CHAPTER XII. 



1. RELATION BETWEEN THE ROCKS AT CAPE GRAY, MADIGAN 

 NUNATAK, AND AURORA PEAK. 



We have shown in our descriptions that the two rock types found at the Madigan 

 Nunatak correspond closely with the two chief types at Aurora Peak. If we subtract 

 the epi zone metamorphism from the Madigan Nunatak rocks, and the meso zone 

 metamorphism from the Aurora Peak rocks we get, in both cases, kata zone meta- 

 morphic types. The basic rocks of the two localities then become identical and the acid 

 rocks are analogous ; but they all agree in possessing a granulitic structure and the 

 mineral hypersthene. 



The basic rock at Aurora Peak is reported as a dyke cutting across the foliation, 

 but, though the basic rock at Madigan Nunatak appeared to form a band, nothing 

 definite could be observed in the field. Still, from general considerations, it has been 

 considered to be probably a metamorphosed basic igneous rock. The likeness to the 

 Aurora Peak rock renders this more probable, but it receives striking confirmation 

 by comparison with the undoubted dyke at Cape Gray. 



We place here, side by side, the mineral proportions, determined by the Rosiwal 

 method, of No. 773 (the plagioclase pyroxene gneiss from Cape Gray), of No. 794 (the 

 plagioclase pyroxene gneiss from the Madigan Nunatak), and of No. 759 (the hornblende 

 plagioclase pyroxene gneiss from Aurora Peak). We add, for the sake of comparison, the 

 mineral proportions of a hornblende norite from St. Thomas Mount, Madras, determined 

 by Washington.* 



The likeness of the proportions of felspar and ferromagnesian minerals in all four 

 cases is obvious. The pyroxene of No. 773 is partly secondary and partly primary ; 

 but the secondary pyroxene is very similar in type to the pyroxene of Nos. 794 and 795. 



* " The Charnockite Series of Igneous Rocks," H. S. Washington, Amer. Journ. Sei., Vol. XLI., 4th Ser., 1916, p. 323. 



