UNUSUAL RHYOLITE FROM THE BLACKALL RANGE. 



207 



The chemical composition of the c^uartzites has been 

 regarded by some as being against an igneous origin, especially 

 on account of the high silica, low alumina, and low alkalies. 



If one compares the two analyses of the quartzites with 

 that of the rhyolite, the only respect in which they differ is in 

 the lime and alkali content ; but in one case — that of the 

 quartzite from the Municipal Quarry — there is really very 

 little difference at all. 



A comparison of the norms of the rhyolite and this latter 

 quartzite is very interesting. 



The quartz, orthoclase, albite-anorthite, diopside-hypers- 

 thene, ilmenite, and apatite percentages are very similar. 

 The rhyolite has almost all of its albite-anorthite material as 

 the soda felspar, while in the quartzite the lime felspar is 

 relatively more abundant. 



The norm of the quartzite selected — from the Municipal 

 Quarry — is much closer to the rhyohte than the other quartzite 

 quoted in the analyses, but at least one may say that it is not 

 unreasonable on the evidence of the analyses to postulate an 

 orignial igneous origin for the Broken Hill quartzites. 



In conclusion, I would like to express my best thanks and 

 apprecirit'on to Mr. J. C. Briinnich, F.C.S. (the Agr-cultural 

 Chemist, Brisbane), for granting facilities for obtaining the 

 chemical analysis of the rhyolite, and particularly to Mr. 

 G. R. Patten for carrying out the same. 



