306 W. F. SMEETH. 



A PERLITIO PITCHSTONE from the TWEED RIVER, 



NEW SOUTH WALES, with remarks on the so-called 



Perlitic Structure in Quartz. 



By W. F. Smeeth, m.a., a.r.s.m. 



Demonstrator in Geology and Lecturer in Metallurgy, University 



of Sydney. 

 [With Plates XLIV. - XLVL] 



[Read before the Royal Society of N. S. Wales, November 7, 1894.~] 



( The specimens from which this rock is described were brought 

 to Mr. G. W. Card, a.r.s.m. of the Mines Department, Sydney, 

 by a prospector under the impression that the phenocrysts of 

 yellow quartz were tin-stone. Mr. Card drew my attention to 

 to them, and, as they appeared to be of an interesting character, 

 we decided to publish conjointly an account of them. I regret 

 that Mr. Card has been unable to find time to carry out this 

 arrangement, and so the task of describing the specimens has 

 devolved upon me 



Macroscopical Characters. 

 In external appearance the rock presents a black glassy matrix 

 uniformly studded with numerous phenocrysts of quartz and 

 felspar averaging two to three mms. in diameter. The ferro- 

 magnesian constituent is only apparent under magnification. The 

 glassy matrix is black (colourless in thin sections), with a semi- 

 vitreous lustre and a conchoidal fracture somewhat interrupted 

 by the perlites which stand out on a fractured surface. The 

 perlitic structure is remarkably well developed, and is best seen 

 on a finely ground surface of the rock, over which a glass slip has 

 been cemented with Canada balsam. The cracks appear as light 

 coloured lines on a black ground, the lighter material filling the 

 cracks being transparent and of about the same refractive index 

 as the surrounding glass, so that in thin sections the perfection of 

 the structure is not so distinctly evident. 



