Oht^idian Bomb'i in Austvalui. 31 



out that, while the low percentage of silica in No. 2 brings it 

 within the intermediate gi'oup, there is a total absence of soda, 

 which is certainly unusual, considering that the felspars of that 

 group are of the soda-lime type. It is to be regretted that so 

 little has been done in their chemical examination, because it is 

 quite possible that each occurrence may present features in 

 common, while difiei'ing from those of others. We should also 

 be able to ascertain whether any divergence from ordinary 

 obsidian can be established. Tested for fusibility, the edges and 

 corners of the fragments became rounded, and in thin splinters a 

 light-coloured glass was obtained, whereas basic glasses fuse 

 readily to a dark opaque glass. The range observed in the 

 specific gravity, although mostly inclining towards rhyolite, is 

 still high for rhyolite-glass, and is more consistent with a trachyte 

 or andesite glass, but the maximum obtained by Clarke is 

 undoubtedly that of a basic glass. 



He found them to vary from 2-42-2'7, Stelzner from 2-41-2-52, 

 Twelvetrees and Petterd from 2-45-2 -47 ; and from a number of 

 specimens from different parts of the colonies I obtained from 

 2-42-2-48. All these experiments were evidently made on whole 

 or fragmentary samples, so that, in some instances, the presence of 

 gas vesicles may have slightly influenced the result. Microscopic 

 examination proved it to be a pure glass, with at times scattered 

 vesicles, but no microliths were observed. Vertical and hori- 

 zontal sections cut from a button specimen revealed, even 

 without aid of the microscope, a peculiar structure. It consists 

 of a number of cloudy, narrow, more or less contorted bands at 

 places closely intermingling. Mr. A. W. Howitt, who was good 

 enough to examine these slides for me, says that under crossed 

 nicols they slightly admitted light, and may therefore be due to 

 strain. Mr. E. G. Hogg expressed the same opinion. Messrs. 

 Twelvetrees and Petterd also observed this structure, and 

 attribute the same cause to it. If it really represents the 

 presence of internal strain, it must have been produced by rapid 

 cooling, but obsidianites certainly do not exhibit any unusual 

 degree of brittleness ; in fact, rather the reverse for substances 

 of their nature. It is open to question whether this is a natural 

 structure or an artificial one produced in the preparation of the 

 section ; some of it is undoubtedly due to the latter cause. 



