54 



NOTES ON A SPECIMEN OF BASALT-GLASS (TACHY- 

 LYTE) FROM NEAR MACQUARIE PLAINS 

 TASMANIA, WITH REMARKS ON OBSIDIAN 

 " BUTTONS." 



By T. Stephens, M.A., F.G.S. 



Read September 14, 1897. 



The absence of any reference iu the records of this Society 

 to the class of volcanic product? to which basalt-glass belongs 

 may justify a brief account of the history and character of 

 the specimen now exhibited. It was originally about two 

 and a half inches long and two inches thick, irregularly 

 angular in shape. The colour is a dark olive-green, but a 

 weathered face is bright blue ; the fracture is conchoidal. 



This specimen was obtained by me many years ago from a 

 heap of basalt and scoriae, excavated from a temporary road- 

 cutting on the right bank of the Derwent between Macquarie 

 Plains and Fenton Forest. Near the same spot I found, on 

 a block of basalt, thin bands of a dull black glass, bearing 

 some resemblance to the glassy selvages of basalt dykes, 

 which have often been described as constituting the chief 

 sources of obsidian and basalt-glass. The basalt of this part 

 of the district is probably of late tertiary age. 



In the early days of geology, the terms " obsidian," 

 " pitchstone,"" volcanic glass," etc., were often used indiscrimi- 

 nately for different varieties of glassy lava without reference 

 to the essential conditions of the rocks of which they formed 

 a part. With the advance of the science of petrology it soon 

 came to be recognised that they may be divided into two 

 principal classes, representing respectively the acidic and 

 basic types of volcanic rocks. These are readily distin- 

 guished by chemical analysis, and by the test of their density 

 or specific gravity ; but a further test of structure was 

 required, and this has of late years been admirably supplied 

 by microscopical examination, without which no test of a 

 volcanic rock is now considered complete. The name 

 "obsidian" is now restricted to glassy varieties of rhyolitic 

 and trachytic rocks, while basalt- glass is usually classed as 

 tachylyte — a term first introduced by Breithaupt in 1826, in- 

 dicating its ready fusibility before the blowpipe. 



The specimen from Macquarie Plains has this quality, 

 and it is distinctly magnetic when pulverised, another charac- 

 teristic of tachylyte. A small piece was sent to Mr. Twelve- 

 trees for comparison with the specimens of obsidian which 

 have been lately examined in Launceston, and he reports the 

 specific gravity as 2-74, while the maximum for obsidian was 



