OCCURRENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 359 



porphyritic by augite and olivine, and contains numerous enclosures 

 of granite an inch or more in diameter. A further description of 

 one of the local basalts is given later in this paper. 



There is thus nothing in these basalt dykes, except their fresh 

 state of preservation, strong intrusion into the granite and approxi- 

 mate parallelism of strike (with that of the tinguaite) which 

 connects them with the last mentioned rock. It is, however, 

 possible that the dykes may be complementary to one another. 



The tinguaite of Kosciusko is brownish-grey in colour with a 

 faint tinge of green, and in this respect differs from the "con- 

 spicuously green" phonolitic nephelinite of S. Antao. The 

 Kosciusko rock has not the sonorous ring of some phonolites. 

 It breaks readily under the hammer with an uneven to sub-con- 

 choidal fracture, and has a somewhat greasy lustre. Macroscopic 

 whitish-grey to pale reddish-grey crystals of nepheline can be seen 

 on freshly fractured surfaces, giving the rock a somewhat pseudo- 

 porphyritic appearance, though probably the rock is not really 

 porphyritic in the strict sense of that term as used by Rosenbusch. 



III. Microscopic Character. — The specimens in the following 

 description were taken from the following portions of the dyke: — 

 No. 1 from northern side of dyke within six inches of its plane of 

 junction with the granite. No. 3, from the centre of the dyke. 

 No. 4, from the southern side of the dyke within six inches of its 

 plane of junction with the granite. 



No. 1. In thin section the rock is seen to possess a hyalopilitic 

 texture. A considerable amount of residual glass is present. It 

 is colourless, but contains fairly numerous dusty crystallites. A 

 very marked flow structure is apparent. None of the minerals show 

 definite evidence of occurring in more than one generation, though 

 the rock is rendered pseudo-porphyritic by occasional largely 

 developed nephelines. The reasons for considering these large 

 phenocrysts of the same generation as the small individuals of the 

 base are : — (a) there is a perfect gradation from the largest crystals 

 to those of sub-microscopic dimensions ; (b) there is no distinction 



