OCCURRENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 363 



felspathic inclusions there are one or two included fragments of 

 greenish-brown biotite, which are corroded though not to the same 

 extent as the felspar. These fragments, derived probably from 

 the granite, intruded by the dyke, are to be distinguished from 

 the accessory authigenic biotite of the dyke rock. The chemical 

 analyses given below show the presence of some chlorine. This 

 indicates sodalite as a constituent mineral, but there is no con- 

 firmatory microscopic evidence as to its presence. It is quite 

 probable that some of the material noted as "glass" should be 

 referred to this mineral. 



No. 3 specimen from the centre of the dyke. This rock is so 

 distinct from those hitherto studied as to belong to quite a different 

 rock-type. It is holocrystalline, though finely so. Flow structure, 

 if present, is very obscure. Nepheline is apparently absent. By 

 far the most abundant mineral in the rock is the felspar, which 

 occurs in the form of irregular sections, which are not twinned. 

 It is exceedingly difficult to determine which felspars belong to 

 the dyke rock proper, and which to the included granite. Some 

 definite laths of sanidine occur, but the greater part is distinctly 

 a plutonic acid felspar. With this felspar is associated abundant 

 muscovite in large irregular plates. Quartz is also present in 

 small quantities. The segirine here differs from that already 

 described only in its habit and in the comparative absence of 

 decomposition. 



The section as a whole is very difficult to interpret. We are 

 inclined to think it does not represent a fair sample of the dyke 

 rock, but rather belongs to a part of it which has had its characters 

 entirely altered by almost complete solution of a large fragment 

 of included granite. The plutonic character of such a large per- 

 centage of the minerals, and the occurrence of abundant muscovite 

 is in favour of this hypothesis. The fragments which can be 

 definitely made out to be inclusions are very numerous. Others, 

 exceedingly like them, shade off insensibly into the groundmass. 

 The peculiar habit of the segirine, namely, its occurrence with the 

 ends of the prism round or conical as if due to solution is sugges- 



