462 PEOP. J. W. JTJDD AND G. A. J. COLE ON THE 



and distinctive of all its characters, however, is its easy fusibility 

 and the nature of the product resulting from its fusion. 



The hardness of basalt-glass is perhaps generally rather less than 

 that of the obsidians ; but the difference is not sufficiently great to 

 afford a ready means of distinction between these two types of rocks, 

 though it may serve to distinguish basalt-glass from the hydrated 

 substances and altered forms known as palagonite, which are some- 

 times confounded with it. 



In chemical composition basalt-glass agrees with the rock, of 

 which it is, in all cases, merely a local variation. The proportion 

 of silica varies from 45 to 55 per cent., just as in the basalts ; but 

 those forms of basalt with the higher proportion of silica appear 

 most frequently and most readily to assume the vitreous condition. 

 The varieties of basalt containing an exceptionally large quantity 

 of silicates of the alkalies seem also to pass more easily into glass 

 than any other. 



In their microscopic character the basalt-glasses appear to be 

 generally distinguished by their great opacity. When cut into sec- 

 tions sufficiently thin to be transparent, the abundance of crystallites 

 and skeleton crystals of magnetite serves at once to distinguish 

 them from the obsidians. Like other vitreous rocks, they frequently 

 exhibit the porphyritic, the pumiceous, the banded, the fluidal, the 

 spherulitic, and the perlitic structures. 



In the Western Isles of Scotland basalt-glass has only been found 

 as a selvage to dykes of basalt. In other districts, however, it has 

 been observed in various situations where rapid cooling has taken 

 place, as in fragments ejected from volcanic vents, and the surfaces 

 of basaltic lava-streams. 



[Note, July 12, 1883. — During the discussion on the foregoing 

 paper, and subsequently, our attention has been directed to several 

 examples of similar materials occurring in different parts of the 

 Western Isles of Scotland. Professor T. G. Bonney, F.E.S., has very 

 kindly placed in our hands, for the purpose of study, a glassy mate- 

 rial forming a selvage less than half an inch in width to a dyke 

 which is seen near the stable in the Castle grounds at Brodick in 

 the island of Arran. This appears to be a true basalt-glass ; it has 

 a specific gravity of 2*83, and a silica percentage of 53'96. In its 

 microscopical characters the rock very closely resembles the basalt- 

 glass of the Beal in Skye. Treated in the same way as the other 

 specimens, 83-69 per cent, of the rock was dissolved in hydrochloric 

 acid. It is very fusible, and its powder is strongly magnetic. 



At the time when our paper was written we had not seen Pro- 

 fessor Heddle's analysis of and notes on a Tachylyte from the 

 Qniraing in Skye (Min. Mag. vol. v. p. 8). In an erratum to this 

 article, published with Part 23 of the Mm. Mag., the same author 

 calls attention to a very interesting note on the basalt-glass of the 

 Beal in Skye, published by Necker in 1840 (Edinb. Phil. Journ. 

 2nd ser. vol. xxix.), wherein the true nature and properties of the 

 material are very clearly defined. 



