UNUSUAL RHYOLITE FROM THE BLACKALL RANGE. 197 



Whether the rhyohte was effused from a central point or 

 from a fissure cannot yet be stated, but there appears to have 

 been a centre of effusion near Portion 143v in Skene's Creek, 

 below the Falls, for we find breccia, agglomerate, and tuffaceous 

 material of a rhyolitic nature. 



GENERAL PETROGRAPHY. 



There is much variation, both in colour and in structure, 

 in the rhyolite. Fluxion and spherulitic structures are usually 

 present, while the colour ranges from pink to grey. 



Spherulites an inch in diameter occur in places, but they 

 are nearly always altered either to kaolin or secondary silica, 

 while often there is a kernel of sihca inside a white fibrous 

 aggregate. (See Plate V., figs. 2 and 3). 



The occurrence of hollow spherulites is not uncommon, 

 and at " The Narrows " on the Obi Obi Creek the rhyolite 

 appears to be composed of a mass of lithophysse showing 

 concentric coats and hollow interiors and about 2-5 mm. in 

 diameter. (See Plate V., fig. 1). 



All the rhyolite occurrences appear to be slightly porphy- 

 ritic, and both pink orthoclase sho\Wng simple twinning and 

 about 2 mm. in length and occasional quartz crystals occur as 

 phenocry.'ts. 



The dens'-t}^ of the rhyohte analysed was 2-545. 



The rock, under the microscope, shows occasional lath- 

 shaped phenocrysts of orthoclase and albite up to 1 mm. in 

 length, set in a groundmass usually cryptocrystaUiiie, but 

 sometimes microcrystalline. The groundmass shows fluxion 

 structure very frequently, while evidence of spheruhtic structure 

 is very common. Much alteration has gone on and both 

 " secondary " quartz and kaolin occur very freely throughout 

 the sections. 



The original minerals were phenocrysts of pink orthoclase, 

 albite, quartz, and very occasionally a ferro-magnesian mineral, 

 possibly biotite, but invariably altered into chlorite, while in 

 the glassy groundmass were developed rod-hke aggregates of 

 felspar. 



The phenocrysts of felspar are moderately well preserved, 

 and the extinction angle of the albite may be determined with 

 but little trouble. 



The groundmass in the rock at present appears to be made 



R.S. — 0. 



