NOl^ ON JACUPIRANGMTE IN TASMANIA. 



By W. H. TWELVETREES, F.G.S. 



(Read September 14th, 1903.) 



The wonderful aptitude of the alkaline magmas for differ- 

 entiation is strikingly exhibited by the nepheline rocks at 

 Port Cygnet. Tlie promontory alt the Regatta Ground south 

 of the jetty consists of a central spur of elaeolite syenite, 

 varying into alkali syenite, a light coloured alkaline erup- 

 tive of the character called leucocratic by Brogger, or salic 

 in the new American terminology. The mairgins, especially 

 the southern one, consist of the dark elaeolite-pyroxene rock 

 known as jacupirangite. This locality name was given by 

 Derby in 1891 to rocks in Brazil occurring as differentiation 

 products in association with elaeolite syenite, usually lami- 

 nated in habit and intersected by small dykes of the latter 

 rock. The nepheline-pyroxene varieties pass into magnetite- 

 pyroxene varieties and the latter into nearly pure titaniferous 

 iron. This ore is found in Alno (Sweden) connected with 

 elaeolite syenite. 



Megascopically, the Port Cygnet jacupirangite is a dark, 

 medium-grained rock, speckled with elaeoMte, and glistening 

 with small brilliant crystals of augite. Under a magnifying 

 glass a little iron pyrites is visible. The colour of the rock 

 gTows lighter as it merges into elaeolite syenite. The specific 

 gravity is 2.89. Microscopically, the respective quantities 

 of augite and elaeolite present do not differ much. The 

 augite is green, slightly pleochroic, c : c 35°. The elaeolite 

 is in large hypidiomorphic plates. Sphene in fair quantity 

 in wedge-shaped crystals. Melanite garnet, which is charac- 

 teristic of all the Port Cygnet eruptive^ is not absent from 

 this, and is occasionally rather plentiful. Apatite is present 

 in the forms of prisms and grains. Magnetite is scattered 

 grains. A little brown biotite. In order of quantity, the 

 minerals are elaeolite, augite, sphene, garnet, apatite, mag- 

 netite, biotite. 



Professor H. Rosenbusch, in mentioning that this is a 

 quite typical jacupirangite, says that search ought to be 

 made in it for the rare mineral baddeleyite (dioxide of 

 zirconium), which has been found in a Brazilian occurrence 

 of the corresponding rock. 



Both Rosenbusch and Zirkel treat this rather peculiar 

 class of rock as a modification of elaeolite syenite. Mr. H. 



