Art. XLVI.—The Geology of the Kermadec Islands.
[Read before the Philosophical Institute of Canterbury, 19th October, 1910.]
The first account of the geology of the Kermadec Islands is contained in a paper by Mr. S. Percy Smith, F.R.G.S., written after a short visit to the group in 1887, in the Government steamer “Stella,” for the purpose of annexing the islands to the Colony of New Zealand (Smith, 1888). Specimens of rocks collected by Mr. Smith were described by Professor A. P. W. Thomas, M.A., F.L.S. (1888, p. 311). Some general information regarding the group is given in Mr. Smith's report on the Kermadec Islands (Smith, 1887). The only other notes on the geology of the Kermadecs that I am aware of are the descriptions by Mr. R. Speight, M.Sc., F.G.S., of a few rocks collected by Professor Park on Macauley and Sunday Islands (Speight, 1896).
As a member of the scientific expedition which was camped for ten months on Sunday Island in 1908, among other things I made a collection of rocks, and noted as well as I was able the geological structure of the island. On the return journey to Auckland, Macauley Island, Curtis Island, and French Rock were visited in turn, when an opportunity was afforded of obtaining a few specimens from these islets. The collection of rocks has been described by Mr. Speight, who in the same paper discusses the geological evidence for the existence of a subtropical Pacific continent (Speight, 1910). I propose now to describe the physical features and structure of the various islands of the Kermadec Group, and, in the case of Sunday Island, the order in which the different series of tuffs and lavas, were laid down and the island thus built up by volcanic action on a submerged base. The names used here for the rocks are taken from Mr.
Speight's paper, and the corresponding numbers of the specimens are quoted in every, case. For much help freely given I beg now to tender my sincere thanks to Mr. Speight.
Sunday Island—Physical Features.
Sunday Island is roughly triangular in shape. Its greatest length is 10–5 km., and its area 29·25 sq. km. The present crater, oblong in shape, the longer diameter being about 3·3 km., occupies a large portion of the island, and contains three lakes (fig. 1 and Plate XXIII), From the craterrim there branch off three main ridges—one runs north-west from Expedition Hill to Hutchison Bluff, another south-west from Mount Junction to Smith Bluff, the third south-east from Moumoukai to the east coast. These, with their ramifications, form practically the whole of Sunday Island.
With the exception of some level ground in Denham Bay, in the crater, and on Low Flat and the adjoining Terraces, the whole of the surface of
Sunday Island consists of a series of narrow ridges and deep ravines. High cliffs are frequent both along the coast and inland. This extremely rough island, which, though small, is difficult to travel over, has by the action of sea and rain been modelled out of a volcanic cone composed mainly of pumice and other andesitic tuffs.
Green Lake appears to have been the centre from which the last outbursts of volcanic activity proceeded. The water is of a dark bottle-green colour, and has a strong alkaline taste.
At the foot of Expedition Hill is a small sheet of fresh water known as Tui Lake, hidden in the forest. It is difficult to give reasons for the presence of this lake, as, on account of the porous nature of the soil, nowhere else on Sunday Island does water lie in small hollows.
Blue Lake, which is nearly circular in outline and very shallow, lies between the small or inner crater-ridge surrounding Green Lake and the
north side of the large or, as I will refer to it later, Sunday crater ridge. The bottom is a blue volcanic mud, and the water is clear, fresh, and good drinking.
South Bay Gorge bears witness to the vast amount of material removed by subaerial denudation in Sunday Island. The ravines which intersect the hills in all directions are deep, with steep, often precipitous, sides; while the tops of the ridges are narrow, usually 2 m. or 3 m. wide, and sometimes, where not protected by vegetation, razorback edges. The boundaryridges of South Bay Gorge are pushed back by the action of rains constantly removing the tuffs; hence towards Denham Bay and Scenery Bay they present convex outlines. The largest valleys on Sunday Island—namely, those which descend to sea-level in South Bay and on the north coast, where one has pushed its way right back to the Denham Bay ridge—intersect the older tuffs of the Expedition volcano. Their basins are wide at first, but narrow to gorges at the coast. Hence the depth and shape of the valleys indicate their relative age.
The greater part of the coast-line is formed of sea-cliffs. At their bases are boulder beaches formed of fragments of lava set free by the disintegration of volcanic tuffs. There is a beach composed of coarse gravel in Denham Bay, while at the base of the Terraces and on Low Flat are some sandy beaches.
A group of seven rocky islets lying to the north-east of Sunday Island, and known collectively as the Herald Islets, may be divided according to their geological character into three sections—(1) Meyer Island, (2) Napier and Nugent Islets, (3) Dayrell and the Chanter Islets.
