of the dykes at the 

 Beal, Gribun, and Some is perfectly vitreous ; but it graduates to 

 resinous, horny, and glimmering in passing inwards into basalt. 

 The lustre of the Lamlash and Screpidale basalt-glass is never more 

 than resinous. 



The. hardness of basalt-glass appears to be not very different 

 from that of orthoclase, varying from 5*5 to 6*5. Though probably, 

 as a rule, softer than obsidian, the difference is not sufficient to 

 distinguish it readily from that rock. But, on the other hand, the 

 hardness of basalt-glass is so much greater than that of the pala- 



* Beitrage zur chemischen Kenntniss der Mineralkorper, vol. iy. p. 190. 



t Die Gesteine der Sababurg in Hessen, (Cassel) p. 40. 



X Ibid. 



§ Ibid. 



|| Neues Jahrbuch fur Min. &c. 1880, vol. ii. p. 41. 



*f[ Petrographiscke Studien an den Basaltgesteinen Bohmens, pp. 182 and 210. 



** A. Geikie, Proceedings of the Eoy. Phys. Soc. of Edinburgh, vol. v. p. 253. 



ft Geol. Mag. dec. 2, vol. ii. (1875) p. 308. 



