TRACHYDOLERITE IN TASMANIA. 

 By W. H. TwELVETREES, F.G.S., Government Geologist. 



[Read 1st December, 1902.] 



In 1889 evidence was laid before this Society by Mr. W. 

 F. Petterd and myself, demonstrating the existence in 

 Tasmania of two centres or districts characterised by 

 eruptive rocks derived from elseolitic and theralitic magmas. 



I now submit further localities for rocks belonging to the 

 same great felspathoid series. One of these is the Table 

 Cape Bluff, near Wynyard, on the North- West Coast, and 

 another is the Nut at Circular Head. A third is One 

 Tree Point, Sandy Bay, near Hobart, where melilite, basalt 

 has been identified, but only the first two will be dea,lt with 

 in these notes. 



The tertiary basaltic rock which forms the headland near 

 Wynyard came many years ago under the notice of the 

 late Professor XJlrich, who thought at the time that he 

 could recognise the small water-clear hexagons v\^hich are 

 abundantly visible in prepared slides as sections of the 

 felspathoid mineral nepheline. Both my colleague and I 

 were, on the other hand, disposed to diagnose the mineral 

 as apatite, and we learned from the lamented Professor 

 shortly before his death that he had arrived at the same 

 opinion. Mr. Thos. Stephens often communicated to me 

 his idea that this coarsely crystalline rock was a distinct 

 flow from our common 'olivine basalt. The mere difference 

 in texture did not appear to me a valid reason for assuming 

 any further diff'erence between the two rocks. However, 

 in drawing up a classification scheme, the occurrence of a 

 dyke of limburgite on the Em.u Bay Raihvay Line, nine 

 miles from Burnie, led me to hesitate, in view of the fact 

 that Rosenbusch classes^ limburgite as belonging to the 

 theralitic magmas. Renewed examination of the doleritic 

 rock at Table Cape and Circular Head showed the presence 

 of a felspathoid. The optical tests were confirmed by stain- 

 ing, and a specimen was sent Home to Professor Rosenbusch 

 It was referred by him to trachydoierite, the effusive form 

 of essexite. I wish here to acknowledge the readiness 

 with which the professor has always aided in solving 

 difhculties in Tasmanian petrology. , His letter reads as 

 follows: — "The compact basaltic rock of the Table Cape 

 Bluff and Circular Head is best referred to the trachydo- 

 ierite group. Besides labradorite, augite, olivine iron ore, 

 and abundant apatite, it contains a colourless mineral with 



