40 



middle, measuring 2|in. long by fin., narrowest diameter, 

 and fin. at the ends. Its sides are grooved vertically, and 

 what may be called its base is bounded all round by a sharp 

 edge. In both of these respects it resembles a type of button 

 found in West Australia. Its sp. gr. is 245. 



The occurrence of specimens both on the east and west 

 sides of the island makes their non-discovery hitherto in the 

 central portion remarkable. 



The specific gravity of the glass ranges in different speci- 

 mens from 2*45 to 2*47. This definitely excludes it from the 

 basaltic glasses (tachylyte, etc.). Tasmanian tachylyte, 

 which we have subjected to a specific gravity determination, 

 varies from 272 to 277. Perhaps the lightest basalt glass 

 recorded is that of the Siisebuhl, 2*50 to 2-54. The Icelandic 

 sideromelane is also very light, viz., 253. These are very 

 exceptional figures for the density of the vitreous form of 

 basalt, which in general probably averages near 27. We 

 have been asked whether the buttons in question may not 

 consist of tachylyte, but a glance at their specific gravity is 

 sufficient to dispel all doubt, irrespective of their micro- 

 scopical appearance, to which reference will be made directly. 

 The density, indeed, is that of obsidian. Under this term is 

 understood the glassy form of the acid and sub-acid rocks, 

 namely, rhyolite — and trachyte-glass, the specific gravity of 

 which varies between the extremes of 2 26 and 2'55. 

 Ehyolite glass is stated by Teall to reach a density as high 

 as 2-41*. 



Without a chemical analysis it is hazardous to say precisely 

 whether the Tasmanian glass is rhyolite— or trachyte-obsidian, 

 but that it is one of the two admits of no reasonable doubt. 

 Judging by the specific gravity, it is highly probable that it 

 belongs to the trachytic variety. 



Its microscopical characters may be summarised as follows : 

 —Transparent, colourless glass, puckered by irregularly 

 sweeping curves in its substance, the result of cooling, and 

 exerting a faint indefinite action on polarised light, indicative 

 of strain. Under a low power the section looks like clear 

 glass, but dusted with minute specks, which under a one- 

 eighth or a one-twelfth objective, are resolved into tiny gas 

 pores and globulitee. The latter are sometimes stringed 

 together in threes and fours, forming the linear groups called 

 margarites. These are crystallites, and have no reaction on 

 polarised light ; short crystallitic rods also occur. All these 

 incipient devitrification products are most numerous near the 

 margin of the button. The gas pores may be distinguished 

 by their dark refraction border, due to the difference between 

 the refractive index of the empty cavities, and that of the 



* British Petrography, p, 49. 



