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Volume 45, 1912
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Art. XL.—On a Collection of Rocks from Norfolk Island.

By R. Speight, M.Sc., F.G.S.

[Read before the Philosophical Institute of Canterbury, 6th November, 1912.] (See sketch-map in previous paper by R. M. Laing.)

The specimens of rocks referred to in this paper were collected by Mr. R. M. Laing during a visit to the group in the summer of 1911–12, and were subsequently handed to me for identification. They include representatives from all parts of the island, and also a few specimens collected during a hurried visit to Philip Island, a small and rapidly disappearing islet situated about three miles south of Norfolk Island, The only reference that I can find to the nature of the rocks in the group is that contained in Appendix II of the Annual Report of the Department of Mines, New South Wales, for the year 1885 (Sydney, 1886), where there is a short descriptive account of the geological features of the islands by J. E. Carne, and a petrological description of a few rock-specimens by T. W. E. David. The

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collection made by Mr. Laing is as complete as the circumstances of his visit allowed, and gives a thoroughly accurate idea of the general geological structure and origin of the islands. As it includes numerous specimens from localities not referred to by Carne or David, I think it right to publish a short description of the collection, especially as interest always attaches to apparently trivial facts bearing on the geological history of islands remote from large land-masses, since they are the only means at our disposal for acquiring a knowledge of vast tracts of the earth's surface now covered by the ocean, and since a careful consideration of their formation may throw light on the troublesome questions of laud connections and the origin of faunas and floras. This is especially true in the case of Norfolk Island, because from a consideration of the biological evidence it can be concluded with certainty that this isolated spot of land once had a fairly close connection with New Zealand on the one hand and with Australia and New Caledonia on the other. This has been admitted by all zoologists who have studied the question, among whom may be mentioned Wallace, Forbes, Hutton, Hedley, and Oliver. The last-named, from a consideration of the distribution of the birds inhabiting the Lord Howe Group, the Norfolk Islands, and the Kermadec Islands, came to the conclusion that Norfolk Island had never formed part of an actual land connection, but existed as an island off the coast of a land stretching from New Zealand to New Caledonia, and including Lord Howe Island as an integral part of its western shore-line.*

The fact that plutonic rocks occur in the Kermadec Group has been recorded by Thomas† and the present author,‡ thus proving that a landmass of continental character did once exist over a part of the earth in that region which is now covered by sea. These islands occupy a position between New Zealand and Oceania analogous to that of Norfolk Island between New Zealand and New Caledonia, so that the rocks of Norfolk Island were examined with a special object of determining, if possible, whether or not they would yield similar material of deep-seated origin, especially as the biological evidence of a continental connection is so much stronger in its case than in that of the Kermadecs. I have, however, been somewhat disappointed in the result, there being only the very slightest indication, afforded by an inclusion in the tuffs of Philip Island, that below the volcanics which now compose almost the entire island there exists a rock of different character, if not of different origin. This evidence will be given in more detail later. Apart from this occurrence, the rocks as a whole are very monotonous, being basalts of varying degrees of coarseness and basaltic tuffs, almost entirely the product of surface volcanic action, a doubtful dyke being recorded from Norfolk Island, although Carne reports that they are common on Philip Island, The reports by Carne and David, referred to previously, contain descriptions of only two specimens from Norfolk Island—viz., the coral-sandstone from Emily Bay and a basaltic lava from Anson Bay. Mr Laing's collection includes numerous samples of the former, and from them it appears to be a yellowish-brown rock composed largely of clear colourless calcite, and containing numerous fragments of corals and tests of Forminifera of various genera, and a small amount of volcanic matter. In treating of the volcanic

[Footnote] * Trans. N.Z. Inst., vol. 44, p. 217; 1912.

[Footnote] † Trans. N.Z. Inst., vol. 20, p. 315; 1888.

[Footnote] ‡ Trans. N.Z. Inst., vol. 42, p. 244; 1910.

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rocks it will be most convenient to refer in sequence to the various localities whence specimens have been obtained, and these will be taken in order, going round the coast in a general clockwise direction, starting from Emily Bay (see map, p. 323).

Headstone, Hundred Acres.

The two specimens from here may be taken as typical of many of the rocks of the group. In the hand-specimen they are of dark-greyish colour, vesicular, somewhat coarse-grained basalts, with numerous weathered olivines clearly visible. The specific-gravity determinations are unsatisfactory, owing to their vesicular nature, but the value given by one rock was 2–60, and the other 2–50.

