336 Ernest W. Skeats : 



By the courtesy of Professor Spencer I was enabled to make an 

 examination of the two specimens, Nos. 24 and 24a. There are 

 two specimens of No. 24. The smaller is partly scoriaceous, partly 

 dense, and is a dark grey-green in colour. 



The larger specimen is similar to the smaller, but shows a 

 sporadic development of spherical scoriaceous areas constituting 

 pseudo-spherulites. The rest of the material is a dense glass. 



No. 24a is dense and mostly dark blue in colour. Some black 

 spherulitic areas occur with cracks, filled with ])i-own limonite. 



Both 24 and 24a have all the appearances characteristic of tachy- 

 lyte, as Mi-. Walcott has previously described. I determined the 

 specific gravity of the two pieces of No. 24 by Walker's balance. 

 The small piece gave a value of 2.36. 



The specific gravity is, of course, quite unreliable, as the speci- 

 men is very vesicular. The true specific gravity would be much 

 higher. 



The larger piece of 24 gave a value of 2.50. 



This specimen is also vesicular, so the result is also too low. A 

 small chip from the larger specimen of No. 24 Avas fairly compact, 

 but contained some vesicles. By the Joly's spring l)alance the 

 specific gravity of the chip was determined at 2.60. 



We may conclude that the true specific gravity exceeds this latter 

 figure. The glass must therefore be basic in composition, i.e., the 

 specimen is tachylyte. This is in agreement with Mr. Walcott's 

 silica deteiinination of 5'3.2 % 



Specimen No. 24a (" Blue obsidian ") is larger and quite com- 

 pact. By Walker's balance the specific gravity is 2.74. This also 

 indicates clearly that the specimen is tachylyte. I am quite at a 

 loss to explain how Mr. Cosmo Newbery could have obtained the 

 results quoted for the chemical analysis of these specimens or the 

 figures for their specific gravities (No. 24 = 2.41, No. 24a = 2.36). 

 It is clear, however, from Mr. Walcott's Avork and my own deter- 

 minations of specific gravity that ])oth specimens are tacliylyte and 

 not obsidian, as described. 



Further, it is uiif<u-tunately cleai- tlint [\w oldor clu'itiical analyses 

 of rocks, etc., published by the Cieological Survey of Victoi-ia, must 

 be regarded as quite untrustworthy. 



This criticism must include in tliis ((.nncrtion not only the an- 

 alysis of the " obsidian " from Geelong. but also tlie analysis of 

 Specimen No. 21, an australite from Horsham, in which less than 

 5 per cent, of alumina is recorded, and over 10 per cent, of 



