142 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Comparison of other Specimens with the Type Specimen. 



Other specimens from this locality tend to emphasise the subsequent meso zone 

 impress. Specimen No. 758 possesses more prominent schistosity, partly due to the 

 presence of prominent quartz lenticles on the weathered surface. In section the example 

 is noteworthy for its more granoblastic character and its coarse perthite. In addition 

 there are clusters of coarse brown biotite associated with ilmenite and with odd grains 

 of hypersthene. The brown biotite with a green transition stage can be found developing 

 from the hypersthene. Garnet is present, and biotite, without the ilmenite, is developing 

 from the garnet, probably by reaction with felspar. These observations, therefore, 

 tend to confirm the meso zone changes reported from the type example. 



Two other specimens (Nos. 756, 757) were collected and reported by the sledge 

 party to be variations in the gneiss, but less plentiful than the type example. These 

 variations are found to consist of the gneiss with attached portions of metamorphosed 

 aplitic veins. In each case the boundary is more or less destroyed by metamorphic 

 diffusion, and consequently they are now all part of the gneiss. Specimen No. 757 

 consists of a granoblastic mass with weak crystallisation schistosity and well-developed 

 diablastic structure. Here quartz and cloudy felspar (orthoclase, perthite, and a sodic 

 plagioclase) form the bulk of the rock. Garnet is present and associated with biotite 

 with alteration to chlorite. Ilmenite and zircon are common accessories, and a green 

 spinel, probably hercynite, is present. 



Specimen No. 756 is a coarser quartz felspar vein. It is more massive and possesses 

 more cataclasis. The big crystals have produced in part a mortar structure. The 

 junction with the gneiss is not noticeable in thin section. In a section across 

 this junction it is simply noticed that one part of the slide carries garnet and biotite 

 clusters with a little hypersthene, while these are absent in that part which represents 

 the original vein. 



There remains one other specimen from Aurora Peak. It has been described by 

 the collector as a specimen illustrating the transition between the dark band (plagioclase 

 pyroxene gneiss) and the hypersthene alkali felspar gneiss. It is rather a dark-coloured, 

 banded specimen. Some of the bands are white, others consist of coarse quartz blebs 

 set in a fine matrix, and others again of very fine black material. The white parts 

 consist of quartz, felspar, more biotite than usual, occasional garnet and hypersthene 

 with accessory ilmenite, zircon, and apatite. There has been considerable cataclasis in 

 which big crystals have frequently assumed a lenticular shape. The crystals may be 

 surrounded by a granulated zone, and biotite crystals may be set in that zone and tend 

 to wrap themselves around the crystal. The dark bands appear to be slaty bands, 

 out of which oval-shaped crystals of secondary quartz have arisen. On closer examina- 

 tion this is not so. Some of the apparently secondary quartz consist of crushed 

 granulitic aggregates. Some are relic felspar crystals, while occasionally we find pale 

 relic crystals of hornblende. Wrapped around them is a fine, dark pleochroic aggregate 



