FROM THE MAINLAND OF SOUTH VICTORIA LAND 



227 



THE CxABBROS 



No occurrences were met with in situ. Erratic specimens, however, are not rare in 

 the McMurdo Sound region. These are often sphene-bearing, and in other ways 

 approach the sphene diorites so closely as to suggest a mutual relationship. A 

 specimen of this kind from the East Fork of the Ferrar Glacier, Upper Kukri Hills, is 

 an allotriomorphic -granular rock with average grain size about 3 mm. Plagioclase, 

 biotite, and uralite can be distinguished with the naked eye. The felspars, which are 

 basic -lab radorites, are partly saussuritised ; they are twinned in broad lamella? after 

 the albite law, with occasional carlsbad combinations and frequent pericline twins. 

 Colourless idiomorphic diallage is present, but largely converted to uralite. The 

 uralite is colourless to greenish -yellow to bluish-green. Rarely small hyper sthenes are 

 met. Occasional idiomorphic crystals of hornblende, dark greenish-yellow to light 

 yellow. Abundant biotite, light yellow to red-brown. Accessories are apatite, magnetite, 

 and grey sphene. 



Very similar to this, but more highly altered, is a sphene-bearing amphibolite from 

 the Dry Valley Moraines. 



Hyper sthene Gabbro. An erratic from Dry Valley. The constituents are : Basic 

 labradorite showing carlsbad, albite, and pericline twins ; some individuals are zoned ; 

 uralite after diallage, remnants of which only remain ; hypersthene changing to 

 bastite ; biotite in scattered flakes ; magnetite in large original grains, and also as 

 secondary dust ; apatite in medium-sized rods ; chlorite in connection with some of the 

 biotite. 



Another specimen almost identical with this comes from the same locality. 



Uralite Porphyry. A third specimen is more porphyritic and the plagioclases are 

 largely of a coarse lath-shaped variety. The hand-specimen is of a general dark grey 

 colour with scattered porphyritic black uralites. In many cases kernels of diallage 

 and hypersthene yet remain. Occasional flakes of biotite are present. 



Bronzite Gabbro. This is a striking rock found in the lower moraines, McMurdo 

 Sound. Idiomorphic honey -yellow crystals of bronzite up to 6 mm. diameter are 

 studded through a white labradorite base. 



II 2 K 



