198 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 



up of kaolin and secondary quartz, some of the latter occurring 

 as allotriomorphic grains up to -2 mm. in length in the cavities 

 of the hollow spherulites. 



The spherulites appear originally to have been both of 

 the compact type (with radially arranged rods of felspar) and 

 of the hollow type (htliophysae). The former have been altered 

 into kaolin and are frequently stained brown by ]imonit(i, while 

 the latter have had their ca\ ities filled by secondary quartz. 

 The concentric rings of rod-like material forming the hollow 

 spherulites have also in some cases been altered to quartz, for 

 one frequently sees optical continuity throughout an allotrio- 

 morphic piece of quartz extending from the centre across the 

 spherulitic ring and into the surrounding original groundmass. 



The glassy groundmass has been thoroughly devitrified and 

 now consists of a cryptocrystalline aggregate in which it is 

 difficult to resolve the separate crystals. 



The groundmass of the lavender- coloured rock on the 

 south side of Skene's Creek in Portion 143v is very deeply 

 coloured by reddish-brown flakes and rods of iron oxide. 

 These appear thickly studded throughout the whole rock, and 

 the rod-like crystallites which were formed in the glassy 

 groundmass have been either replaced or deeply stained by 

 iron oxide. (See Plate VI., fig. 2). 



Professor G. A. J. Cole,^ in his very interesting paper in 

 1885 on Hollow Spherulites, etc., describes an identical occur- 

 rence in the groundmass of the rhyolite from Beaver Lake, 

 Yellowstone Park. 



PETROGRAPHIC DESCRIPTIONS OF THE VARIOUS 



TYPES OF RHYOLITE. 

 Specimen 281 A. « 



This specimen, from which the material for the chemical 

 analysis was obtained, was collected from the bed of Skene's 

 Creek, about 100 yards above the highest falls in Reserve 546. 



Megascopic Characters. — The rock is a very fine grained 

 pink and grey rock, showing marked fluxion structure owing 

 to the different colour bandings. Phenocrysts of pink ortho- 

 clase occur sparingly, but small rounded brown spherulites up 

 to 0.6 mm. in diameter are abundant. (See Plate VI., fig. 1). 



The specific gravity of the rhyolite is 2-545. 



5 Q.J.G.S. 1885, p. 165. 



* This and succeeding numbers apply to the collections in the 

 University of Queensland. 



