THE METAMORPHIC ROCKS OF ADELIE LAND. STILLWELL. 133 



composition that the rock was either a gabbro or a diabase. In view of the large number 

 of metamorphosed diabase dykes that are present in this region one would be inclined 

 to consider that this rock is a diabase dyke which, suffering different metamorphic 

 conditions, has been converted into a different rock type to the amphibolite at Cape 

 Denison. Such is in accord with the field observation that the rock seemed to be a band, 

 and such conclusion will be subsequently supported by correlative argument. 



In the kata zone, then, the primary rock suffered its first and thorough 

 recrystallisation, and the pyroxene and plagioclase and ilmenite were formed. In 

 the transition from the kata zone to the epi zone we naturally find some evidence of the 

 passage through the meso zone. The evidence is yielded by the alteration of the 

 pyroxene into biotite and hornblende. It has been pointed out that the biotite flakes 

 are often twisted and bent, and the hornblende sometimes granulated ; hence, like the 

 pyroxene and plagioclase, they are secondary metamorphic relic minerals in the epi zone 

 metamorphism. 



The obscurity in the field concerning the boundary between the epi plagioclase 

 pyroxene gneiss and the epi hypersthene alkali felspar gneiss may be readily explained 

 by the presence of a metamorphic diffusion type which would cause a transition from 

 one type to the other. Near the boundary the lighter coloured constituents become 

 more prominent, and blue quartz may appear. At the same time the rock assumes a 

 more schistose character. 



Specimens showing the junction between the basic gneiss and the acid gneiss are 

 present in the collection (No. 795). In these the junction is partly indefinite, and its 

 position cannot be precisely marked in certain places. Small pieces of the dark rock 

 are seen to be apparently detached from the parent mass and lie enclosed in the lighter- 

 coloured rock. These junction specimens were no doubt collected from those parts 

 where the junction was most obvious in the field. 



HYPERSTHENE ALKALI FELSPAR GNEISS. 



The second type of gneiss at the Madigan Nunatak is a coarse-grained grayish-white 

 rock, in which the gneissic structure can be detected. Like the preceding type, it 

 weathers to a brown colour. Microscopically, one can see blue quartz, felspar, and 

 smaller amounts of black hypersthene. There is considerable variation in the grain 

 size of different specimens, and this is especially noticeable with respect to the dark 

 hypersthene. 



In thin section (No. 797) the schistosity is not noticed, but there is abundant 

 evidence of crushing and cataclasis. Mortar, cataclasic, and diablastic structures are 

 common. Quartz, orthoclase, and plagioclase form the bulk of the slide. Hypersthene 

 biotite, and ilmenite are important, though in a lower order of abundance. Zircon, 

 apatite, and pyrite are accessory minerals. 



