346 



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



lithological and penological characters to those of the 

 rhyolite-porphyry near the Grand Arch that there can be 

 no doubt that they have been derived from one and the 

 same magma. The outcrops of this second occurrence 

 along the line of section are such, however, that their 

 stratigraphical relations with the adjoining slates are 

 not clear, and neither time nor opportunity have allowed 

 of our investigating this matter further at this locality. 



It might also be expected that if this rock were a con- 

 temporaneous lava flow which had been subjected to the 

 same folding as the Silurian sediments, that it would show 

 some evidence of metamorphism, and this is actually the 

 case. Micro-slides show that the rock has been subjected 

 to considerable pressure ; many of the biotite crystals are 

 bent; the quartz crystals are shattered and fractured ; and 

 in some slides the rock shows distinct foliation, with the 

 development of sericite and secondary quartz. These 

 features are not shown by any of the undoubtedly intrusive 

 rocks. 



Much more field work would be necessary to settle the 

 intrusive or effusive origin of this rock beyond doubt, but 

 the balance of evidence at present available seems to be in 

 favour of the latter view. The occurrence of rhyolite lava- 

 flows of Silurian age is not uncommon, as they have been 

 recorded for the Orange District, 1 the Yass District, 2 and 

 the Canbelego District. 3 



(c) The Igneous Intrusions. 

 Not the least interesting feature of the geology of this 

 region is the number and variety of the intrusive igneous 



1 The Silurian and Devonian Rocks of the Orange District, by C. A. 

 Sussmilch, this Journal, Vol. xl, page 130. 



2 The Geology of the Yass District by A. J. Shearsby, Report of the 

 Thirteenth Meeting of the Australasian Association for the Advancement 

 of Science, Sydney, 1911, p. 106. 



3 The Canbelego Budgery and Budgerygar Mines by E. C. Andrews, b.a. 

 Mineral Resources, No. 18, Department of Mines, N. S. Wales, 1915. 



