302 



AUSTRALASIAN ANTAECTIC EXPEDITION. 



A portion of the rock, which was very rich in tourmaline, was sectioned in a 

 direction at right angles to the schistosity. A Rosiwal analysis resulted in the following ; 



Quartz, cordierite, and sillimanite ... ... 47-5 



Tourmaline 30-1 



Magnetite 21-4 



Apatite ... 1-0 



The tourmaline is a strongly pleochroic variety, changing from green to a 

 reddish- or brownish- violet. The flakes show a rather infrequent cleavage and contain 

 gas, magnetite, and minute zircon or monazite inclusions, sometimes with a definite 

 alignment. Where these minute zircons or monazites are present, there is occasionally 

 a distinct lightening of colour in the form of a halo around the inclusion. The colours 

 of the tourmaline sometimes vary locally in a zonal fashion. The mineral is associated 

 with magnetite, both occurring in bands throughout the rock. 



Cordierite is abundant and exhibits a great number of well-formed pleochroic 

 halos around inclusions of either zircon or monazite (Plate II, fig. 6.) While the 

 experience of Joly and Fletcher* has indicated that halos in cordierite cannot be used 

 for accurate measurement, the halos in this rock are remarkably distinct and capable 

 of measurement. By means of a Beck micrometer eye-piece and a 4 mm. (x 45) objective, 

 the following results were obtained, the radii given being always the distance from the 

 outer edge of the halo to the edge of the inclusion. All measurements are in millimetres. 



TABLE VIII. 



* " Pleochroic Halo." Phil. Mag., 6th series, vol. xix, 1910, p. 635. 



