CARBONIFEROUS AND PERMO-CARBONIFEROUS ROCKS, N.S.W. 289 



The minerals visible microscopically in the rhyolites are 

 as a rule quartz, albite, biotite, and a little ilmenite; some- 

 times quartz is absent except in the groundmass. The 

 felspar is as a rule much decomposed, but lias been gener- 

 ally determined as fairly pure albite. Some of the rocks 

 appear quite devoid of phenocrystic orthoclase, in others 

 its presence is doubtful, while in others again it is about 

 equal to albite in amount. 



The most common ferromagnesian mineral is biotite, 

 generally a good deal altered; in one slide, however, from 

 the Mount Gilmore section hornblende has been recognised 

 in amount comparable with biotite. No soda pyroxenes or 

 amphiboles have been detected. 



The groundmass is generally cryptocrystalline, often 

 spherulitic or axiolitic, but sometimes partly glassy, and 

 traces of flow-structure are frequent. 



Two examples of tuffaceous types have been examined, 

 one from a mass interbedded in the. Burindi Series, and 

 the other from the Mount Gilmore section. In hand speci- 

 men the latter is a very dense felsitic-looking rock of a 

 greenish-grey colour with inconspicuous phenocrysts of 

 felspar and little or no quartz. Microscopically the only 

 recognisable phenocrysts in these rocks are albite and 

 ilmenite with very little quartz. The groundmass is very 

 full of little cuspate fragments of devitrified pumice, 

 with glass, secondary quartz and felsitic material. 



Brogger's name Dellenite may perhaps be employed to 

 describe a rock associated with rhyolite on the east side 

 of the Williams River, Seaham. In this orthoclase is 

 present in notable proportion, but not to such an extent as 

 the plagioclase, which appears to be oligoclase. The rock 

 is otherwise indistinguishable from rhyolite, and may be 

 regarded as a type transitional between rhyolite and 

 toscanite. 



S— December 3, 1919. 



