288 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Joly and Fletcher* give the radii of halos in biotite as follows : 



Uranium Ra C Ra A Thorium Th C. Th X 



0326 -0222 -0397 -0263 



On the basis of these measurements the first six halos in the above Table IV are probably 

 due to Th C and the origin of the others is doubtful. 



The structure of the rock is heteroblastic and it may be called a Magnetite- 

 Garnet-Gneiss. 



No. 147. Part of this specimen is a dark magnetite gneiss and part is a light- 

 coloured acidic rock. It exhibits a pronounced example of lenticular structure. 



The acidic part shows large crystals of quartz, fibrous sillimanite, magnetite, 

 biotite, plagioclase, and orthoclase. The dark banded part contains abundant magnetite, 

 garnet, quartz, cordierite, and felspar. Magnetite is mere abundant in the dark part 

 than in the light, but is especially plentiful at the junction. 



In the dark gneissic portion the magnetite encloses cordierite, apatite, and 

 eillimanite. Ilmenite and rutile are also present. Garnet, which is absent from the 

 acidic part, contains inclusions of quartz, magnetite, and apatite, and also shows evidence 

 of resorption. The felspar has an extinction angle of 12 and a refractive index usually 

 greater than quartz. It thus suggests andesine. Quartz shows strain polarisation. 

 Biotite shows evidence of resorption and sometimes contains radioactive inclusions; 

 one perfect example has a halo with a radius of -037 mm. and is ascribed to Th C. f 



Cordierite is abundant; at times it is altered to a pinitic mica with an absence 

 of any basal lamination, and at others it has given rise to a serpentinous product with 

 a radial arrangement and brush polarisation. Sillimanite is usually intergrown with the 

 biotite and appears to have developed from it, more especially in the acidic part. It 

 occurs in the fibrous form and also in stouter prisms, with the slowest ray parallel to the 

 prism axis. Orthoclase and andesine occur in the acidic part. Some crystals of epidote 

 are present with a darker margin which shows higher polarisation colours and stronger 

 pleochroism than the central portion. They usually adjoin magnetite crystals and the 

 darker margin contains a greater percentage of iron. The structure of the gneissic part 

 is heteroblastic and the rock is a Magnetite-Garnet-Schist. 



No. 827. This is a gneissic type with a discontinuous banded structure, the 

 various bands differing greatly in mineral content. The following minerals can be 

 identified in the hand specimen : Quartz, magnetite, blue cordierite, felspar, sillimanite, 

 biotite, and pink garnet. The distribution of the magnetite is especially irregular. A 

 central band through the rock is composed of magnetite, altered cordierite, pink garnet, 

 and quartz. Biotite is plentiful on both sides of this band. 



* Pieochroic Halos." Phil. Mag., 6th Series, Vol. XIX, p. 633, 1910. 

 t Jolly and Fletcher, op. cit., p. 633. 



