212 PETROLOGY OF ROCK COLLECTIONS 



Chcmico-mineralogical Classification : Class 1, Order 4, Rang 1, Subrang 3 (Liparose). 



The dyke -rock "following" includes granite porphyries, aplitic pegmatite, and ker- 

 santite. These are fully described in a later section. Metamorphism affecting this 

 rock is also referred to under the general subject of metamorphism. 



Similar granites are to be met with at intervals between McMurdo Sound and 

 Mount Nansen. A red granite is observed extensively exposed on the mountain scarps 

 to the north-west of Granite Harbour. A whale-backed promontory (practically an 

 island) between Gregory Point and Ross Cape is composed entirely of this rock and 

 its dyke following. At other points erratic specimens were met. Certain outcrops 

 of a grey granite are texturally similar to the pink type, and it appears that they are 

 but variations of it. The grey granites in the neighbourhood of Mount Larsen are to 

 be mentioned in this connection. 



Dr. Prior has already dealt with the pink granitic rocks of the Ferrar Glacier region. 

 Priestley's collection contains abundant specimens also, one from a moraine in the East 

 Fork of the Ferrar Glacier proves to be identical with that already described ; the 

 others are more in the nature of granite porphyries and descriptions will appear later. 



THE GREY GRANITES 



In the present state of our lack of knowledge concerning the field relations of these 

 rocks, the only other obvious subdivision into which we can split the granites is the 

 group of grey granites. 



Sufficient has already been said to make it clear that these do not all belong to one 

 period of igneous activity ; furthermore, it seems clear that some of the grey granites 

 are closely connected with the pink granite already described. Such is the grey granite, 

 to be referred to immediately, occurring in situ to the south-east of Mount Larsen. A 

 comparison of the analyses shows the kindred characters of these rocks. The difference 

 lies mainly in increased proportion of plagioclase in the Mount Larsen rock. A 

 quartz porphyry dyke which crosses the grey granite has a chemical composition 

 almost that of the pink granite, being richer in potash felspar than the parent grey 

 granite. 



Evidence culled from observations relating to field occurrence, and especially 

 chemical composition in the differentiation sequence, suggests that much of the 

 original granitic magma has solidified as a more highly biotitic and plagioclastic grey 

 granite, followed by the separation of an orthoclastic hornblendic magma solidifying 

 as a pink granite occupying subsidiary bosses and dykes. 



In this connection the Mount Larsen grey granite is specially worthy of study. 



Biotite Granite from near Mount Larsen. (Fig. 6, Plate I) 



Granite in situ at the foot of the Backstairs Passage Glacier, the sharp ascent to the 

 Larsen Glacier. A ridge composed of broken masses of this granite extends for a distance 

 of several hundred yards. In the hand-specimen it appears as an even fine-grained 

 light grey granite. Quartz, plagioclase, and biotite are the obvious constituents. This 

 rock is quite different in appearance to the Cape Irizar granite ; most noticeable is the 

 want of orthoclase. 



Microscopic Characters. The grain size in the slide averages 15 mm. diameter. The 

 texture is hypidiomorphic granular. 



