Olmdidvitea. 437 



a certain amount of provincial distribution of the obsidianites. 

 It will be seen that tlie ave.rai.e .specitic gravity of the Hainilton 

 specimens is 2.40t), an<l Mr. Simpson gives 2.448 as the average 

 S])ecific gravity of the s{)ecimens determined by him. Judging 

 from the above values, it seems that the Lake Eyre District 

 and Western Victoria arc characterised by a more acid type of 

 obsidianite than Kalgoorlie District. 



For convenience in recording the distribution of the different 

 types, I would suggest the following divisions, according to 

 specific gravity : — 



A.— Under 2.390. Peake Station type. Analysis No. III. 



B.— 2.391— 2.410. Hamilton type. Analysis No. II. 



C— 2.411— 2.440. Mt. Elephant type.i Analysis No. I. 



D. — 2.441—2.470. Kalgoorlie type. Analyses Nos. IV., V., 

 VI., VIII. 



E.— Over 2.470. (?) type." No analysis. 



The gravities recorded in this paper give the following re- 

 sults : — • 



Hamilton 6B. 



Balmoral lA, 3B, IC, ID. 



Peake Station 3A, IB, 2C. 



Kalgoorlie 2C, 3D, IE. 



I hope in time to collect a sufficient number of records of 

 specific gravities to thoroughly test this apparent distribution 

 according to chemical composition. 



Judging from records taken from Mr. Walcott's paper, viz., 

 Clarke, 2.42—2.7 ; Stelzner. 2.41—2.52 ; Twelvetrees and 

 Petterd, 2.45—2.47 ; and Walcott, 2.42—2.48, it seems that 

 the Peake Station and Hamilton types are rarely met with, 

 except in the type localities. As, however, these are the pre- 

 vailing types about Lake Eyre and part of Western Victoria, 

 it appears that we certainly have two areas on which the more 

 acid type fell most abundantly, whereas about Kalgoorlie no- 

 thing but the more basic types are recorded. 



1 The Mt. Elephant analysis is taken as the standard as it is inoie niiinial than the 

 Pieman analysis. 



2 The Uralla specinieii may belong here, l>ut unfortunately no specific ^^i-avity is. 

 recorded. 



