198 



PETROLOGY OF SOME LIMESTONES FROM THE ANTARCTIC 



Microscopic characters of the green material. — Examination under a one-inch 

 objective showed the green material to be composite in character, and to be distributed 

 in streaks which passed through the rock in an irregular way. A thick band tails 

 out in a short distance to a thin filament or branches into a number of threads 

 traversing the limestone. It is difficult to decide whether this is an original arrange- 

 ment due to irregularity in deposition or whether the irregularity has been sub- 

 sequently induced by pressure. Under a ?,-inch objective the most prominent mineral 

 is a colourless to pale greenish micaceous mineral, showing sometimes fairly bright 

 polarisation colours. A good deal of the greenish area, however, shows low polarisation 

 colours in the first order, and in places minute felspar crystals appear to be present. 

 In addition minute brownish specks represent oxide of iron, possibly an alteration 

 product of a ferro-magnesian mineral, and some rather highly refracting fragments 

 appear to be isotropic. 



Nature and origin of the green material. — 'Two alternatives present themselves as to 

 the nature of this material. It may be very fine-grained detrital material washed out 

 from a shore- line and deposited with mica among the calcareous matter of the 

 limestone. On this view its irregular distribution through the rock would be accounted 

 for by the subsequent irregular squeezing of the limestone during earth movements. 



On the other hand, it may be a submarine tuff. On this view its peculiarities of 

 distribution might be to some extent original and connected with explosive discharge 

 and irregular settling from a neighbouring vent. 



If the material consists of tuff fragments its chemical composition should help to 

 place it. 



In the American classification the norm calculated from its chemical composition 

 is as follows: 



Its position in the classification is : 



Class II., Dosalane. 



Order V., Germanare. 



Rang I., Peralkalic-Umptekase. 



Subrang III., Sodi-potassic Ilmenose. 



This is a subrang including a few rocks such as representatives of the porphyrites, 

 kersantites, and rather basic alkali trachytes. The analysis of a porphyrite from 

 Thuringia comes closest to it. 



In determining the probable nature of the green material we have, then, the choice 

 between a tuff from a rather alkali intermediate rock such as a porphyrite or somewhat 

 basic trachyte on the one hand and fine-grained micaceous detrital material on the 

 other. The available evidence is not sufficient to enable a positive choice to be made, 

 but the presence of a considerable amount of the colourless micaceous mineral 

 suggests that the detrital view of its origin may be the correct one. 



