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TRACllYDOLEKITE IX TASMANIA. 



a very low refractive index and abnormal double refraction, 

 which gelatinises in weak acid and allows much Na ^ O to 

 go into solution. I regard it as analcime. It is always 

 allotriomorphic, and it is highly probable that this mineral 

 is derived from nepheline. In a special slice which I 

 prepared I saw distinctly in an isotropic spot the inter- 

 ference figure of a negative uniaxial mineral (nepheline)." 



The hornblende, haiiyne. and sphene, which are common 

 in typical trachydolerite. are absent, so the rock is not a 

 normal member of the family. 



In the Table Cape rock, olivine is abundant as pheno- 

 crysts ; augite as prisms, also in grains of the second 

 generation ; labradorite in slender twinned prisms ; apatite 

 in vertical and hexagonal sections, iron ores in numerous 

 grains and cubes. There is a great deal of the feebly 

 refractive analcime in plates of extreme tenuity. In some 

 slides thin rectangular sections of the mineral determined 

 by Professor Rosenbusch as nepheline are present. 



In the Circular Head rock, the augite is in larger crystals 

 and plates, and exceeds the olivine in quantity. It is the 

 violet-tinted variety of diopside so common in nepheline- 

 bearing rock. Apatite is abundant in the slides in the 

 form of short columns, spindle-shaped, or hexagonal 

 sections. Titaniferous iron or magnetite has separated out. 

 The interstitial groundmass abounds with microlites and 

 with isotropic or feebly refractive material. It is 

 apparently saturated with analcime, and in one instance 

 natrolite could be detected. 



The macroscopic aspect of these rocks is doleritic. The 

 Circular Head variety is somewhat coarser in grain than 

 that of Table Cape. 



Briefly, the abundance of apatite and analcime warn us 

 that we are not dealing with an ordinary basalt, and the 

 presense of nepheline. in however small quantity, confirms 

 this belief. 



It is difficult to suoforest what relation the rock of these 

 Blujff bears to the ordinary Tertiary olivine basalt of the 

 coast. Both are of Tertiary age. but each is the product of 

 a different magma, and such rocks hitherto have not been 

 found associated. The two families would be represented 

 as under — 

 Normal olivine basalt Trachydolerite 



Diabase (Dolerite) 



Gabbro 



Essexite (not yet discovered) 



