462 , H. S. Washington — Rhyolites of Lipari. 



ian from Monte Arci in Sardinia and of one from the 

 island of Milos. As there are very few data on this 

 feature of volcanic glasses, and as Merwin's results show 

 some points of interest, it may be worth while to devote 

 a few words to discussion of the subject. The data 20 are 

 as follows : 



Obsidian, Bocche Eosse 1.488-9 



Obsidian, Forgia Vecchia 1.490 



Pumice, Monte Pelato 1.499 



Obsidian, Monte Arci 1.487-9 



Obsidian, Milos 1.490 



The refractive indices of the two Lipari obsidians are 

 almost identical, and that of the Monte Arci obsidian is 

 also almost the same ; this correspondence being quite in 

 accord with the very similar chemical compositions of the 

 three glasses. But the distinctly higher index of the 

 Lipari pumice glass is remarkable. As this contains 

 about three per cent of water, much more than do the 

 corresponding obsidians, it would be expected that the 

 pumice glass would have a lower refractive index, while 

 the contrary is true. It is difficult to account for this, 

 and the matter must await further study and the 

 accumulation of more data. 



Martelli 21 determined the refractive index of the 

 Nisyros obsidian glass as between 1.478 and 1.496. The 

 mean, 1.487, is very close to that of the Lipari obsidian. 

 Taking the difference in chemical composition into 

 account, the close approach in refractive indices may be 

 explained by the effect of the lime in raising and the 

 silica in lowering, the index. 



20 Wiilfing gives the refractive index of obsidian glass as 1.484-1.495 

 (Bosenbusch-Wiilnng, Mikros. Physiog., I, (1), Table I, 1905). 



21 A. Martelli, Mem. Soc. Ital. Sci. dei Quaranta, Some, 20, 66, 1917. 



Geophysical Laboratory, Carnegie Institution of Washington, 

 Washington, D. C, July, 1920. 



