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



In the descriptions of the various rocks, embraced in the 

 area, the subject of the present paper, the writer has 

 endeavoured to give, as far as possible, a general descrip- 

 tion of each rock type. For this purpose numerous sections 

 of specimens collected from different parts of the same 

 occurrence have been examined. The result is, therefore, 

 more n account of the average rock type, than a detailed 

 description of individual sections. The material taken for 

 analysis was that least likely to be affected by absorptions 

 of the intruded rocks, and therefore, occurring as far as 

 possible from such contacts, and as representative as 

 possible of each occurrence. 



In the calculation of the norms all the lime has been 

 included, its combination with C0 2 as calcite etc. not being 

 taken into account, as it is considered that the calcite 

 present would represent in large part, the alteration in situ 

 of the lime-bearing minerals. The analyses were under- 

 taken in the Geological Survey Laboratory, through the 

 kind permission of Mr. J. O. H. Mingaye, f.i.c, f.c.s., 

 Analyst in Charge. I wish also to express my indebtedness 

 to Messrs. G. W. Card, a.r.s.m., of the Mines Department, 

 N. S. Wales, and W. N. Benson, b.Sc, b.a., f.g.s., Linnean 

 Macleay Fellow in Geology, for kindly advice. 



A. The Rhyolite Porphyry (Quartz-porphyry). 

 The detailed description given below and the analysis 

 are of specimens taken from the massive outcrop near the 

 Grand Arch. Specimens from other localities will be dealt 

 with more briefly and comparisons made. 



Megascopic Description. — Colour is creamy-white with 

 darker blebs due to phenocrysts of quartz. The rock is 

 aphanitic and porphyritic, the phenocrysts present being 

 quartz, felspar, and rarely mica. The quartz is plentiful 

 and represents approximately 10% by volume of the rock ; 

 some of the crystals reach 9 mm. in diameter, but the 



