316 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Regional metamorphism has been the chief determinative factor in the constitution 

 of this rock. The high temperature-pressure conditions which prevailed were followed 

 by thermal metamorphism where pressure no longer was one of the dominant factors 

 operating. This throws new light on the metamorphic transformations which have taken 

 place in the rocks of the island. 



The large garnet crystals which dominate the slide have been formed under the 

 first set of conditions mentioned above; but the masses of strongly coloured brown 

 biotite and little accompanying flaky chlorite scattered throughout their whole mass 

 are more likely products of the second set of circumstances. As is true in the case of the 

 red garnets of the Scottish Highlands (Harker, p. 324), so here the formation of biotite 

 which has assumed a, decussate arrangement in some places and rosette forms in others 

 has from the garnet liberated ilmenite, which is seen in skeletal crystals, and some 

 interstitial cordierite which, though clear, is usually intensely crushed. The cordierite is 

 recognised by the presence of intensely pleochroic yellow haloes and its biaxial nature 

 (2V large, approximately 80). The alteration of the chemical nature of the minerals 

 of the rock has proceeded, pari passu, with the disintegration of the crystal form. 

 Smaller, more or less skeletal, masses of garnet are separated from one another by the 

 products of alteration, ilmenite, cordierite and biotite. 



The pleochroism of the biotite (apparently titania-rich) is intense, the colours 

 being Z deep red-brown, and X very pale brown. The absorption as usual is Z>X. 

 Other features of the biotite are the presence of numerous pleochroic haloes with zircon 

 nuclei and the bent nature of the laminae. The bending shows that a shear stress has 

 been operative to a perceptible degree. 



In the coarser parts of the rock the acidic plagioclase present occurs in large grains 

 which, however, have been considerably crushed, but more especially is this noticeable 

 in the neighbourhood of the biotite and the garnet. The plagioclase is not so clear in this 

 rock as in most rocks from this area, nevertheless it shows albite and pericline twins 

 very well developed. Bends in the twin lamellae are further evidence that shear has 

 occurred in the history of the rock. Quartz grains are associated with the plagioclase, 

 particularly in the parts where granulitisation has been most intense. 



In the finer parts of the rock practically no garnet is seen at all. In its stead 

 is a granulitic aggregate of cordierite and clear acid felspar shot through with flakes of 

 brown biotite and scales of very green chlorite. The disintegration which is noticeable 

 in the very large garnet crystals of the coarser part of the rock has been able completely 

 to change the nature of the smaller garnets and this mass of cordierite, acid plagioclase, 

 biotite and chlorite, maintaining the position and size of the earlier garnet, is 

 paramorphous after the garnet. The granular nature of the aggregates is realised more 

 clearly under crossed nicols, for then the heterogeneously oriented granules of 

 cordierite form a definite mosaic. 



