CHAPTER IV. 

 1. THE GRANODIORITE GNEISS AT CAPE DENISON. 



The chief rock type at Cape Denison is a coarse-grained, grey-coloured gneiss with 

 a granitic appearance. It is foliated, and the strike of the foliation is N. 24 W. The 

 dip of the foliation is at a high angle, sometimes to the east and sometimes to the west. 

 By observation of these dips the axes of folds seem to be determined, but no evidence 

 of folding is forthcoming from the study of the black amphibolite bands that traverse 

 the area parallel to the strike of the foliation. In the description of this gneissic type 

 we exclude reference here to the patches of dark-coloured gneiss that may appear 

 enclosed in the granitic gneiss. 



In the hand specimen the grey gneiss has a varying amount of schistosity. 

 Foliation is well marked in some specimens, while only faint in others. Beautiful 

 examples of contorted gneiss are found in some places where the crystallisation 

 schistosity is marked by bands of quartz and felspar (Plate XX., fig. 3). Yet the 

 character of the gneiss is fairly constant across Cape Denison. Quartz, felspar, and 

 biotite are always visible to the naked eye ; magnetite is sometimes well developed, 

 and there are patches where the same is true of pyrite. In some parts black vitreous 

 allanite is obvious and developed in flat prismatic individuals. The largest allanite 

 crystal obtained is an impertect one, |in. long, Jin. broad, and -]Vn. thick. Apatite 

 may also be abundant in the same areas as the allanite. Rarely large orthoclase 

 crystals are found as white or reddish- white porphyroblasts up to 2in. in breadth. 



In the hand specimens the normal texture of the rock is dominantly massive, but 

 a tendency to the schistose types can always be detected. The biotite flakes may bend 

 round large crystals of quartz and felspar, and then there is a tendency to augen gneiss 

 and a rough lenticular texture. These lenticles may become flattened and more 

 granular and then a distinct banded appearance is evident (Plate XL, fig. 5). Also 

 the parallel bands of quartz and felspar against mica may develop the columnar 

 appearance of wood gneiss (No. 143 A). 



The structure is granoblastic, due to the approximately isometric character of 

 the quartz and felspar grains. Blasto-granitic structure is present, because the original 

 big crystals of felspar and quartz in the granite can often be reconstructed in the 

 cataclastic areas. Cataclastic structures are common when quartz and felspar crystals 

 have been crushed. Mortar structure is common, but is usually best developed along the 

 j unction of two felspar crystals. Diablastic structure is frequently seen in the crush areas. 



Specimen No. 1 1 has been selected and analysed as the normal type. It was collected 

 from the site of the main hut at Cape Denison. No. 11 will, therefore, be described 

 first, and then the other types can be dealt with in a relative manner. 



The chief minerals present are quartz, microcline, orthoclase, andesine, perthite, 

 and biotite. In smaller amounts are epidote, muscovite, sphene, chlorite, and calcite. 

 As accessories are apatite, zircon, magnetite, pyrite, and hematite. 



