392 PROCEEDINGS OF SECTION C. 



eufcectic mixture has crystallised as a granophyric intergrowth of quartz 

 and felspar. The very small proportion of quartz and the almost com- 

 plete absence of magnetite are noteworthy. 



SEGREGATION IN GRANITE FROM SUMMIT OF STATION PEAK, 

 SECTION 361 (See Plate I, Fig. 2). 



Megascopic Examination. — This ditters trom the last rock in hand 

 specimen mainly on account of the porphyritic structure produced by 

 the development of large quartz and felspar aggregations. 



Microscopic Examination. — The structure of the rock is hypidio- 

 morphic. The minerals, iii order of abundance, are alkali felspar, 

 quartz, biotite, oligoclase (abganj), hornblende, apatite, zircon. 



The felspars appear to be of three types — an alkali felspar, in large 

 irregular crystals, and showing undulose extinction and either obscure 

 lamellation, or none at all, and extinction angles from 0° up to 11°. 

 Another type consists of smaller, sometimes idiomorphic alkali felspars ; 

 more often, however, allotriomorphic, untwinned or simply twinned ; 

 these are probably orthoclase. A third type is a plagioclase, probably 

 oligoclase, in which symmetrical extinctions from twin lamellae give 

 angles of 9° and 15° in different crystals. 



Quartz, both interstitial and also included in plagioclase, is more 

 abundant than in section 390, and shows slight undulose extinction. 



Biotite and Hornblende both occur as phenocrysts, and as smaller 

 elongated crystals included in felspar and quartz. The hornblende has 

 irregular boundaries, is of a pale-green color, and has colorless patches 

 in the centres of the crystals ; while the margins are in places altered 

 to chlorite. 



Apatite and zircon occur as small inclusions in the biotite and the 

 felspar crystals. 



SPECIMEN FROM BASE OF DYKE ON SOUTH-EAST SLOPE OF 

 STATION PEAK, SECTION J. (See Plate II., Fig. 2). 



Megascopic Examination. — This is a dark-grey close-grained rock, 

 with occasional phenocrysts occurring as a broad dyke 20ft. to 30ft. 

 wide, and extending for some hundreds of feet up the south-east slope 

 of Station Peak. 



Microscopic Examinatioyi. — The structure is porphyritic, with a 

 pan-idiomorphic even-grained ground mass. The minerals present, in 

 order of abundance, are plagioclase, hornblende, biotite, alkali-felspar, 

 quartz, uralite, magnetite or ilmenite. 



Two or three large porphyritic kaolinised crystals of a plagioclase, 

 probably a basic andesinc occur with indistinct lamellae extinguishing 

 at 20°. 



Another crystal shows lamellae in one-half of an irregular twin and 

 no lamellae in the other half, and showing undulose extinction. The 

 felspars of the ground mass are mainly idiomorphic, zoned, imtwinned 

 sections of plagoclase, the central zone extinguishing in one crystal 

 at + 30°, diminishing in outer zones to 0°, and at the margin being 

 — 1'^. The central part is therefore labradorite, the middle parts 



