154 AUSTKALASIAN ANTARCTIC EXPEDITION. 



on the triangular projection close to the mean position of Group II., the group of the 

 aluminium silicate gneisses. No. 772 also lies in the same area, but in a direction tending 

 towards the mean position of the plagioclase gneisses. 



Classification. 



The gfoup values, considered collectively, place both rocks in the group of the 

 aluminium silicate gneisses. In all cases the values fall within the assigned limits of 

 this group, except the value of T in No. 772, which is just below the lower limit (3-0). 

 The other types from this region, viz., Nos. 770, 777 from Garnet Point, No. 917 from 

 Stillwell Island, No. 785 from the Cape Pigeon Rocks, probably belong to the same 

 group, though certainty is unattainable without a chemical analysis. 



The dominating garnet and cordierite with sillimanite in No. 784 means that the 

 recrystallisation of the sediment took place under conditions of very high temperature 

 and great hydrostatic pressure, i.e., under the conditions of the kata zone of 

 metamorphism. No. 784 is therefore placed in the kata division of the aluminium 

 silicate gneisses. In the rocks Nos. 772, 770 from Garnet Point it has been noticed 

 that the garnet and felspar with, perhaps, sillimanite have been replaced by biotite 

 and quartz. This change is considered to occur in transition from the kata zone to the 

 meso zone. When the meso zone metamorphic conditions are dominant, the areas of 

 sillimanite, cordierite, and garnet become areas of secondary relics and indicate the 

 double phase of metamorphism of the original sediment. 



No. 777, the second type from Garnet Point, containing enormous garnets, also 

 shows trace of the meso zone conditions. The large garnets show considerable alteration 

 to biotite and quartz, and some of the felspar is dissociated into perthite and myrmikite. 

 The small development of sericite and saussurite brings in an epi zone element. Hence, 

 though the abundant large garnet rocks at Garnet Point are indicative of kata zone 

 metamorphism, there is also the impress of meso zone conditions, which is sufficient 

 to place these examples in the meso division of the aluminium silicate gneisses. 



The garnet felspar gneiss, No. 917, from Stillwell Island, is like the garnet felspar 

 gneiss from Garnet Point, and shows evidence of meso zone conditions. In this case 

 cataclasis is present, and we find a portion of the quartz and garnet granulated. The 

 felspar has become more cloudy, owing to further sericitisation and saussuritisation. 

 Hence, while the evidence of meso zone conditions is greatest, there appear the initial 

 stages of epi zone metamorphism. 



THE ACID HYPERSTHENIC GNEISSES OF STILLWELL ISLAND AND THE CAPE PIGEON 



ROCKS. 



In addition to the garnet felspar gneisses a second type, related to the acid 

 hypersthene gneisses of Madigan Nunatak and Aurora Peak, was discovered on Stillwell 

 Island and the Cape Pigeon Rocks. In one case on Stillwell Island this type of gneiss 

 appears in dyke form. The same form of occurrence is strongly suspected at the Cape 

 Pigeon Rocks, and similar rocks can be remembered at Garnet Point, though no specimens 

 are in the collection. 



