THE METAMORPHIC ROCKS OF ADELIE LAND. ST1LLWELL. 147 



to sieve structure, and the most common inclusions are ilmenite and biotite and, to a 

 lesser extent, blebs of quartz and felspar. Biotite is present, both in large flakes and 

 very small crystals. It is pleochroic from a reddish brown to a very pale straw. The 

 small biotite crystals appear abundantly in cordierite. Cordierite is very prominent 

 with its pleochroic yellow spots, and has the appearance of a fine granulitic aggregate 

 produced by the crushing of a large crystal (Plate III., fig. 4). In addition to the biotite, 

 small garnets, ilmenite and sillimanite are frequent inclusions in the cordierite, and the 

 whole gives the appearance of a hornfels structure. Sillimanite is associated with the 

 cordierite, both in the form of matted fibrous aggregates and prismatic needles. But 

 it is not uniformly distributed, being more abundant in some slides than in others. 

 Monazite is present, and when included in biotite or cordierite is surrounded by strong 

 pleochroic haloes. Ilmenite is abundant, though more commonly included in the biotite 

 and cordierite areas. Pyrite is also present. 



The garnet and the cordierite provide the dominant characteristic of the rock, 

 which may be called a garnet cordierite gneiss. 



Garnet, Point. 



On Garnet Point there are two dominant types of gneiss in which are incorporated 

 felspar veins bearing abundant garnet. The first type is rather a dark-coloured gneiss 

 with abundant biotite. The second type is rather light-coloured and carries the large 

 garnets which give the mottled appearance to the outcrop. 



Specimen No. 772, collected from this locality, is a dark-coloured rock with feeble 

 schistosity in the hand specimen. The abundant glistening biotite is sometimes 

 aggregated in circular bunches, and felspar and garnet are visible. In thin section the 

 rock is heteroblastic, and garnet is much more abundant in some sections than in others. 

 In part it presents a granoblastic aggregate of biotite and plagioclase with some quartz, 

 but there are, in addition, circular aggregates up to a quarter of an inch in diameter, 

 consisting wholly of brown biotite. There are also granoblastic areas with grain size 

 smaller than the average, consisting largely of biotite and quartz ; and there are lenticles 

 of quartz and felspar in which the diablastic structure may be prominent. The biotite 

 is the most abundant mineral in all sections and usually has the same reddish-brown 

 tint as in the previous case. It has a tendency to a parallel arrangement, except in 

 the circular aggregates. It is remarkable in the possession of numerous and well- 

 developed pleochroic haloes ; and the nuclei of these haloes are sometimes large, and 

 seem to be monazite rather than zircon. Apatite inclusions are also present in biotite 

 but they are not surrounded by pleochroic haloes. The radius of the halo was measured 

 by a micrometer eyepiece, and found to be 0-040mm. in several cases, thus agreeing 

 with the ionisation range ThC and furnishing proof of thorium haloes. In several 

 cases the haloes show the structure, described by Joly*, of an inner dark and an outer 

 and lighter corona. The pupil of the halo is nearly always a bit fuzzy at the edge, 



* " Pleochroio Halow." Joly and Fletcher, Phil. Mag., 1910, p. 630. 



