130 PROCEEDINGS OF SECTION C. 



Megascopic. — The rock is grey and specklsd and rather coarser 

 in grain than No. 43. The light minerals show bright lustre in 

 reflected light. The dark mineral is aegirine. 



Microscopic. — The rock contains the same minerals as in No. 43, 

 but with rather different habit. i?igirine occurs in more stumpy 

 prismatic crystals and in small allotriomorphic fragments in the 

 ground-mass, which have separated out after the felspar. The 

 felspar occurs in clear lath-shaped crystals, between which are 

 moulded rather cloudy to clear material polarizing in colours ranging 

 from low neutral to yellow in the first order. This mineral may be 

 felspar, but may possibly on further examination turn out to be 

 cancrinite. In places an isotropic mineral with very low refractive 

 index is moulded on other minerals and sometimes shows a faint 

 purphsh colour. I am not yet prepared to give a name to this mineral. 

 Nepheline occurs rather less abundantly than in No. 43, and 

 generally is present in rectangular to hexagonal crystals. 



Texture.— The rock is porphyritic in habit, owing to well 

 developed crystals of nepheline and gegirine, and is also trachytic in 

 respect to the fluidal arrangement of the lath-shaped felspars in the 

 ground-mass. 



Microchemical evidence. — The rock is very readily attacked and 

 gelatinised by 20 per cent, hydrochloric acid, and the nepheline is 

 stained by malachite green. Some areas are dissolved out, especially 

 the interstitial material, which may be cancrinite. The rock is best 

 described as a nepheline phonolite. 



Summary and Conclusions .-—Fvevious reported occurrences of 

 felspathoids in Victoria are shown to be either undemonstrated or 

 inaccurate. The rocks with which this communication deals form 

 part of a collection of dyke stones from Omeo, collected by the late 

 James Stirling and sent to the late Dr. Howitt. Pre-Ordovician 

 schists lie to the east of Omeo and intrusive granites and quartz 

 diorites occur to the west of the schists. Associated with the 

 plutonic rocks are acid dykes and quartz veins. Later than these 

 is the intrusive mass of Frenchman's Hill, composed of trachytes 

 and solvsbergites. Radiating from this hill are a series of trachytic 

 dykes. Further south, near Omeo, a series of dykes, some diorites 

 or diabases, others trachytes, trend nearly east and west. Some of 

 the trachytic dykes have associated lenses or veins of quartz. The 

 youngest rocks, which lie more than a mile to the south of Omeo, 

 appear to be two dykes trending north and south and cutting a 

 diabase and a trachytic dyke respectively. These youngest dykes, 

 examined microscopically are shown to contain abundant pheno- 

 crysts of nephehne associated with aegirine and lath-shaped crystals 

 of soda orthoclase. One of these dykes on analysis yielded 5.25 per 

 cent, of potash and 10.33 per cent, of soda. Further field work is 

 necessary before it will be possible to discuss fully the age and field 

 relations of these dykes and the rocks of Frenchman's Hill and their 

 bearing on the genesis of the alkali rocks. 



Plate IX. — Sketch Map of Omeo. 



