220 PETROLOGY OF ROCK COLLECTIONS 



felspar-mica rock, sometimes containing much black tourmaline ; quartz veins carrying 

 pyrites ; also more basic formations containing abundant biotite and hypersthene. 



From the Beardmore Glacier are specimens of reef quartz and coarse quartz - 

 microcline -biotite rock. 



THE LAMPROPHYKES 



Kersantites (Camptonitic) 



Associated with the granites are frequent narrow basic dykes (Fig. 4, Plate IV). 

 That described at length herewith is a kersantite, but others appear to tend towards 

 camptonites and tinguaites. The dykes are usually irregular in length and narrow in 

 width. When as narrow veins they are very close textured, in part devitrified glasses. 

 In almost all cases a marginal zone, a few centimetres wide, appears to have solidified as 

 glass. Where the dykes are some feet in width, the central portions are occupied by 

 material in which the crystallisation is evident to the unaided eye. It is not unusual to 

 find a sprinkling of small porphyritic plagioclase crystals. At all times fragments of the 

 enclosing granite or of the minerals therefrom may be expected, partially corroded, 

 suspended in the dyke rock. 



The lamprophyres are always very dark in colour, practically black. The weathered 

 surfaces often exhibit dark bluish and greenish tints. As it is more readily weathered 

 than the enclosing granite, it is usual to find the dykes occupying depressions in the 

 surface. On this account, also, lichens are more frequently noted adhering to its 

 surface than to the granite. 



The effect of these intrusions upon the neighbouring granite is marked to the naked- 

 eye inspection by an increased depth of colour in the pink granite. The large orthoclase 

 crystals are converted to a bright red colour. Examined under the microscope the 

 changes are seen to lie chiefly in notable sericitisation of the felspar and chloritisation 

 of the hornblende and biotite. In addition the quartzes often appear smoky, and 

 there is generally present a larger content of accessory minerals. Slides of the granite 

 cut in close proximity to the contact show numerous planes of fracture approximately 

 parallel with the main line of fissure. These average not more than - 75 mm. in width 

 and are occupied by the crushed and recemented constituents of the granite. 



Specimens from different locations are found to vary very much in the freshness 

 of the constituents. In some uralisation, chloritisation, and serpentisation are far 

 advanced and the original characters of the more readily affected minerals entirely 

 obliterated. 



Kersantite from Cape Irizar 



This is one of the denser types. In it minute plagioclase laths can be distinguished, 

 also frequent grains of strongly pleochroic (yellow to dark brown) hornblende. Grains 

 of ilmenite and pyrites are present. The base is very dense and may have partly 

 solidified as a glass. It is now pervaded with a chloritic alteration. The hornblendes 

 are also attacked by chloritisation. 



Specimens from other portions of the same dyke throw more light upon the mineral 

 composition of this rock. There is a dominance of plagioclase (acid labradorite to 

 andesine) which is present both as small laths and larger porphyritic zoned crystals. 

 Carlsbad, albite, and pericline twins are present. Hornblende (usually basaltic) is 

 present in idiomorphic rods. Light -coloured pyroxene partly uralitised is present in 

 this dyke but seldom in others. Rarely flakes of biotite appear. Ilmenite, usually 

 leucoxenised, is always abundant, so also is apatite. The finer material of the base of 

 holocrystalline samples appears to contain some fine granular orthoclase. In these 

 cases particles of quartz are evenly distributed through the section. 



