178 AUSTKALASIAN ANTAECTIC EXPEDITION. 



The Rosiwal analyses of two slides gave the following results : 



I. II. 



Felspar : 45-3 .... 38-2 



Pyroxene 25-8 .... 26-7 



Hornblende 19-1 .... 24-1 



Garnet 3-2 .... 2-9 



Iron ore 4-3 4-9 



Biotite 2-1 .... 2-9 



Sphene, apatite 0-2 .... 0-3 



The first of these is cut parallel to the schistosity, and its higher felspar percentage 

 is due to the fact that the schistosity is marked by strings of felspar in the hand specimen. 

 The second slide is cut in a haphazard direction, and the measurement is made to 

 determine the variation in the garnet percentage. This variation proves to be less than 

 anticipated. 



The rock has a finely granoblastic structure. The felspar consists of water-clear 

 grains which sometimes show diablastic structure. The pyroxene, which occupies 

 one*quarter of the rock volume, includes plates of relic, dusty augite ; but it mostly 

 forms small granular crystals of augite and hypersthene, aggregated in areas which 

 originally represent large primary augite crystals. The clear recrystallised augite has 

 a pale green colour as before, and is practically free from the ilmenite inclusions. The 

 more pleochroic hypersthene is again present among the recrystallised pyroxene. The 

 percentage of green hornblende is not much less than that of the pyroxene, and indicates 

 the prominent degree of hornblendisation of the pyroxene. The garnet appears in small 

 pink crystals and is usually set in felspar areas ; this can be taken as evidence that it 

 has formed in the same way as in the basic rocks of Stillwell Island. The brown biotite 

 is usually associated with the pyroxene and hornblende areas. 



An interesting feature in this rock is the presence of a shear line which cuts across 

 the schistosity. This line is marked chiefly by a decolouration of the hornblende and 

 by broken strings of pyrite. The hornblende may assume a pale green colour, and, 

 if the bright polarisation are absent, it may look like chlorite. Sometimes the shear line 

 may cut straight a crystal of green hornblende and then there appears a belt of colourless 

 hornblende in the green crystal, and this belt is even more noticeable in polarised light. 

 Sometimes there is a pale green mineral with high polarisation colours in the shear 

 zone, and as it has straight extinction it is looked upon as a pale biotite. In addition, 

 there is a very fine granular aggregate of highly polarising mineral, which is possibly 

 talc. The felspar becomes saussuritised and, in general, there is a fuzziness in the 

 neighbourhood of the line. Conditions along a shear plane would correspond in some 

 measure with the conditions of the epi zone of metamorphism ; and the pale hornblende, 

 the chlorite, the talc, and the saussurite are, in general, looked upon as epi zone 

 products. 



