124 AUSTRALASIAN ANTARCTIC EXPEDITION. 



The separation of the anorthite part of the molecule of the plagioclase, and the 

 consequent production of the highly sodic felspar, has liberated more lime and probably 

 accounts for the unusually high epidote percentage. In this respect this rock is different 

 from the average Cape Denison amphibolite, but agrees with the exceptional case, No.5. 

 At Cape Denison we find a high percentage of epidote with the high percentages of 

 biotite and the low percentages of hornblende, whereas in this case, as in No. 5 before, 

 we get a high percentage of epidote even with a low percentage of biotite. This 

 difference is no doubt due to slightly different conditions during recrystallisation, which 

 are reflected both in the composition of the plagioclase and in the epidote percentage. 



No. 983. No. 983 is an example of the chief rock type. It looks like a coarse, 

 massive biotite granite in the hand specimen, showing quartz and felspar and biotite. 

 The biotite flakes of the normal granite are replaced by aggregates of small biotites. 

 The pink colour of the felspars is inclined to dominate the colour of the rock, and its 

 general appearance is different from that of the grey granodiorite gneiss of Cape Denison. 



Under the microscope there is little doubt that this rock has been subjected to 

 metamorphic agencies similar to those interpreted in the Cape Denison rock, and it 

 must be classed as a gneiss, not as a granite. The large crystals of quartz show strong 

 cataclasis. Large crystals of felspar, probably orthoclase, have been replaced by 

 granoblastic aggregates of microcline. A large crystal of orthoclase, which is cloudy 

 with the development of sericite, encloses areas, with more or less rounded outline, of 

 perfectly clear microcline which is certainly due to the recrystallisation. The crystals 

 of plagioclase have not been found with a refractive index above that of basal quartz. 

 They are interpreted as an oligoclase, being less calcic than in the granodiorite gneiss. 

 Diablastic structure is often developed in the plagioclase while -rounded and vermicular 

 pieces of quartz may be set in the felspar. The felspar crystals become more noticeably 

 cloudy in the crush areas. 



The crush areas, produced by the grinding movement, can be recognised between 

 two large crystals. Mortar structure, however, is not obvious, because recrystallisation 

 has proceeded in the crush zones and caused the development of comparatively large 

 granular crystals. The development of these even-sized crystals in the crush zones 

 and the replacement of large crystals by granulitic crystals may be considered as a 

 stage in the development of granoblastic structure in a completely recrystallised rock. 

 Biotite is abundant in these areas and it is often accompanied by muscovite. These 

 two minerals are nearly always arranged around the contours of the relic minerals. 

 Sericite, epidote, and muscovite are intergrown with the biotite. 



Large crystals of allanite, apatite, and sphene are accessory constituents. 



The rock may be called a granite gneiss or an epi orthoclase gneiss. Compared 

 with the granodiorite gneiss of Cape Denison there seems to be more orthoclase (or its 

 equivalent) and a less calcic plagioclase. 



