THE METAMORPHIC ROCKS OF ADELIE LAND. STILLWELL. 191 



Orthorhombic and monoclinic forms are present in each case. The primary augite in 

 No. 773 contains abundant minute inclusions of ilraenite, and these were neglected in 

 the count of iron ore. There is, therefore, reason for the smallest iron ore percentage 

 in No. 773. 



Thus the proportions of felspar and pyroxene in the plagioclase pyroxene gneiss 

 at Cape Gray with average absolute grain size approximately 0-05 mm., whose primary 

 dyke origin is beyond all possible doubt, are practically identical with the proportions 

 in the epi plagioclase pyroxene gneiss (pyroxene granulite) at Madigan Nunatak with 

 average absolute grain size approximately 0-30mm. The minerals are in each case 

 the same, and there is no important difference in the chemical composition, and they are 

 not far removed from one another in the Ozann triangular projection. The chief 

 difference between these two rocks is the grain size. The fine grained type, No. 773, 

 possesses the relic structure and is most like the primary dolerite. Hence we can 

 conclude that the metamorphic conditions at Madigan Nunatak were longer continued 

 and caused certain crystals to enlarge themselves at the expense of other crystals, thus 

 producing fewer and larger crystals. The fine-grained facies has been replaced by a 

 coarse-grained facies. 



We have the direct evidence that the felspar and pyroxene of a coarse-grained 

 dolerite at Cape Gray have been replaced by a fine granoblastic aggregate. Secondary 

 enlargement of these fine grains can proceed till we get an aggregate many times coarser. 

 If the pyroxene in the aggregate be then partially converted into hornblende we get 

 the meso plagioclase pyroxene gneiss at Aurora Peak which is identical with the rock 

 called hornblende norite from St. Thomas Mount, Madras. If the hornblendisation 

 of the pyroxene be completed, an amphibolite, comparable to those at Cape Denison 

 would result. If the conditions of hornblendisation are replaced by those of the epi zone 

 of metamorphism a certain amount of granulation appears, and we find some crystals 

 are fractured, some are crushed, and some have granulated selvages as in the Madigan 

 Nunatak example. The direct transition stages between dolerite and amphibolite 

 have not been directly traceable in these areas as it has been done in other parts of the 

 world. 



The two-phase metamorphism of these basic rocks at the nunataks is considered 

 to rest on direct and sure evidence. Therefore it is not reasonable to doubt the inter- 

 pretation of a two-phase metamorphism of the acid hypersthene gneisses which are 

 associated with and intruded by the basic rocks. Subtracting respectively the effects 

 of epi and meso zone metamorphism, we find a family likeness which is exhibited by 

 the granoblastic structure and the presence of hypersthene. We attribute this family 

 likeness to kata zone metamorphism. 



As we have shown that the hypersthene is a metamorphic product in the plagioclase 

 pyroxene gneiss, it is little assumption to assert that the hypersthene is also a meta- 

 morphic mineral in the acid hypersthene felspar gneisses which have suffered similar 



