NOTES ON ANTARCTIC ROCKS. 477 
of brownish-green augite moulded on the felspars, and magnetite 
dust. 
The porphyritic constituents are :—Plagioclase (chiefly labro- 
dorite), some of the crystals being zoned and others appear to be 
made up of patches of felspar of different composition, forming 
a large lath-shaped section. Some of these patches have all the 
appearance of sanidine. Augite, pale brown in section with 
idiomorphic contours. Olivine, generally in rounded grains, occa- 
sionally idiomorphic, and showing no alteration to speak of. 
There are also some large grains of- magnetite. Some of these 
have a coarsely arborescent structure, the spaces between the 
branches being occupied by felspar and augite similar to the 
ground-mass. This would seem to point to the constituents of 
the ground-mass having started to crystallize before the grains of 
magnetite had completely separated out. (Plate 14, fig. 2.) 
Olivine Basalt, (No. 7). 
This specimen is a vescicular pebble of a reddish-brown colour. 
Sp. gr. 3-07. 
Under the microscope the rock is seen to consist of a fine-grained 
ground-mass much stained with ferruginous decomposition pro- 
ducts, and carrying numerous phenocrysts of augite, olivine and 
Magnetite. The scarcity of felspar is at once apparent, no crystals 
of any appreciable size being present. The ground-mass however, 
contains a considerable number of very small felspar microlites. 
These together with much very finely granular augite and 
magnetite dust make up the ground-mass, little if any glass being 
discernible, 
At first the comparative scarcity of felspar would suggest that 
the rock approached a limburgite in composition. The following 
analysis shows that it must be retained in the basalt group : 
SiO, = 45°137 
Al,O = 1813 
Fe,O, = 12:94 
CaO = 11°23 
