FROM THE MAINLAND OF SOUTH VICTORIA LAND 215 



THE GRANITE PORPHYRIES 



A notable example of this class is that occurring in situ at the western end of the Kukri 

 Hills, Ferrar Glacier. There is little doubt but that this is a dyke rock of the Cape 

 Irizar granite magma. It is an intermediate stage between the granites already 

 described and the quartz and felspar porphyries to follow. As porphyritic ingredients 

 it contains large pink orthoclase crystals 2-5 cm. long, less noticeable smaller white 

 oligoclases, small idiomorphic quartz crystals, and abundance of hornblende prisms. 

 These coarse crystallisations are scattered through a uniform even fine-grained pink 

 base composed of granular quartz, orthoclase, and plagioclase, with some biotite and 

 subordinate hornblende. 



Other specimens nearly related thereto, representing variations in the solidification 

 of the magma, occurring under conditions ranging from those which lead to the 

 development of typical granite, and those characterising the solidification stages of 

 porphyritic aplites, are represented. 



One of these from the Knob Head Moraine carries a phenocryst of orthoclase 7 cm. 

 in length. This crystal and even the smaller individuals of ground-mass show indications 

 of dynamic force by fracture. Occupying a fissure in the orthoclase is a vein of 

 hornblende. 



Another example from the Stranded Moraines, East Fork, Ferrar Glacier, shows 

 hornblende prisms 1 cm. long, and these enclose flakes of biotite. 



In another specimen from this latter locality the hornblendes contain poikilitically 

 included felspar. 



THE FELSPAR AND QUARTZ PORPHYRIES * 



Quartz Porphyry from near Mount Larsen 



Quartz Porphyry occurring in situ as an injection in the even granular granite already 

 described outcropping to the south-east of Mount Larsen (Fig. 3, Plate II). 



The hand-specimen shows idiomorphic crystals of quartz and faintly pink orthoclase 

 distributed through a dense grey base. The orthoclase crystals reach a length of 

 7 mm. and ±he quartz individuals a diameter of 4 mm. Occasional planes of fracture 

 traversing the massive rock are filled with pistacio-green epidote. These veins are 

 never more than 2 mm. across. 



The microscopic examination shows the presence of some plagioclase, biotite, 

 magnetite, and sphene, in addition to the above. Quartz phenocrysts of idiomorphic 

 form are prominent. The hexagonal bipyramids, which are perfect in form, reach a 

 diameter of 4 mm. The orthoclase is very fresh. Carlsbad twins are combined with 

 delicate cross twin-striae indicating the felspar to be a soda variety of the anorthoclase 

 type. In one case a Baveno twin makes its appearance. Plagioclase crystals are present 

 though much less frequent and smaller than the orthoclase. Extinction determinations 

 indicate them to have the composition of oligoclase. They are yellowish to greenish - 

 yellow in colour, due to secondary alteration, which is much more evident than in the 

 case of the orthoclase. An idiomorphic oligoclase individual was noted embedded in 

 an anorthoclase. Biotite as irregular fragments showing strong absorption, pleochroic 

 in yellows and greens. Magnetite : a few idiomorphic grains. Sphene of grey colour 

 in scattered irregular aggregates, sometimes encased in the mica. Apatite as minute 

 prisms noted only very rarely. Allanite. Rare scattered prisms are met with floating 



* Rocks of this class from Cape Adare are described by Dr. Prior, loc. cit., p. 324. 