Under the microscope both rocks appear to be alike, and are somewhat weathered. They are doleritic in character, and of an even-grained texture. The constituent minerals are plagioclase (medium labradorite) in broad laths, augite sometimes ophitically intergrown with the feldspar, but usually in grains; magnetite and other iron ores; but the most important constituent appears to be the olivine, which occurs in grains up to 1.5 mm. in diameter, and much stained with brown hematite as a decomposition-product; occasionally, however, it is perfectly clear.

Bumbora Bay, South Side Of The Island.

Three specimens were obtained from this locality. All are dolerites of dark-grey colour, slightly vesicular, and with numerous small olivines visible to the eye; specific gravity = 2.86. In section they are formed of broad laths of plagioclase (labradorite); faintish-green augite with high extinction angle, ophitically intergrown with the plagioclase; large olivines up to 1 mm. in diameter, edged and seamed with brown limonite, some crystals being idiomorphic; titaniferous magnetite in grains, laths, and broken-comb forms. Two of the specimens are much stained by chloritic matter derived from the augite, these rocks being decidedly diabasic in character.

Included in the collection from this locality is a piece of pumice, but it is in all probability of drift origin, as it is much wave-worn, and differs mineralogically from the other rocks of the island.

Anson Bay, North-West Of The Island.

The sequence of rocks in this locality appears to be as follows: The underlying bed consists of scoriaceous basalt, and then come beds of stratified tuff, which are succeeded by a coarse basalt exhibiting well marked columnar structure.

The rock from the lowest bed is dark grey in the hand-specimen, very scoriaceous, with the cavities partly filled with a yellowish-green substance. Under the microscope it appears to be a somewhat coarse-grained dolerite, the feldspars (med. labradorite) are in larger size than is usual with other rocks of similar species from the islands; the olivines are occasionally serpentinized, but sometimes quite clear; the augite is much altered, and passes into a green chloritic material which colours a great part of the section; and there is much secondary magnetite and limonite. This rock appears to be of a less basic type than the remaining specimens, and more diabasic in character.

The tuffs, which according to Mr. Laing are distinctly stratified, are of two kinds—the lower is a coarse tuff of reddish tint composed of fragments

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up to 2 cm. in diameter imperfectly cemented together. The higher tuff is much finer in texture, and of yellowish-brown colour. These rocks are so friable that it is impossible to make sections from them, but a microscopical examination of fragments shows that they are composed of essentially the same material as the overlying rocks, and grains of olivine, decomposed augite, and feldspar form the bulk of their substance. Between the upper and lower portions of these tuffs is a variety, largely composed of volcanic material, greenish-grey in colour, soapy to the feel, but microscopically presenting no marked features to differentiate it from the overlying beds.

The uppermost member of the series in this locality consists of a darkgrey moderately coarse-grained rock in which the feldspars and olivine grains are clearly visible to the eye. It is even-grained in texture and slightly vesicular; its specific gravity is 2.60.

The microscope shows that it is composed of broad lath-shaped feldspars (med. labradorite), which are usually perfectly clear and unaltered, and frequently enclosed ophitically by crystals of faintly coloured augite. The olivines are numerous in large grains, 1.4 mm. in diameter, often arranged in aggregates, with many smaller pieces scattered through the section; they are occasionally clear, but usually stained with iron oxide. There is a considerable quantity of titaniferous magnetite in grains and broken-comb shaped aggregates; some of this is certainly secondary. The rock is therefore a dolerite of distinctly basic character.

Duncombe Bay, North Side Of The Island.

At Duncombe Bay, on the north of the island, tuffs of similar character occur, overlaid by both fine-grained and coarse-grained basic rocks. The tuffs are similar in mineral composition to those at Anson Bay, the lower beds being yellow in tint, while the upper beds are distinctly red.

The rock immediately overlying the tuffs is a dolerite of somewhat open texture, grey in colour, with abundant olivine showing in the hand-specimen; its specific gravity is 2.74. In section it appears to be principally composed of plagioclase (basic labradorite) in broad laths, exhibiting at times a rude parallel arrangement; augite in grains 0.5 mm. in length, much altered; and olivine in grains up to 0.6 mm. in diameter, occasionally fresh and clear, but usually stained brown with iron oxide round the edges or in cracks; a considerable amount of magnetite is also present. Associated with this is a finer-grained type of similar mineral composition. No glass is present, but the olivine phenocrysts are exceptionally prominent. The specific gravity of this rock is 2.87.

