278 AUSTRALASIAN ANTARCTIC EXPEDITION. 



A coarser-grained type of the same class of rock is No. 726, in which the pink 

 bands of felspar are 2^ cm. wide. No. 726 is a handsome rock, whose colour is dominated 

 by the pink felspar, but contains disseminated patches of bright green epidote. In 

 addition to the pink bands, there are bands of green chloritised hornblende with a silky 

 sheen. In section, there is a little cloudy untwinned felspar, which may be orthoclase, 

 and lamellar-twinned oligoclase-albite. The development of epidote within some 

 of the plagioclase indicates that it has been partly derived from felspar. Pale 

 decolourised hornblende is present, and the abundant chlorite and some of the epidote 

 has been derived from it. Quartz is absent in this case, but large grains of apatite 

 and zircon are present. 



Of different appearance is specimen No. 598, a pink and green massive boulder 

 containing large porphyroblasts of pink microcline up to 3cm. in length and over 1cm. 

 in width, set in a rather coarsely-crystalline base of pink felspar, bright green epidote, 

 and dark-green chlorite. The general appearance is somewhat similar to a pink 

 porphyritic granite, except that there is a general absence of quartz. The resemblance 

 is more strictly to a felspar-porphyry, from which it has probably been derived. The 

 large porphyroblasts show the irregular wedge-shaped twinning of microcline, with 

 small inclusions of quartz, plagioclase and sphene. The character of porphyroblasts 

 has been confirmed by a determination of the alkalies by Mr. J. C. Watson, in the 

 Victorian Geological Survey laboratory, which gave : 



K 2 14-04 per cent, 



Na 2 -66 



The felspar in the base of the rock consists of lamellar-twinned plagioclase 

 and subordinate untwinned felspar. Sometimes it shows cataclasis and a partial 

 development of myrmekitic structure, and sometimes bent lamellae. Its extinction 

 angle measures up to 14 and it is probably an oligoclase. The green chlorite and 

 epidote are bunched together in aggregates, and of these two minerals, epidote is the 

 more abundant, forming large idiomorphic crystals. Sphene and apatite are accessory 

 minerals, and the rock can be classed as an epidote-felspar-gneiss. The presence of 

 large crystals of microcline indicate a gradation towards the group of the alkali felspar 

 gneisses (Group I). 



A related rock is No. 954, in which the porphyroblasts of microcline are much 

 smaller. The grain size is less than half that of No. 598, and the appearance is more 

 granulitic. The epidote and chlorite are more or less evenly distributed through the 

 section, producing the darker colour as opposed to the pink colour of the preceding 

 specimen. The aggregated amount of epidote and chlorite is less than in previous 

 examples, and the chlorite is more abundant than epidote. Crystals of sphene 

 accompany the epidote. Microcline occurs in base as well as forming the porphyro- 

 blasts, and contain numerous inclusions of albite, with a ragged and irregular shape, 

 after the manner of graphic structure. There is a considerable amount of untwinned 



