Art. XLIV.—Geological Notes on South-west of Otago.
Read before the Otago Institute, 14th November, 1905.]
All geologists who have visited the fiord region of New Zealand have united in ascribing the peculiar physiographical features of the district to the effects of glacial erosion. The most recent writer on the subject, Mr. E. C. Andrews, of New South Wales, has given a fuller and more satisfactory description of the probable development of the land forms that any earlier author. His paper on the district, published in the “Transactions of the Australasian Association for the Advancement of Science,” vol. x, contains references to previous literature on the subject. It is therefore not proposed to reconsider the origin of the fiords in this paper, though it must always be remembered that many eminent authorities regard ice as a preservative rather than a destructive agent of a land surface. By such geologists crustal movements, warping, and faults are most frequently invoked
to account for the peculiarities of fiord depressions and the valley-lakes of alpine regions. The remarks and descriptions of Mr. Andrews have been found by the author to hold satisfactorily for the various sounds and inland valleys visited by him during the past few years. As stated in the “Geography of New Zealand,” the peculiarities of the valleys of the Mount Cook region still occupied by glaciers reproduce in their main features the peculiar curves and mountain and valley forms found in such abundance in the fiord region. The beautiful illustrations in Mr. Andrews' paper depict satisfactorily the features of most importance in these Sounds, with the exception perhaps of those smoothed and rounded rock forms that are in all countries the most striking result of glacial erosion. Such rounded rock forms are present in abundance in some of the fiords, but nowhere more frequently than in Thompson's Sound, near Dea's Cove. Plate XXII gives an illustration of this, where the wall of the fiord rises to a height of 4,000 ft., while the water is 1,700 ft. deep.
We are extremely fortunate in New Zealand in having a well developed fiord region in which all the features characteristic of such areas are to be found typically developed, and at the same time a glacial region where the facts and principles of the erosion that ice performs can be studied in detail. Even at the present day the Fox Glacier has its terminal face only 600 ft. above sea-level.
It cannot be doubted that every one who compares the features of the two regions will come to the conclusion that glacial erosion is responsible for all the main characteristics of the fiords of the south-west of New Zealand.
It has long been known that the Sounds region consists of crystalline rocks, but very few descriptions of the actual rocks have hitherto been given. Captain Hutton, in his “Geology of Otago and Southland,” includes all the Sounds rocks in his Manapouri system of probable Archæan age. He mentions granite, gneiss, granulite, syenite, and various other types, but gives no descriptions of them. In another paper* he still includes these rocks in his Manapouri system, and states that their age is Archæan. His third paper on the general structure of New Zealand† makes no specific reference to these rocks, though he apparently regards them as intrusive masses of Maitai (Carboniferous) age.
[Footnote] *Quart. Journ. Geo. Soc., 1885, p. 191.
[Footnote] †Trans. N.Z. Inst., 1899, p. 159.
Hector, in his “Outline of the Geology of New Zealand,” in 1886, describes the rocks as crystalline schists, but he does not give a definite age to them, though he suggests a correlation with Humboldt's gneiss-granite series of South America. He says that the basement rocks are foliated and contorted gneisses associated with granite, syenite, and diorite, while round them wrap hornblende schist, clay slate, and other rocks, probably metamorphic representatives of Devonian age.
The only actual descriptions I can find are due to Hutton,* who mentions granitite from Port William, Stewart Island; syenite from Preservation Inlet and Wet Jacket Arm; biotitepyroxenite from Dusky Sound; chloritic pyroxenite from Martin's Bay; and serpentine from Big Bay.
Seeing that so few accurate rock-descriptions have hitherto been published, it has been considered advisable to put on record the following notes on rocks collected at various points in the Sound region indicated in the map (Plate XXI). The results of the examination of the various specimens collected justifies the following general statements.
