GEOLOGY OF THE JENOLAN CAVES DISTRICT. 347 



rocks; these include the following: — 1. The Andesites, 2. 

 The Quartz-Porphyrites, 3. The Felsites, 4. The Diorites. 

 The andesites are limited in their occurrence to the region 

 lying to the west of the Gave Limestone and intrude the 

 radiolarian cherts only; all the other intrusions occur to 

 the east of the Cave Limestone and intrude the Silurian 

 strata. 



1. The Andesite,— This occurs in the form of a dyke 

 striking nearly north and south, and, as may be seen from 

 the geological map, parallel to and very close to the junction 

 of the radiolarian cherts and limestones. Good exposures 

 have been found at two localities only, viz., (1) immediately 

 adjoining the Cave House, and (2) some distance up 

 McEwans Creek at the place marked A on the geological 

 map. At the former locality this rock was extensively 

 exposed by the quarrying done in making the foundations 

 for the Cave House. The dyke here is upwards of 150 feet 

 in thickness and careful examination shows it to contain 

 two distinct rock types, one in which there are abundant 

 black phenocrysts, the other in which such phenocrysts are 

 absent; the former will be referred to as the porphyritic 

 type and is a hornblende-augite-andesite, the latter will be 

 referred to as the non-porphyritic type and is an augite- 

 andesite. 



The augite-andesite is an aphanitic rock, greenish-grey 

 in colour and is much jointed; many of these joints are 

 slicken-sided and contain films of calcite, chlorite, etc. It 

 is traversed in places by very thin veins of quartz, some of 

 which contain a little arsenopyrite; this latter mineral 

 and pyrite also occur as occasional minute crystals 

 sporadically scattered through the rock. This non-por- 

 phyritic type of andesite occupies by far the larger part of 

 the intrusion. The porphyritic hornblende-augite-andesite 

 contains abundant black phenocrysts of augite and horn- 



