38 AUSTRALASIAN ANTARCTIC EXPEDITION. 



both types may appear in one and the same crystal, and the green biotite appears to 

 be a transition stage between green chlorite and brown biotite. The micaceous 

 constituent, in two of the described examples, is green chlorite. The chlorite plates 

 always, and the biotite sometimes, present a poikiloblastic structure. The included 

 minerals are epidote, felspar, sphene, and magnetite. Pleochroic haloes may surround 

 inclusions in biotite. 



The green hornblende is also fresh and clear. It is characteristically bluish green 

 in the Z direction, indicating an admixture of the glaucophane molecule, and in one 

 case a bright-blue grain of glaucophane appeared. More commonly there are streaks 

 and fringes of pale hornblende through the more deeply coloured hornblende. Inclusions 

 are not abundant in the hornblende. Sometimes rounded blebs of quartz and felspar 

 produce the sieve structure. The prism and clino pinacoid faces are the best developed, 

 and cross sections with two cleavages may be idioblastic. At times chlorite and epidote 

 seem to develop from the hornblende. 



In contrast to the fresh biotite and hornblende is the felspar, which may be quite 

 turbid and decomposed. The decomposition has been referred to throughout as 

 saussuritisation as epidote, lawsonite, zeolites, calcite, a secondary plagioclase, and 

 micaceous products have been recognised. , It is the same type of alteration as appears 

 on a large scale among the xenoliths which are to be described later. In some cases a 

 clear transparent felspar dominates, and in others the decomposed felspar, but both 

 often appear together. The discrimination of the plagioclase is not simple, because 

 lamellar twinning is not well developed. The lamellae are often confused and inter- 

 mittent, and many sections are not twinned at all. Such untwinned sections can be 

 proved by their refractive index not to be orthoclase. Labradorite, with broad lamellae 

 and large extinction angle, has been detected. This labradorite is at times partly 

 saussuritised. A second sodic plagioclase has been found with a smaller extinction 

 angle, fine lamellae, and lower refractive index. This second plagioclase is sometimes 

 andesine and sometimes nearer oligoclase. It is a product of recrystallisation in 

 which saussuritisation is but a stage, and the composition varies with the conditions 

 of recrystallisation. 



Quartz is a minor constituent and detected in several cases. It is abundant in 

 examples where an ingress can be suspected. 



Epidote appears throughout the series, but is more important in the mica schists 

 and mica amphibolites. Generally, its percentage varies with the percentage of mica. 

 It is found in characteristic honey-yellow or yellow-green grains frequently with crystal 

 outlines. It may also be found in aggregates of little grains which may appear like 

 small heaps. It may be intergrown with biotite or hornblende, and it may appear as 

 poikiloblastic grains in chlorites, biotite, or hornblende. It may rarely be intergrown 

 in parallel with zoisite (or clinozoisite). Even when the percentage of epidote is very 

 small we may still find large grains enclosed in biotite or chlorite. The epidote crystals 

 are occasionally found with nuclei of calcite and allanite. 



