OCCUKRENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 379 



have shown that these rocks have intruded the strata of Pernio 

 Carboniferous age. It is probable that they are older than the 

 Tertiary basalts of Tasmania, but definite evidence on this point 

 as well as on the exact relation of this soda series to the vast 

 intrusive masses of diabase of the same island has not yet been 

 obtained. On the whole, until more accurate evidence is available, 

 it may be concluded that the nephelinitic tinguaite of Kosciusko 

 is probably of Upper Mesozoic age, but may possibly be Eocene. 



VII. Summary. — The evidence so far collected proves that a 

 highly interesting nepheline rock, for which we propose the name 

 nephelinitic tinguaite var. muniongite, is developed as a dyke rock 

 at Kosciusko. Its chemical composition proves that it is on the 

 border line, as regards silica percentage (51 J%), between the inter- 

 mediate and the basic groups of eruptive rocks, and tested rigidly 

 by its silica percentage it falls within the latter group. At the 

 same time it is obvious that its general physical characters ally 

 it more to the phonolites than to the nephelinites. We should 

 have no hesitation in classing it at once as a phonolite were it not 

 for the absence of distinct and well formed crystals of potash felspar 

 and for the fact that it is hypabyssal. As already stated minute 

 lath-shaped crystals of soda-sanidine or soda-orthoclase are 

 present in some numbers in the base, but they are of the nature 

 of microliths. They constitute perhaps about 42% of the whole 

 rock. Nepheline is very abundant, occurring in sharply idiomor- 

 phic crystals varying in size up to about 3 mm. in length by nearly 

 the same in breadth ; in one case a crystal of sanidine was 

 observed to be enclosed in the centre of one of these nepheline 

 crystals. (See Plate i, fig. 4.) 



^girine is also very abundant, and forms over 13% of the 

 whole rock as compared with the 35% formed by the nepheline. 

 The presence of 07% of chlorine implies that a little sodalite is 

 also developed. The analcite observed was in every case of 

 secondary rather than of primary origin. The same remark 

 obviously applies to the tufted aggregates of natrolite. A little 

 biotite is present, but much at all events is of derivative origin. 



