GEOLOGY OF THE JENOLAN CAVES DISTRICT. 363 



some crystals have an almost colourless central zone with 

 an outer zone of yellowish-green. The crystals in general 

 have a very fresli appearance, but some show various stages 

 of alteration into serpentinous material and calcite. In 

 those sections in which the augite only occurs it occupies 

 up to 40% of the volume of the rock. 



Felspar, This occurs in small idiomorphic crystals ranging 

 up to 1 mm. in diameter, the average being about 0*5 mm. 

 The quantity present varies considerably in different sec- 

 tions, reaching as much as 30%. These felspars are always 

 much altered; any twinning noticeable is always multiple. 

 A few approximate extinction determinations place it as 

 being a basic andesine. 



Iron ores. — Pyrite is the most plentiful, there being but 

 little magnetite or ilmenite present, and this is mainly 

 associated with the alteration of the augite. 



Apatite is only sparingly developed. 



Secondary minerals are abundant and include kaolin and 

 sericite as alteration products of the felspars; serpentine, 

 calcite and iron ores as alteration products of the augites; 

 secondary silica due to infiltration. 



Serpentine is the main product of the alteration of the 

 augite and is sometimes completely pseudomorphous after 

 that mineral. In this connection it is often associated 

 with calcite, which in some sections is fairly abundant, 

 and in patches up to 2 mm. in diameter. The serpentine 

 also occurs filling oval or circular shaped cavities often in 

 association with secondary silica, the latter occupying the 

 central zone. Some of the cavities are filled completely 

 with secondary silica largely chalcedonic in character, and 

 showing well marked concentric structure. Pyrite is 

 sometimes associated with the serpentine and secondary 

 silica often forming a complete border zone. 



