BY REV. J. MILNE CURRAN, F.G.S. 975 



shai'ed my views on the matter with ]Mr. W. Anderson, and 

 decided to examine the locality, when we were rewarded by 

 discovei'ing a patch of bome live square miles of a leucite-basalt 

 similar in every respect to the rock I found at Byerock. At the 

 latter place the basalt rests in part on a silurian slate and an 

 intrusive granite, while near Cobar the underlying rocks are 

 silurian slates and conglomerates, and a silicitied tertiary drift. 



The texture of the leucite-rock varies somewhat in different 

 localities. Macroscopically there is nothing in the external appear- 

 ance of the rock to indicate its richness in leucite It occurs as a 

 dark fine-grained rock with patches of a reddish-brown mineral — 

 probably mica. The dark, almost black colour is due to the great 

 abundance of magnetite the rock contains. When a thin slice is 

 examined the leucite is found to be the dominant ingredient in 

 the mass. It is distributed in closely packed, rounded or roughly 

 octagonal grains between which no vitreous matter can be 

 detected. The leucite is more abundant than in any of the 

 typical leucite-basalts figured by Fouque and Michel Levy*, or 

 Rosenbuschf, and almost as plentiful as in the American rocks 

 already I'eferred to J. In a few of the slices I have detected the 

 well-known and characteristic radial and concentric interpositions 

 so common in the Vesuvian leucites. 



As far as is known at present leucite rocks occur only among 

 tertiary and recent volcanic productions. The Australian example 

 proves no exception, for both Byerock and El Capitan basalts 

 belong certainly to tertiary, and to my mind latter tertiary times. 



For detailed microscopic examination I have sent some slices to 

 our eminent Australian petrologist, Mr. A. W. Howitt, of Sale, 

 ■Gippsland. 



*Mineralogie Micrographique. Planches 48, 49, 50, and .51. 

 tMicro. Phys. wichtigen Mineralien, Taf. 14. (Zweite Auflage). 

 + Micro. Petrography, by F. Zirkel— (Geol. of Fortieth Parallel). 



