i8 2 DISCOVERY REPORTS 



Two other specimens in their richness in almandine garnet are reminiscent of the 

 gneisses of the Cape Bruce area. They are: (i) a microperthite -quartz gneiss carrying 

 biotite, garnet and accessory xenotime; (2) a quartz plagioclase {andesine) garnet gneiss 

 with spinel enclosures in the garnet. Both these rocks are of igneous origin. 



The remaining rocks are of more basic character. They include : 



(1) Hypersthenite and garnetiferous hypersthenite reported as forming a small outcrop 

 15-20 ft. in diameter towards the western end of Bertha Island. In these the coarse- 

 grained hypersthene has translucent exsolution lamellae of ilmenite and accessories 

 include plagioclase, biotite and opaque ores including magnetite, pyrrhotite, and 

 chalcopyrite. 



(2) Mangerite passing into quartz norite. This is a coarse-grained banded rock of 

 variable composition containing hypersthene, augite, plagioclase (andesine-antiperthite) 

 and microperthite with andesine interpositions characteristic of the mangerite type of 

 rock. Hornblende is in variable amount and in one band is the dominant coloured 

 mineral. Garnet is sporadic. In the absence of field data the significance of this variable 

 composition is difficult to assess. 



(3) Finally, a group of basic rocks recorded as bands in the country rock, examples 

 of which are reported to range in width from a few inches to several feet. 



Three rocks of this type are available for study. They may be described as meta- 

 morphosed gabbros or norites exhibiting as they do clear signs of secondary changes. The 

 principal constituents are augite, hypersthene, hornblende and labradorite, while there 

 is a very variable content of late formed garnet. They all show some features of foliation 

 usually in the orientation of elongate crystals of the chief ferromagnesian minerals, 

 while signs of stress are evident in undulose extinction and a patchy and interrupted 

 development of twinning in the felspars. As a group one distinctive feature is the 

 occurrence of a diablastic intergrowth of plagioclase and pyroxene (augite) fringing and 

 often embaying the hornblende crystals. (Plate XXXVIII, figs. 1 and 2.) 



Garnet noted above as a variable constituent appears chiefly in the form of coronae 

 around (a) hypersthene, (b) augite, (c) hornblende, (d) iron ores, usually then abutting 

 against plagioclase felspar. But garnet may also appear isolated within felspar or as 

 grains in an augite-garnet granulitic zone, surrounding or crossing larger pyroxene 

 crystals. The commonest enclosure in the small garnets is monoclinic pyroxene forming 

 a minute vermicular intergrowth and in some examples garnet development is accom- 

 panied by the separation of quartz. The presence of quartz is especially characteristic 

 of the coronae around augite and hypersthene, and the mineral then appears as a narrow 

 zone intervening between the pyroxene and the garnet (Plate XXXVIII, figs. 3 and 4). 

 The variable development of garnet in one and the same specimen is strikingly revealed 

 in a rock showing one face with a coarse development of hornblende (± augite) and 

 plagioclase (± quartz) which passes sharply into the main body of the rock. Sections 

 taken at right angles to this face show that garnet increases in amount as this face is 

 approached. Sporadic in its development at points most remote (3 in.) it becomes an 



