GEOLOGY OF THE JENOLAN CAVES DISTRICT. 359 



As already described in Part I, two distinct types of 

 andesite occur, these will be described separately: — 



(a) Andesite (Non-porphyritic Type). 



Megascopic Description. — The colour is dark grey with 

 a tinge of green; phenocrysts are always small and rarely 

 visible to the eye, but on a smooth surface one can, with 

 a pocket lens, see that the rock is crowded with small 

 felspar phenocrysts. Inclusions of black chert are fre- 

 quently noticed, besides the more abundant inclusions of 

 the porphyritic andesite referred to in Part I. 



Microscopic Description. — The rock is noncrystalline 

 and microporphyritic, with a microcrystalline to crypto- 

 crystalline and somewhat turbid groundmass. The pheno- 

 crysts are mainly plagioclase with a subordinate amount of 

 augite and hornblende. 



Felspar. — The phenocrysts are for the most part small, 

 averaging about 0*4 mm. in diameter, but crystals up to 

 1 mm. in size occur. These crystals are for the most part 

 rounded or irregular in shape, only occasionally idiomorphic 

 and often encircled by a ring of dark kaolin-like material. 

 These felspars constitute about fifty per cent, of the rock, 

 but are so much altered that any exact determination is 

 impossible. Multiple twinning can be detected in some 

 crystals. The analysis indicates that both orthoclase and 

 plagioclase are present, the latter largely predominating. 



Augite. — This is not plentiful. In colour it is pale-green 

 to almost colourless, and is fairly fresh. The crystals vary 

 from 0*25 to 0*50 mm. in size, some few crystals reaching 

 1 mm. in diameter. 



Hornblende. — This is less plentiful than the augite, is 

 yellowish-green in colour and quite fresh. It occurs in 

 typical prismaticand basal sections with strongpleochroism, 

 but the crystals are rather smaller than the augites. Both 



