276 AUSTRALASIAN ANTARCTIC EXPEDITION. 



subordinate to hornblende, and epidote has developed from the crushed biotite. Some 

 of the epidote is finely granular, and some of it occurs in grains and crystals. Quartz 

 is subordinate to felspar, which consists of an acid oligoclase and untwinned felspar. 

 The felspar possesses myrmekitic structure, but it is free from products of decomposition. 

 Quartz and some of the felspar shows strong undulose extinction. Accessory grains of 

 apatite, allanite, and sphene are present. 



Specimen No. 240 is a biotite-plagioclase-gneiss, and another example with 

 affinities to the amphibolite group. It is a grey schistose rock, in which the dark 

 constituents, biotite, and hornblende are subordinate to the white minerals. In 

 section, the flakes of brown biotite are arranged parallel with the schistosity, and are 

 in large excess over the green hornblende. There is a small amount of green chlorite 

 and colourless lawsonite interlaminated with the biotite. Quartz is present, and the 

 andesine felspar is quite clear, except for a few crystals which possess cloudy micaceous 

 alteration products. 



An exceptional biotite-hornblende-felspar-gneiss is specimen No. 956. The 

 hand specimen is a dark schistose rock, showing glistening biotite, and is finer grained 

 than most of these rocks. In section, the hornblende is quite distinct from the 

 granular green variety of the typical amphibolites. It occurs in ragged, acicular 

 crystals, and the colour of the prisms is mostly green and pale green. Some prisms 

 show a pleochroism from greenish-blue to blue, indicating a change to glaucophane. 

 In addition to the large crystals, it also occurs as fine acicular needles. Its polarisation 

 colours are unusual. In many cases there is no position of absolute extinction 

 between crossed nicols ; but a position of 30 to 40 to the cross wires shows ultra-blue 

 colours. While the polarisation colours appear to be low, they are interfered with 

 by the colour and dispersion of the crystal. The biotite possesses an unusual colour, 

 which changes in cross-section from a reddish-brown to a biscuit-brown. The crystals 

 possess an unusually ragged outline, and contain minute dots of discharged ilmenite. 

 The amount of felspar appears to be a little greater than in the typical amphibolites. 

 The felspar crystals are mostly untwinned, some possess traces of microcline twinning 

 and some are lamellar twinned crystals of acid oligoclase. Calcite, ilmenite, and epidote 

 are minor constituents, and pyrite and apatite are accessory minerals. While the 

 rock is more basic than typical members of this group, it is at the same time distinct 

 from the amphibolites. 



A different type of hornblende-quartz-plagioclase schist is No. 51. This is a 

 grey schistose rock, in which numerous prismatic needles of hornblende and occasional 

 small crystals of ilmenite are set in a grey groundmass. In section, the rock consists 

 of thin, elongated crystals of green hornblende, set as porphyroblasts in a finely 

 granulitic groundmass of quartz and felspar. The hornblende crystals are ragged 

 with the pronounced prismatic habit. Its colour is dark green to bluish-green. 

 Numerous small and isolated flakes of biotite have developed along some of the 

 schistose layers. The ground mass, in which the quartz appears to predominate is 



