THE METAMORPHIC ROCKS OF ADELIE LAND STILLWELL. 149 



This identification has been confirmed by the preparation of more sections, in which 

 we learn that the alteration of garnet to quartz and biotite is not the complete story 

 of the change. Granoblastic areas are found which consist of cyanite and the pale green 

 mica which is developed from the garnet. It seems, therefore, that the normal reaction, 

 which produces quartz and biotite, may be replaced by one which produces cyanite 

 and biotite. In the latter case there has been an excess of AJ 2 S present, and possibly 

 corundum has been involved. Ilmenite is not as abundant as in the Cape Gray gneiss, 

 and monazite and apatite occur as accessories. The rock may be called a cyanite 

 biotite gneiss produced from a garnet cordierite gneiss. 



Specimen No. 770, obtained from the same locality, is similar to the preceding, 

 though cyanite is not found in it. The hand specimen consists of the biotite gneiss 

 with a piece of felspar garnet vein attached. In the slide the vein consists of colourless 

 areas of orthoclase, perthite, soda plagioclase with abundant myrmikite and its diablastic 

 structure. Occasional areas of ilmenite (with its alteration product leucoxene) are also 

 associated with the biotite. As hypersthene has been found in similar veins in a similar 

 locality (Stillwell Island) it is not at all impossible that these may represent the decom- 

 position of hypersthene. In addition to the colourless areas there are large garnet 

 areas in the hand specimen of the vein, with which biotite is associated. The biotite 

 fills up the cracks and surrounds detached pieces of garnet, while the outline remains 

 that of a large crystal. In the slide of this rock the relic areas containing sillimanite, 

 garnet, and cordierite are more prominent than in No. 772. The sillimanite is found in 

 coarse prismatic needles as well as in fibres, and is occasionally in parallel position with 

 the biotite. There are also the aggregates of biotite and quartz which have certainly 

 developed in the same way as No. 772. Sometimes the normal brown biotite is replaced 

 by a much paler mica crowded with opaque magnetite dust. Some reaction has caused 

 the separation of the iron content of biotite as magnetite. Associated with the biotite 

 are numerous needles and grains of a yellow-brown mineral with high refractive index 

 and double refraction, and with a tendency to be opaque. It is frequently included in 

 biotite and is never surrounded by pleochroic haloes, and is considered to be a variety 

 of epidote. Aggregates of muscovite are occasionally found with the biotite, while some 

 of the biotite flakes are bent, crushed, and broken. 



Specimen No. 777 is an example of the second type of gneiss from Garnet Point, 

 and contains the large porphyroblastic garnets. The hand specimen is massive, and 

 shows felspar and quartz, as well as the pink garnet. In the section the porphyroblastic 

 garnets are found in skeleton form and penetrated by quartz and biotite. These may 

 appear as inclusions in the garnet or else along the cracks and edges developed by inter- 

 action with the felspar. The biotite, as before, contains the pleochroic haloes, and is 

 here again found to develop through a yellowish-green micaceous mineral. Areas of 

 aggregated biotite and quartz with ilmenite are present, as in the preceding examples. 

 Apart from the garnet areas, the rock consists of a granoblastic aggregate of cloudy 

 plagioclase, orthoclase, microperthite, and quartz. The felspar is cloudy, partly 



