1886.] Alteration hy Heat in certain Vitreous Rocks. 437 



The specimen had, however, quite lost its vitreous lustre, and had 

 changed from black to a pale brown colour. Another fragment of 

 basalt-glass from Kilauea, very vesicular, but less so than the previous 

 sample, was kept at a high temperature, about 750° to 1200° C, for a 

 period of 960 hours. 



The effect of this heating has been to completely destroy all glassy 

 lustre. The specimen is still vesicular, but the colour has, like that 

 of the Mokua specimen, changed from black to purplish-grey or light 

 brown. 



Fig. 1, Plate 5, shows the general character of a section of the 

 unaltered basalt-glass of Kilauea, cut from the same specimen as that 

 submitted to the furnace. The drawing, made under an amplification 

 of 25 linear, shows portions of three vesicles, several crystals of 

 olivine, some small spherulites, and numerous crystallites, in a clear 

 brown glass. The minute crystallites in this lava are extremely 

 beautiful, and especially worthy of careful study. They frequently 

 assume delicate pectinate forms, which one would think quite as 

 likely to be preserved or to be re-formed after the fiery ordeal as the 

 little trichites in the obsidian previously described. A glance, how- 

 ever, at the section prepared from the altered specimen at once dispels 

 all hope of seeing them again, or indeed of seeing anything which is 

 not translucent, and does not occupy the entire thickness of the thin 

 section which, in spite of its thinness, appears of a deep brownish- 

 black and perfectly opaque. When examined by reflected light under 

 a power giving about 30 diameters it is then seen to be of a deep 

 reddish-brown colour with paler streaks, often cracks, which circle 

 round the porphyrinic crystals of olivine, which are still present and 

 seem to have undergone little or no change (fig. 2, Plate 5). These 

 cracks represent perlitic structure, evidently due to unequal tension in 

 the glass surrounding the olivine crystals. The opacity of the altered 

 rock must probably be attributed to the passage of some of the prot- 

 oxide of iron in the normal rock into the state of peroxide, thus 

 giving rise to the formation of magnetite. The normal rock is 

 scarcely, if at all, magnetic, but the altered specimen causes a strong 

 deflection of the needle. 



This passage of a clear basic glass into a perfectly opaque substance 

 is a point of considerable interest, since it seems highly probable that 

 some of the opaque and extremely scoriaceous lapilli in certain 

 volcanic tuffs of palaeozoic age are simply fragments of basic glassy 

 lavas, like those of the Sandwich Islands, which have undergone a 

 change similar to that here described. That in course of time the 

 magnetite in some of these ancient lapilli should pass into limonite is 

 also a point worth bearing in mind. 



It seems probable that some of the opaque lapilli and dust in the 

 volcanic ejectamenta of Snowdon and Brent Tor, especially those of 



