THE METAMORPHIC ROCKS OF ADEL1E LAND- BTILLWELL. 141 



This analysis is very similar to that of the granodiorite gneiss of Cape Denison. 

 There is a little less iron and magnesia in the case, but, except for the relation of one 

 alkali to the other, there is no important difference. This rock is notable for its excess 

 soda, though the alkali total is approximately the same in both cases. The analysis 

 is more similar to the Cape Denison granodiorite gneiss (No. 11) than to the hypersthene 

 gneiss (No. 797) of the Madigan Nunatak, to which it is closely allied in structure and 

 mineral composition. Compared with this hypersthene gneiss there is more iron, 

 magnesia, and lime, corresponding probably with the garnet and the different felspar. 

 The alkalies furnish the most striking difference. Whereas there is a large excess of 

 potash and orthoclase in No. 797, there is a large excess of soda and albite in No. 754. 



The Classificatory Position. 



The group values place the rock among the alkali felspar gneisses of Group I. These 

 values illustrate the acidity of the rock and its high alkali value. The projection values, 

 when plotted, give a position not greatly different from that of the mean group value of 

 Group I., and intermediate between that of the hypersthene gneiss of the Madigan 

 Nunatak and the granodiorite gneiss of Cape Denison (fig. 10). 



Mineralogically this rock differs from No. 797 in the presence of garnet and 

 the dominance of soda felspar over potash felspar. These differences do not carry 

 the rock into a different schist group, because the relation between the alkalies does not 

 enter into the classification. They are both hypersthene alkali felspar gneisses, while 

 No. 754 is, in addition, garnetiferous. Cataclasis is not important in the Aurora Peak 

 example, and we thus lose the dominating epi zone character found at Madigan Nunatak. 

 Assuming for the present that the formation of the hypersthene and garnet belongs 

 to the kata zone of metamorphism, we can infer that the rock is a kata zone rock. As, 

 however, we have admitted the meso zone modification of its neighbour, the plagioclase 

 pyroxene gneiss No. 759, we must consider what evidence of the meso zone conditions 

 might be found in this rock. It is possible that some of the biotite has been derived 

 from the reaction of garnet with felspar, or the biotite and ilmenite from the hypersthene 

 and felspar. There is a significant association of these minerals, but it is not possible 

 to give a sure interpretation from the study of this specimen alone. A breaking up of 

 the garnet or the hypersthene might be viewed as a modification due to meso zone 

 conditions. The abundance of perthite may be looked upon as further evidence. 



We may, therefore, describe this rock as a garnet hypersthene alkali felspar gneiss 

 developed in the kata zone of metamorphism and somewhat modified by the meso zone 

 of metamorphism. 



Without correlative evidence we must depend on the chemical evidence to indicate 

 the nature of the primary rock. As far as can be determined there is no reason to suspect 

 metamorphic differentiation or metamorphic diffusion, and the chemical criteria are 

 valuable. The chemical composition is that of a well-known rock type, viz., a 

 granodiorite, and this pointe to a primary igneous origin. 



