BY H. C. RICHARDS. 133 



Microscopical and chemical considerations show that 

 there are two well-defined series of acid volcanics, namely 

 the alkaline and sub-alkaline, but in addition there are 

 representatives of a series intermediate between these two. 



Sub-alkaline Acid Rocks. 

 Among the sub-alkaline rhyolites we find both porphy- 

 ritic and non-porphyritic varieties. The porphyritic rhyo- 

 lites may be white, yellowish, or reddish in colour, and have 

 phenocrysts of quartz and felspar. The phenocrysts of 

 quartz are always transparent, and in the case of felspars 

 which may be orthoclase, sanidine, or anorthoclase, they 

 are typically colourless and glassy and not white. The 

 relative proportions of phenocrysts and groundmass do not 

 vary very much, and the phenocrysts always play a very 

 subordinate part. 



The groundmass varies from holoerystalline to holo- 

 hyaline, as in the case of the pitchstones, and all intervening 

 stages are represented in the various types. 



The lithoidal varieties have a characteristic colour 

 ranging from light grey to lavender. Occasionally one may 

 see in the hand-specimen short, stout, lath-shaped crystals 

 of orthoclase. Fluxion structure is very pronounced and 

 is particularly evident on the weathered surface. 



As described by Harker,-* the banding of the rhyolites 

 is seen on microscopic examination to be the result of slight 

 differences in the nature and texture of the groundmass. 

 While spherulitic structure on a microscopic scale is 

 generally present in the rhyolites from this field, in one 

 place only has it been found sufficiently coarse to be easily 

 seen in the hand-specimen. This occurrence is on the 

 banks of the Coomera River west of the Gin's Leap, and 

 there one finds coarsely spherulitic rhyolite with spherulites 

 averaging perhaps half an inch in diameter but ranging 

 up to 114 inches. The rock is very much altered and 

 chalcedony occurs abundantly between the spherulites 



The microscopic characters may be summarised thus : — 

 " Tertiary Igneous Rocks of Skye. 



