THE METAMOKPHIC ROCKS OF ADELIE LAND. STILLWELL. 



87 



The analysis of the Cape Denison gneiss is strikingly similar to that of a grano- 

 diorite, and an analysis of a Macedon granodiorite is, therefore, inserted for illustration. 

 The similarity is strong in all essential features, and both rocks occupy the same division 

 in the American classification. Analyses of a banded biotite granite gneiss from the 

 Rainy Lake region in Canada, and of a grey biotite gneiss from the Scottish highlands, 

 are also quoted, and these show general similarities to the Cape Denison gneiss. Such 

 comparisons, which could be readily multiplied, are interesting in emphasising the 

 lithological uniformity in the Archaean terraines in all parts of the world. Similar 

 rocks are known to exist in Australia and in South America and in South Africa. 



The ratio of the potash to the soda is abnormal in the Canadian rock, while the 

 alkali percentage of the Scottish rock is lower than that of the Antarctic rock. The 

 differences in total alkali percentage are made important in Grubenmann's classification 

 of schists. The group values and projection values of these three gneisses are : 



THE CLASSIFICATORY POSITION. 



The Cape Denison gneiss occupies a position on Osann 's triangular projection 

 (fig. 9), which is midway between the mean group values of Groups I. and III. The 

 Canadian rock enters Group I. and the Scottish rock Group III. The Cape Denison 

 gneiss should be considered as an intermediate type, and it occupies a position on the 

 triangular projection halfway between the positions of these Canadian and Scottish 

 rocks. Since, however, it is the metamorphic equivalent of a granodiorite, and since 

 granodiorites are well known and definite rock types, it must be acknowledged that 

 the metamorphic equivalents of granodiorites should be recognised. The Cape Denison 

 gneiss is, therefore, best named as a granodiorite gneiss. 



The mechanical effect of the metamorphism upon the original granodiorite is evident 

 in the jundulose extinction of the quartz, the cataclasis of the quartz and felspar, the 

 prominent mortar structure, and the tendency of the colourless minerals to .be arranged 

 in layers of aggregated fragments. Evidence for the following transformations have 

 also been noted : 



1. Partial decomposition of primary biotite into epidote, sphene, and ilmenite. 



2. Partial decomposition of primary biotite into chlorite. 



