128 PROCEEDINGS OF SECTION C. 



inmore detail on Stirling's manuscript map, which he sent to Howitt 

 with the collection of dyke stones. Among these dykes are some 

 which contain a little quartz, sometimes granular, sometimes in 

 graphic intergrowth with felspar. One of the dykes in section is 

 mineralogically and chemically in agreement with the typical 

 bostonites. Analyses of these rocks are given in Howitt's paper, 

 and some of them I have quoted in the address referred to above. 



Further to the south in the township of Omeo and to the south of 

 it are a number of dykes, most of which strike almost east and west. 

 Their positions are shown on Stirling's map, and some of them are 

 indicated on the map accompanying this paper. They include, 

 relativety, acid and basic types, to which Stirhng has given the 

 field names of felspathic dykes and diabases respectively. It is 

 interesting to note that Stirling describes lenses and veins of quartz 

 as being in places directly associated with and forming part of the 

 felspathic dykes. The youngest dykes in the district, according ta 

 Stirhng. are two (No. 3 and 3a and No. 43 on Stirling's list), which, 

 striking north and south, cut. in the one case a diabase, in the 

 other a felspathic dyke. 



It is with these two north and south dykes that this paper is 

 principally concerned. On examining sections of these two rocks a 

 few months ago I was interested to find that nepheline is present in 

 each in considerable amount and that the rocks have the habit 

 and texture of phonolites, although occurring as dykes. 



Petrographic description of the Phonolite' Dykes. — No. 43. Phonolite 

 dyke, south of Omeo. (See map, Plate IX.). 



Megascopic. — The rock is fine-gi"ained and grey in colour, with 

 dark crystals. The grey areas consist of nepheline and felspar and 

 show minute areas with bright lustre. The dark areas consist of 

 irregular crystals and patches of Aegirine. The .specific gravity of 

 the rock is 2.59. 



Microscopic.~-T\\e rock is seen to consist of three minerals, 

 which are identified as nepheline, aegirine and soda-orthoclase. 

 The minerals crystallised in that order since the felspar is moulded 

 on the two other minerals, while the aegirin;?, idiomorphic to the 

 felspar is in places moulded on the nephehne. A determination of 

 the quantitative gravimetric proportions of the constituent 

 minerals by Rosiwal's method^ gave the following percentages by 

 weight : — 



Soda Orthoclase . . . . . . . . =46.4 



Nephehne . . . . . . . . =28.5 



Aegirine . . . . . . . . . . =25.1 



Total =100.0 



The texture of the rock is nephehnitoid and trachytic. The 

 nepheline occurs mainly as fairly equi- dimensional rectangular 

 phenocysts with some hexagonal sections. The aegirine is partly 

 elongated and lath shaped, but partly as irregular masses. Its 



1 Rosiwal: Vcrhandl. d. K.K. Gfol. Reichiustalt ,1898, p.p. U:.',, el seq. 



