104 
G. T. PRIOR 
Hornblende-basalts were found on the 1300-ft. knoll on Mount Terror (218), 
at the Sulphur Cones near Winter Quarters (382, 385, 391), and also on Brown Island 
(G08). The latter specimen is almost precisely identical in microscopic characters with 
the rocks from the Sulphur Cones. 
Olivine-basalts. 
To the basalts with porphyritic crystals of augite and olivine belong, apparently, 
most of the more basic lavas of the Ross Archipelago. They are dark-gray to black 
rocks, showing to the naked eye numerous crystals of these minerals. By increase of 
glass and decrease of felspar in the base they pass into limburgite-like rocks, in some 
of which the clear yellowish-green olivine-phcnocrysts reach a considerable size (up to 
1 in. in length). 
Under the microscope (Plate VIII. Fig. 1) the olivines are seen to be mostly of 
very irregular outline and to be often deeply indented and corroded : they are clear, 
colourless and fresh, showing generally only a few inclusions of magnetite. A marked 
exception to this rule, however, is presented by the phenocrysts of a basalt from 
Harbour Heights (366), which resemble closely the pseudomorphs after hornblende 
described above (Fig. 59), since they are rendered almost opaque with magnetite 
arranged in parallel lines. They may possibly mark a stage in the conversion of 
hornblende into olivine.* In some of the more limburgite-like rocks ( e.g ., 176 from 
Cape Crozier) the olivine shows small brown octahedral inclusions of picotite, and 
also numerous vermiform inclusions similar to those in the augite and olivine of the 
nodules described in the following section (see Fig. 60, p. 108). 
The augite-phenocrysts are of the pale purplish-brown titaniferous variety common 
to basaltic rocks. As in the hornblende-basalts, variations of composition are indicated 
by changes in tint and by zonal structure exhibited between crossed nicols. 
In the ground-mass occur olivines and augites of the same character as the 
phenocrysts, together with magnetite and ilmenite in grains or rod-like skeletal 
crystals, sharply defined felspar-laths, and, in many cases, brown glass. 
The structure varies from pilotaxitic to hyalopilitic ; in some of the rocks fluidal 
structure round the larger phenocrysts of olivine is well marked. 
The felspar-laths are of labraclorite, giving symmetrical extinctions of about 25°. 
Only rarely is a much-corroded small phenocryst of felspar seen in these rocks. In 
some fine-grained basalts {e.g., 180, from the top of the 900-foot knoll at Cape Crozier, 
and 311, from White Island) the phenocrysts are very sparingly distributed. In these 
rocks olivine occurs very plentifully in the ground-mass, either as small rhombic sections 
or (in specimen 180) as long prismatic crystals, not easily to be distinguished in ordinary 
light from the lath-shaped felspars. In some specimens (222) augite-phenocrysts are 
seen to have been formed round magnetite-pseudomorphs after hornblende. In others 
(335) unaltered hornblende occurs as phenocrysts in addition to the augite and olivine. 
* See Mtigge, Petrog. Untersuch. an Gest. v. tl. Azoren. Neues Jalirb. 1883 (ii), p. 224. 
