102 
Chapter I. 
VOLCANIC ROCKS. 
Basalt appears to be the prevailing lava which has been erupted by the volcanoes 
of this Antarctic region. The ‘ Southern Cross ’ collection showed that Cape Adare 
and, in all probability, also the islands off the north coast of South Victoria Land, 
are mainly composed of this rock ; but the presence, in that collection, of one or 
two specimens of a phonolitic character was sufficient to suggest that in this Antarctic 
region there is a similar association of basalts with more-acid rocks rich in alkalies to 
that which prevails in East Africa and along the Atlantic volcanic chain generally.* 
This suggestion is amply confirmed by the specimens brought back by the 
‘ Discovery ’ from the Ross Archipelago. Basalts of a basic type are the prevailing 
lavas, but accompanying them are phonolitic trachytes and kenytes (trachydolerites of 
Rosenbusch) very rich in alkalies. To this latter type belong perhaps the most 
remarkable specimens in the collection, viz., the rocks from the slopes of Mount Erebus, 
showing conspicuous lenticular crystals of anorthoclase and exhibiting characters almost 
precisely identical with those of the rhomb-porphyries of Norway and the more recent 
kenytes of Mounts Kenya and Kilimandjaro in East Africa. 
Basalts of the Ross Archipelago. 
The basalts of the Ross Archipelago are very similar to those of Cape Adare. They 
present the same two types, viz., hornblende-basalts with few and small phenocrysts, 
and olivine-basalts with plentiful and fairly large porphyritic crystals of olivine and 
augite.y 
Hornblende-basalts. 
The hornblende-basalts are dark gray, slightly vesicular rocks, showing only 
sparingly small black porphyritic crystals of hornblende and augite. They are rather 
more coarse-grained than the very compact rocks of Cape Adare. 
Under the microscope small phenocrysts of pale purplish-brown augite and deep 
reddish-brown basaltic hornblende (or more often magnetite-pseudomorphs after horn- 
blende) are seen in a ground-mass of felspar-laths, magnetite-grains, purplish augite in 
grains and needles, and small olivines. 
The felspar-laths are mostly of labradorite, giving symmetrical extinctions in twin 
lamellae of about 22°. Apatite is plentiful in the ground-mass and as inclusions in the 
hornblende-pseudomorphs (see Fig. 59) ; it has often a pinkish tinge and is cloudy 
with black inclusions arranged in lines parallel to the sides of hexagonal sections. 
In most of these basalts the hornblende is only represented by pseudomorphs, but 
* Prior, Rep. ‘ Southern Cross ’ Collections (British Museum), 1902, p. 328, and Mineralogieal Magazine, 
1903, vol. xiii, p. 261. 
t Prior, Rep. ‘ Southern Cross ’ Collections (British Museum), 1902, p. 326. 
