MAGNETITE GARNET ROCKS-COUL80N. 301 



In the Broken Hill occurrence no associated limestone is reported, neither does 

 the garnet of the Adelie Land specimens correspond to andradite. Yet it is interesting 

 to note that the same moraines have yielded a number of specimens of metamorphic 

 limestones and calc-silicates rocks.* It is known, however, that similar garnet rocks 

 can be produced without limestone. Kemp,f in commenting on the processes of garnet 

 formation in contact rocks of intrusive zones, concludes that, where limestone is absent, 

 the emissions from the deeper parts of the eruptive supplies the lime and iron oxide for 

 the garnet occurrences in such rocks as quartzites, schist, and gneiss, just as the reverse 

 contributions of silica and iron and alumina to the limestone would lead to the same 

 result. In the Broken Hill type it is clear that there has also been an important addition 

 of phosphorus. 



In the garnet-magnetite types from Cape Denison the garnet is a variety 

 containing magnesium, iron, manganese, and alumina. Felspar, as noted above, is 

 often an important constituent though it fails in the quartz-garnet-magnetite rock 

 No. 102. The presence of aluminous garnet in addition to felspar suggests that the 

 garnet- magnetite formation has occurred in a rock in which there has been sufficient 

 alumina and silica to satisfy all the alkalies and available lime for the formation of 

 felspar, and further that the garnet-magnetite formation has occurred in the presence 

 of excess alumina rather than excess lime. Where P 2 8 is present or introduced, the lime 

 would be incorporated in apatite in preference to felspar. Aluminous garnet is present 

 in the analogous type at Broken Hill which occurs in a sedimentary series with excess 

 alumina, expressed in abundant sillimanite. It is therefore possible that the Cape 

 Denison types have developed in aluminous sediments similar to those which are related 

 to the magnetite schists, Nos. 912 and 294, and the related types Nos. 296, 989 and 933. 



In conclusion, it is therefore likely that the garnet-magnetite rocks of Cape 

 Denison have resulted from the raetamorphism of sediments, the composition of which 

 has been modified by igneous emanations. 



V. TOURMALINE-BEARING MAGNETITE GNEISS. 



In view of the probable role of igneous emanations in the formation of the 

 garnet-magnetite rocks the following description of No. 252, a tourmaline-bearing and 

 garnet-free type, has added interest. 



No. 252 is a banded variety, in general fine grained, but with coarsely crystalline 

 bands of quartz and bluish cordierite. Tourmaline is very abundant and long needles 

 of sillimanite and black grains of magnetite are plainly seen in the hand specimen. 

 The proportions of cordierite, magnetite, tourmaline and sillmanite are very variable. 

 The specific gravity of the rock is 3-09. 



* " The MeUmorphic Limestone* of Commonwealth By, Adelie Land." C. E. Tilley. Aurt. Ant. Ezp. Sci. RpU. 

 Series A., vol. iii. pt. 11, pp. L'3 1-243. 



t " Ore Depoaitft at the Contacts of Intnuive Rocks and Limetone." J. F. Kemp. Journ. Qeol., vol. ii, pp. 1-13. 



