Art. XV.—Description of Arid Island, Hauraki Gulf.
[With a Map.]
[Read before the Auckland Institute, August 3, 1868.]
Arid Island lies about three miles from the north-east coast of the Great Barrier. It is rather less than one and a half miles long, by about three-quarters of a mile broad, and contains nearly 600 acres. Its highest point is about 700 feet above the sea level.
In December, 1867, in company with Mr. A. J. Allan, we spent several hours in examining this island, which, owing to its out-of-the-way
position, and its rugged cliffs, is seldom visited, except by Maories, in the Mutton-bird season—the end of November. The accompanying map, which we then made, will serve to show its principal features; although being merely an eye-sketch and made without any instruments, it has no pretensions to strict accuracy.
Arid Island certainly does not deserve the name bestowed on it by Captain Cook. The high, rugged, desolate-looking cliffs, that encircle the greater part of the island, hide within them, beautifully sheltered valleys, covered with luxuriant fern and bush, and watered by streams which, uniting, empty themselves into the small boat harbour on the west coast. This harbour although too small for a cutter to swing at her anchor, and exposed to north-westerly winds, is well adapted for boats, as it terminates in a fine, sandy beach, up which they can be easily hauled. It is the place where the Maories always land when they come to eat Mutton-birds. These Mutton-birds, or Oil as they call them, are a kind of petrel of a dark-brown color, belonging we suspect, to the genus Puffinus. We were not able to procure a specimen, the season being too far advanced, and saw nothing but the feathers lying about, where the Maories had been eating them. These birds live in holes which they burrow into the soft hill sides. We were informed by the natives of the Great Barrier, that formerly they used to be very numerous, but that, latterly, they had become scarce, having been killed off by the rats. The other birds that we saw on the island were, the Tui, (Prosthemadera Novæ Zelandiæ), the Bell-bird, (Anthornis melanura), the Fan-tail (Rhipidura flabellifera), the Ground-lark (Anthus Novœ Zelandiœ), the New Zealand Titmouse (Certhiparus Novœ Zelandiœ), the little Miromiro (Petroica toi-toi), and the Pigeon (Carpophaga Novœ Zelandiœ). The greater part of the island is surrounded by high precipitous cliffs; the harbour, one point on the eastern, and possibly another on the northern side, being the only places from whence the island seems to be accessible. On the north and south, the sea has eaten back the cliffs into mere ridges, only a foot or two wide in some places.
The high ridge that surrounds the depressed interior of the island, proclaims, at once, that it is the summit of an old volcanic cone, and an examination of the rocks confirms the supposition, and shows that it belongs to the trachytic class of volcanoes. The shape of the crater is singular, its length being more than twice its breadth, and the northern part being divided into two, by a ridge running from the edge of the crater towards the centre of the island, and directed nearly to the boat harbour, or that point where the wall of the crater is lowest. This configuration is probably owing to there having been two craters, the southernmost of which was the last in activity, and filled up the northern one with ashes; and the two valleys have been subsequently scooped out by subærial denudation. The direction of the dividing ridge, and the termination of the north-western valley in a narrow gorge prove the correctness of this view.
Nearly the whole of the island is composed of trachytic tuffs and breccia generally either white, or of a pale yellow or violet color, and enclosing here and there, fragments of trachyte and obsidian. These
tuffs are arranged in the cliffs that formed the wall of the crater, more or less horizontally, although of course much confused in places, and are but little intermixed with lava streams. To find these latter, we must go to the adjacent coast of the Great Barrier, about three miles distant, where, at the south side of Wangapoua Bay, we see thick beds of trachyte, and trachy-dolerite, interstratified with tufa, dipping away, from Arid Island at an angle of 35°; and farther inland, on the top of the dividing ridge of the island, we find tufa and agglomerate, most probably derived from Arid Island, at an elevation of 1550 feet from the sea level.
Now, these facts lead to some interesting deductions, which bear on one of the great questions of the day, in Geology, viz.:—Are volcanoes connected with a central fluid interior of the earth, or are their lavas derived from comparatively shallow depths below the surface?
It is evident that Arid Island, in its present condition—only 700 feet above the sea—could not have distributed tufas at an elevation of 1550 feet, neither could lava streams flowing from it now, have, at the same level as the crater, and at a distance of three miles, a dip of 35°. It follows therefore that either some other crater, in the direction of Arid Island, and much higher than it, but which has now quite disappeared, was the origin of these lavas and tufas; or, that Arid Island was once at a considerably higher elevation, not only above the sea, but with respect to the main land of the Great Barrier. The depth of the intervening sea, 12 to 17 fathoms, makes the latter supposition much the most probable; and assuming that it was so, we see that Arid Island must have sunk down, at least, 2000 feet below the level of the Great Barrier, because the tufa, at a level of 1550 feet, is evidently a submarine formation; while the shape and preservation of the crater of Arid Island shows that it was formed under the air, so that the bottom of the crater must have been above the highest level of the tufa. Now, it appears most probable, that the sinking of Arid Island was produced either by the breaking of the roof of the cavity, from which the lava and ashes had been extracted, or by a slipping down in mass of that part between it and the Great Barrier. If, therefore, we suppose that Arid Island was the centre of the subsided portion, we have a district six miles in diameter, which has broken, or sunk through by its weight, into a cavity below; and it appears to be impossible that such should have been the case, unless the thickness of the crust broken, or sunk down, was considerably thinner than the diameter of the cavity. For, if not, it would have been strong enough to have resisted the fracture, and the friction along the sides would have been too great to allow it to slip; so that it seems impossible that the region from which Arid Island derived its lava was so deep as six miles below the surface, and therefore it would be unreasonable to, infer that it was connected with a fluid interior; for most astronomers and geologists are now pretty well agreed that the solid crust of the globe is at least a thousand miles thick.