Cascades, North-East Side Of The Island.

The collection from this locality includes specimens of columnar basalt, a' coarse-grained basalt, a fine-grained basalt, and a reddish vesicular rock from a dyke. The specific gravity of the specimen from the boulder is 2.74. In section these rocks are all of the same general type, with varying degrees of coarseness. The columnar basalt and the specimen from the boulder are remarkably fresh rocks, the freshest in the collection. The groundmass appears in section to be composed of laths of plagioclase (labradorite) with fluxion arrangement, numerous grains of augite, much magnetite with rectangular outlines. The phenocrysts are exclusively of fresh olivine; occasionally the crystals show idiomorphic outlines, but

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are frequently packed together in nests of irregular grains. No glass is visible in the slide. In another specimen the feldspar laths and augite crystals are of larger size, and the structure is occasionally ophitic. The vesicular rock is noted by Mr. Laing as coming from a dyke, but I think it has in all probability come from a flow, judging from its texture and from its close resemblance to the dolerite from flows in other parts of the island. On inquiry from Mr. Laing, I find that he is not certain on the point, as the exposure was a small one, and not clearly visible.

Ball Bay, East Coast of the Island.

Two specimens come from this locality. They are grey in colour, closer grained than usual, showing to the eye phenocrysts of olivine and augite. Under the microscope they appear to be the ordinary coarsegrained basaltic type found elsewhere on the island. They have a specific gravity of 2.82.

Mount Pitt and Middle of the Island.

A number of specimens come from the neighbourhood of Mount Pitt, the highest point in the island, situated towards its north-west corner. Two samples are from near the summit; one of these is a fairly compact rock and the other very scoriaceous, the former having a specific gravity of 2.65. They are both pinkish-grey in colour. In section the compact rock appears to belong to the doleritic type, with large olivine phenocrysts, fresh or with a brown fringe. The scoriaceous rock contains the same minerals, and, in addition, there are broken-comb forms of ilmenite and an occasional phenocryst of feldspar (med. labradorite).

A specimen from what is known as the Pop Rock, a prominent physical feature situated about half-way up the southern slope of the mountain, is a dark grey, vesicular, and with specific gravity 2.71. Under the microscope it exhibits large phenocrysts of olivine, some fresh, others stained with iron oxide. Augite occurs in small grains in the groundmass, never in large crystals nor with idiomorphic outlines, but usually packed in between the feldspar laths; these appear to be a medium labradorite. Some brown glass full of inclusions is also present.

A specimen from Ghost's Valley, on the western side of Mount Pitt, closely resembles this in general features, but it is very scoriaceous, with the vesicles filled with alteration-products.

Specimens similar to the usual type come from boulders scattered on the surface of the middle of the island.

Philip Island.

The rocks collected by Mr. Laing at this interesting remnant of a larger land-mass prove in every case to be tuffs. One of these is largely composed of lapilli of the size of a small pea, which under the microscope prove to be formed largely of yellowish glass full of vesicles, and containing crystals of clear olivine and augite and laths of plagioclase (med. labradorite) with an occasional feldspar phenocryst. The vesicles are full of decomposition-products; in some cases this is calcite, and in others it appears to be stained with chlorite.

The most interesting feature of these tuffs is an inclusion of pinkish-brown colour and fine-grained texture. In section it is composed of comminuted fragments of quartz, plagioclase, some orthoclase, biotite, and

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a greenish hornblende, minute zircons, and an irresoluble base full of grains of iron-ore probably derived from mica. There is not sufficient evidence to say definitely what was the nature of the parent rock—whether it was volcanic or plutonic; but the evidence that exists points perhaps in the direction of the latter. If this is the case, it would afford some slight evidence that the present volcanic mass of these islands was built up on a foundation of plutonic rocks, and even if the fragments are not derived from rocks of deep-seated origin there is undoubted evidence that the islands are built up on a foundation of igneous rocks entirely different from those now exposed everywhere on the surface.

In general facies the rocks of the group appear to be more closely connected with those of Lord Howe Island, and have no relationship to those of the Kermadecs. It is impossible to give any idea of their age, except that the extent to which nearly all are weathered and the diabasic character of some suggest an early Tertiary date for their extrusion.

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