294 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Felspar is an important constituent, though it is totally absent in the magnetite- 

 schist No. 912, in the quartz- or epidote- or mica-magnetite-schists Nos. 296, 989, 933, 

 and in the quartz-garnet-magnetite rock No. 102. Orthoclase is a common constituent 

 but microcline only occurs in the magnetite-garnet-schist No. 245, and the garnet- 

 magnetite-schist No. 288. The plagioclase ranges from albite to labradorite and i^ 

 commonly happens that an acidic and a basic plagioclase are present. Generally the 

 felspars are much kaolinised and sericitised. 



Cordierite is not common but is observed in four magnetite-garnet schists and 

 gneisses No. 889, 147, 827, and 304. Such crystals as are seen in these sections do not 

 exhibit any measurable pleochroic halos. 



Biotite is a very common constituent, the usual variety being green in colour. 

 Muscovite is present in some sections, notably in the chlorite-magnetite-schist No. 

 527 A, the mica-magnetite-schists Nos. 55, 933, and the epidote-magnetite-schist No. 889. 

 Chlorite is an important mineral and usually owes its origin to the alteration of biotite 

 or garnet. Epidote is abundant in some of the magnetite schists but it loses importance 

 as the ratio of garnet to magnetite increases. Apatite is a common accessory, as also is 

 zircon. 



Magnetite usually has ilmenite or hematite or rutile intergrown with it. Limonite, 

 pyrite, and sphene also occur but the latter two minerals are not common. The 

 percentage of iron ore ranges from 33-0 in the magnetite-schist No. 294 to 6-2 in the 

 garnet-magnetite-schist No. 288. In most cases the crystals of magnetite are xeno- 

 blastic rather than idioblastic. In Nos. 181, 288, and 348 there are magnetite 

 porphyroblasts and in the example in Plate I, fig. 2, a pronounced linear structure has 

 developed from the porphyroblasts. As the porphyroblasts of magnetite have their 

 pnger axes parallel to the schistosity and contain numerous inclusions of garnet, quartz, 

 and biotite, all of which originated during, or were recrystallised by, the metamorphism, 

 these porphyroblasts evidently arose at the time of major metamorphism, when the 

 conditions of pressure and temperature probably corresponded to Grubemann's meso 

 .zone of metamorphism. The chloritisation of the biotite and the kaolinisation of the 

 felspars probably arose under later conditions of epi zone metamorphism. Still well * 

 has applied a conception of metamorphic differentiation to clots of biotite and epidosite 

 in amphibolite dykes at Cape Denison. It is possible that the same conception can be 

 applied to the magnetite porphyroblasts, in which case they are due to local segregation 

 of iron ore arising as a direct consequence of the metamorphism. In many instances in 

 the suite of rocks described, magnetite has been considered as arising partly from the 

 result of epi zone processes on the garnet and mica. 



Garnet is a less constant constituent than the magnetite. It is absent in some 

 specimens and ranges up to 40 per cent, in No. 102, where it dominates the colour of the 



* " The Metamorphic Rocka of Adelie Land." F. L. Stillwell. Aust. Ant. Exp. Scientific Reports, Series A, Vol. Ill, 

 Part 1 (1918), p. 58-71. 



