afhil, 



There was a laree attendance at the 

 monthly meeting of the Royal Society of 

 Tasmania (the first of the 1899 session) at 

 the Museum on Tuesday, 11th April, when 

 Mr. Napier Bell's and other papers were 

 read. His Lordship the Bishop of Tas- 

 mania (the Right Rev. Dr. Montgomery, 

 D.D.) Vice-President, presided. 



Apologies were read from the President 

 (His Excellency the Governor), regretting 

 his inability, through ill-health, to be pre- 

 sent ; Sir James Agnew, V.P., Mr. T. 

 Stephens, V.P.. and Hon. F. W. Piesse, 

 M.L.C. 



HISTORICAL SECTION. 



The Chairman said the Society had 

 seen fit to inaugurate a historical and 

 geogTaphical section, so as to obtain all 

 that could possibly be gathered concerning 

 the past history of the colony before it be- 

 came too late. The section had now been 

 formed, and he hoped every member inter- 

 ested in historical and ge'ographical know- 

 ledge would give their names to Mr. 

 Morton, so that the fii'st meeting of the 

 section might be called as soon as possible. 

 (Applause). 



PORT CYGNET ROCKS. 



The Secretary (Mr. Alex. Morton) read 

 a paper by Messrs. W. H. Twelvetrees, 

 F.GI-.S., and W. F. Petterd, C.M.Z.S., on 

 Haiiyne-trachyte and allied rocks in the 

 district of Port Cygnet and Oyster Cove. 

 Igneous rock at Port Cygnet, in Southern 

 Tasmania, has been well known for a long 

 time by the name of felspar porphyry. 

 The authors submit in this paper the 

 results of field work and misroscopical 

 examination, showing these porphyries to 

 be soda trachytes containing the sodic 

 minerals haiiyne, aegirine, analcime, cata- 

 phoritic-hornblende, with melanite-garnet, 

 sphene, zircon, and apatite as nearly 

 constant accessories. It is pointed out 

 that as haiiyne in the first British 

 phonolite (wolfrock, Cornwall) was dis- 

 covered by the late Mr. Allport in 1871 by 

 means of the microscope, the same instru- 

 ment has led to the discovery of the same 

 mineral in Tasmania. Geological sections 

 round Lovett show the haiiyne and gar- 

 netiferous trachytes to be interbedded 

 with the permo-carboniferous beds, and 

 consequently, geologically, contemporane- 

 ous with these. Beside these volcanic 

 sheets, intrusive haiiyne and other dyke 

 rock have been found, as well as syenites 

 containing elacolite and analcime. The 

 group is thus represented bv effusive, in- 

 trusive, and plutonic members, and the 

 complete series forms a unique set of 

 rocks, so far as Tasmania is concerned, 

 as well as being apparently the first 

 authentic occurrence in Australia. Thev 



1899. 



are geologically 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 Lymington, and found 

 in the gullies on Mount Mary. The 

 authors also refer to sand from a creek a 

 mile back from Little Oyster Cove, which 

 contains flaky gold, numei'ous melanite- 

 garnets, zircons, and small crystals of 

 sphene. This sand is the detritus of the 

 garnetiferous trachytes, and the occur- 

 rence 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. At 

 present there is no trustworthy evidence 

 to show that the Port Cygnet quartz 

 veins are auriferous, while on the other 

 hand there is some reason for believ- 

 ing 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 tuffaceous and siliceous. 

 zones for exploration. 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-aurifei'ous. Appearances 

 are against the quartz veins being true 

 lodes, and the quartz is so closely asso- 

 ciated with and banded with trachyte 

 that the assay results are inconclusive 

 upon the question of the auriferous nature 

 of the quartz. Thei^e is a remarkable 

 development of melanite garnet in the 

 trachyte and syenite, and this mineral 

 runs through the whole series of rocks. 

 The trachyte rock on the Living- 

 stone Hill, N.E. of Lovett, crowded with 

 large tabular crystals of orthoclase, fur- 

 nishes the largest and most remarkable 

 felspars in the island. Detached fi'om the 

 matrix they form fine cabinet specimens. 

 The dark green, sometimes fissile, meta- 

 morphic looking rock on Mount Mary, 

 west of Lovett, has been ascertained to be 

 an aegirine trachyte, owing its colour to 

 the needles of green soda pyroxene 

 (aegirine) with which it is crowded. The 

 syenitic looking rock on the back road 

 N.E. of Lymington called "granite" 

 locally, is shown to be a typical trachyte, 

 while the dark rock on Crannj^'s farm, 

 known as "basalt," is a dyke-diorite (mala- 

 chite), a somewhat abnormal occurrence in 

 this group of rocks. The Port Cygnet 

 series extends to Oyster Cove. This small 

 and peculiar petrogTaphical province is a 

 purely local one, confined, so far as is 

 known to the authors, to this part of 

 Tasmania. Its unexpected discovery maj'' 

 be placed to the credit of the young 

 and expanding science of microscopical 

 petrology. 



