314 AUSTRALASIAN ANTARCTIC EXPEDITION. 



commonest consists of blebs of quartz included in areas of orthoclase. The felspar, 

 when plagioclase, is very highly saussuritised. Some lamellar twin forms, both albite 

 and pericline, are present. The orthoclase and microcline are, in part at least, 

 developments due to the concentration of the potassium from earlier plagioclase which 

 must have been highly potassic. A further development has been the production of 

 considerable sericite which is almost invariably associated with secondary calcite, the 

 mineral which has been formed by the contemporaneous concentration of the lime 

 molecule of the original felspar. The existence of quartz and calcite together is evidence 

 of the dynamothermal metamorphism to which the rock has been subjected. 



Zircon occurs as nuclei of intense pleochroic haloes in the biotite and also as 

 isolated fragments in the orthoclase. Its brown colour is suggestive of monazite, but 

 its positive uniaxial nature differentiates it from that mineral. 



The structures developed are porphyroblastic, shown by the garnet, granoblastic 

 shown by almost the whole of the remainder of the rock, and granulitic shown by the 

 calcite. 



No. 59. Grarnet-Plagioclase-Pyroxene-Hornblende Gneiss. 



A note with this specimen made at the time of collection (by Dr. Mawson) records 

 that it forms a broad band about 20 feet wide with ill-defined borders where the boat 

 was kellicked when a landing was made upon the island. 



The rock itself is gneissic. It is composed of black pyroxene and amphibole 

 crystals set in a mesh-work structure of brown matter, whose content is made up of pink 

 garnet, felspar and a very little quartz. 



The mineral content of the rock is pyroxene, hornblende, biotite, ilmenite, garnet, 

 andesine, quartz and pyrites. 



The pyroxene is non-pleochroic. It always shows some indication of alteration. 

 There is exhibited a serial change in different parts of the rock. The simplest is the 

 separation of ilmenite as fine dust particles which make the mineral very dark and patchy. 

 There is usually, but not always, an accompanying development of green hornblende ; 

 particularly is this true in the peripheral areas where a border of hornblende round the 

 pyroxene is quite common. This last development sometimes occurs with grains of quartz 

 (or very acid felspar), thus showing that these two constituents have been formed 

 contemporaneously by the mineralogical alteration of the pyroxene. 



The rest of the mineralogical changes are of interest for they show the progressive 

 stages in the formation of garnet by the interaction of felspar and hornblende as a 

 variation there is also seen the formation of garnet from the reaction between biotite 

 and felspar. The usual arrangement of these minerals consists of a central core of skeletal 

 ilmenite interwoven with brown biotite which gives way to a surrounding zone of 

 hornblende the ilmenitiferous content is much lower and the hornblende, in turn, 

 frequently terminates in phalanges which project into reaction areas of highly acidic 



