129 
Chapter III. 
DYKE-ROCKS (LAMPROPHYRES, etc.). 
The dyke-rocks in the crystalline limestones and granites form an interesting series 
ranging from kersantites to rocks allied to the banakites of Wyoming. Some of the 
dykes show numerous phenocrysts of basaltic hornblende and approach to camptonites 
such as occur generally in association with nepheline-syenites. No specimens, however, 
of the latter rocks appear to have been met with in this Antarctic region. 
Camptonites. 
A typical camptonite is the dark-gray rock (839) found in situ by Dr. Wilson at 
the south end of the Southern Foothills, near the Koettlitz Glacier. To the naked eye 
this rock shows only a few small phenocrysts of hornblende. Under the microscope 
(see Plate IX, Fig. 5) the rock is seen to consist of small prismatic crystals of reddish- 
brown hornblende thickly distributed in a base of lath-shaped felspars with a little 
quartz. The hornblende has the same optical characters as that in the diorite described 
above (p. 127). The felspar-laths are of labradorite with refraction markedly greater 
than that of Canada balsam, and symmetrical extinctions as high as 19°. Magnetite 
and ilmenite are virtually absent. The section shows one or two foreign enclosures of 
quartz containing liquid-inclusions with bubbles. 
To the camptonites must also be referred a specimen (G29) brought by Lieut. 
Armitage from the foot of Cathedral Rocks. It is a dark greenish-gray rock showing 
to the naked eye no porphyritic crystals. Under the microscope small green uralitic 
hornblendes are seen in a base of felspar-laths and thickly-distributed ragged prisms 
and minute needle-like microlites of basaltic hornblende with a little biotite. The 
felspar-laths appear to be of oligoclase : one rhombic section cut nearly parallel to 
b (010) gave a positive extinction of about 10°. The hornblende-prisms show well- 
marked flow-structure. 
A specimen (498) from a moraine at New Harbour Height (S. foot) shows the 
junction of a gray hornblende-biotite-granite and a black fine-grained camptonitic rock. 
The latter consists of numerous small sharply-defined prismatic crystals of basaltic 
hornblende, in a base showing a few felspar-laths and a little quartz but rendered 
dense with long hornblende-microlites. Besides the hornblendes a few prismatic 
crystals of colourless augite are also present. None of the small phenocrysts exceed 
0 1 3 mm. in length. Near the junction the base becomes more dense and glassy and 
shows no well-defined hornblende-microlites. 
VOL. I. 
S 