Our visit to Arid Island was not made under favourable conditions for the investigation of its botany; most of the vegetation of the open land having been burnt off by a party of Maories, a few days before we landed;
and our stay being limited to a few hours by the unsettled state of the weather.
Although it was impossible to make even an approximate list of the plants of the island, sufficient was observed to show, it possessed a flora which comprised a greater number of forms than could be collected on the islets off the west coast of the Great Barrier; and that the general character of its flora approximated closely to that of the Little Barrier Island, which it so nearly resembles geologically. A complete examination of Arid Island and the Little Barrier, would probably result in the discovery of other plants common to both, but absent from the Great Barrier, besides those observed by us.
The flora of the island may be roughly divided into, Ericetal,—or plants of the open land;—Sylvestral,—or forest plants;—Littoral,—or beach plants; and Uliginal,—or marsh and swamp plants. It need scarcely be remarked, that these terms are not always capable of precise application.
The greater portion of the central area of the crater, and its rim, is occupied by Ericetal plants; in the lower parts, a dense growth of Pteris esculenta, which often attains the height of six feet, intermixed with occasional tufts of Phormium tenax, renders all progress slow and laborious. On higher parts and in open places, the fern is supplanted by Leptospermum scoparium and Pomaderris phylicifolia, sparingly intermixed with bushes of Coprosma robusta, C. lucida, Carmichœlia australis, Leucopogon fasciculatus, Veronica salicifolia, Coriaria ruscifolia, and other small shrubs. Agrostis œmula, Leucopogon Frazeri, Drosera auriculata, Lobelia anceps, Haloragis micrantha, Lagenophora Forsteri, etc., etc., were common amongst open fern, together with the ubiquitous introduced plants, Erigeron canadensis. The pretty Ophioglossum lusitanicum was seen on tufaceous ledges, and was afterwards observed, in exactly similar habitats, on the Little Barrier, but appeared to be entirely absent from the Great Barrier. Many specimens had two or more scapes springing from same root, a peculiarity it shares with other forms of the genus in New Zealand, although all the forms collected in the northern hemisphere have invariably solitary scapes. On the highest points of the island, as, in fact, of all islands and headlands in the North of New Zealand, Astelia Banksii and Metrosideros tomentosa were invariably found. Astelia Banksii, we may remark, is always rupestral in its habitat, never epiphytal,—nor is it found at any great distance from the sea, so far as our experience extends. From personal observation, we can testify, it is abundant on rocks at Mercury Bay, where we sought for it in vain “on the limbs of trees,” as reported in the Handbook of the N. Z. Flora. Astelia Cunninghamii, is both epiphytic and rupestral, and is most frequently found inland.
The sheltered open spaces at the base of the cliffs on the exterior of the crater, and large portions of the northern and southern sides of the interior, are occupied by the Sylvestral portion of the flora in the crater itself, forming a somewhat open bush, although few of the trees attain large dimensions. As might be expected, the Pohutukawa (Metrosideros tomentosa), is abundant, although greatly inferior in size and appearance to the fine specimens of this tree on the Great Barrier, and which are probably unsurpassed. In the crater, this tree resembles M. robusta, in its free and erect habit of growth, but on the cliffs it presents the dis-
torted appearance so commonly seen about Auckland. The principal trees of large size are the Rimu (Dacrydium cupressinum), and the Tarairi (Nesodaphne Tarairi). Amongst smaller trees and shrubs are Dysoxylum spectabile, Melicytus ramiflorus, Corynocarpus lœvigatus, Dodonœa viscosa, Coprosma grandifolia, Panax Lessonii, P. arborea, Shefflera digitata, Corokia buddleoides, Olearia Cunninghamii, Brachyglottis repanda, Geniostoma ligustrifolia, and many others, but perhaps the most remarkable is Pisonia umbellifera a few trees of which were found growing amongst young and large-leaved specimens of Corynocarpus lœvigatus, which it closely resembles in the shape and colour of its leaves; and in the absence of its flowers and fruit, presented an anomalous appearance,—“like, yet unlike.” Some of its leaves measured fully eighteen inches in length, and seven inches in breadth. It was subsequently collected, in a curiously similar habitat, on the Little Barrier Island, but was not found on the Great Barrier.
The Littoral and Uliginal plants present nothing worthy of special notice, nor indeed had we sufficient time to examine them closely, Samolus repens, Salicornia indica, Selliera radicans, and others of rupestral habitat, are abundant at the base of the cliffs. Amongst the Arenarian plants are Coprosma acerosa, Convolvulus Soldanella, and Spinifex hirsutus, the last named curious grass formed large tufts, with prostrate culms, sometimes thirty feet or more in length, which throw out roots at the joints, and aid in binding the shifting sands. The cultivated radish, Raphanus sativus, is also found growing with the above on the sands at the head of the little harbour.
The Raupo, Typha latifolia, var. angustifolia, and other uliginal plants, find a suitable habitat, although of limited extent, near the centre of the island.
The notes just read must be considered as merely a contribution to the botany of Arid Island. We venture to express the hope that some member of the Institute may visit the island under more favourable circumstances, than fell to our lot, and be able, at least, to make a complete catalogue of its phœnogamic plants and ferns; not only on account of the interesting nature of the locality; but because of the positive value possessed by an exact and minute knowledge of the local distribution of plants, as an element in the ultimate circumscription of their specific limits.