The prevailing rocks are gneisses containing but little quartz, much feldspar, some pyroxene hornblende, or biotite, and often garnet. At Half-moon Bay, Stewart Island, there is abundant hornblende. Irregular basic patches are numerous. The rock is here a diorite gneiss. At Golden Bay, a mile distant; granite outcrops, and there is a thick vein of graphic granite with microcline—microperthite. At Ruggedy Point there is a large intrusive mass of granophyre whose resistant nature causes it to form outstanding rugged pinnacles and cliffs. At Preservation Inlet there is a pink granite (syenite, Hutton) which apparently is intrusive in the spotted slates of that locality. At Dusky Sound there is a muscovite gneiss with little quartz, but with muscovite in plates often 2 in. across. At Duck Cove pyroxene gneiss with much garnet is the most frequent rock, but there are basic secretions of amphibolite and an altered crushed pale-green rock in which epidote and quartz form with a little feldspar the whole rock. At Breaksea Sound the pyroxene gneiss is the most abundant rock. In Doubtful Sound an amphibole schist occurs at Blanket Bay, and the same rock is found at Dea's Cove, Thompson's Sound, where a typical gneiss also constitutes a large rock-mass. At Milford Sound there is a peridotite intrusion† with gneissic rocks all round it. The enstatite of one of the hartzbergites is here in places entirely altered to a carbonate, and the rock appears a pure-white marble, though in section olivine is found to constitute a fourth of it.
[Footnote] *Trans. Roy. Soc. N.S.W., vol. xxiii, p. 112.
[Footnote] †Marshall, Trans. N.Z. Inst., 1904, p. 481.
In the gneisses of Milford Sound the feldspar is often pierced by epidote needles formed apparently by secondary action. The Darran Mountains appear to be composed in large part of a mica norite, while near the Mackinnon Pass a wehrlite was found.
In many of these rocks, but more especially in the pyroxene gneisses, rutile is a frequent constituent, but sphene is practically absent. As noticed by Hutton, there appears to be a general west to north-west dip; usually the amount of dip is considerable, often over 45°.
The rocks described from Anita Bay surround the intrusion of dunite and hartzbergite described in a previous paper,* but their relations to the magnesian rocks are not clearly seen.
Golden Bay, Paterson's Inlet, Stewart Island (A 12): A white rock on the fractured surfaces, but golden-yellow on weathered surfaces. Section: Microcline microperthite, forming a coarse graphic intergrowth with quartz. The feldspar is somewhat decomposed.
Preservation Inlet (A 7): A coarse-grained pink granite, with little mica. Section: The feldspar is slightly decomposed; some of it is orthoclase, but the greater part is oligoclase. The quartz contains minute inclusions. The mica is biotite, and is somewhat weathered.
Ruggedy Point, Stewart Island: A dull-pink rock showing no development of crystals. Section shows an intergrowth of feldspar and quartz of a granophyric nature. The feldspar is somewhat decomposed.
Cleddau River, Milford Sound (H 25): Hand-specimen a pale-grey rock showing several crystals with schiller surfaces imbedded in feldspars. Section: Feldspar abundant, sometimes with curved lamellæ, labradorite. Large plates of biotite. Some hypersthene and much diallage, both with schiller structure. Occasionally a little quartz intergrown in feldspar in granophyric fashion.
Mackinnon Pass (H 26): A dark heavy rock showing much hornblende in the hand-specimen. Section: Brownish-green hornblende showing strong pleochroism constitutes half the
[Footnote] *Marshall, Trans. N.Z. Inst., 1904, p. 481.
section, Hypersthene with faint pleochroism rather frequent. Olivine less abundant, and some diallage. Iron-ore (magnetite) distributed as inclusions in most of the minerals.
Resolution Island (G 13): Hand-specimen with marked schistose structure showing biotite and muscovite. Section: Structure cataclastic. Quartz highly irregular but plentiful. Muscovite and biotite in about equal quantity. The plates of the former often contain numerous highly refringent needles with straight extinction. They have negative elongation, evidently sillimanite.
