444 PEOF. J. W. JUDD AND G. A. J. COLE ON THE 



24. On the Basalt- Glass (Tachylyte) of the Westeen Isles of 

 Scotland. By Prof. John W. Jttdd, F.R.S., Sec. G.S., and 

 Geenville A. J". Cole, Esq., F.G.S. (Eead May 23, 1883.) 



[Plates XIII. & XIV.] 



In a previous paper * it has been pointed out that the Tertiary vol- 

 canic rocks of the Western Isles of Scotland offer beautiful examples 

 of materials of every variety of composition, from the most acid to 

 the most basic, and of every type of structure, from the holocrystal- 

 line to the vitreous. The detailed description of these varieties of 

 volcanic rocks was reserved for a future occasion ; and in the present 

 paper we propose to give the first of such a series of descriptions. 

 As the more acid vitreous rocks have during recent years been dis- 

 cussed in considerable detail in numerous papers read before this 

 Society, it may not be inappropriate to direct attention to the rare 

 but equally interesting glasses of basic composition, which have, up 

 to the present time, received far less notice in this place. The 

 studies on which this paper is based have been carried on in the 

 Geological Laboratory of the Normal School of Science and Eoyal 

 School of Mines. 



1. History of Previous Opinion on the Subject. 



By the older writers on petrography rocks of the kind of which 

 we now treat appear to have been classed as " pitchstones." Jame- 

 son and the other followers of Werner, who endeavoured to intro- 

 duce the precision of nomenclature and the exact methods of their 

 master into the study of British rocks — though they recognized these 

 materials, not as minerals but as rocks — do not seem to have dis- 

 cerned the difference between the acid and basic varieties. Mac- 

 culloch, who may be justly regarded as the father of British petro- 

 graphy, as early as 1819 pointed out that, though basalt occasionally 

 passes into glass, yet examples of such a transition are exceedingly 

 raref . He particularly records two such cases in the Western 

 Isles of Scotland. One of these instances of a transition from 

 basalt into " pitchstone " is given as occurring in the Isle of Lamlash 

 (Holy Isle), near Arran, and is illustrated by a detailed description 

 and drawing J. The other example is at Garbsbeinn, in Skye ; but 

 the specimen described was not found insitu§. 



In 1827 Sedgwick and Murchison found a basalt dyke at the 

 Beal near Portree, in Skye, the sides of which are seen passing into 



* Quart. Journ. Geol. Soc. vol. xxx. p. 233. 

 t Western Islands of Scotland, vol. i. p. 402. 

 X Ibid. vol. ii. p. 437. 

 § Ibid. vol. i. p. 402. 



