298 J. A. THOMSON. 



known refringence prove the maximum refractive index to 

 be in the neighbourhood of 1*658. This combination of 

 properties precludes identification with any well known 

 uniaxial mineral. To test the possibility of the miuerai 

 being phenacite, Mr. G. J. Burrows very kindly undertook 

 a qualitative examination of the rock for beryllium, but 

 with negative result. There is no common biaxial mineral 

 of low axial angle which agrees in all the above characters, 

 and as the amount of material was too small to permit of 

 isolation and chemical analysis, the mineral must be left 

 unidentified for the present. 



The other minerals present are quartz and orthoclase in 

 large amount, magnetite in smaller quantity and occasional 

 crystals of zircon. The yellow colour of the rock is due to 

 staining by limonite, the orthoclase in particular being 

 striated by plates of this mineral along the cleavage planes. 

 The quartz and orthoclase form an uneven grained mosaic 

 with a limonitic cement, a structure which in many respects 

 suggests a clastic origin. On the other hand the nearly 

 constant orientation of the unnamed mineral, and an alter- 

 nation of bands of clear mosaic with other bands containing 

 magnetite and limonite points more strongly to a parallel 

 structure developed in situ (Fig. 3, Plate XIV). It is 

 reasonable to suppose that the rock is a member of the 

 gneissose series. 



There is only one rock that resembles the rocks of the 

 auriferous areas of Western Australia, and it is from the 

 Oavanagh Range. The hand specimen is a light green 

 aphanatic rock, which shows when wetted a few veinlets 

 of lighter colour. The section shows that it consists pre- 

 dominatingly of fine grained albite and clinozoisite with 

 smaller amounts of a pale actinolite, chlorite, sphene and 

 accessory apatite. The felspars, in bundles of sub-radiating 

 prisms, form a network within which the other minerals 



