170 AUSTRALASIAN ANTARCTIC EXPEDITION. 



omphacite in the Otz Valley* eclogites. The development of the secondary pyroxene 

 involves a change in the double refraction and many augite crystals show uneven polarisa- 

 tion colours. If the primary augite is showing blues and greenish blues of the second 

 order, the recrystallised augite may show the higher greens ; but, on the other hand, 

 the degree of colour is often lowered to the reddish purples and violet at the top of the 

 first order. In the latter, grains of hypersthene have been seen as a nucleus. No 

 corresponding change in the extinction angles of the augites has been noted. 



A second type of alteration of augite that can be traced in this rock is the passage 

 into green pleochroic hornblende. The 3-4 per cent, hornblende in this rock has developed 

 in this way. The hornblende grains possess the same average size as the secondary 

 augite or felspar, but their distribution is not uniform. It is found in sporadic patches 

 which sometimes seem to indicate the outline of a prismatic crystal of pyroxene. The 

 4-2 per cent, of secondary brown biotite is distributed more uniformly throughout the 

 rock, and some of it may be reliu. Ilmenite abounds in small crystals and as minute 

 inclusions in the augite. It is definitely recognised as some of the larger crystals show 

 alteration to greyish leucoxene. Occasional grains of pyrite are also present. 



The metamorphic character of this rock certainly dominates the igneous character, 

 and, therefore, we call the rock a plagioclase pyroxene gneiss. 



No. 766. This example is a modified variety of No. 773, and shows distinct schis- 

 tosity. The secondary felspar and the secondary pyroxene are arranged approximately 

 in layers, producing a crystallisation schistosity. It is possible that the original rock 

 had a coarser grain size than the original rock of No. 773, because the plates of relic 

 augite are much larger in this example. The average grain size of the recrystallised 

 individuals is about twice as large. Relic dusty felspar is still present, and is surrounded 

 by a granulitic mass of clear felspar. In one case sharp lamellar twinning is present, and 

 the lamellae have extinction angles of 36 and 37, again indicating labradorite. The 

 recrystallisation of the primary pyroxene is more diagrammatic than in No. 773. Plates 

 of primary augite may form the nucleus of beautiful granoblastic zones of clear secondary 

 pyroxene whose growth in the direction of the schistosity may produce long tails (Plate 

 VII., fig. 1). Many of the grains can be identified as hypersthene ; in many cases 

 where the primary augite is completely replaced, the layer of granular pyroxene may 

 enclose areas, sometimes circular, of fine vermicoidal pyroxene set in an aggregate of 

 felspar producing a diablastic structure. At other times the vermicular pyroxene 

 forms a fringe around the outline of a primary pyroxene in the same manner as is more 

 prominently exhibited in No. 951 from Stillwell Island. This diablastic pyroxene has 

 character different from the granular pyroxene, and is not unlike the intermediate 

 stage m the formation of garnet which is seen in the Stillwell Island rocks. The 

 suggestion, therefore, is that these are incipient garnet areas. 



' The Percentage of Alumina in the Omphacite in the Otz Valley Eclogite is 10-91 per cent. " Beitrag zur Kenntnis 

 der Eklogite und Amphibolite," Laura Hezner. Wein, 1903, p. 10. 



