Art. LXIV.—Notes on some Rocks from the Kermadec Islands.
[Head before the Philosophical Institute of Canterbury, 2nd October, 1895.]
By the kindness of Captain Hutton I have been allowed to examine some specimens of rock in the Canterbury Museum which were brought from the Kermadec Islands by Mr. Park. The specimens are all small, and I have been unable to do more than examine them microscopically. I have been unable to identify them completely with any of those described by Professor Thomas in the “Transactions of the New Zealand Institute,” vol. xx.,* though some of them correspond in part. They are all volcanic, and belong to the intermediate division, with the exception of the first, which is a doubtful tachylyte. They all exhibit rather higher specific gravities than is usual in such rocks.
Tachylyte (Macaulay Island).
This specimen is from Macaulay Island. It is black in colour, and has the lustre of pitchstone. Its specific gravity is 2.49, which is rather low for a basalt-glass but corresponds to the value for a tachylyte. It fuses under the blowpipe, but does not dissolve to any great extent in hydrochloric acid; though after digesting the powdered rock for several days with this acid a quantity of iron was dissolved out. It thus corresponds with tachylyte as regards specific gravity and fusibility, but differs as regards solubility. However, the last does not appear to be at all constant in the case of rocks which are undoubtedly tachylytes.
Under the microscope in ordinary light it is brown in colour, with numerous small grains of magnetite and microliths of feldspar, the arrangement of which shows well marked fluxion structure. It is rather opaque, and only very thin sections transmit light.
With polarized light crystals of augite, feldspar, and magnetite are visible, though none of them are large enough to be
[Footnote] * See Trans. N.Z. Inst., vol. xx., art. xli., “Notes on the Rocks of the Kermadec Islands,” by Professor A. P. W. Thomas, M.A., F.L.S., &c.
seen with the naked eye. The feldspar is probably labradorite or anorthite, as the method of determination by the extinction of twin lamellæ gave very high angles. Some of the crystals exhibit zonal structure. The crystals of augite are small and occasionally twinned. The ground-mass consists almost wholly of glass, in which are feldspar microliths and grains of magnetite. The rock must therefore be classified as a volcanic glass, and the presence of magnetite and augite shows that it belongs to the basic series. From its chemical and physical properties it would probably be called a tachylyte. This rock appears to be the same as that described by Professor Thomas, but I did not notice the pleochroic mineral mentioned by him.
Andesite (Macaulay Island).
This rock is dark in appearance, and has a specific gravity of 2.87. Small crystals of porphyritic feldspar are visible to the naked eye. On examining it microscopically this appears to be the only porphyritic mineral, though the specimen was so small that others might very well exist. The angles of extinction were again high, so that it is probably labradorite or anorthite. The feldspar is not weathered, and contains numerous small inclusions, and often shows undulose extinction. The ground-mass is semicrystalline, and contains microliths of feldspar and other dark material. The rock appears to belong to the andesite group, both from its specific gravity and its microscopic appearance; but the absence of any ferro-magnesian mineral renders its accurate classification difficult.
Augite-andesite (Macaulay Island).
The external appearance of this rock is dark-grey, with feldspar crystals plainly visible. The specific gravity is 2.7. On examining it microscopically it appears to be composed of a semicrystalline ground-mass, with porphyritic crystals of feldspar and augite. The feldspar is in moderately large crystals up to ⅓in. in length. They are clear and free from inclusions. The extinction-angles render it probable that it is labradorite or anorthite. The crystals of augite are small. The ground-mass is full of grains of magnetite and feldspar microliths; very little glass is present. The rock therefore appears to be an augite-andesite.
Andesite (Sunday Island).
This specimen was much weathered, and an accurate description is therefore difficult. It is light-grey in colour. Specific gravity, 2.55. Crystals of feldspar are visible with the naked eye, and under the microscope show the twinning of plagioclase. From the extinction-angles it is probably lab-
radorite, though they are by no means so high as those of the feldspar in the other rocks. The only other porphyritic mineral observed was augite, but the crystals were very small. The ground-mass is semicrystalline, and full of dark grains, probably of magnetite. The general appearance of this rock and its specific gravity render it probable that it is an augite-andesite.