THE METAMORPHIC ROCKS OF ADEL1E LAND.-8T1LLWELL. 13 



In the Cape Gray dyke series the metamorphism of the primary dolerite has occurred 

 under very different conditions from those which operated at Cape Denison. The 

 mineral changes have occurred under conditions which have not destroyed the dyke 

 form. The dykes branch and send forth little tongues into the garnet gneisses, in the 

 same manner as in unmetamorphosed regions. Minerals and structures of the primary 

 dolerite may be recognised, but in all cases the metamorphic features dominate the 

 igneous character. The mineral changes can be directly traced and prove to be very 

 interesting. The relic pyroxene of the dolerite is usually found to be crowded with 

 minute, dusty inclusions of ilmenite (schiller inclusions). On recrystallisation the 

 pyroxene may pass into a granular aggregate of clear secondary pyroxene, including 

 both hypersthene and augite, while the minute ilmenite inclusions coalesce into crystal 

 units. If the pyroxene then passes over to hornblende, amphibolites are produced. 

 The pyroxene may also react with the anorthite molecule of the labradorite and produce 

 garnet and quartz. The formation of the latter may be preceded by the development 

 of a diablastic intergrowth of felspar with vermicular pyroxene which may extend as 

 a reaction rim around the pyroxene crystals. If part of the pyroxene is amphibolised 

 at the same time, a garnet amphibolite is produced. 



The grain size of the recrystallised rock depends entirely upon the metamorphic 

 conditions. An early stage of the recrystallisation of a dolerite may be the production 

 of a fine-grained aggregate consisting of felspar and augite. A later stage involves the 

 growth of large crystals at the expense of smaller ones, and the production of a 

 moderately coarse grained rock. Rosiwal analyses have demonstrated the similarity in 

 percentage in mineral composition between a fine-grained type at Cape Gray, a coarser 

 type at Aurora Peak, and still coarser type at Madigan Nunatak. As the dyke origin 

 of the Cape Gray type is certain, sound evidence is thus brought forward concerning the 

 origin of the plagioclase pyroxene gneisses at Aurora Peak and Madigan Nunatak. In 

 this case, and in many others in this series, there are similarities between these Antarctic 

 rocks and the rocks called pyroxene granulites in Saxony, and norite in India. Evidence 

 is therefore provided for the metamorphic nature of these rocks. 



The acid hypersthenic gneisses of Madigan Nunatak, Aurora Peak, Stillwell Island, 

 and Cape Pigeon Rocks possess affinities due to similar conditions of complete recrystal- 

 lisation. The primary igneous nature of the first two is determined by their chemical 

 composition, which closely resembles that of granites, while the igneous nature of one 

 example from the third occurrence is indicated by its dyke form. These rocks are found 

 to be very closely related to the hypersthene rocks in India, which have been described as 

 charnockite, and which have been looked upon as igneous rocks that have consolidated 

 under phenomenal conditions. The metamorphic nature of the Antarctic rocks is 

 maintained, and argument is found to show that the charnockites are also metamorphic 

 rocks which have been completely recrystallised under deep-seated metamorphic 

 conditions. 



