FROM THE MAINLAND OF SOUTH VICTORIA LAND 217 



Pink and Grey Porphyries from the Stranded Moraines, East Fork of the Ferrar 

 Glacier. The porphyritic constituents are white or pinkish felspar, quartz, and 

 frequently mica. There is a marked orientation of the felspars and flow is further 

 indicated by parallel dark streaks. These rocks are very dense and of the halleflinta 

 type, and may possibly be of much older age than those described earlier. They very 

 closely resemble certain Pre -Cambrian porphyries of Southern Australia. In the case of 

 other porphyries from the East Fork of the Ferrar Glacier, the bases are microscopically 

 crystalline and in other respects they resemble the aplitic porphyries of the Cape Irizar 

 granite following. 



Another interesting ortlwclase porphyry (Fig. 2, Plate II) is a grey hornblendic 

 variety occurring in situ at the Kukri Hills, Ferrar Glacier. This is rather more basic 

 than usual as it contains much plagioclase. It is rendered remarkable on account of 

 the white orthoclase crystals 12 mm. in length, studded throughout the hand-specimens. 

 The ultimate base consists of quartz and orthoclase, which frequently exhibit a micro- 

 pegmatitic structure. 



THE APLITIC GRANITE PORPHYRIES 



There are now to be considered a large number of specimens illustrating granite 

 porphyries in which an aplitic base is the most conspicuous part. They always form 

 part of the granite masses but are usually of small size and very irregular in shape. In 

 the more regular dyke -like occurrences, the porphyritic constituents diminish in bulk 

 and typical aplites are arrived at. The aplitic condition is perfect when the rock is 

 an even-grained panidiomorphic mixture of quartz and orthoclase with inconspicuous 

 amounts of accessory minerals. The base is either grey or pink in colour ; as a rule 

 the more aplitic types are the pinker in colour. 



Aplitic Granite Porphyry ; Cape Irizar 



A typical example of this aplitic granite porphyry occurs at Cape Irizar, about a 

 quarter of a mile west of the extreme point. There, there are two parallel outcrops 

 emerging from amongst the granite ; the junctions are unfortunately obscured. In 

 the hand-specimen it appears grey, with rather large pieces of porphyritic orthoclase 

 as much as 9 mm. in diameter. 



Hornblende prisms up to 6 mm. diameter are not infrequent. Aggregates of varying 

 size consisting of porphyritic quartz, felspar, mica, and even hornblende are scattered 

 through the finer base. 



In the microscopic section (Figs. 6 and 8, Plate III) porphyritic orthoclase, anortho- 

 clase, oligoclase, hornblende, and quartz are distinguished. The hornblende and some of 

 the felspars are in sharply defined crystals, whilst other minerals are corroded. Horn- 

 blende is plentiful, pleochroic in light yellow, yellowish-green, and blue-green. Viewed 

 in ordinary light it is of a blue tint. Some are true reibeckite whilst others appear to 

 be intermediate between the latter and common hornblende. The base is a fine 

 granular mixture of quartz and orthoclase and some oligoclase. The grain size averages 

 0-15 mm. The felspar is hypidiomorphic or idiomorphic in form. Graphic inter- 

 growths of the quartz and felspar are often present in this class of rock, but are not 

 marked in this particular outcrop. Allanite : several pieces noted. There are a few 

 grains of ilmenite, leucoxene, and of pyrites. 



The granite, near the junction of this aplitic porphyry, contains a larger proportion 

 of hornblende, entirely replacing the biotite. Zoning is noticeable in some of the 

 plagioclase crystals. 



