GEOLOGY OF THE JENOLAN CAVES DISTRICT. 3G9 



Microscopic Characters. — Slide x 888 is described in 

 the main as being typical of the rock type. This rock is 

 noncrystalline and porphyritic with medium to fairly coarse 

 micro-crystalline groundmass. It is much altered, in some 

 places to such an extent as to render description difficult. 

 The phenocrysts in their order of abundance are felspar, 

 augite, quartz, and would occupy between 30 and 40% of 

 the rock by volume. 



Felspar. — Is in an advanced stage of alteration and it is 

 often difficult to defkie its boundaries, except between x 

 nieols, as it merges into the groundmass in ordinary light. 

 Both plagioclase and orthoclase are present, the former 

 predominating; only occasionally is the multiple twinning 

 of the plagioclase clear enough to obtain measurable extinc- 

 tions, and approximate determinations place it about 

 andesine. A faint zoning is sometimes seen. The colour 

 is variable depending upon the type of alteration, being 

 brownish, greenish, grey, clouded etc. The decomposition 

 products are kaolin, sericite and chlorite mainly, with 

 calcite and epidote. In some sections the phenocrysts 

 are either completely sericitised or kaolinised; others show 

 an alteration to both kaolin and sericite, often having a 

 zonal arrangement, the outer zone being kaolin with the 

 central part sericite; and others still are a mixture of 

 kaolin, sericite, chlorite and calcite. 



Quartz occurs only sparingly as phenocrysts and is of 

 small size compared to the felspar; generally speaking it is 

 more a constituent of the groundmass. Large and numer- 

 ous phenocrysts as in the quartz-porphyrite being typically 

 absent. It is often interstitial in habit, separated grains 

 being often optically continuous. 



Augite — Slide x 889, shows a very pale green augite 

 somewhat sparingly in patches, not yet completely altered. 

 In other slides, no trace of an original ferro-magnesian 



X— December 1, 1915. 



