166 AUSTRALASIAN ANTAKCTIC EXPEDITION. 



Cape Pigeon Rocks, is also a hypersthene gneiss. It is also probable that similar rocks 

 occur at Garnet Point. These rocks are related to the acid hypersthenic gneisses of 

 Madigan Nunatak and Aurora Peak. One example from the summit of the island is 

 found in dyke form, and is no doubt of igneous origin. This rock is a granulitic 

 aggregate of quartz, orthoclase, plagioclase, and hypersthene. Biotite and ilmenite are 

 developed by the reaction of the hypersthene with the felspar. The hypersthene also 

 changes to a greenish serpentinous mineral, as in the rocks at Madigan Nunatak and 

 Aurora Peak. The rock is identical in kind with the normal charnockite of the Indian 

 charnockite series. 



Two other examples of hypersthene alkali felspar gneisses are described from Still- 

 well Island and are distinguished from the first by a higher specific gravity and by the 

 coarse-grained character in the hand specimen. In one of these there is considerable 

 quartz, but only fragmentary garnet, and the brown biotite is found developing through 

 the stage of pale-green biotite. The hypersthene may lose its iron content and change 

 into enstatite. In the second considerable garnet is found in part, but very little quartz. 

 The quartz in the first is replaced by orthoclase in the second. The garnet appears 

 not only as large crystals but also as granular zones surrounding ilmenite, biotite, and 

 hypersthene. As zones of biotite may surround ilmenite and hypersthene, these garnet 

 rims may be explained in each case by a reaction between biotite and plagioclase and 

 quartz, producing garnet and orthoclase. This reaction is found to be highly localised, 

 being absent from a second section cut at the other end of the specimen, and distant 

 Sin. from the first section. A third section cut from the middle of the same specimen 

 shows some garnet. The garnet-forming conditions are, therefore, very limited. 



Remarkable variation of a similar kind is found in the specimen of hypersthenic 

 gneiss from the Cape Pigeon Rocks. The specimen was not more than 3jin. long before 

 the slicing, and four sections have been studied. In the first of these there is very little 

 garnet and no hypersthene. At the other end of the specimen there is considerable 

 hypersthene and the garnet rims are equally developed as in the preceding example 

 from Stillwell Island. Here, in addition to the ilmenite, biotite, and hypersthene nuclei, 

 we find the garnet rims enclosing curious areas of small biotites and quartz. These areas 

 are looked upon as formed by the reaction of hypersthene with orthoclase. In another 

 case the garnet penetrates the hypersthene crystals in the form of thin seams. In the 

 fourth there is a curious aggregate of hypersthene and altered hypersthene in which the 

 outlines of the crystals are marked by garnet borders. The garnet rims are in most cases 

 explained, as at Stillwell Island, by a reaction between biotite, plagioclase and quartz ; 

 but in the hypersthene aggregate, it is supposed that the hypersthene, containing some 

 A1 2 S , changes in part into garnet. 



A very noticeable feature in this type of rock is the presence of biotite both in the 

 form of platy crystals and in fan-shaped sprays. The fan-shaped sprays of biotite are 

 constantly associated with an intergrowth of felspars, and a genetic connection is assumed 

 between them. It is considered likely that these biotite fans are produced from garnet 

 by a reversal of the biotite-plagioclase-quartz reaction. 



