240 AUSTRALASIAN ANTARCTIC EXPEDITION. 



the ground mass is constituted of tremolite, zoisite (clinozoisite), quartz, felspar, 

 titanite, and apatite. Epidote is present in idioblasts measuring up to 1-5 mm. in 

 length. It shows a light yellow tint, but is seldom noticeably pleochroic. A basal 

 cleavage is strongly developed. The double refraction does not exceed 0-027. The 

 optic axial plane is perpendicular to the elongation and the cleavage, and the optic 

 sign is negative. Twinning on the 100 plane is rarely developed, and is seen in sections 

 cut parallel to the optic axial plane. The extinction is 3 to 4 degrees from the plane 

 100. From the basal cleavage the angle Z A a is 30. 



In sections nearly perpendicular to the optic axes, the interference tints are 

 often anomalous. A few of the porphyroblasts are optically positive. These are 

 clinozoisite. It is clear that the porphyroblasts are types comparatively poor in 

 Fe. 2 3 , and grade in composition from clinozoisite (optically positive) with increasing 

 iron content to the more birefrigent epidote (negative). 



The porphyroblasts of felspar are dominantly plagioclase. They often show 

 both albite and pericline twin lamellae. In composition they grade from albite- 

 oligoclase to oligoclase. Both optically positive and negative varieties are present, 

 and the refractive index is usually equal to or greater than that of canada-balsam. 

 The refrigence is, however, always less than that of quartz. Potash felspar is sparingly 

 represented, optically negative and has a refringence less that that of canada-balsam. 



The ground mass is constituted of quartz and alkali felspar, in which are set 

 fibres of colourless tremolite, prisms of zoisite, and less frequently highly refringent 

 grains of titanite and apatite. The porphyroblasts of felspar are remarkably free 

 from the constituents of the ground mass. 



III. REVIEW OF THE METAMORPH1C PROCESSES INVOLVED. 



The mineralogical variation of the suite of rocks described in the foregoing is 

 clearly resultant of two independent factors, viz. : 



(a) The range of chemical composition of the original sediment, 



(b) The grade of metamorphism which the several classes have experienced. 



Accepting throughout that an additive metamorphism has been inoperative, 

 the rocks of classes (1), (2) and (3) are examples of metamorphosed dolomites. Classes 

 (4) and (5) represent types in which a gradation to a more calcareous sediment is 

 apparent, whilst in class (6) the metamorphosed equivalent of calcareous limestones 

 are revealed. A varying grade of metamorphism is apparent. 



The forsterite marbles bear witness to a metamorphism of essentially thermal 

 type. The silica and alumina of detrital origin have reacted with the carbonate 



