PROCEEDINGS OF SECTION C, 391 



A determination in duplicate of ttie alkalies in the porphyritic felspar, 

 kindly made for me by one of my senior students (Mr. A. J. Robertson) 

 in the Chemical School, gave the following result : — 



Na^O = 241 

 K,0 = 11-85 



Total alkahes = 14-26 per cent. 



This result confirms the microscopic determination, and shows that 

 the bulk of the felspar is orthoclase, with some albite and probably 

 some anorthoclase. The smaller alkah felspars are generally allo- 

 triomorphic, and moulded on the quartz. 



Plagiodase. — The plagioclase crystals are much less abundant than 

 those of the alkali felspars. They are generally idiomorphic, show well- 

 defined narrow albite lamellse in some sections (100), and exhibit, with 

 respect to these lamellae, symmetrical extinctions which range in 

 different crystals from 4° to 14°, indicating oligoclase to andesine or 

 albite oligoclase. 



Some sections parallel to (010) show no lamellae, but marked zoning 

 in polarised light, and show extinction angles ranging from + 10° in 

 the centre, + 5° in an intermediate zone, and — 16° at the boundary, 

 indicating changes in composition from oligoclase-albite in the centre, 

 through oUgoclase (ab^anj) to albite or albite-oligoclase on the outside. 



Biotite occurs in ragged crystals, with pleochroism, ranging from 

 straw-yellow to nearly black. It has many enclosures of apatite, and 

 a few of magnetite, and shows pleochroic halos round them. 



The rock differs markedly from the normal Victorian grano-diorites 

 in the large excess of alkali felspar over plagioclase and the almost com- 

 plete absence of magnetite. It belongs, therefore, to the granite family. 

 If the identification of anorthoclase is correct, it would be best des- 

 cribed as a natron or soda granite, and in any case it may safely be 

 described as an alkali-granite. 



SEGREGATION IN GRANITE, SOUTH-WEST FLANK OF STATION 

 PEAK, SECTION 390 (See Plate II., Fia. 1). 



Megascopic Examination. — The hand specimen containing more 

 biotite is darker, and also finer and more even-grained, than the normal 

 granite. 



Microscopic Examination. — The structure of the rock is even-granied 

 and panidiomorphic. 



The minerals present, in order of abundance, are biotite, alkali 

 felspar, plagioclase, quartz, hornblende, apatite. 



Alkali Felspar.— Much is untwinned or simply twinned, and is 

 probably orthoclase. Some shows very fine obscure lamellation, and 

 may be anorthoclase. One or two porphyritic crystals are of this 

 character, while others are oligoclase, with extinction angles of 15°, 

 while (010) sections show zoning. The small amount of quartz is 

 moulded on the felspars, while in places a small amount of a residual 



