232 AUSTRALASIAN ANTARCTIC EXPEDITION. 



erode more rapidly than the tougher schists and gneisses which appear in the 

 visible outcrops thereabouts. There is no doubt that the rocks of this series 

 represent phases of alteration of the same sedimentary series. The alteration has 

 been effected, it would appear, by the intrusions of extensive granitic magmas 

 now appearing as gneiss typically developed at Cape Denison and the Mackellar 

 Islets." 



The carbonate sediments from which these rocks have been derived were 

 characterised to a greater or less degree by the presence of detrital material, which in. 

 the process of metamorphism has reacted with the carbonate minerals. This group of 

 rocks, however, with two exceptions, still possesses a content of free carbonate mineral. 

 Its quantitative amount is dependent in some cases on the degree of metamorphism of 

 the rocks concerned, and in others on the quantity of foreign material present in the 

 original sediment, capable of chemical reaction with calcite or dolomite. Amongst 

 these rocks, there is no example which suggests any extensive addition of material 

 from magmatic sources, other than purely volatile constituents. 



According to their mineralogic content, these rocks may be divided into the 

 following classes : 



(1) Forsterite-Marbles. 



(2) Tremolite-Marbles. 



(3) Diopside-Tremolite-Marbles. 



(4) Pyroxene-Garnet-Marbles. 



(5) Pyroxene-Epidote-Marbles. 



(6) Epidote-Marbles. 



(7) Carbonate-free Calc-silicate Rocks. 



The description of these various classes can now proceed seriatim. 



II. PETROGRAPHY. 



(1) FORSTERITE-MARBLES. 



The rocks characterised by the presence of magnesian olivine, comprise the 

 following : Nos. 135, 137, 307, 318, 392, 395, 402, 653, 992, 993, 994. 



As a class, they are medium-grained, white to grey rocks. The majority are 

 characterised by the presence of yellowish-green pseudomorphs of serpentine after 

 forsterite, and these project on weathered surfaces. In No. 395, serpentine veins the 

 rock in two parallel bands, in the centres of which narrow venules of chrysotile asbestos 

 are developed. Nos. 135 and 137 are characterised by the presence of orange-coloured 

 crystals with vitreous lustre, and these on examination prove to be chondrodite. In 

 No. 135, this mineral is largely developed along a plane surface. Flakes of light- 

 coloured mica are sparingly distributed in a number of these rocks. 



