THE METAMORPHIC ROCKS OF ADELIE LAND STILL WELL. 143 



which is found to consist of minute biotite in which one can detect the incipient appear- 

 ance of large biotite crystals mixed with some ilmenite dust. In places the dark bands 

 and the finely granulitic material form a set of parallel bands and sometimes they 

 penetrate the relic crystals. These dark bands of slaty appearance may possibly be 

 formed by a continuation of the early stages of the same processes which cause the 

 biotite to wrap itself around the crystals in the white portions of the rock. If, in addition, 

 the dark zones represent zones of shear it may become possible to understand why they 

 should be zones of excessive cataclasis and granulitisation. No large individuals exist 

 in the crush zone which do not show strain polarisation and are not surrounded by a 

 zone of granulitised material. 



The early examination of this rock gave the impression that it represents the 

 remains of a recrystallised sediment, and that the dark bands were originally slate. 

 If this were so, and the field report is correct, it means either that the plagioclase 

 pyroxene gneiss is a recrystallised sediment a conclusion directly opposed to the study 

 of the rock or that there is a recrystallised sedimentary gneiss at Aurora Peak which 

 escaped the observation of the sledging party. The specimen is not a transition type 

 between the plagioclase pyroxene gneiss and the hypersthene alkali felspar gneiss as it 

 is reported to be. The only possible explanation is that it represents a shear zone in 

 which finely powdered biotite and ilmenite dust have dominated the colour. Large 

 crystals of biotite could form, and it may be a stage of the process in which biotite 

 crystals wrap themselves around crystals of quartz and felspar in the uncoloured portion 

 of the rock. 



