448 H.' S. Washington — Rhyolites of Lipari. 



and some specimens are almost wholly gray. These 

 gray varieties are so common and so distinctive that 

 they might be worthy of being considered as a distinct 

 type, were a detailed stndy of obsidians being under- 

 taken. Lithophysae and sphernlites occur, in some 

 places rather abundantly, though generally they are not 

 common. The fayalite from cavities in these obsidians 

 has been described by Iddings and Penfield. 9 



The refractive indices were determined by Dr. Merwin, 

 who found, for the obsidian of Rocche Rosse, n = 1.488-9, 

 and for that of Forgia Vecchia, n = 1.490. 



Microscopic characters. — In thin section the black por- 

 tions of the specimens show a perfectly clear and color- 

 less glass, with no perlitic cracks. In this are sparsely 

 scattered very minute microlites, some of them prismoids 

 and some irregular grains. Under low powers the latter 

 appear black, but higher powers show that they are color- 

 less; both appear to be slightly birefringent. It is 

 assumed that the prisms are of orthoclase and the grains 

 of quartz. It is perhaps worthy of note that these micro- 

 lites are not as thickly scattered as would be expected 

 from the blackness of the hand specimen. Flow structure 

 is not marked. 



In thin section the gray streaks show very many 

 yellowish spherules, which appear white (by reflected 

 light) between crossed nicols. Dr. Merwin suggests that 

 this effect is due to diffraction. The glass in one of the 

 spherulitic streaks (from Forgia Vecchia) is thickly 

 crowded with extremely minute prismoids of a colorless 

 mineral, of low refractive index, which is assumed to be 

 orthoclase. 



The pumice of Lipari may be mentioned with the 

 obsidians, as it is but a highly vesicular form of these. 

 Most of this comes from Monti Chirica and Pelato, with 

 some from the Fossa delle Rocche Rosse; the better 

 qualities being found mostly at Monte Pelato. In the 

 store-houses at Canneto it is separated into three main 

 grades. The first quality is the most finely vesicular, 

 the whitest, and the lightest in weight ; the lower grades 

 showing more of the non-vesicular glass. The first grade 

 floats readily in water ; the second floats, but less readily ; 



9 This Journal, (3), 40, 75, 1890. Similar crystals of fayalite from the 

 trachyte of Cuma, in the Phlegrean Fields, have been described by vom 

 Eath (Zs. d. geol. Ges., 18, 609, 1866). The Cuma crystals are tabular paral- 

 lel to b (010), while those of Forgia Vecchia are tabular parallel to a (100). 



