mS Ernest W. Hkeat.^ : 



volcanic rocks of Victoria. Until it has been found " in situ," and 

 its relation to the basaltic rocks of the district has been determined 

 it would be premature to give a detailed discussion of its characters. 



A glance at the analysis, however, shows at once that it is not 

 ■obsidian, as the relatively low silica percentage and the six per cent, 

 of alkaline earths preclude this possibility. On the other hand it is 

 clearly not a basalt in view of the high silica percentage, and the 

 fact that the total alkalies exceed 7 per cent. 



Through Mr. Herman, Director of tlic (Geological Survey of 

 Victoria, I obtained a small fragment of the material, from which 

 the analysis was made, and had two rock sections made. The rock 

 is clearly a volcanic glass and contains besides brown glass, globu- 

 lites, trichites and scattered phenocrysts of olivine, augite and 

 plagioclase felspar. 



The minerals present show affinities with the basalts, but the 

 large amount of glass is evidently high in silica and the alkalies. 



It has long been known that in a molten basic magma in which 

 crystals are floating the composition of the molten ground mass is 

 generally more acid than that of the phenocrysts or of the whole 

 rock. In Teall's British Petrography, pp. .399-401, analyses by 

 Lagorio are quoted which show this effect, which is especially 

 marked in rocks of intermediate composition and less marked in 

 basic rocks. It is interesting to note that the glass of such rocks is 

 richer in silica and in the alkalies than is the parent rock. It is 

 this richness in silica and the alkalies which makes the Taradale 

 rock interesting, and suggests that it may represent the glass from 

 a basaltic magma from which the bulk of the phenocrysts are want- 

 ing. 



The closest approach to the composition of the Taradale rock i.s 

 an analysis of a diorite-porphyry from Montana, quoted above. 



The norms of the Taradale rock and of the Montana loek. and 

 of the Uralla australite are as follow : — 