Meyer Island is composed of beds of a compact yellow andesitic tuff (No. 5*), dipping to the north-west at an angle of about 40°. Lava dykes traverse these beds in various directions, but the general trend is from between north and north-west to between south and south-east. A large dyke composed of dark-grey augite-andesite (No. 8) runs in a due north-and-south direction in the northern islet. The soil on Meyer Island is the loose weathered surface of yellowish andesitic tuffs (No. 10), which, from being continually overturned by burrowing shearwaters (Puffinus assimilis and P. chlororhynchus), is constantly moving downhill. In a small gully on the western slope is a bed of hard yellowish-grey fine-grained tuff (No. 18), containing impressions of leaves, apparently belonging to the species Rhopalostylis Baueri and Corynocarpus laevigata. A few plants of Corynocarpus are still to be found on Meyer Island, and both species occur on Sunday Island.
Napier Islet is composed almost entirely of beds of lava, enclosing here and there blocks of coral, dipping to west-north-west at a high angle. Fragments of coral, calcite incrustations, and water-worn stones were collected on the summit ridge about 60 m. above sea-level. I am indebted
[Footnote] * These numbers refer to the specimens described by Mr. Speight, who uses the numbers originally given to them by me at the time of collecting (Speight, 1910).
to Mr. Speight for the following description of a specimen of lava collected on Napier Islet :—
“No. 11.—Augite-hypersthene-andesite. Forms a large part of Napier Islet, and encloses blocks of coral.
“Macroscopic: A dark-grey rock with small phenocrysts of feldspar clearly visible throughout. Specific gravity, 2·83.
“Microscopic: The groundmass consists of basic labradorite microlites, grains of augite, and iron-oxides, with a little brownish glass present. Phenocrysts of basic labradorite are very common, and contain numerous glass inclusions. The dominant F.M. mineral is hypersthene in large crystals with characteristic cleavages and pleochroism; augite grains are common, frequently twinned, and with oblique extinction. There is a little olivine present in the form of small grains stained with iron-oxides.”
Dayrell Islet consists mainly of beds of submarine origin, dipping to the north at a low angle, about 5°. The lower beds are composed of hard sandy tuffs containing fossils, the upper ones of a white calcite rock. These are covered by andesitic tuffs (No. 20), whilst lava dykes pierce the islet in various directions. The Chanter Islets are similar in structure to Dayrell Islet. (Plate XXIV, fig. 1.)
On one occasion only I was able to land for about half an hour on Dayrell Islet, when I collected a number of fragments of molluscs and corals. The corals have not yet been examined. Of five species collected, I recognize only one among those found living in the group. The molluscs include the following: Turbo argyrostomus L., Pupura* Clanculus, Risella, Trochus, Amalthea, Spondylus ostroides Smith,* Pecten kermadecensis Watson,* Chama, Chione, Arca decussata Sow.* Those species marked with an asterisk (*) also occurred among the recent shells collected on Sunday Island. I am indebted to Mr. C. Hedley, F.L.S., of Sydney, for the identification (though he expresses some doubt) of Turbo argyrostomus L. Recent examples of this species have never been obtained on Sunday Island, though M. R. S. Bell knows the fauna well.
Structure of Sunday Island.
Long lines of high cliffs afford splendid opportunities for observing the structure of Sunday Island. Here will be described the arrangement of the various volcanic beds as seen in typical sections in different portions of the island. Further on in this paper an attempt will be made to describe the order in which the material was ejected from the different centres of eruption.
At the south end of Denham Bay a horizontal stream about 10 m. thick of greyish andesitic basalt (No. 38) is exposed for a considerable distance. Unfortunately, one end is buried under fallen cliff débris, but evidently it must terminate abruptly at the base of Expedition Hill (fig. 2). Above the basalt are beds of tuffs extending to the top of the crater-ridge and Mount Junction. These are arranged in two distinct series. The lower beds, about 60 m. thick, are arranged in perfectly horizontal thin even beds, which, I should say, have undoubtedly been deposited under water. They are composed of small fragments of andesitic rocks in a finer matrix, and include many larger rock-fragments. They contain no pumice, and will be
referred to in this paper as the “newer submarine beds.” The upper beds are irregular, and doubtless of subaerial origin. They are composed almost entirely of andesitic pumice, including fragments of lava rocks. At the summit of Mount Junction (472 m.) a small angular fragment of hornblendegranite was collected, and high up on one of its southern spurs, over 200 m. above sea-level, a larger boulder was noticed.
In the cliffs beyond the beach the basalt-flow is seen to thin out and finally disappear. Entire cross-sections of other flows are also exposed. They are horizontal, of nearly uniform thickness for most of their length, and taper at both ends. A specimen from one of these beds proved to be a dark-coloured augite-olivine-andesite (No. 26). At the base of the cliffs is a boulder beach composed mainly of fragments released from the volcanic tuffs above as they weather away. At one spot was observed a large waterworn boulder of hornblende-granite about 50 cm. long.
The sections of Big and Expedition Hills as exposed to view in Denham Bay show a series of lava-flows dipping at an angle of about 5° to the northwest. A specimen from one of these is a grey basalt (No. 37); another from a flow lying above is a dark compact andesite (No. 21, Above the lava-flows are volcanic tuffs.
