v 



236 AUSTEALAS1AN ANTARCTIC EXPEDITION. 



Biotite is present in Hakes showing a brown to pale-yellow pleochroism. 



A crystal of a mineral showing parallel intergrowth of several individuals with 

 good crystal outline, is developed in this rock. It is pleochroic in bluish-green to yellow- 

 green with brown tints, is optically negative, and an optic, axial plane perpendicular 

 to the elongation, the sign of which is negative. The pleochroic tints are those of soda 

 amphiboles, and the extinction is oblique (14 degrees). The position of the optic axial 

 plane cannot be the symmetry plane, however, if prismatic habit obtains. The only 

 metamorphic soda amphibole in which the optic plane is perpendicular to the 010 face 

 is crossite. In the absence of further sections, no definite determination is possible. 



(4) PYROXENE-GARNET-MARBLES. 



Under this class come the rocks Nos. 658, 730, and 849. They are essentially 

 types which would be grouped under the term " Kalksilikatgneise " by German and 

 Scandinavian petrographers. 



No. 658 is a dark-coloured banded rock, consisting of layers of pink calcite in 

 which silicate minerals are sparingly distributed, alternating with bands rich in greenish- 

 black pyroxene, white felspar, and thin layers of brown garnet. 



No. 730 is also a banded calc-silicate-gneiss, showing layers of flesh-coloured 

 calcite containing greenish-black pyroxene, and porphyroblasts of garnet intimately 

 associated with epidote. Such a band is separated from finer-grained bands rich in 

 felspar and pyroxene, by narrow layers of pyroxene and epidote. 



No. 849 shows large grains of dark-brown garnet, pyroxene, and yellowish- 

 green epidote in addition to calcite and colourless silicates. 



These banded rocks with varying mineralogies 1 composition attest the changing 

 composition of the original carbonate sediment during its deposition. 



The constituent minerals of these rocks are calcite, pyroxene, garnet, epiflote, 

 scapolite, microcline, titanite, actinolite, quartz, apatite, magnetite and plof/iodase. 



The pyroxene of No. 658 is green in colour and noticeably pleochroic : X ; 

 yellow-green, Y and Z = sea-green. The value of the extinction Z A c > 48 degrees, 

 and this figure when considered with the colour of the mineral in thin section indicates 

 that the hedenbergite molecule must dominate, but that a content of sesquioxides is 

 also present. The pyroxenes of Nos. 730 and 849 are also green in thin section, and 

 have a high extinction-angle corresponding to a content of hedenbergite. The garnet 

 of these rocks has a clove-brown colour very similar, to that of the associated titanite. 

 It is not idioblastic, and is often moulded on the green pyroxene. It is always isotropic. 

 In No. 658 it is associated with green pyroxene, and may enclose the other minerals. 

 In No. 730 it usually forms a core to the epidote-garnet aggregates, the central garnet 

 being enclosed by a narrow corona of pleochroic epidote, 



