GEOLOGY OF THE JENOLAN CAVES DISTRICT. 373 



(b) The Quartz-porphyrite (Green Variety). 



Megascopic Description. — The colour at a distance is 

 dark green, near at hand it is dark green speckled white. 

 The rock is porphyritic with an aphanitic groundmass. On 

 casual inspection the rock appears to be phanerocrystalline, 

 but on closer study the aphanitic character is distinctly 

 seen; the phenocrysts of felspar and quartz being set in a 

 matrix of chloritic material, the latter obscuring somewhat 

 the aphanitic nature, and giving to the rock its prevailing 

 green colour. The quartz phenocrysts are very plentiful 

 and have an average size of from 1 to 2 mm., occasionally 

 up to 4 or 5 mm. It is of a dark glassy to faint milky 

 colour. With the aid of the lens the groundmass is seen 

 to invade some quartz crystals. 



The felspar is white to faint pink in colour, with an 

 average size of 1 to 2 mm., occasionally ranging up to 4 mm. 

 It is somewhat more plentiful than the quartz and shows 

 various stages of alteration, some being fairly fresh in 

 appearance, the majority, however, have a more or less 

 dull lustre. Traces of twinning are consequently seldom 

 noticed. Chloritic material of a greasy lustre and dark 

 green colour comprises the balance of the rock. A yellowish 

 green mineral of a secondary nature which appears to be 

 epidote, is noticed here and there up to 3 mm. in size. 

 Small fragments of foreign rocks, viz., limestone, slate, etc., 

 are seen to be included occasionally, these are probably 

 the remnants of larger lumps and blocks which have not 

 been completely absorbed by the magma. 



Microscopic Description. — The rock is noncrystalline 

 and porphyritic with a microcrystalline base. Like the 

 granophyric-quartz porphyrite, it is much altered, and 

 description becomes rather difficult, especially in connection 

 with the groundmass. Although chlorite is abundant and 

 undoubtedly represents the alteration of a ferro-magnesian 



