192 AUSTRALASIAN ANTARCTIC EXPEDITION. 



conditions of recrystallisation. Indeed, the definite parallel arrangement of the hyper- 

 sthene crystals would be difficult to explain on any other hypothesis. In many 

 metamorphosed granites and similar rocks the ferromagnesian percentage is expressed 

 in a content of biotite or chlorite. If the temperature of the metamorphism should 

 exceed that at which biotite is capable of holding its water of combination, what will 

 be the product? Pyroxenes are high temperature minerals and it would not be 

 unreasonable, on a priori grounds, to expect biotite to be replaced by pyroxene under 

 such circumstances. If there were sufficient lime in the rock we might equally well 

 expect garnet, provided the pressure factor is suitable. Biotite has been proved to be 

 an alteration product of hypersthene in the zone of the gneissic dacites at Belgrave, 

 Victoria.* Hence the metamorphism of a granite under conditions of high temperature, 

 high uniform pressure, and weak stress, might produce a hypersthene alkali felspar 

 gneiss. Apart from such considerations, however, or any correlative evidence, the 

 study of the rock relations has made it clear that hypersthene is a metamorphic mineral 

 in the basic rocks and, as these and the acid hypersthenic gneisses have suffered complete 

 recrystallisation under similar conditions, it is unreasonable to deny the metamorphic 

 character of the hypersthene in the acid gneisses. 



To this complete recrystallisation under similar conditions we must assign the 

 family likeness of the Madigan Nunatak and the Aurora Peak rocks, in spite of chemical 

 differences. On the other hand we must ascribe the differences between the gneisses 

 at Aurora Peak and at Cape Denison to dissimilar conditions of recrystallisation, in spite 

 of marked chemical likeness. The Cape Denison granodiorite gneiss is almost a product 

 of epi zone conditions. 



We think that it is impossible to deny the metamorphic character of these acid 

 hypersthenic rocks, and yet we find that they correspond very closely with the descrip- 

 tion of the hypersthenic rocks of Peninsular India, which have been called the Charnockite 

 Series. We have already demonstrated the similarity in chemical composition of the 

 acid and basic types at the Madigan Nunatak to members of the Charnockite Series ; 

 and we have just found that the mineral composition of the hornblende plagioclase 

 pyroxene gneiss at Aurora Peak is very close to that of a hornblende norite, a basic 

 charnockite from the type locality, St. Thomas Mount, Madras. But the charnockites 

 have been considered by Holland f to be igneous rocks which have consolidated under 

 phenomenal conditions. Can, then, these Antarctic metamorphic rocks be strictly 

 compared with igneous rocks, or is it possible that the charnockites are really 

 metamorphic rocks ? It behoves us to critically examine the evidence. 



* " Gneisses and Dacites of the Dandenong District," E. W. Skeats, Q.J.G.S., vol. LXVL, 1910, pp. 450-469. 

 t " The Charnockite Series," T. S. Holland, Mem. G.S. India, XXVIII., pt. 2. Numbers in brackets in the following 

 refer to pages in this publication. 



