32 Proceedi ii<ji:i of the Royal Society of Victoria. 



Some confusion has existed in the part between the occurrence 

 of obsidian and tachylyte in Victoria, and in some of the 

 mineral catalogues localities are given for the former when it 

 should unquestionably have been the latter. In the 1866 

 catalogue already referred to, two other analyses are given of 

 obsidian from near Geeloiig, which show respectively 72-23 and 

 6845 per cent, silica. Professor Ulrich informs nie that 

 Daintree also analysed some volcanic glasses from that neigh- 

 bourhood and found them to vary in silica from 50 to 70 per 

 cent. As natural glasses are only forms of lava which have 

 cooled rapidly, they must in composition be identical with the 

 devitrified or crystallised forms with which they are associated, 

 and from the amount of silica shown by their analyses we would 

 at least expect to find trachyte in that part, but instead of this 

 being so it is stated that they occur in a basalt quarry in patches 

 and irregular veins of an inch or more in thickness, of generally 

 a black to brown, sometimes a bluish-grey colour. The specimens 

 marked 24 and 24(7 are in the National Museum Collection of 

 Victorian Minerals, and it is from a portion of these that the 

 analyses were made. No. 24 is opaque and of a dark grey 

 colour. It contains peculiar spherules of a deyitrified vesicular 

 character, the glass itself being quite homogeneous and fi'ee from 

 vesicles. It is readily fusible to a rather frothy glass. By 

 analysis I obtained 53-2 per cent, of silica, and this, with its 

 easy fusibility and other characteristics, should place it within 

 the tachylyte group. Mr. O. R. Rule informs me that there 

 was only a small quantity of this variety found, although he 

 had spent much time in search of it. I think I might therefore 

 be justified in saying that, so far as we know at the present 

 time, with the exception of obsidianites, acid volcanic glass, or 

 obsidian, does not exist in Victoria. 



Olisidianites may be briefly described as small bodies of dense 

 obsidian, of regular but varying form, which are found in 

 alluvial drifts and scattered over vax'ious parts of Australia and 

 Tasmania. The largest I have seen of the spherical ones 

 measures 59 mm. in its greatest diameter, and of those with an 

 elongated form, one in the Warrnambool Museum measures 90 

 mm. in length. They are, however, mostly of a smaller size. The 

 most characteristic form is that bearing a marked resemblance to 



