148 AUSTKALAS1AN ANTARCTIC EXPEDITION. 



and accurate measurement is therefore impossible. Some measurements give the 

 radius of the pupil as O031mm., corresponding to the ionisation range of RaC, and this 

 halo is to be considered as a compound thorium radium halo. Other haloes have been 

 found to be 0-027mm., corresponding best with the range of ThX, while one case was 

 found in which there was a suggestion of two coronas, and the radius of the pupil was 

 0-021mm., corresponding with the ionisation range of RaA. There are also small haloes 

 with radius 0-01 3mm., which Joly accounts for by the slower moving ray of ionium, 

 radium, or uranium. The structural features are not always very distinct, but the 

 measurements indicate that haloes exist in the rock which are thorium haloes ; others 

 are radium haloes ; and others are a mixture of thorium and radium. It is certain 

 that the thorium haloes predominate. If monazite is the common nucleus in this rock, 

 we should expect a mixture of thorium and radium in one halo, because monazite may 

 contain up to 18 per cent, of Th0 2 as well as some radium. 



The felspar is usually in clear grains with granular outline. It frequently shows 

 good sharp twin lamellae and is found to be andesine. In the lenticles, which are com- 

 paratively free from biotite, the felspar is often more cloudy and shows conspicuous 

 sieve structure as well as diablastic structure. Some untwinned orthoclase may be 

 present. Quartz is clear and most abundant in the areas associated with garnet and 

 biotite. The garnet has a very pale pink colour and is found in part as rounded grains 

 with corroded outline, and in part as skeletal crystals noticeably associated with quartz 

 and biotite. The larger garnet grains, which have suffered less alteration, may be 

 surrounded by a pale greenish mica, distinct from the normal brown biotite. This 

 pale mica may follow all the cracks that penetrate the garnet crystal, and it may pass 

 by direct transition into the brown biotite. Pleochroic haloes are equally abundant 

 in the two types of mica, but they seem to show more often the structure zones in the pale 

 green type, i.e., they are less often over-exposed. Moreover, the circular zone of the 

 halo, situated in the pale green mica, is often changed to the brown type of biotite. 

 Matted fibres of sillimanite may also be present in the quartz biotite areas. It seems 

 evident that the garnet has reacted with the felspar, and possibly sillimanite, and has 

 produced biotite and quartz. Such a change is quoted by Grubenmann* as an example 

 of a zonal change in passing from the conditions of the kata zone of metamorphism 

 to those of the meso zone of metamorphism. In other examples it will be considered 

 that sillimanite is not a necessary factor in this reaction, but in this case sillimanite 

 has been seen associated with the reaction areas. 



Cordierite, with its pleochroic yellow spots, is also associated with the same areas 

 of relic garnet and sillimanite. Within the granoblastic area of biotite and plagioclase 

 coarse crystals of a colourless mineral may be found. It has a moderately high refractive 

 index and oblique extinction in a section showing cleavage. Sections with imperfect 

 cleavage are normal to a bisectrix, and the mineral is negative. There appears to be a 

 simple twin whose two halves show a marked change of colour in parallel polarised light 

 without difference in extinction. These characters cause the identification of cyanite. 



* Grubenmann, op. cit., vol. I., p. 52. 



