FROM THE MAINLAND OF SOUTH VICTORIA LAND 



213 



Quartz in regular allotriomorphic grains. It is one of the last products of crystalli- 

 sation. Strain shadows are universal. Orthoclase is present only in very small quantities 

 as compared with the plagioclase. It usually shows the faint strife of anorthoclase. 

 Plagioclase is very abundant, and is distinguished by the fine lamellae of the albite 

 twin ; Carlsbad twins are also rather frequent. The plagioclase crystals are hypidio- 

 morphic and occasionally idiomorphic. Both orthoclase and plagioclase are compara- 

 tively fresh. Biotite is abundant ; pleochroism light yellow to deep yellowish-green. 

 Allanite occurs in stout prisms embedded in the biotite. It is of a yellowish-brown 

 colour and well zoned. Apatite in colourless stout prisms is also frequently observed 

 enclosed in the biotites. 



The chemical analysis determined by A. B. Walkom, B.Sc, is as follows : 



COMPOSITION OF THE NORM 



99-35 



10014 



Chemico-mineralogical Classification : Class 1, Sub-class 1, Order 4, Rang 3, 

 Subrang 3 (Amiatose). 



An even-grained grey granite collected as an erratic specimen from Dry Valley, 

 New Harbour, is very similar to that from Mount Larsen. 



Fine-Grained Granite ; Beardmore Glacier. (Fig. 7, Plate I) 



A fine even-grained light grey granite composed of quartz, white felspar, and fine 

 fragments of biotite from the Lower Glacier Depot, Beardmore Glacier. 



Microscopic Characters (Fig. 3, Plate III). Allotriomorphic granular texture. The 

 minerals are chiefly quartz, orthoclase (microline and anorthoclase), plagioclase (oligo- 

 clase chiefly), biotite, apatite, and magnetite. 



The quartz is present as clear grains of irregular or rounded boundaries ; these 

 seldom exceed 1 mm. diameter. Rounded blebs of quartz (quartz de corrosion) are 

 frequently embedded in the microcline. The orthoclase is chiefly microcline and 

 anorthoclase though clear simple-twinned orthoclase is present. Gitter structure is 

 beautifully shown in the microcline, which is always fresh. Plagioclase is generally 

 dusty with change products which include flakes of secondary white mica. Measure- 

 ments of the optical constants show the range to be between acid andesine and acid 

 obgoclase. Biotite, pleochroic in green and yellow, is present in very small amount 

 only ; a change to chlorite is observable. Muscovite in small quantity is present also. 

 Scattered grains of magnetite and tiny rods of apatite complete the list of constituents. 



