314 Transactions. — Geology. 



and there are to be seen quite pure and free from microliths. 

 Octahedral crystals of magnetite occur in the grey spots, the 

 glass being, as it were, cleared and bleached around the magnetite, 

 suggesting that the bleaching of the glass in spots is due to the 

 withdrawal of the iron oxide from an originally brown glass. 

 In this ground-mass are embedded a small number of crystals 

 of plagioclase, some sanidines, augite, and magnetite. These 

 secretions, however, form but a small proportion of the whole 

 mass of the rock. Eeduced to powder, and digested with hydro- 

 chloric acid, the rock yields a quantity of iron in solution, but 

 is otherwise only imperfectly decomposed. Eosenbusch has 

 divided the basic glassy rocks into tachylite soluble in acids, and 

 zyalomelane insoluble in acids. The present variety will there- 

 fore be a zyalomelane, and not a normal tachylite. 



Several varieties of glassy lava are found in the pumiceous 

 tuff on Macaulay Island. Amongst these is a pitch-stone, much 

 resembling the one above, but richer in microliths. It shows 

 a yellowish-brown glass, in which are colourless spots : in the 

 coloured glass the pellucid microliths are fairly abundant, but 

 the colourless spots consist of dense nests of microliths and 

 magnetite dust. In other specimens the devitrification has 

 extended further, so that the nests of microliths join with 

 neighbouring patches, giving the rock a mottled or cloudy 

 appearance, and finally converting it into a grey mass of felted 

 microliths and magnetite dust imbued with glass. 



In the recent crater of Sunday Island were found masses of 

 finely-porous rock of a dark-brown colour. The rock is almost 

 porous enough to be called a pumice, and under the microscope 

 shows a brown glass with very numerous steam cavities. The 

 glass contains vast numbers of slender pellucid microliths, the 

 largest of which polarise, and a few minute magnetites. The 

 larger crystals are very few in number, and are like those in the 

 last mentioned rock. 



Another kind of glassy rock from the pumiceous tuff of 

 Macaulay Island has the full vitreous lustre and black colour of 

 a true obsidian. It shows, however, none of the conchoidal 

 fractures of the typical obsidian related to acid rocks, but breaks 

 into irregular little pieces with uneven surface. Towards the 

 edge this glass becomes porous, and passes into a brownish-grey 

 pumice. Microscopic sections show a glass which is yellowish- 

 brown even in their section. This glass is nearly pure, con- 

 taining only a small number of pellucid microliths of felspar, and 

 probably augite. Some crystals of felspar (both plagioclase and 

 sanidine), augite, and magnetite are embedded in the glass, as 

 well as slender columns, which are rather strongly dichroic in 

 longitudinal section, and appear to be hypersthene. Specimens 

 of a pale-reddish or yollowish-grey pumice from the recent 

 crater of Sunday Island are the frothy condition of a volcanic 