Pigeon Island, Dusky Sound (G 5): Rock very similar to that from Resolution Island.
Pigeon Island, Dusky Sound (A 13): A perfectly white rock showing cleavage planes of feldspar on the fractured surface. Section: Oligoclase forms nine-tenths of the rock. There is a little interstitial quartz and a few small plates of muscovite.
Dea's Cove (A 9): A white rock with many feldspar cleavage surfaces and some small specks of mica. Section: Chiefly microcline, perfectly fresh, and finely twinned; some biotite, very dark-coloured, and a good deal of interstitial quartz.
Bowen Falls, Milford Sound (G 12, G 11): Hand-specimen shows large hornblende aggregates imbedded in a white feld-spathic mass. Section shows abundant crystals of oligoclase, the lamellÆ much bent and broken and often having undulose extinction. Throughout the oligoclase small needles and idiomorphic crystals of epidote, colourless in section, have formed in abundance (Plate XXIII, fig. 2), but otherwise the feldspar is perfectly clear and fresh. Hornblende abundant, often showing secondary corrosion in perfection. Except in the corroded cavities of the feldspar there is but little quartz in the section. Brown mica in large plates often associated with the hornblende. Some rutile, both brown and colourless. The epidote needles in the feldspar usually have their longer arcs parallel to the basal plane or to the brachypinacoid.
Road one mile from Beech Huts (G 22): A pale-grey rock with distinct gneissic structure. Section shows interlocking grains of triclinic feldspar (albite), some microcline and quartz. There is a small quantity of epidote in rounded grains and some rutile. Thin plates of muscovite are abundant.
Anita Bay, Milford Sound (G 4, G 18): Hand-specimen a dark rock showing much hornblende on the broken surface, large garnets, and a greyish-green epidote. Section: Hornblende bright-green with strong pleochroism to pale-brownish green,
often intergrown with quartz, in which case the hornblende is optically continuous in large plates, and the thin arms of intergrown quartz are also continuous, so together they form a graphic structure. The boundaries of the hornblendes are highly irregular. Inclusions of brown rutile rather frequent. Quartz frequent, but in rounded and irregular grains, filling up spaces between the other minerals. Epidote almost colourless with no evident pleochroism, often 1.5 mm. by 0.5 mm. in size; apparently an older constituent than the hornblende. Garnet pale-pink, in large grains, highly irregular and discontinuous, with the interspaces filled with quartz, hornblende, or rutile. Very small amount of triclinic feldspar (Plate XXIII, fig. 3). In another section there is a quantity of colourless rutile sometimes surrounding the brown grains.
(G 27): A rock showing very distinct banded structure. The bands are coloured light-brown and white. In section the brown bands are seen to be composed of minute plates of biotite with some granular quartz. The white bands are compcsed entirely of quartz. The individual grains are small and angular. Occasionally larger rounded grains of quartz and round garnets colourless in section. The structure is completely cataclastic.
Anita Bay (G 31): A black rock evidently composed of hornblende. Section entirely hornblende, with intensely strong pleochroism—c, bluish-green; b, grass-green; a, very pale-yellow.
Island Beach Harbour, Breaksea Sound (G 15): Hand-specimen shows large granular groups of hornblende crystals imbedded in a white feldspathic mass. Section: Very little quartz; orthoclase and oligoclase plentiful. Larger irregular crystals of brown hornblende, slightly pleochroic, in many places changed into green fibrous hornblende. Some finely granular epidote interspersed with the fibrous hornblende. A good deal of granular brown rutile.
Beach Harbour (G 8): Very similar in hand-specimens and sections to the garnetiferous rock from Duck Cove. Oligoclase is much more frequent; quartz less frequent; and there is a little greenish-brown hornblende.
