OCCURBENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 377 



Muniongite, "Muniong" being the native name for the Kosciusko 

 Plateau. 1 



VI. Age. — Lastly as regards the age of the nephelinitic tinguaite 

 of Kosciusko, it is obvious that it is of newer date than the 

 gneissic granites which it intersects, and that it is much newer also 

 than the sedimentary rocks containing radiolaria and graptolites, 

 which have been intruded by the granites. These sedimentary 

 rocks, as already stated, are of Lower Silurian (1) (Ordovician) 

 age. The granite therefore, is Post-Ordovician, and it is obvious 

 that the tinguaite is very much newer than the granite. This is 

 proved by the fact that the minerals in the nepheline dyke rock 

 are mostly remarkably fresh and free from decomposition, and in 

 the second place by the fact that there is an entire absence of 

 planes of foliation or cleavage from the dyke rock. 



As already stated these nepheline dyke rocks of Kosciusko 

 appear to be of approximately the same age as the basalt dykes 

 of the same neighbourhood. There can be little doubt that the 

 latter belong to some part of the Tertiary Era. If, as seems not 

 improbable, the eruptions which produced the nepheline rocks of 

 Kosciusko were synchronous with those which produced the 

 solvsbergite lavas of Mount Macedon, Victoria, they would then 

 slightly antedate or be nearly contemporaneous with the " older 

 basalts," so called, of Victoria, as according to Professor Gregory, 

 the latter are near in age to perhaps a little newer than the Mount 

 Macedon nepheline rocks. 2 



1 Since the recent important discoveries by Mr. J. E. Carne of at least 

 eleven mountains of nepheline-orthoclase-segirine rocks in the neighbour- 

 hood of Barigan, near Lue, it has become apparent that the Kosciusko 

 rock is more or less closely related mineralogically and chemically to 

 these rocks of Barigan. The Barigan rocks pass, at their margins, into 

 a fine-grained selvage very similar to the Kosciusko rock, Possibly at a 

 depth the Kosciusko rock would also pass into ' Bariganite/ For the 

 present, however, we think it better to retain for it the name given above. 



2 Proc. Eoy. Soc. Vict., Vol. xiv., (New Series) Pt. ii., pp. 185 - 217, 

 -Pis. xi. - xvii., 1901 [published 1902.] 



