THE MBTAMORPHIC ROCKS OF ADEL1E LAND. STILLWELL. 165 



crystals of garnet. Cordierite and sillimanite are most prominent at Cape Gray, but 

 are found at the other localities, where they are interpreted as secondary relics. With 

 the recession of garnet and cordierite, biotite with quartz and felspar become prominent. 

 Biotite and quartz have been produced by the reaction of garnet and felspar. The 

 biotite so produced is a pale green variety which develops later into the normal brown 

 biotite. The alteration from the green to the brown colour may be effected by the 

 radio-active rays which produce the pleochroic haloes in the biotite. 



The junction between the cyanite biotite gneiss and an amphibolite dyke has been 

 described at Garnet Point. Near the junction there is considerable garnet and cyanite 

 in the gneiss. The cyanite is not normal and shows prominent lamellar twinning, but 

 its double refraction is estimated to be about 0-019 a value higher than recorded values 

 for cyanite. There is perfect crystalline continuity across the junction, which is only 

 indefinitely marked by the gradual appearance of hornblende in the section. The 

 cyanite and quartz travel further into the amphibolite than the garnet or biotite, and 

 the cyanite has been noticed in the amphibolite at a distance of lin. from the apparent 

 junction. The cyanite may be intergrown with the hornblende in the amphibolite. 

 It is not considered possible to explain these features by assimilation of the sedimentary 

 rock by the igneous rock prior to the metamorphism. The characters of the complex 

 of sediment and dyke are solely due to the recrystallisation, during which it is supposed 

 that a limited migration of material occurred across the pre-existing junction, tending 

 to efface it. It is viewed as another example of metamorphic diffusion. 



The chemical composition, as well as the mineral composition, shows that these 

 gneisses are sedimentary in origin. 



The gneisses in each case are placed among the Aluminium Silicate Gneisses in 

 Grubenmann's classification of the crystalline schists. The rock at Cape Gray, the 

 most northerly outcrop, is placed in the family of the cordierite gneiss in the kata 

 division. Kata zone metamorphism is found in each of the other outcrops but is 

 modified first by meso zone metamorphism and, later, by additional traces of epi zone 

 metamorphism. At Madigan Nunatak, situated on the ridge which terminates in 

 Cape Gray and 18 miles due south of it, we have already described the rocks as examples 

 of kata zone metamorphism modified by strong epi zone features. We now find that 

 of these four localities, the nearest in point of distance from the Madigan Nunatak is 

 the locality in which traces of epi zone metamorphism have been described as super- 

 imposed upon kata zone metamorphism. Cape Gray, the furthest in point of distance 

 from the Madigan Nunatak, possesses the least modified kata zone metamorphism. 

 The intermediate localities possess kata zone metamorphism modified by meso zone 

 metamorphism to a degree sufficient to place the rocks in meso division of the schist 

 group. In the latter case the specific families of cyanite gneiss and meso garnet gneiss 

 are represented. 



In addition to the garnet felspar gneiss on Stillwell Island, acid hypersthenic gneisses 

 occur. The only specimen of gneiss, apart from the altered dykes, collected from the 



