COLLECTED AT CAPE ROYDS 171 



438. Syenite (Plate I, Fig. 1 ) 



Macroscopic Character s. — Dark grey mottled rock, holocrystalline, medium 

 to coarse. Consists of colourless felspathic material, and still more abundant horn- 

 blende. The latter mineral is mostly granular but occasionally is in crystals up to 

 8 mm. by 1 "5 mm. 



Microscopic Character s. — The texture is hypidiomorphic granular with fairly 

 even grainsize averaging about 1*6 mm. Hornblende and oligoclase-andesine in about 

 equal proportions make up roughly 80 per cent, of the rock ; orthoclase accounts 

 for about 15 per cent., the remainder being biotite, sphene, apatite, and quartz. 



Hornblende is light-greenish in colour, the pleochroism being ft =very light greenish- 

 yellow, b =bronze-green, C=brownish-green, absorption ft<t)>C. While the bulk of 

 the hornblende is no doubt primary, the nuclei of some of the grains suggest 

 uralite, but no unaltered pyroxene can be observed. Maximum extinction angle 

 measured in the vertical zone 15°. 



Oligoclase-andesine occurs in idiomorphic and subidiomorphic sections twinned 

 after the Carlsbad, albite, and pericline laws. It is somewhat clouded by decom- 

 position products, chiefly calcite and sericitic mica. In some instances these products 

 are zonally arranged. 



Orthoclase is allotriomorphic and untwinned. It is somewhat kaolinised but is 

 less altered on the whole than the plagioclase. 



Biotite in bleached flakes is mostly included in the hornblende. 



Sphene is present as greyish grains sometimes moulded on the plagioclase. 



Apatite in minute, sharply defined prisms is moderately abundant. There is very 

 little quartz, what there is is graphically intergrown with the orthoclase. 



E. 36. Sodalite Syenite with Wohlerite (Plate I, Figs. 2 and 3) 



Microscopic Characters. — This is a most peculiar rock. It is holocrystal- 

 line and consists chiefly of felspar, some of the sections of which have diameters of 

 several millimetres. The larger grains are subidiomorphic. The extinction of this 

 felspar is cloudy, reminding one of moiree microcline. In some instances excessively 

 fine crossed twin lamellae can be seen, while in others what looks like perfect microcline 

 " grating" is present. The extinction angles of the lamellae are 11° to 13°, but the 

 refractive index is decidedly higher than that of cooked Canada balsam, so that the 

 felspar cannot be microcline, and must be a plagioclase near andesine, with quite 

 abnormal development of twin lamellae. The values for extinction angles given 

 above are not at all satisfactory as the extinction is very cloudy. 



The grains of felspar are crowded with markings like those producing " schiller " 

 structure in hypersthene. These are really plates of an isotropic substance perfectly 

 colourless and with a refractive index much below that of Canada balsam, and which 

 I believe to be sodalite. This regular intergrowth of felspar and sodalite forms a 

 veritable sodalite microperthite. In sections cut at right angles to the plane of the 

 plates of sodalite the structure appears laminated. In addition to this intergrowth 

 other felspars without twinning, but with the same relatively high refractive index, 

 show perthitic intergrowth with the microcline- like felspar above described. 



Scattered through the rock, and included in the felspars, are fairly abundant stout 

 prisms up to 1 "2 mm. by - 3 mm. of a bright orange- coloured mineral. This has a positive 

 elongation and gives extinctions up to 32 ° from the length. There is a decided cleavage 

 parallel to the length. The refractive index is considerable and birefringence strong. 



