214 PETROLOGY OF ROCK COLLECTIONS 



The approximate composition of the mode as determined by the Rosiwal method 



is as follows : 



Per cent. 

 Quartz 26 



Orthoclase 



Microcline aud anorthoclase 



Plagioclase 



Mica .... 



12 



39 



19 



4 



100 



Stress is indicated by the optical characters of the constituents. It closely resembles 

 certain Pre-Cambrian granites of South Australia and is likely to prove of considerable 

 antiquity. The arkose already described from this locality contains an abundance 

 of microcline likely to have been derived from this and similar rocks. 



From this locality was got a specimen of a like granite stained yellowish due to 

 weathering. 



Porphyritic Biotite Granite ; Beardmore Glacier. (Fig. 5, Plate I) 



A grey granite from the Lower Glacier Depot. It contains porphyritic white ortho- 

 clase crystals 2-5 cm. diameter twinned after the Carlsbad law. The texture of this 

 rock resembles that of some of the porphyritic pink felspar granites of the Cape Irizar 

 type. 



The orthoclase, largely anorthoclase and microcline, is almost exclusively comprised 

 in the porphyritic crystals. The plagioclase is an acid andesine and oligoclase and 

 constitutes a large part of the finer material of the rock. Biotite is present, pleochroic 

 from light yellow to deep brown. Associated with the biotite is a little sphene and 

 abundant tiny prisms of apatite. 



There is no evidence opposed to a possible relationship of this granite with those 

 described from more northerly areas. 



From the same locality comes a still coarser porphyritic granite similar in other 

 respects. This bears distinctly pegmatitic characters. One of the large orthoclases 

 contains scattered quartz fragments in a semi-graphic arrangement. Coarse black 

 biotite flakes reach 5 cm. diameter. Embedded in the biotite in one place is a red 

 brown crystal of sphene 3 mm. diameter. 



A grey porphyritic granite very similar to the local pink variety is met with in the 

 form of erratics at Cape Irizar and at the rocky cape a few miles to the south. These 

 all contain sphene. In a more even-grained variety allanite is prominent, pleochroic 

 (light yellow to red-brown) and twinned. 



Grey granites are described by Prior and Ferrar as occupying large areas in the 

 Ferrar Glacier region, at Granite Harbour, and at Cape Adare. Frequent erratics of 

 more or less metamorphosed grey granites were noted by us along the coast-line. We 

 came upon extensive outcrops of such at Cape Roberts, Cape Ross, Depot Island, and 

 in the Mount Larsen neighbourhood. Lamprophyric dykes and inclusions are frequent 

 amongst them. 



