86 HORN EXPEDITION — PETROLOGY. 



high niagnitication. As in the case of No. 220 the rock has a granular structure 

 which is well-marked "under crossed nicols." A few lines have been put in the 

 drawing (Plate I., Fig. 2), to indicate the outlines of the grains. The quartz and 

 felspar components of the micro-pegmatitic grains are frequently arranged in 

 parallel lines presenting tlie appearance of innumerable fine striations, which is 

 well seen in Plate IV., Fig. 'U\ where one of the constituents is extinguished. At 

 other times they completely inter-ramify, giving a liomogeneous appearance, the 

 complexity of which is recognisable only under considerable magnification. The 

 mass of the rock is made up of compacted grains of tlie above nature. 



In addition to this there are a few small granules of quartz, and a fair number 

 of kaolinised idiomorphic crystals of felspar. Some of the latter are surrounded 

 by micro-pegmatite arranged in irregular radial sectors. 



A i^ale pyroxene is distriljuted in small grains through the rock as well as a 

 fair amount of magnetite, and a little red oxide of iron. Much of the magnetite 

 as well as a pleochroic bluish-green mineral appears to have resulted from the 

 decomposition of part of the pyroxene. 



An important feature of the rock is the presence in most of the micro-pegma- 

 titic grains of numerous needle-like inclusions, which are aggregated in diverging 

 groups. Tliese inclusions are very often absent from the centre of the grains, 

 apparently not having made their appearance till part of the grain had crystallised 

 out. Under a high power these needle-like inclusions are seen to be composed of 

 longulites of a pale green colour, which are irregularly aggregated along straight 

 lines, and appear to have been formed contemporaneously with the micro-pegmatite. 

 This specimen was also obtained from a pebble occurring in the Mount Olga 

 conglomerate. 



Granulitic pyroxene diorite. No. 25. Plate I., Fig. 3, and Plate IV., 

 Fig 1^/-/;, and Fig. 5(;. Sp. gr. 3-09. 



This is a fine grained rock with a sulj-conchoidal fracture, not unlike a basidt 

 in appearance, but ratlicr lighter in colour. Small dark grains are distinguishable 

 by the naked eye, and colourless particles of about the same size ; the former 

 prove under the microscope to be augite and hornblende crystals, and the latter 

 poi-phyritic plagioclase crystals. It is composed of an aggregate of granular 

 crystals of felspar, hypersthene, augite, ;uid magnetite, which with the exception 

 of a few of the felspar and augite crystals are of an uniform size. There are, 

 however, some porphyritic crystals of plagioclase in lathe-shaped sections, and of 

 a pale gr-ecn augite. 



