BY W. H. TWELVETRBES AND W. F. PETTERD. 2i^ 



may mean sanidine or oligoclase. The porphj-ritic felspars 

 are large and scattered ; zonal structure common. A large 

 orthoclase crystal, giving a section parallel to the clino- 

 pinacoid, showed a characteristic extinction angle of 22°. 

 The hornblende is green, basal sections brownish green ; 

 extinction angle about 14^', and the absorption that of 

 cataphoritic hornblende, t) > C > a. 



The preceding form a complete series of a unique set of 

 rocks so far as Tasmania is concerned. To the geologist 

 they are important as being the youngest matrix of gold 

 in the colony. The trachytes appear to have shed the 

 gold which has been won on the alluvial field at Lyming- 

 ton and found in the gullies on Mt. Mary. They are in 

 places veined with quartz, but it is not at all clear that the 

 quartz itself is auriferous, and the veins have not the 

 characteristics of true fissures. We have seen some sand 

 collected from a creek about a mile back from Little 

 Oyster Cove towards' Port Cygnet, which contains flaky 

 gold, more or less waterworn, with numerous melanite 

 garnets, zircons, and small crystals of sphene. It is a 

 sand which is evidently the detritus of the garnetiferous 

 trachytes, and the occurrence in it of gold associated with 

 the minerals just named supports the idea that the 

 trachytes are the source of the gold throughout the whole 

 province. There is nothing intrinsically inconsistent in 

 the occurrence of gold in the quartz veins of trachytes. 

 It is found in various parts of the vs^orld in much younger 

 trachytes and andesites than these, though the gold- 

 bearing reefs in the rest of Tasmania are of older date 

 and traverse Silurian slates and sandstones. But, so far as 

 "we can see at present, there is no trustworthy -evidence to 

 show that the Port Cygnet quartz veins are auriferous, 

 v^rhile, on the other hand, there is some reason for believing 

 the trachytes themselves to contain sparingly disseminated 

 gold, especially where they are silicified and brecciated. 

 The miners are pursuing the right course in selecting 

 these tufEaceous and siliceous zones for exploration. 

 Unfortunately the mining work which is being carried on 

 has not so far proved the existence of the precious metal 

 in the matrix in anything like payable quantities. 

 Whether the gold has been concentrated anywhere to a 

 greater extent than in the parts hitherto exploited, remains 

 for future search to decide'. The pyrite in these rocks has 

 so far proved non-auriferous. 



