MAGNETITE GARNET ROCKS COULSOX. 



As in the halos measured in No. 149 in biotite, corona effects are generally 

 absent, being noted only in two cases, and Joly's correction for nuclei, based on the 

 assumption that they are zircon and sensibly spherical, obviously cannot be made. 

 By a comparison with the list of radii given by Joly and Fletcher, it seems probable, 

 as the dimensions of the halos in cordierite will not differ markedly from those in 

 biotite, that No. 1-8 owe their origin to Th C, No. 9 to Th X and Nos. 10-12 to Ra A. 



The cordierite is much intergrown with long prismatic needles of sillimanite 

 which make the measurement of halos sometimes very difficult. In places the halos 

 are so numerous that the cordierite remains brown throughout a complete revolution 

 of the stage. As has been noted above, the radioactive inclusions are not confined to 

 cordierite, some occurring in tourmaline. Monazite appears to be the more common 

 nucleus in No. 252, while zircon was the more frequent in No. 149. The cordierite is at 

 times altered to a yellowish-brown form of biotite and at others the change proceeds, 

 from a centre, giving a chloritic mass. 



Quartz is abundant and distinguished from cordierite by the absence of halos 

 cleavage and decomposition. Apatite is frequent but it is restricted to certain bands 

 in which it is associated with magnetite. The structure of the rock is homoblastic, 

 and though the nature of the different bands is very variable a general name would be 

 Tourmaline-Magnetite-Sillimanite-Cordierite-Gneiss. 



The absence of felspars and the abundance of sillimanite and cordierite clearly 

 indicate that the rock has been derived from a sediment. The abundance of tourmaline 

 indicates the action of igneous emanations and tends to confirm the similar view in 

 regard to magnetite and apatite. 



In Grubenmann's classification it would occupy a position in the Kata division 

 of the group of aluminous silicate gneisses (Group II). Within this division there is a 

 family of the sillimanite gneisses and the family of the cordierite gneisses, but the Cape 

 Denison rock is clearly distinct from typical members of these families in the presence 

 of abundant tourmaline and magnetite. 



. VI. GENERAL SUMMARY AND ACKNOWLEDGMENTS. 



In all twenty-two specimens have been described from Adelie Land and treated 

 as crystalline schists. Two contain abundant magnetite and no garnet. Seven contain 

 dominating magnetite and, together with the preceding two, are associated with the iron 

 oxide group, No. XI, of Grubenmann's classification of the crystalline schists. In ten 

 apecimens garnet is approximately as important as the magnetite. The chemical 

 characters of these types are widely different from any defined group of crystalline 

 schists and a new group of garnet-magnetite rocks is suggested. 



36640 I) 



