FROM THE MAINLAND OF SOUTH VICTORIA LAND 



211 



represent the latest separations of orthoclastic material. There has probably been no 

 lapse of time between these successive modes of separation, the variation being merely 

 consequent upon the changing physical-chemical conditions existing in the magma. 



Microperthitic intergrowths are common. Some part of the orthoclase showing 

 mottled markings under crossed nicols appears to be soda orthoclase. Generally 

 speaking the mineral is very fresh. Dusty change-products become prominent in the 

 redder felspars of certain outcrops where metamorphosing external agencies, referred 

 to later, exerted an influence. Plagioclase, chiefly oligoclase but occasionally as basic 

 as andesine. Twinning is usually after the albite law only, though occasionally noted 

 compounded with the Carlsbad type. It is always more dusty than the orthoclase. 

 Amongst the secondary minerals white micaceous flakes are recognisable. The 

 plagioclase has crystallised earher than the quartz and orthoclase. The mica is a 

 biotite, pleochroic (yellows and browns). The laminae are often slightly bent. The 

 hornblende occurs in irregular prisms. At a hasty glance it is not easily distinguished 

 from the biotite. It is pleochroic from light yellow to dark green. Chloritic decom- 

 position products are sometimes observed. Magnetite in irregular grains only rarely 

 seen. Sphene is still rarer and observed almost always adhering to magnetite grains. 

 Apatite is very rare : it appears always in the tiniest crystals adhering to magnetite 

 or embedded in the biotite. Zircon appears to be more frequent in some specimens 

 than others. It is seen as minute grey crystals almost always embedded in the biotite 

 mica and surrounded by pleochroic haloes. Allanite is by far the commonest of the 

 accessory minerals appearing in nearly all the granites in our collections. In slides 

 from this particular rock grains "5 mm. diameter were met with. It is generally 

 associated with biotite and is always idiomorphic. It is of a brown colour, slightly 

 pleochroic and distinctly zoned. It is weakly doubly refracting and has high refractive 

 indices. 



An approximate Rosiwal mineral analysis gave : 



Mode 

 Quartz 295 



Orthoclase, including soda orthoclase 



Microcline 



Plagioclase, chiefly oligoclase 



Biotite 



Hornblende 



Accessories . 



32 

 7-2 



250 

 4-0 

 2-0 

 0-3 



The following is a chemical analysis of a fresh sample made by A. B. Walkom, B.Sc. 



Si0 2 



Ti0 2 



A1 2 3 



Fe 2 3 



FeO 



MnO 



MgO 



CaO 



Na a O 



K 2 6 



H 2 + 



H 2 0- 



P 2 O s 



C0 2 



71-87 

 0-25 



1516 

 0-62 

 210 

 04 

 0-29 

 0-84 

 3-88 

 4-72 

 0-35 

 015 



trace 

 09 



COMPOSITION OF THE NOEM 



99-S5 



ii 



100-36 



2h 



