17 2 AUSTKALASIAN ANTAECTIC EXPEDITION. 



The bulk of the felspar is in clear transparent crystals in which the twinning is some- 

 times indefinite and irregular. A maximum extinction angle of 28 has been measured, 

 and the felspar is, therefore, interpreted as labradorite. Both augite and hypersthene 

 are present and are free from the dusty inclusions of ilmenite. Brown biotite is present 

 both in large platy crystals and in small secondary crystals. The ilmenite is abundant. 



The large pyroxene and biotite crystals, as well as the ilmenite, are almost invariably 

 bordered by a zone which follows the outline of the crystal, no matter how irregular 

 and ragged it may be (Plate VII., fig. 5). The zone may be described as a diablastic 

 intergrowth of vermicular pyroxene and felspar ; but the pyroxene is different from 

 the normal pyroxene, and the felspar is not the relic labradorite. The vermicoidal 

 pyroxene has a lighter colour than the normal pyroxene, suggesting that part of the 

 iron may have separated out to form ilmenite. The felspar is a more sodic felspar 

 and its development from the calcic felspar is very noticeable. Sometimes small 

 secondary biotites are seen in these diabiastic fringes, as well as small ilmenites. Ilmenite 

 crystals, large or small, are always associated with the pyroxenic parts of the slide 

 though there is no direct evidence to show here, as in No. 773, that they form during 

 the recrystallisation by the coalescence of minute inclusions in the primary pyroxene. 

 The same diablastic fringe is also found surrounding large ilmenite and biotite crystals ; 

 but it does not accompany these with the same regularity as it accompanies the pyroxene. 

 Ilmenite crystals may be found with a rim of pyroxene, and if this rim should pass into 

 the vermicoidal type we should get the ilmenite crystal surrounded by the diablastic 

 zone in the way we have often observed in this section. Some of the iron ore has the 

 appearance of pyrrhotite. 



There can be no doubt that this diablastic zone is a product of a reaction between 

 labradorite and pyroxene, or between labradorite and biotite. Stages may be observed 

 between augites surrounded by a thin rim and small augites surrounded by a thick zone. 

 In the latter the remaining augite is mouse eaten and has nearly disappeared. 



The formation of biotite in the vermicoidal zone must be associated first with a supply 

 of K 2 from the felspar, and secondly with the temperature factor during metamorphism. 

 The temperature factor must be high to permit the formation of secondary pyroxene, 

 and it may have been, in the first stage of metamorphism, too high for biotite. Biotite 

 may have been formed only after a lowering of the temperature, and the appearance 

 of the biotite is quite in agreement with the suggestion that the biotite is subsequent 

 to the initial formation of the rim. A study of the phenomena in No. 935 shows that this 

 reaction is the initial stage in the formation of garnet. 



The rock may be described as a plagioclase pyroxene gneiss, which shows the 

 incipient stages of garnet formation. 



No. 942. No. 942 is another example which still retains normal igneous structures. 

 It occurs in dyke-like bands, up to 10ft. wide, crossing the garnet gneiss. It is a much 



