BASALT-GLASS OE THE WESTEKN ISLES OF SCOTLAND. 457 



Silica 51-71 



Alumina 11-44 



Oxides of iron 12-62 



Lime 10*75 



Magnesia 7*59 



Potash 0-67 



Soda 3-47 



Water 0-98 



This result is in such close agreement with the average composition 

 of common or felspar basalts that we cannot hesitate to regard 

 these lavas of the Sandwich Islands as referable to " basalt-glass." 



Another very interesting discovery of Cohen's is the fact that a 

 lava of Niuafou in the Friendly Islands has a glassy structure, and 

 a composition almost identical with that of the Sandwich-Island 

 lavas *. While, then, basalt-glass is, on the European continent and 

 in North America, confined to small and local occurrences, as the 

 selvages of dykes, the small fragments thrown out of volcanic vents 

 and cooled rapidly in passing through the air, &c, we have in the 

 Sandwich Islands, and perhaps also in the Friendly Islands, ex- 

 amples of glass of the same composition as basalt constituting the 

 whole mass of lava-streams. 



The basalt-glass of the Pacific Islands differs, however, from that 

 of Europe by being clear and transparent. In this respect it agrees 

 with the fused Rowley Rag and similar artificial products, in which 

 the iron is united with the silica, and is not separated in the form 

 of magnetite, the reverse being so very commonly the case in the 

 locally-developed basalt-glass of Europe. 



The lavas of the Sandwich Islands appear to be no less peculiar 

 and exceptional in their behaviour during ejection. Their extreme 

 fluidity permits of their being thrown into fountains 400 or 500 feet 

 in height (a circumstance nowhere witnessed in the case of ordinary 

 basalts), and of their being drawn out by the wind into the delicate 

 threads known as Pele's hair. 



All these facts point to the conclusion that the Hawaiian lavas 

 are ejected at a much higher temperature than that at which the 

 basalts of Europe and North America issued. The only mineral, 

 indeed, in the Hawaiian glassy lavas which is not fused appears 

 to be olivine. In this peculiarity of their lavas the great Pacific 

 volcanos present another and most interesting exceptional feature 

 in addition to many which have been already pointed out. 



7. Microscopical Characters. 



The microscopical characters of many varieties of basalt-glass 

 have been described by Zirkelf, Vogelsang J, Mohl§, and other 



* Neues Jahrb. fur Min. &c. 1880, vol. ii. pp. 36 and 41. 

 t Untersuchungen iiber die mikroskopische Zusainmensetzung der Baealt- 

 gesteine, p. 182. 



j Die Krystalliten, p. 111. 



§ Die Gresteine der Sababurg in Heasen, (Oassel) 1871. 



