134 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Quartz units show strong undulose extinction, and are sometimes crushed so that 

 a unit in ordinary light becomes a fine fragmentary aggregate in polarised light. In such 

 cases a rude schistosity is evident, because many of the fragments are elongated in one 

 direction. Every large quartz unit is separated from neighbouring crystals by crush zones. 



Most of the felspar is orthoclase with perthitic inclusions of albite. The albite 

 forms lenticular layers in at least two directions in the crystal. In cross section the 

 albite is rectangular, and in longitudinal sections it appears as thin needles, while in 

 some sections two sets can be seen crossing at an acute angle. In addition to the 

 orthoclase and perthite there is a small amount of plagioclase with lamellar twinning 

 and a comparatively low refractive index. It is considered as an albite oligoclase or 

 an oligoclase, and it may contain perthitic inclusions of orthcclase. The lamellar 

 twinning may only appear indefinitely in one corner of the crystal, or the laminae may 

 be bent and irregularly wedge out. In such cases undulose extinction is present, and 

 there may be a poorly developed microcline structure. These observations are suggestive 

 of secondary pressure twinning. However, the plagioclase is definite where the crush 

 zones cut across the lamellae, for in such cases the lamellae must have existed prior to the 

 crushing. The felspar shows the crush phenomena even more markedly than the quartz. 

 The best examples of mortar structure are exhibited in felspar crystals which have 

 straight fractures. A single crystal may contain one or more fractures, and each fracture 

 filled with pulverised material. Mortar structure exists between a quartz crystal and 

 a felspar crystal, but it is always less noticeable between two quartz crystals. Strings 

 of minute inclusions are common and may extend into neighbouring crystals. Shear 

 zones of sericite are often present, and become iron-stained during weathering. Sericite 

 also appears along cleavage planes. Diablastic structure is very common in the crush 

 areas, and there appear vermicular interlacings of felspars and of quartz and felspar 

 (Plate III., figs. 1 and 2). Sometimes it is coarse and sometimes it is very fine, but in 

 many instances it is obviously a secondary structure produced during metamorphism. 



The hypersthene is present with its characteristic pleochroism and straight extinction. 

 Like the quartz and felspar, it has suffered mechanical deformation, and one crystal is 

 broken with the two pieces separated by a fracture zone. The crystals are sometimes 

 bordered with a crush rim. It shows considerable alteration to a greenish, fibrous, 

 serpentinous mineral, with moderately low polarisation colours, usually masked by 

 the green colour. This mineral is similar to the alteration product of hypersthene 

 in the Indian charnockites, which is described by Holland as resembling delessite*. 

 The greenish mineral is a very constant associate of hypersthene in all the acid 

 hypersthenic gneisses of Adelie Land. It is found in other cases to be intimately mixed 

 with a pale-green biotite showing brilliant polarisation colours, and this rather suggests 

 that it is delessite, a chloritic mica. On the other hand, cases have been noticed where 

 the hypersthene passes through bastite into serpentine, whose appearance is quite similar 

 to this green mineral. In part brown biotite and ilmenite seem to be developed from 

 it. The biotite and ilmenite are practically confined to the hypersthene areas, and 



* " The Charnockite Series," T. S. Holland, Mem. 28, pt. 2, G.S. India, p. 141. 



