BASALT-GLASS OF THE WESTEKN ISLES OF SCOTLAND. 455 



with a high percentage of silica and an unusually large proportion 

 of the alkalies. 



The basalt-glass of the Beal near Portree seems to be an illustra- 

 tion of this rule, as does the similar rock of Lamlash, Delesse's 

 analysis of which has been already cited. An analysis of the Beal 

 rock, made in Dr. Frankland's chemical laboratory, under the super- 

 intendence of Dr. Hodgkinson, F.I.C. &c, was as follows : — 



Silica 52-59 



Alumina 17*33 



Ferric Oxide 11-14 



Manganous Oxide 0-66 



Lime 6-47 



Magnesia 2*62 



Soda 4-24 



Potash 2-40 



Loss on ignition 3*27 



100-72 



Traces of copper and barium were also detected in this rock. 

 The high percentage of silica and the large proportion of the alkalies 

 are especially noteworthy in this analysis. 



But certain of the varieties of basalt-glass found in the Western 

 Isles of Scotland are especially interesting to the petrographer, as 

 showing that even the basalts with a low percentage of silica do 

 sometimes, though rarely, pass into the vitreous condition. 



We have already pointed out that the varieties of basalt-glass of 

 Gribun in Mull, Screpidale in Kaasay, and Some in Mull, differ 

 from those of the Beal in Skye and Lamlash near Arran by their 

 high specific gravity. The density of the three first-named rocks 

 is 2*82, 2-84, and 2-89 respectively, and that of the two last-named 

 2*72 and 2-78. All the foreign basalt-glasses recorded have 

 densities ranging from 2'5 to 2*7. It became therefore a matter of 

 interest for us to determine whether these exceptionally dense 

 basalt-glasses of the Western Isles of Scotland were not vitreous 

 forms of more basic basalts than had yet been found in this con- 

 dition. 



Mr. J. J. H. Teall, F.G.S., has kindly undertaken the partial 

 analysis of these interesting materials ; and his results are quite in 

 accordance with our anticipations. Mr. Teall's analyses show that 

 the basalt-glass of Gribun in Mull contains 50*51 per cent, of silica 

 and 10-05 of metallic iron, the basalt-glass of Screpidale in llaasay 

 contains 46-68 per cent, of silica and 10-80 of metallic iron, and 

 that of Some in Mull 47*46 of silica and 12*47 of iron. It is 

 evident that the exceptionally high specific gravity of the last- 

 mentioned rock is due to the large quantity of the oxides of iron 

 which it contains. 



The only basalt-glass which has been hitherto described with a 

 composition approaching that of the Scottish rocks is the siderome- 



2x2 



