BY D, MAWSON. 601 



Zircon — Only very exceptionally are crystals answering to 

 zircon met with. These are developed as microscopic laths with 

 high double refraction and straight extinction. 



Apatite is wanting in the vein-products, though in the syenite 

 it is often exceptionally well developed as idiomorphic hexagonal 

 prisms. 



Small colourless to faintly greenish crystal fragments with 

 optical properties very close to acmite were occasionally noted, 

 and are probably a variety of pyroxene inclined towards lavenite. 

 In one case, where a comparison was possible, the crystallisation 

 period was determined as preceding that of the segirine. 



Secondary Minerals. — Bitumen, as described under veins of 

 type (a), is found tilling microscopic fissures and neighbouring 

 minute cavities. It is no doubt a condensation from vapours 

 originating with the distillation of the coal beds at a depth 

 below. 



Examined microscopically, it appears black by reflected light 

 and also by transmitted light except in the thinnest sections, 

 when it is red-brown, resembling heematite. It is non-pleochroic 

 and isotropic. 



Chalcedony lining cellular spaces in a zonal arrangement of 

 parallel fibrous bands; also as spherulitic radial aggregates, 

 especially so in the larger veins. 



Quartz, in comparatively large crystals, often forms the central 

 filling of original cavities; frequently they remain drusy. Much 

 of the quartz exhibits shadowy extinction and is sometimes built 

 up of roughly arranged sectors; after a detailed examination, it 

 was decided that this is a case of optically anomalous quartz and 

 not tridymite as might have been suspected. 



In several instances, parallel fibrous material resembling chal- 

 cedony was proved by its positive character to be quartz. 



Especially where metachemisation has been extreme, a large 

 bulk of the vein is occupied by a mixture of radial fibrous chal- 

 cedony and quartz; in such cases the secondary products always 

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