NEPHELIXE IN PHONOLITE DYKES. 129 



•extinction angle ranges from 0° to 7°, and the mineral is markedly 

 pleochroic. The felspar forms the background to the rock and 

 occurs as lath shaped crystals, rather cloudy from alteration and 

 showing a defined fluidal texture. The extinction is practically 

 straight. 



Nepheline.- — It is rather surprising to note that the crystals of 

 nepheline, except at their margins, are generally clear and unaltered, 

 while the felspar of the ground mass is cloudy. The recognition 

 of the mineral was based on the following considerations : — 



Form. — Sections are mostly rectangular, but some are hexa- 

 gonal in outline. 



Extinction and Polarization colours. — The rectangular sections 

 give uniformly straight extinctions and polarization colours, not 

 rising above white of the first order. Hexagonal sections are 

 isotropic. 



Figure.- — Hexagonal sections give a uniaxial negative figure 

 determined by the selenite plate in convergent polarized light. 

 Rectangular sections give partial uniaxial figures. 



Alteration. — The phenocrysts are surrounded by a narrow 

 margin of a colourless material which is isotropic, apparently 

 secondary in origin, and which may be analcite. In places it extends 

 towards the centre of the crystal and then shows a fibrous habit. 



Refractive Index. — The refractive index of the nepheline is about 

 the same as that of the canada balsam, considerably higher than 

 that of the altered rim, and slightly higher than that of the lath 

 shaped felspars of the ground-mass. 



Micro -chemical tests. — When attacked with hydrochloric acid 

 of 20 per cent, strength for five minutes most of the nepheline 

 crystals are gelatinised and stain with malachite green. In some 

 ■crystals the attack appeared to be feeble, while in others the crystals 

 were entirely or partially dissolved out of the section. 



Chemical composition.- — No complete chemical analysis of the 

 rocks has yet been made. 



To one of my assistants, Mr. F. L. Stillwell. B.Sc, Kernot 

 Research Scholar in the Geological Department. I am indebted for 

 a determination of the percentages of the alkalies in the rock. The 

 result is as follows : — 



Na20. = 10-33 

 K2O. = 5-25 



Total = 15-58 



The high total alkalies, and especially the very high soda 

 content of the rock in conjunction with the microscopical and micro- 

 chemical evidence, serves to show that the rock is rich in nepheline. 

 Its occurrence as a dyke suggests a comparison with the tinguaites 

 tut its texture shows that it is perhaps better described as a 

 nepheline phonolite. 



No. 3a. — Phonolite dyke from south of Omeo {See map, Plate IX.). 



