GEOLOGY OF THE JENOLAN CAVES DISTRICT. 355 



average is about 3 mm. The felspar phenocrysts, somewhat 

 more plentiful than the quartz, are creamy- white in colour 

 and usually with the dull appearance produced by kaoliniza- 

 tion. The crystals range up to 8 mm. in diameter, but 

 average about 2 mm. The mica, which is only sparingly 

 present, is yellowish in colour and averages about 1 mm. 

 in diameter. 



Microscopic Description. — The rock is noncrystalline 

 and porphyritic, with a microcrystalline groundmass. The 

 minerals present are quartz, felspar, biotite and a little 

 apatite. Oalcite, chlorite, limonite, magnetite, and possibly 

 ilmenite are present as alteration products. 



Quartz. — This is sometimes idiomorphic, but more often 

 shows rounded or irregular outlines due to corrosion, typical 

 examples of which are present. It occasionally encloses 

 crystals of biotite. 



Felspar. — This is mainly orthoclase, seldom shows twin- 

 ning, but exhibits corrosion in some crystals, while others 

 are quite idiomorphic. Some phenocrysts exhibit a micro- 

 scopic intergrowth with plagioclase (probably albite). 

 Plagioclase crystals are not common. Alteration is present 

 and varies from crystals which are almost fresh to those in 

 an advanced stage of alteration. The secondary products 

 are kaolin, sericite, chlorite and calcite. 



Biotite. — This is not plentiful. It is somewhat contorted, 

 has a bleached appearance, is almost colourless and only 

 faintly pleochroic. Iron oxides have separated out as 

 magnetite and limonite along the directions of cleavage, 

 while cloudy patches of calcite are frequent. 



Apatite. — This occurs sparingly as minute crystals, both 

 in the felspar and biotite phenocrysts and in the groundmass. 



Calcite. — This occurs as an alteration product of the 

 felspar and biotite crystals and also in the groundmass. Its 

 appearance indicates the presence of magnesium carbonate 