Duck Cove (G 24): Hand-specimen shows abundant garnets associated with a black ferro-magnesian mineral. These groups are imbedded in a white base. In section the garnet groups are granular, and the ferro-magnesian mineral is a pale-green faintly pleochroic pyroxene. The white minerals are quartz with highly irregular margins, and oligoclase. Much brown rutile associated with the garnet-pyroxene groups (Plate XXIII, fig. 4).
Duck Cove (G 19): A fine-grained pale-green rock. Sec-
tions completely cataclastic. Smashed grains of epidote and quartz constitute the whole mass of the rock.
Cutting north side Mackinnon Pass (G 17, G 10): A pale-green rock, with no distinct crystals showing. In thin slices, much crushed. Quartz frequent; oligoclase equal in quantity to quartz. Twin IamellÆ curved and broken in many grains. Fine granular epidote frequent, and a few grains of rutile.
Blanket Bay, Doubtful Sound (G 14): A dark greenish-grey rock, showing on the broken surface abundant cleavage planes of hornblende. In thin slices pale-green hornblende composes more than half the rock. Shows some secondary corrosion, as evidenced by the rounded inclusions of quartz. Around and between the hornblendes fine granular quartz and oligoclase angular grains completely interlocked. A few small plates of brown mica and some crystals of clear rutile. A few crystals of pale-pink garnet.
Dea's Cove, Thompson Sound (G 26): A finely foliated rock, with conspicuous hornblende, with cleavage planes parallel to the foliation planes. Section shows abundant pea-green hornblende, not highly pleochroic. It constitutes five-sixths of the rock. It contains some rounded quartz inclusions, but less noticeably than the rock from Anita Bay and Bowen Falls. The hornblende also contains inclusions of brown rutile. The colourless minerals are quartz showing undulose extinction and oligoclase with bent lamellÆ. In some specimens (G 25) there are inclusions of rounded grains of colourless rutile in great abundance.
Anita Bay (G 20, G 23): Hornblende schist. A dark rock with conspicuous cleavage planes of hornblende. In thin slices the hornblende crystals are pale-green, not strongly pleochroic, sometimes with thin laths of brown mica round the margin. The rest of the rock is fine-grained, consisting of a mixture of hornblende and quartz, which often shows evidence of a flow movement round the larger crystals, producing an eye structure. There is a little magnetite in the fine-grained part. Other specimens from near the same locality have no large crystals of hornblende, but sphene is rather plentiful in them.
The rocks from Anita Bay described in a previous paper* were hartzbergite and dunite. The hartzbergite showed an apparent change from silicate to carbonate in some of the crystals, especially in those of enstatite. A further inspec-
[Footnote] *Marshall, Trans. N.Z. Inst., 1904, p. 481.
tion of specimens which before examination by microscope were thought to be marbles showed that these rocks were only a further and more completely changed hartzbergite, for they contained a large quantity of unchanged rounded grains of olivine (Plate XXIII, fig. 1). These white rock-specimens were found on the beach; none have yet been found in situ. The margins of crevices penetrating the hartzbergite showed, however, a far more complete carbonation than other parts of the rock.
Explanation Of Plates XXI-XXIII.
Map of fiord region of Otago, showing localities from which rocks were obtained.
Face of rock on east side of Thompson Sound, near Anita Bay, showing the effects of glacial erosion on the rocks.
Fig. 1. Altered hartzbergite. Anita Bay. Rounded grains of olivine are surrounded by grains of magnesium-carbonate.
Fig. 2. Gneiss. Bowen Falls. A large plate of triclinic feldspar—andesine—showing albite twinning and undulose extinction is penetrated by needles of epidote, with their long axes parallel to the basal plane of the feldspar. To the left small grains of quartz and larger ones of hornblende.
Fig. 3. Gneiss. Anita Bay. On the right a large crystal of hornblende penetrated by epidote crystals, which extend to the left of the figure. Some of the crystals of epidote contain a central core of garnet.
Fig. 4. Pyroxene gneiss. Duck Cove, Dusky Sound. The pyroxene shown on the left of the figure is associated with much garnet, feldspar, and a little quartz.