136 AUSTRALASIAN ANTARCTIC EXPEDITION. 



The analysis is similar to that of a potash granite and it is considered that the 

 chemical evidence of igneous origin is very strong in this case. The analysis bears 

 witness to the high silicity of the rock with corresponding low percentages of iron, lime, 

 and magnesia. The latter is absorbed in the hypersthene and its alteration products 

 and is, therefore, some indication of the small quantity of hypersthene in comparison 

 with the amounts of quartz and felspar. The relative amounts of Ti0 2 and Fe ? 3 

 confirm the record of ilmenite in the rock. The abundance of alkali felspar is reflected 

 in the high percentage of alkalies, and the great excess of orthoclase is similarly reflected 

 in the large excess of potash over soda. 



Among the group values the high value of K corresponds with the high silicity, 

 and the high value of A corresponds with the high alkali percentage. The projection 

 values are such as to place the rock in the area of Group I. in the triangular projection 

 (% ID- 



Like the associated basic rock, the analysis shows considerable resemblance to the ' 

 quoted analysis of acid charnockite. The silica percentage of the charnockite is nearly 

 5 per cent, greater ; but this is not important, as Washington has drawn attention to 

 such a range of variation among the acid charnockites themselves. The relative pro- 

 portions of ferrous and ferric iron, of magnesia and lime, and of soda and potash are 

 similar in each analysis. 



The Classificatory Position. The group values and projection values bring the rock 



into Group I., the group of Alkali Felspar Gneisses. The epi zone metamorphism is 



important and is revealed by the cataclastic and mortar structures. Before the epi zone 



metamorphism the rock consisted of a granular aggregate of quartz, orthoclase, a little 



plagioclase, and small amounts of hypersthene, biotite, and ilmenite with accessory 



apatite and zircon. As the schistosity is chiefly marked by the parallel arrangement 



of the hypersthene crystals it is obvious that the rock was schistose before the epi zone 



imprint. Further, if the inference that the biotite and ilmenite has been formed from 



hypersthene and felspar is correct, then such alteration took place before the epi zone 



metamorphism, because both biotite and ilmenite show marked mechanical effects. 



We must, therefore, recognise two metamorphic phases in the development of the meta- 



morphic character of this rock. Parallel with the plagioclase pyroxene gneiss it will 



be subsequently affirmed that the primary rock was first recrystallised in the kata zone 



of metamorphism. As the rock ascended from the depths of the earth's crust and 



became subject to meso zone condition the hypersthene reacted with the felspar and 



produced ilmenite and biotite. Possibly here also some of the microperthite was formed 



and the diablastic structures produced. These metamorphic results were completed 



in the epi zone where the excessive mechanical effects were produced. 



The rock may, therefore, be described as an epi hypersthene orthoclase gneiss, 

 produced by the superposition of epi zone metamorphism upon a kata hypersthene 

 orthoclase gneiss. The primary equivalent of the latter, judged from the chemical 

 composition, was probably a granite. 



