OCCURRENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 361 



It occurs in exceedingly slender lath-shaped sections about -125 

 mm. long by -007 mm. broad. Some of these are distinctly singly 

 twinned, as observed with a ^th inch objective, have a refractive 

 index somewhat lower than that of Canada balsam, and exhibit 

 the characteristics of sanidine. On the other hand, as observed 

 by Mr. Gr. W. Card, some of the felspar microlites extinguish at 

 an angle of about 10° measured from the clinodiagonal or brachy- 

 diagonal, on the assumption that the microliths have been elongated 

 in the direction of that axis. This points to the probability that 

 some, if not all, of the felspar microliths are not normal sanidine. 

 They may be soda-bearing varieties or even albite. The principal 

 ferromagnesian constituent of the rock is segirine. In reflected 

 light most of the pyroxene is seen to be somewhat decomposed, 

 possessing a light greenish-yellow colour, but where undecomposed 

 it is blackish-green. In transmitted light the colour is dark green. 

 Many of the individuals are almost opaque owing to decomposition 

 products. The habit is prismatic, the larger prisms being con- 

 spicuously frayed out at the ends. Besides the prisms, there are 

 numerous tufty aggregates with a slight tendency towards radial 

 spherical arrangement. The pleochroism is very strong in brown 

 and dark blue-green tints. The extinction angle is small, not 

 more than 4° or 5° from the axis of the prism, but exact measure- 

 ments are rather difficult to make. The maximum length is 

 about -25 mm., and the smallest individuals sink to microlitic 

 dimensions. A very small quantity of biotite is also present of 

 same order of size as the larger aegirines, and its pleochroism is in 

 dark greenish bronze to opaque. A little magnetite is scattered 

 through the section. Mr. G. W. Card suggests that some of the 

 curved microlites may be melilite. 



The most remarkable feature of the rock is the abundance of 

 included fragments. In ordinary light these are colourless but 

 somewhat clouded by decomposition products which appear opaque 

 white in reflected light. The double refraction of these is stronger 

 than that of nepheline. The refractive index is lower than that 

 of Canada balsam in the cases where a comparison is possible 



