160 AUSTKALASIAN ANTARCTIC EXPEDITION. 



the Ti0 2 content. The hypersthene may contain ilmenite inclusions, which are situated 

 either irregularly or in planes. Occasionally the hypersthene loses its colour and 

 pleochroism, assumes the lower polarisation of enstatite and changes into enstatite. 

 Sometimes it is partially replaced by a platy brown mineral with the deep red brown 

 colour of biotite but with very low polarisation colours. This brown mineral is an 

 iron-stained serpentine. 



Biotite is again abundant and appears in large platy crystals, in close aggregates 

 of smaller crystals surrounding ilmenite, and as small crystals set in quartz. The fan- 

 shaped biotite sprays may appear in the zone around ilmenite or with the biotite plates. 

 Some of the biotite flakes are crushed and bent. Sometimes the biotite plates possess 

 a dark border in which the integrity of the plate is broken. A slight perforated appear- 

 ance develops and the dark colour is due to the separation of minute crystals of iron ore. 

 It is an alteration which is either associated with the crush phenomena or else with the 

 reaction which produces the biotite sprays. A further state is noticed where the biotite 

 has completely lost its brown colour, though still surrounded by a fragmentary garnet 

 rim. It has assumed a pale greenish colour and is dotted with small magnetites (or 

 ilmenites) but still retains its bright polarisation colours. Residual patches of brown 

 biotite may remain in the pale biotite, and as chlorite is present in the slide this may be 

 interpreted as the passage of biotite into chlorite. 



A feature of this slide is the presence of garnet rims similar in nature to those in 

 No. 947. Apart from the rims, garnet only occasionally appears in moderate sized 

 crystals. The garnet rims may surround biotite and hypersthene, and are usually 

 composed of small, idioblastic crystals. The rims have not been observed around ilmenite 

 as in No. 947, but an excellent example is found of an ilmenite nucleus, surrounded by 

 biotite, which in turn is surrounded by a garnet rim (Plate VI., fig. 1). We also find 

 the hypersthene surrounded by biotite and this in turn by garnet (Plate V., fig. 2). A 

 thin layer of orthoclase may exist between the garnet and the biotite, but it is often 

 absent. The garnet may come into direct contact with the hypersthene, and may 

 even penetrate the hypersthene in seams. As biotite is often intimately mixed with 

 the hypersthene it is possible, in many cases, to still explain the presence of the garnet 

 in the hypersthene by a biotite-plagioclase-quartz reaction as in No. 947 ; but the 

 examination of the fourth slide of this specimen proves that this explanation is 

 inadequate in certain cases. The biotite-plagioclase-quartz reaction still explains the 

 garnet rims on the biotite, but all the garnet is not so formed. 



We find here, also, that the garnet rims surround aggregates of biotite and quartz 

 (Plate V., fig. 1). Some of these have a definite rectangular outline and others may be 

 irregular or approximately hexagonal or octagonal. The aggregates are very often 

 without ilmenite, but they may enclose fragments of enstatite. The definite outline 

 indicates that they formerly surrounded a single crystal, and that they existed before 

 the biotite-quartz aggregate. The presence of a portion of an enstatite crystal suggests 

 that the original mineral was a pyroxene. As, in addition, we may find the biotite- 

 quartz aggregate containing an ilmenite nucleus and scattered fragments of garnet, 



