THE MKTAMORPHIC ROCKS OF ADEL1E LAND. 8T1LLWELL. 155 



Stittwett Island. 



No. 949 is an example of this rock type, and was collected from a fine-grained band 

 several inches wide, which crossed the gneiss irregularly near the summit of the island. 

 This specimen is a rather dark-coloured rock with a vitreous lustre. Quartz and felspar 

 are visible with a lens, and specks of pyrite are sprinkled unevenly through it. 



The section consists of an even-sized granoblastic aggregate of quartz and felspar, 

 through which grains of pyroxene and its associated biotite and ilmenite, pyrite and 

 apatite are scattered (Plate III., fig. 3). The average absolute grain size is approximately 

 0-20mm. There is a general absence of crystal boundaries, and the manner in which 

 blebs of quartz are set in the felspar is clearly metamorphic. The felspar is very clear 

 and unaltered, and includes orthoclase and plagioclase. Lamellar twinned individuals 

 have a refractive index often above basal quartz. The large extinction angle, measured 

 from the trace of the lamellae, is 20, and hence the felspar is andesine. The pyroxene 

 is largely hypersthene, and only a few grains do not possess straight extinction*. The 

 pale-pink to pale-green pleochroism is very marked. The same green serpentinous 

 alteration product which appears associated with the hypersthene in the Madigan 

 Nunatak and Aurora Peak gneisses is found in this rock. In part biotite and ilmenite 

 are developed in its alteration. The biotite may be mixed with the green alteration 

 product, but probably the green mineral has developed after the biotite, as the latter 

 may be associated with perfectly fresh hypersthene. Whenever the green mineral 

 appears the alteration is more advanced. The association of the ilmenite with the 

 biotite is fairly constant. Apatite, zircon, and pyrite are accessory minerals. The 

 rock may be called a hypersthene felspar gneiss. 



This rock has not suffered the subsequent crushing that is evident in the Madigan 

 Nunatak gneiss, and the absence of garnet makes it different from the Aurora Peak 

 gneiss. It therefore possesses, without any modification, the characters of the Indian 

 charnockitet. A rough determination of its specific gravity gave the value 2-67, which 

 is the same as that for normal charnockite, and greater than the specific gravity of the 

 Madigan Nunatak gneiss, and just a little less than that of the Aurora Peak gneiss. Its 

 composition would not be very different from that of the Aurora Peak rock, and would, 

 therefore, possess the igneous characteristics which are in agreement with the dyke 

 form of its occurrence. 



No. 979. Another example of gneiss, related to the preceding charnockite-like 

 rock, is No. 979, which was collected about 150yds. from the boat moorings at Stillwell 

 Island. In the field it was noticed to be unusually free from garnet. 



* Grain* of pyroxene with apparently oblique extinction hare been shown by Washington (Amer. Journ. Sci., vol. XLI., 

 4th ST., 1916, p. 323) to possess the optical character of hypersthene. This effect is ascribed to the development of a cleavage 

 other than the usual prismatic cleavage. 



t The charnockite series will be discussed later. A special rock name is desirable for the acid hypersthenic gneisses, 

 yet Holland has definitely asked that the name chamockito should not be used for extra-Indian rocks. Still, if it be acknow- 

 ledged that the Indian charnockite series does not consist merely of phenomenal igneous rocks but of definite metamorphic 

 types, then it may be suggested that, with Holland's permission, " charnockite " should supply the need. 



