234 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Spinel. The spinel of these rocks is usually the colourless magnesia spinel 

 developed in rounded grains or more rarely subidioblastic with octahedral outline. It 

 occurs isolated in the carbonate minerals or may be intimately associated with 

 forsterite, occasionally enclosed in the outline of the latter. 



In No. 992, the spinel has the green colour of the pleonaste type, and is there 

 associated with magnetite which is developed peripherally and along cracks. It is 

 usually quite free from alteration, but in some examples has developed a peripheral 

 ring of colourless serpentine. 



Hornblende. A colourless amphibole is sometimes abundantly developed in 

 these rocks. This is especially the case in rocks No. 318. In this rock, the horn- 

 blende is developed as subidioblastic crystals enclosing grains of forsterite, and also 

 as narrow corona-like rims to the same mineral. The mineral shows the typical 

 amphibole cleavages, and cross-sections show the emergence of an optic axis, with the 

 optic axial plane bisecting the obtuse intercleavage angle. These grains are optically 

 positive, and there can be no doubt that the mineral is edenite, and not the normal 

 tremolite. Moreover the extinction angle exceeds the value for this latter type. 

 Tremolite is, however, not absent from these rocks, but is sparingly developed. 



Diopside is present as an accessory constitutent in clear colourless grains, with 

 prismatic habit. In No. 395 it is present with tremolite fringing a band of serpentine. 

 There is no definite evidence however to suggest that the serpentine has a pyroxenic 

 derivation. The bands in this rock, with their accompanying chrysotile venules are 

 essentially of forsterite derivation. 



Pfdogopite. The colourless mica which is a frequent member of these rocks is 

 a type with very small optic axial angle, approaching uniaxiality, being the magnesia- 

 rich phlogopite variety common to metamorphosed dolomites. 



(2) TREMOLITE-MARBLES. 



This class includes the following rocks : Nos. 306, 306a, 355, 406, 673, and 707. 

 The distinct habit of crystalline schists is given to these rocks by the abundant develop- 

 ment of fibrous amphibole. They are grey to green-grey rocks in which the amphibole 

 is present in light-green fibres often with a parallel orientation. This, however, is not 

 a constant feature, the porphyroblasts of tremolite being developed as in No. 406 in 

 diverse orientations. 



A radiate arrangement appears in the lighter-coloured rock No. 707. On 

 weathered faces the more resistant amphiboles usually project from the general surface. 



The constituent minerals are dolomite, calcite, tremolite, pUogopite, biotite, 

 (chlorite), magnetite, and apatite. 



Dolomite is again revealed by the characteristic type of twinning, and both it 

 and the calcite are universally twinned on a polysynthetic scale. 



