BASALT-TUFFS. 
109 
grains showed an obtuse negative bisectrix, being cut parallel to 010. In this case the 
magnetite is seen to be arranged in lines parallel to the (100) cleavage, and also in 
more wavy lines approximately at right angles to the first ; along the latter lines the 
magnetite occurs in blobs, which are flattened out at right angles along the more 
definite cleavage (see Fig. 61). This olivine with included magnetite is very similar to 
that figured by Tschermak in the Sierra de Chaco meteorite.* 
The origin of the so-called “ olivine-nodules ” in basalts has been the subject of 
much discussion.f The presence in these Antarctic rocks of felspathic gabbro-like 
nodules as well as those having the composition of peridotites is of interest, since the 
general absence of felspar in “ olivine-nodules ” has been used as an argument against 
the theory that they are intratelluric separations from the magma. 
Basalts and Dolerites from Auckland Islands and Macquarie Island. 
The specimens brought back from Auckland Islands are olivine-basalts similar 
to those collected by the Ross Expedition and described by the present writer in 
Mineralogical Mag. 1899, vol. xii., pp. 70-73. Those with conspicuous phenocrysts of 
olivine and augite resemble the olivine-basalts of Cape Adare and the Ross Archipelago. 
The dyke-rock (893) from Williamson Point shows under the microscope large and 
much-corroded crystals of colourless olivine and pleochroic (purple to yellow) titani- 
ferous augite in a base of felspar-laths, purple augite, and magnetite in grains and 
skeleton-crystals. 
The basalts from Macquarie Island are of a somewhat different type. They are much 
more altered than the rocks of the Auckland Islands, and appear to be of greater age. 
The more coarse-grained rocks (l, 5, 6) are diabasic in character, and consist of a 
medium-grained aggregate of felspar-laths, colourless augite (sub-ophitic), large 
magnetite-grains, sparingly distributed, and interstitial green chloritic and hornblendic 
alteration -products. The rock (4) from “ 100 yards up the stream” (see p. 96) shows 
large phenocrysts of labradorite and a few chloritic pseudomorphs after olivine. The 
crushed rock (9) is a much-altered andesitic basalt showing large phenocrysts of labra- 
dorite in a very fine-grained altered base. 
Basalt-tuffs. 
Fragmental basaltic rocks were found in most of the localities on Ross Island 
where bare rocks are exposed, viz., Cape Crozier, V-Cliffs Hogsback, Sultan’s Head, 
Castle Rock, The Turk’s Head and Hutton Clifts. 
The pale-yellow tuff from Sultan’s Head contains numerous black fragments of 
vesicular basalt which show under the microscope a few sharply defined felspar-laths 
and grains of olivine in a glassy base dense with magnetite. The tuff, however, 
* Tschermak, Mikr. Beschaff. Meteoriten, 1885, Lief. Ill, FI. XXIII, Fig. 4. 
t See Zirkel, Lehrbuch d. Petrographie, 1894, Bd. ii, p. 931 ; and Abhand. math.-phys. Cl. d. kgl. sacks. 
Ges. d. Wiss. 1903, Bd. xxviii, p. 103. 
