376 T. W. E. DAVID, F. B. GUTHRIE, AND W. G. WOOLNOUGH. 



conspicuously green. This description exactly fits the Kosciusko 

 rock with the exception that (1) it is doubtful whether two 

 generations of nepheline are present in the Kosciusko rock, though 

 at first sight there appear to be. Brogger's remark about the 

 nepheline-porphyry of the Lougenthal, we think, is applicable to 

 the Kosciusko rock, in which we are of opinion, in spite of its 

 porphyritic appearance, that only one generation of nepheline is 

 present. (2) The Kosciusko rock is rather brown tinged with 

 green, like a typical phonolite, than "conspicuously green." 1 



As the microliths of felspar present in the Kosciusko dyke rock 

 belong to an alkali felspar rather than to a lime-soda or lime felspar 

 the rock cannot be classed with the nepheline tephrites, and it is 

 too basic to be grouped under grorudite or solvsbergite. The 

 nepheline-tinguaites, however, have much closer affinities with the 

 Kosciusko rock. At the same time as regards structure the rock 

 is rather nephelinitic than tinguaitic. Some tinguaites which 

 when rich in nepheline approach in character to the Kosciusko 

 rock have been described by G. T. Prior 2 from the neighbourhood 

 of Adowa and Axum in Abyssinia. The Abyssinian tinguaites 

 are, however, more acid than that of Kosciusko containing nearly 

 58% of silica as compared with the 51 \ / o in the Kosciusko rock. 



The provisional conclusion arrived at by us is that this Kosciusko 

 rock does not exactly resemble any definite type of rock known to 

 us from other parts of the world with the exception of the 

 nepheline-segirine-alkali-felspar rock of Barigan in New South 

 Wales. 



As the rock is distinctly hypabyssal and intermediate between 

 the (plutonic) nepheline-syenites and the (volcanic) phonolites we 

 have decided to group it provisionally with the tinguaites rich in 

 nepheline, and propose to call it nephelinitic tinguaite, var. 



1 Kecent observations in the Barigan district near Lue, N. S. Wales, 

 convince us that the brownish tinge of the Kosciusko rock is simply due 

 to weathering and that the rock at a slight depth would be distinctly 

 green. 



* Min. Mag., xn., 57, pp. 268-269, July 1900. 



