APPENDIX TO PART XIII 



PETROGUAPH1CAL NOTES ON SOME 



ROCKS RETRIEVED FROM THE CACHE 



AT DEPOT ISLAND, ANTARCTICA 



BY 



LEO A. COTTON, B.A., B.Sc. 



Dyke Eock from Cape Ross, South op Depot Island 



Crystallinity : holocrystalline. 



Grain size : relative — porphyritic ; absolute — porphyritic crystals, 1 mm. ; 



ground-mass, -1 mm. 

 Minerals present in decreasing order of abundance : hornblende, plagioclase, quartz, 



magnetite. 

 Secondary mineral : kaolin. 



The hornblende is the basic variety, deep brown in colour and strongly pleochroic. 

 The crystals are idiomorphic, excellent prismatic and basal sections being present. In 

 a number of basal sections a colourless kernel similar in shape and similarly situated 

 to the outline of the basal section was observed. This kernel has a refractive index 

 and double refraction like that of felspar, but could not be with certainty determined. 



There is a marked parallelism of the prismatic hornblende crystals. 



The felspar is a plagioclase with very little decomposition. There is a tendency 

 to trachytic structure, but the parallelism of these crystals is not nearly so well marked 

 as in the case of the hornblende. 



A few crystals of quartz were observed. These were rounded and possessed an outer 

 concentric shell. They are probably foreign to the magma and the shell represents 

 the result of chemical interaction. The shell consists of an undeterminable colourless 

 base resembling felspar, studded with minute crystals of basic hornblende radially 

 arranged. 



The ground-mass of the rock consists of fine crystals of plagioclase and basic horn- 

 blende, with a few scattered crystals of magnetite. This rock may be termed a 

 Camptonite. 



Dyke Rock from Cape Ross, South of Depot Island 



Crystallinity :" holocrystalline. 



Grain size : relative — porphyritic ; absolute — phenocrysts, about 1 mm. (constituting 



less than 5 per cent, of the rock) ; ground-mass, -05 mm. 

 Minerals present in their order of abundance : plagioclase, diopside, allanite ( ?), 



quartz, magnetite. 

 Secondary mineral : chlorite. 



II 235 2l 



