344 



C. A. SUSSMILCH AND W. G. STONE. 



geologically above the limestones, but separated from them 

 by a thickness of about 100 feet of slates. No fossils have 

 yet been found in these beds, although very little search 

 has yet been made for them. 



4. The Jenolan Rhyolite-porphyry . — This occurs almost 

 immediately to the east of the Gave limestone, being sepa- 

 rated from it by a thickness of 100 feet of slates only. Its 

 bold massive outcrop may be seen crossing the valley just 

 below the Grand Arch, where it forms a very prominent 

 feature. It has the same direction of strike as the Gave 

 limestone and appears to have a similar dip. Mr. V. 

 Wyburd informs me that this outcrop may be traced for 

 many miles in a southerly direction, always occupying a 

 position parallel to and east of the Gave limestone. 



In the hand specimen, this rock has all the characters of 

 a typical intrusive quartz-porphyry; it is almost white in 

 colour, and is usually markedly porphyritic, containing 

 large phenocrysts of quartz and felspar set in an aphanitic 

 groundmass. It resists weathering, giving bold rugged 

 outcrops, and so resistant is it to the forces of denudation 

 that the valley of Jenolan Greek narrows markedly where 

 it crosses the outcrop of this rock at the " bathing- pool" 

 just below the engine-house. In the valley of McEwans 

 Greek about a mile north of the Grand Arch at a place 

 marked D on the map. This rock is not porphyritic and 

 exhibits a well developed flow structure; the width of 

 outcrop here is much narrower than where it is crossed by 

 Jenolan Greek. 



At first sight it seemed probable that this quartz-por- 

 phyry was an intrusive rock, but its occurrence in the field 

 and its characters as revealed under the microscope suggest 

 that it is more probably a contemporaneous lava-flow. 

 There are a number of undoubtedly intrusive igneous rocks 

 in the immediate neighbourhood, some of which bear a con- 



