XXXVl 



JUNE, 1899. 



The monthly eveiiin.u: iiiieLinii; was held 

 on Tuesday cveniny;, .June 20, Mr. T. 

 SLcphcns, F.G.S., M.A., jji-osidinj;-. 



COKUKSrONDKNCE. 



The Skchktaky read the followinj^ 

 letteis :— From His Exeelleney the Ad- 

 ministrator, Chief Justice J)odds, C.M.G., 

 re,i:;rc'tliM!L:; that, owini:; to his aljsenee from 

 lloliart, he would be unable to in-eside at 

 the meetin.i?. From tlie Hon. Sir James 

 As^new, K.C.M.G., senior vice-president, 

 and the 13ishop of Tasmania, who also 

 forwarded apologies. 



ELECTION OF NEW MEMBEliS. 



The followiuG; gentlemen were unani- 

 mously elected Fellows of the Society :— 

 Kev. 8. Bucknell, Messrs. W. Aikenhead, 

 M.II.A., and Alan AValker. 



PAPERS. 



The Secretary, in the absence of the 

 authors (Messrs. W. II. Twelvetrees, 

 F.G S., and W. F. Petterd, C.M.Z.S.), read 

 a paper entitled " Nepheline and Melilitc 

 Rocks from the Shannon Tier " : — 



This paper is descriptive of some speci- 

 mens of rocks fi"om the Shannon district, 

 received from the Mines Department, and 

 fiom Mr. Geo. Allison, of Hunterston. 

 They have been looked upon locally as 

 indicative of tin and gold. From Mr. 

 Allison's outlines of their occurrence, the 

 geological features of the locality are 

 briefly traced. At Hunterston the Shannon 

 Tier forms a plateau of mesozoic dolorite 

 rising 1,000ft. above the permo carboni- 

 ferous country at its base. On the slope 

 below the Tier are small, round, or conical 

 hills of a dark gi'ey basaltic rock, and on 

 the flanks of these is a coarse zeolitic rock, 

 locally called " tourmaline rocks." Gold 

 is said to have occurred in this pseudo 

 tourmaline rock, but on assay by the 

 Government Analyst did not confirm this. 

 The locality thus yields three varieties of 

 eruptive rock, viz., the mesozoic dolorite, 

 the tourmaline rock, and the basalt. The 

 authors diagnose the so-called tourmaline 

 rock as a nephelinite, and the basalt as 

 melilite-basalt. This age, as far as can be 

 hazarded at present, is probably permo- 

 carboniferous for the nepheline and 

 melilite rocks. T he dolorite is considered 

 to belong to the close of the mesozoic era. 

 This latter rock is the typical ophitic 

 dolorite, which occupies the summits of 

 the Tiers, and of numerous mountains in 

 every part of the island. It is a holo- 

 crystalline plagioclase-augite rock, struc- 

 turally diabasic, and sometimes, where 

 the augite is chloritised, merging into 

 diabase. The well-formed felspar crystals 

 are cemented together by the augite 



mineral, and these two elements have 

 combined to form a non-vitreous massive 

 rock of essentially the same constitutipn 

 as gal)bro and l)asalt ; but as regards 

 grain and structure, int/ermediate between 

 t he two. If one could follow this rock to 

 its dee])-seated roots in the earth's crust, 

 where the pressure was greater, and the 

 process of crystallisation corresjiondingly 

 slower, we should probably find it existing 

 thei'c as coarsely crystalline gabbro. Ad- 

 mitting the intrusive nature of the rock, 

 there are two theories of its occurrence. 

 Seeing that its internal structure agrees 

 closely with that of diabasic sills, has it 

 spread laterally from fissures as an in- 

 trusive sheet? On this hyi)oth(!sis the 

 dolorite on the Tiers and tlie mountain 

 tops would be merely a capping, and shafts 

 sunk through it would pierce the stratified 

 sediments below. The level contours of 

 the sedimentary beds abutting on the 

 faces or sides of the Tiers and simulating 

 infra-position have suggested this explana- 

 tion ; but no actual trial has been made. 

 The second hypothesis is that the dolorite 

 represents the massive intra-telluric part 

 of an immense body of eruptive rock, 

 which as a whole never reached the sur- 

 face, but everywhere thrust out lateral 

 dykes, parts of which can still 1)0 traced in 

 the coal measures. Either explanation is 

 attended with difficulties. The nepheli- 

 nite is a nepheline-augite rock. The long 

 black prisms are not tourmaline, but 

 augite. The interstices between the 

 prisms are occupied by yellowish nephe- 

 line, which has often decomposed into 

 snow-white radiated aggregates of the 

 zeolite natrolite. The nepheline forms half 

 of the entire roek. The microscopical charac- 

 ters of this rock are discussed, and the rock 

 is correlated with the nepheline dolorite 

 of the Katzenbtickel in the Odenwald. 

 The basaltic rock, associated with the 

 nephelinite, contains no felspar in one 

 variety of it. Its melilite is the most 

 interesting element. The other con- 

 stituents are olivine or augite. This basalt 

 has no relation whatever with ordinary 

 basalts. It has proceeded from a different 

 magma, the theralitic magma as defined 

 by Rosenbusch, who groups nephelinite, 

 nepheline-basalt, and melilite basalt as an 

 integral effusive formation the product of 

 this magma. The authors of the paper 

 have not detected nepheline in any of the 

 other Tasmanian basalts. The crystals 

 formerly attributed to nepheline in the 

 tertiary olivine-basalts of Northern Tas- 

 mania are probably apatite. Viewed from 

 a mining point these peculiar basaltic 

 rocks of the Shannon do not offer anything 

 particularly encouraging. As they are 

 unique in Tasmania there is little use in 

 comparing them with mineral-bearing 



