OCCURRENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 365 



zonally arranged. The trichites mentioned in the quartz do not appear 

 to be represented, but otherwise they are similar. 



Biotite is very abundant. It occurs in ragged sections which give very 

 marked evidence of the crushing to which the rocks have been subjected. 

 In ordinary light the colour is bright yellow on vertical sections to 

 reddish-brown on sections near the base. Basal sections are practically 

 opaque. The pleochroism is very intense, absorbtion of light vibrating 

 parallel to the cleavage being almost complete. (In the thick slice this 

 gives rise to a remarkable appearance where a thin film of biotite over- 

 laps a section of quartz and felspar. The biotite acts as an analyser and 

 the polarisation colours of the other mineral appear even though the 

 upper nicol is out of the axis of the tube). The biotite flakes are fairly 

 free from decomposition. In one or two places a little greenish chloritic 

 material is present. One of the remarkable features of the rock is the 

 presence of numerous aggregates of faintly bluish-green talc or sericite. 

 These represent the decomposition products of some previously existing 

 mineral. In one of the aggregates there is apparently a kernel of greenish- 

 yellow epidote, showing that whatever may have been the character of 

 the primary mineral, epidote was one of the alteration products. The 

 aggregates are parallel tufted and irregular. It is obvious that there 

 has been an increase of bulk during the process of decomposition which 

 gave rise to the talcose mineral, since the surrounding minerals have 

 been very much broken up by cracks radiating from the aggregates and 

 the talc (?) has been injected irregularly through these cracks. 



Among the accessory minerals other than the rutile and apatite already 

 mentioned, are apatite (in larger prisms) zircon and pyrites. 



The apatite is fairly abundant, particularly as inclusions in the biotite. 

 In one case an apatite, itself an inclusion in the biotite, includes a well 

 developed zircon. 



Zircon is remarkably abundant, especially in the biotite, where it pro- 

 duces very strongly marked pleochroic halos. 



Pyrites occurs in two distinct habits, (i.) as small dusty grains which 

 accompany the regularly arranged fluid and gaseous enclosures of the 

 quartz; (ii.) larger tufts and perfect cubical (pyritohedral) crystals 

 scattered through the rock. 



Basalt from between Boggy Plains and Pretty Point Kosciusko. This 

 rock has a pilotaxitic base, with marked flow structure composed of felspar 

 and brownish augite microlites with a lot of minute crystals of magnetite. 

 There is no glass present. The felspar microlites are somewhat basic, 

 the refractive index, as tested by Becke's method being distinctly higher 

 than that of Canada balsam, and in some sections they have extinction 

 angles up to about 30°, measured from the Carlsbad twinning plane. 

 Colourless augite is present in idiomorphic stunted prisms with pyramids. 

 They have a small opti-axial angle and good cleavage. They decompose 

 into light green products, apparently chlorite. There are also present 



