132 



AUSTRALASIAN ANTARCTIC EXPEDITION. 



are noticed. The percentages of Si0 2 and A1 2 S are practically the same. The total 

 iron is much the same in each case. The relative proportions of MgO and CaO are also 

 the same, while there is large excess of soda over potash in both cases. 



Classification and Origin. 



The Ozann group values place the rock in the group of Eclogites and Amphibolites 

 in Grubenmann's classification. The projection values assign to the rock a position in 

 the triangular diagram close to the mean group value of Group IV. (fig. 11). 



Fig. 11. 



I. Mean Position of Group I., the Alkali Felspar Gneisses. 

 IV. Mean Position of Group IV., the Eclogites and Amphibolites. 

 797. Hypersthene Alkali Felspar Gneiss, Madigan Nunatak. 

 754. Garnet Hypersthene Alkali Felspar Gneiss, Aurora Peak. 

 794. Plagioclase Pyroxene Gneiss, Madigan Nunatak. 



We admit the marked traits of the epi zone in the assigned name epi plagioclase 

 pyroxene gneiss ; but the rock bears a double metamorphic character. Reconstructing 

 the outlines of the fractured minerals, it can be seen that, before the epi zone imprint 

 was received, the rock consisted of a granoblastic aggregate of augite, hypersthene, 

 plagioclase, and ilmenite, with a little hornblende and pyrite and biotite. As such it is 

 identical with some of the Saxon pyroxene granulites, or the French pyroxene gneisses, 

 or the Indian norites, whose metamorphic character will be subsequently affirmed 

 from a comparative study with the basic rocks at Cape Gray and Aurora Peak. As such 

 it is a member of the plagioclase augite family of the kata division of Group IV. Before 

 the recrystallisation in the kata metamorphic zone we might judge from the chemical 



