GEOLOGY OP THE JENOLAN CAVES DISTRICT. 379 



Both in hand specimens and under the microscope this 

 rock is seen to be distinct in character from the grano- 

 phyric quartz-porphyrite already described. Quartz pheno- 

 crysts are abundant and typical of this rock, and the quartz 

 is practically confined to the phenocrystic stage, and enters 

 only slightly into the composition of the groundmass. In 

 the granophyric quartz-porphyrite, quartz as phenocrysts 

 is only sparsely present, but it enters largely into the 

 composition of the groundmass. The extreme chloritization 

 of the groundmass of quartz-porphyrite is also very charac- 

 teristic. 



The analyses of both these rocks is somewhat similar, 

 except in the percentages of potash present. The amount 

 of potash in each is characteristic and constant, as 

 already indicated by two determinations of the alkalis of 

 material collected from different places. This difference 

 is manifested mineralogically by the proportions of ortho- 

 clase present in each, being much more plentiful in the 

 granophyric- quartz-porphyrite, and giving rise in it, in 

 conjunction with the quartz, to the fairly abundant micro- 

 pegmatitic structures. 



The texture of the groundmass of the latter is also dis- 

 tinctly coarser and the rock generally approaches a more 

 plutonic type. From a petrographical and chemical aspect 

 these two occurrences appear to represent separate intru- 

 sions, as opposed to considering them as different phases 

 of the one intrusion. 



D. The Quartz Felsites. 

 The specimen selected for description is from the road- 

 cutting at the head of Hinchman's Creek, (Slide x 894). 



Megascopic Description. — Colour greyish-green to 

 white. The groundmass has a distinctly felsitic appearance 

 and contains numerous small phenocrysts of quartz and 



