458 PROE. J. W. JTJDD AND G. A. J. COLE ON THE 



writers. These rocks are all found to consist of a glass, usually of 

 a brownish colour, but occasionally colourless or greenish, in which 

 various crystallites are distributed. 



Basalt-glass, even in its most vitreous varieties, is very rarely 

 clear and transparent. Usually opaque inclusions are so abundant 

 as to render the rock non-translucent. In most cases it is only by 

 the most careful grinding of slices of the rock that it is possible to 

 obtain sections sufficiently thin to exhibit its internal structure, and 

 it is necessary to employ the most powerful sub-stage illumination 

 to transmit light through them at all. 



In their great opacity the natural varieties of basalt-glass differ 

 very strikingly from the artificially fused basalts, such as Rowley 

 Rag. These artificial glasses are clear and of a rich yellowish- 

 brown colour by transmitted light; and only faint traces of a 

 " globulitic " structure can be made out in them with the highest 

 powers of the microscope. 



Some of the German varieties of basalt-glass, for example those 

 of Bobenhausen and the Sababurg, have spaces of a similar clear 

 brown glass, the crystallites being collected into skeleton crystals or 

 spherulites ; but in all the British varieties we have studied such 

 spaces of clear glass are rare, and all these rocks are characterized 

 by their great opacity. Indeed, in the most perfect specimens of 

 our natural basalt-glasses, devitrification appears to have gone so 

 far as to have resulted in the separation of the whole or nearly the 

 whole of the magnetite, the minute crystallites of which, scattered 

 through the rock, render it perfectly opaque, even in the thinnest 

 slices which it is possible to prepare. Fortunately this dust of 

 magnetite is occasionally collected into nebulous masses (cumulites), 

 and in the more devitrified varieties into skeleton crystals ; in such 

 cases the nature of the intervening glass spaces can be made 

 out. 



The structures found in these basic glasses appear to be quite 

 similar to those which have become so familiar to geologists from 

 the study of obsidians, pitchstones, and the glassy varieties of the 

 acid class of rocks. In some cases the crystallites and microliths 

 exhibit the parallel arrangement characteristic of the banded and 

 fluidal structures ; in other cases the crystallites are united to form 

 various kinds of skeleton crystals, or the globular and often con- 

 centric concretions known as spherulites. Sometimes the glass is 

 traversed by numerous fine cracks, some of which are curved and 

 concentric, giving rise to the well-known perlitic structure. Per- 

 litic basalt-glass has been already described by Prof. Zirkel from 

 Marostica in the Euganean Hills *, by Mr. Rutley from Slieven- 

 alargy, county Down t, and by Fouque and Levy from Graham's 

 Isle t 



The basalt-glass of the Western Isles of Scotland is usually, to a 



* Zeitschr. d. deutschen geol. Gesellschaft, vol. xix. p. 776 ; also Roseri- 

 busch, Neues Jahrb. fur Min. &c. 1872, p. 141. 

 t Journ. Koy. Geol. Soc. of Ireland, vol. iv. p. 227. 

 J Compte3 Eendus, 1878, March 25. 



