THE METAMORPHIC ROCKS OF ADELIE LAND. STTLL\VELL. 139 



in the section. The formation of this hornblende is distinctly a meso zone character. 

 The small amount of biotite is probably developed by the interaction of pyroxene with 

 felspar. The felspar is again found in two varieties of plagioclase, and even though 

 more than half is untwinned it is all believed to be plagioclase*. There is a calcic 

 andesine or a sodic labradorite, and there is a much smaller quantity of a sodic or 

 albitic plagioclase which in very exceptional cases possesses a refractive index less than 

 Canada balsam. Ilmenite is found in indented and irregular grains. In all cases the 

 pyroxene exerts its crystalline form against the ilmenite, so that in any aggregate of 

 pyroxene and ilmenite grains the ilmenite is pushed into the interstices between the 

 pyroxenes. It can therefore be understood, when the ilmenite is crushed and granulated 

 in subsequent epi zone metamorphism as in No. 794, why the crushed ilmenite should 

 appear as a border to the pyroxene crystals. Red-brown pleochroic rutile is sometimes 

 associated with the ilmenite. 



The absence of cataclastic structures and the important hornblende percentage 

 influence the decision that this rock shows meso zone characters rather than kata zone 

 or epi zone features. Its history will otherwise be the same as the related rock No. 

 794, and it has been a kata zone metamorphic rock consisting essentially of felspar, 

 pyroxene, and ilmenite on which a meso zone metamorphic impress has been super- 

 imposed. It may therefore be called a meso plagioclase pyroxene gneiss (meso pyroxene 

 granulite), or a hornblende plagioclase pyroxene gneiss (Plate IV., fig. 2). 



Had the meso zone metamorphism been complete, all the pyroxene would have 

 been converted into green hornblende and an amphibolite produced. The chemical 

 composition must be similar to that of No. 794, and hence both chemical and micro- 

 scopical characters reveal relation to the amphibolites. As many amphibolites are 

 altered dykes rocks, this plagioclase pyroxene gneiss, similar to some of the Saxon 

 pyroxene granulites, is probably a diabase dyke which has suffered metamorphism 

 under kata zone conditions. The field observation that this rock appears as a band 

 crossing the gneiss is confirmatory of such an argument. 



HYPERSTHENE ALKALI FELSPAR GNEISS. 



Specimen No. 754 is the type example of this rock, and it is a coarse-grey gneissic 

 rock. In some examples the gneissic character becomes more prominent on the 

 weathered surface. Macroscopically one can see thin lenticles of quartz set in a granular 

 mosaic of felspar, and pink garnets and black hypersthene are also drawn out in layers 

 in the direction of the schistosity. 



In thin section the rock is granoblastic, with a tendency to a coarse crystallisation 

 schistosity. Quartz units are built up of interlocking grains. There is little cataclasis. 

 Felspar consists of untwinned individuals and lamellar twinned individuals. The 

 former include both orthoclase and clear albite with perthitic inclusions of orthoclase. 



The frequent absence of twinning lamelle has been noted by Washington u a peculiarity of the hypersthenic roclu 

 of India and allied area* ; but we have noticed it in amphibolitea and other metamorphic roclu. 



