UNUSUAL RHYOLITE PROM THE BLACKALL RANGE. 199 



Microscopic Characters. — There are occasional pheno- 

 crysts of orthoclase and albite up to 0-8 mm. long occurring in a 

 cryptocrystalline groundmass. Spherulites up to 0-6 mm. 

 long and brown in colour are seen and have a well-defined radial 

 structure and for the most part are regular. The felspar rods 

 have been altered into kaolin, which is much stained by 

 limonite. Throughout the slide there are small blotches of 

 kaolin and limonite. 



The groundmass is composed of a de vitrified glass, and it 

 is impossible to resolve it into the individual grains which one 

 presumes to be felspar and quartz. 



Here and there throughout the field there occur irregular 

 patches of a fine quartz mosaic which strongly suggest 

 secondary silicification. 



Specimen 473 from near the top of the Bon Accord Falls. 



Megascopic Characters — This is a salmon-pink in colour 

 and. is very fine grained. Occasional phenocrysts of a pink 

 and a white felspar are noted, while the even surface of the 

 rock shows a structure which much resembles a fine graphic 

 intergrowth. When viewed under the microscope one sees 

 the structure is due to the arrangement in the altered ground- 

 mass of the kaolin, which is stained pink, and of the clearer 

 secondary quartz granules which have developed in the cavities 

 of the small hollow spherulites of which the rock was composed. 



Microscopic Characters. — This rock section differs from the 

 one described above (261a), owing to the possession of small 

 oval and rounded patches of secondary quartz surrounded by 

 thin circular shells which once represented walls of hollow 

 spherulites or lithophysae. (See Plate VI., fig. 3). 



The rod-like aggregates of felspar and quartz which 

 constituted the rock mass have undergone alteration, so that 

 the felspar is either altered to kaolin or is replaced by secondary 

 quartz. 



The diameter of the spherulites appears to average about 

 0'4 mm., but there is much irregularity about the shape and the 

 sizes as they range down to less than O'l mm. and up to 06 mm. 

 in diameter. 



The old walls of the spherulites show a rodded structure 

 and are about 0'05 mm. thick. The walls are now mostly 

 kaolin in nature, but frequently one sees secondary quartz 

 granules optically continuous from the inside right across the 

 walls to the o' mass material outside. The spherulites 



