162 AUSTRALASIAN ANTAECTIC EXPEDITION. 



biotite and quartz can be produced in a reaction in which the hypersthene has taken part. 

 For average compositions of these minerals the reaction may be expressed chemically 

 as follows : 



(KH) 2 0, 2(FeMg)0, A1 2 3 , 3Si0 2 + 5Si0 2 ^ 2J(MgFe)0, Si0 2 i + K 2 0, A1 2 3 , 6Si0 2 . 

 Biotite Quartz Hypersthene Orthoclase 



Excess iron may separate out as iron ore, and K 2 is assumed again to largely dominate 

 over H 2 in the biotite. This reaction has undoubtedly followed the production of 

 the garnet from biotite or pyroxene, and the orthoclase which accompanies the formation 

 of the garnet may react again with the hypersthene. 



In one case a biotite crystal, partly crushed, extends into a hypersthene aggregate 

 (Plate IV., fig. 4). Part of the biotite has a garnet rim, and one corner of the biotite 

 area has a perforated appearance with the development of quartz, and is in intimate 

 relation with the biotite. There is only occasional chloritisation of the biotite and 

 serpentinisation of the hypersthene. 



No. 785 (3). The slide No. 785 (3) is cut from the middle of the specimen. The 

 same general features can be recognised here ; but there is less biotite and more hyper- 

 sthene, most of which is considerably altered to serpentine. 



A new feature appears in this slide in a large aggregate of hypersthene and altered 

 hypersthene in which the outlines of the crystals are marked by thin garnet borders 

 (Plate VI., fig. 2). The garnet also penetrates some of the crystals in thin irregular 

 seams. Similar seams have already been noticed in fresh hypersthene and in the garnet- 

 rimmed areas of biotite and quartz. In this aggregate the original hypersthene crystals 

 have assumed a pale-green colour, are slightly pleochroic, and are in part finely fibrous. 

 The least altered still have the polarisation colours of hypersthene, but in many cases 

 the mottled colours of serpentine appear. Strong pleochroic haloes appear in the ser- 

 pentine. The alteration takes place here through bastite to serpentine. 



During the serpentinisation a considerable amount of magnetite (or ilmenite) has 

 separated out ; and this separation is well illustrated in a crystal of partially altered 

 hypersthene, which is apart from the aggregate. The centre of this crystal is still the 

 unaltered pleochroic hypersthene ; but its low polarisation colours indicate that its 

 iron content is small, and that it is passing over into enstatite. The outer portions 

 have changed to clear enstatite or to serpentine, which is brownish in part ; but along 

 the fringe of the crystal there are numerous, small, opaque crystals formed from the 

 liberated iron. 



In another crystal the hypersthene has completely changed to enstatite in which 

 serpentinisation has freely developed along the cracks and fractures in a manner common 

 in olivine. This enstatite crystal is seamed irregularly with planes of colourless garnet, 



