CHAPTER IX. 

 AURORA PEAK. 



Aurora Peak (Plate XXVI., fig. 3) is situated in Adelie Land in Lat. 67 24' and Long. 

 144 12' and is about 50 miles E.S.E. of Cape Denison. It is a solitary peak or nunatak 

 rising above the snow plain on the west side of Mertz Glacier to a height of 1,750ft. 

 above sea level. It is distant about 25 miles from the Madigan Nunatak, and similar 

 rock types appear on both outcrops. It was visited by Madigan's sledging party in 

 December, 1912, and our information comes from their report and from the examination 

 of the specimens they brought back. 



The similarity in rock types to the Madigan Nunatak is its outstanding feature. 

 Here again there are two principal rock types, viz., a plagioclase pyroxene gneiss and a 

 hypersthene alkali felspar gneiss, which are analogous in mineral content with the two 

 types described at the Madigan Nunatak, differing only in mineral proportion and in 

 structures. Whereas the rocks at the Madigan Nunatak are remarkable for their crush 

 structures, the rocks at Aurora Peak are almost devoid of such features. Whereas 

 the epi zone metamorphism is dominant at Madigan Nunatak, the meso zone meta- 

 morphism is equally dominant at Aurora Peak. Similar primary rocks have been 

 metamorphosed in both instances under different physico-chemical conditions. 



MESO PLAGIOCLASE PYROXENE GNEISS. 



Specimen No. 759 is reported as a black band which cuts across the gneiss. In the 

 hand specimen it is a fine-grained, dark-coloured rock with a weak schistosity produced 

 by lenticles of felspar. The average absolute grain size is approximately 0-1 7mm. 

 Its colour is black when fresh, and it grades up to a light-brown colour as the felspar 

 becomes cloudy and iron-stained by weathering. The same minerals are present in 

 the slide as in the analogous rock No. 794 from Madigan Nunatak. Hornblende is 

 much more abundant, and there is a corresponding decrease in the amount of pyroxene 

 from which it is derived. The proportion of felspar to ferromagnesia is practically 

 the same in both cases. Apatite is also more abundant. The following proportions 

 have been determined by a Rosiwal analysis : 



Felspar 44-8 



Pyroxene 28-6 



Hornblende 15-5 



Iron ore 10-1 



Biotite 0-3 



Apatite 0-7 



Thus there is in this rock nearly five times as much hornblende as in No. 794, and 

 the transformation of pyroxene to green hornblende is correspondingly more obvious 