A large landslip which occurred in Denham Bay some six years ago exposed a section of the cliff up to a height of 300 m. The lower part of
the cliff, however, is covered by the fallen material. The section exposed shows no lava-flows, but a series of irregularly stratified beds of andesitic tuffs, not pumiceous, but containing angular fragments of lava rocks. These beds have a northerly dip. Further on a few beds dip at a high angle—about 40°—as though they had been undermined and slipped down; they are adjacent to the next-described submarine series.
At the north end of Denham Bay the series of beds corresponds in part to those at the south end. At the base is a horizontal flow of andesitic basalt (No. 39), then about 60 m. of evenly stratified submarine tuffs covered by subaerial tuffs, which, however, are not pumiceous. Below the lava at one place a little tuff containing some decomposed wood is exposed.
The extreme point of Hutchison Bluff is quite a bare cliff 300 m. high. It consists entirely of irregular beds of andesitic tuffs. These tuffs consist of a coarse-grained gritty rock, which weathers quickly, containing small fragments of lava. On the north coast, from the Terraces (which cover the bases of the higher cliffs) to near the bluff there are two distinct series of beds, the upper ones superimposed on the eroded surface of the crumpled lower beds. The upper beds are composed entirely of andesitic tuffs (with no pumice), and, dipping generally but slightly to the westward, pass beyond the lower beds near Hutchison Bluff. The lower beds consist of
thin even strata of tuffs which I take to be of submarine origin (referred to as the “older submarine series”). The stratification is perfectly even, though the beds are now tilted in short sections alternately to east and west, as though great lateral pressure had been brought to bear. In two places they are pierced by small intrusions of augite-andesite.
At Hutchison Bluff a core of lava, about 8 m. wide, of burnt and scoriaceous appearance has burst through the submarine tuffs (fig. 3). On the western side a stream of augite-andesite 1 m. to 4·5 m. thick (No. 29) descends from near the top of the core at an angle of about 10°.
Further to the east is another intruson of augite-andesite (No. 30). Surrounding the lava the tuffs are burnt to a brick-red colour.
The Terraces are a comparatively recent addition to the north side of Sunday Island, and at the base of Big and Expedition Hills abut against cliffs which are a direct continuation of the sea-cliffs extending to Hutchison Bluff. They are composed entirely of pumice tuffs arranged in slightly irregular but generally horizontal beds, apparently of subaerial origin. A specimen of pumice, Mr. Speight informs me, is andesitic in character. Fragments of lava rocks, sometimes 2 m. or 3 m. in diameter, are strewn thickly throughout the pumice tuffs. At the base along the shore-line there is for the most part a boulder beach composed of fragments too heavy to be washed away by the sea, which is gradually eating away the comparatively loose tuffs. Among the boulders I gathered samples of hornblende-granite* and andesitic pitch-stone (No. 2), and noted that the largest pieces of lava were identical in appearance with the andesites from a large flow of lava, in the Sunday crater. Logs of charred wood, recognized as that of Metrosideros villosa, the most abundant tree on Sunday Island, were noticed in the tuffs at the base of the cliff. The section exposed at Fleetwood Bluff shows No. Terrace to have been formed during two distinct periods of eruption, in the interval between which two wide valleys some 20 m. deep were formed in the first, deposited beds. On the hillside above No. 1 Terrace, about 80 m. above sea-level, is a large boulder of hornblende-granite.
On the east coast of Sunday Island, at sea-level, is a stratum composed of rounded evidently water-worn boulders of all sizes up to 2 m. in diameter, cemented together by a sandy matrix. Above this and conformable to it are the newer submarine tuffs, intersected in Coral Bay by a large dyke running east and west, and overlaid by a flow of basalt. (No. 36), above which are pumice tuffs.
In South Bay, near D'Arcy Point, at sea-level, is also a bed similar to that on the east coast, containing water-worn boulders. Several dykes with a general north-west trend intersect the newer submarine tuffs of South Bay. On the shore several small rounded stones of hornblende-granite were seen.
[Footnote] * For a description of the hornblonde-granite sao Speight, 1910, p. 251.
The cliffs in Scenery Bay present a series of lava-flows horizontal in section, and cut by several dykes. Between the lower flows at D'Arcy Point are portions of beds of yellow andesitic tuff (No. 13) of submarine origin. A lava-flow at sea-level in Scenery Bay is composed of dark-grey andesitic basalt (No. 41). A specimen of andesitic basalt (No. 40) from the same locality was taken from a dyke 2 m. to 3 m. wide, and running northwest and south-east.
Titi Knob, which is the name given to a small hill above Boat Cove, is composed of a mass of lava and tuff which shows little orderly arrangement, and is in places thoroughly mixed with coral. A typical specimen of the lava is composed of andesite containing many fragments of coral (No. 22). In one spot on Titi Knob is a rock showing planes of stratification and containing fossils (No. 25). It consists of a coarse gravel composed of grains 1 mm. to 3 mm. long cemented by white calcite. Included are fragments of lava up to 2 cm. long, coral, and molluscan shells. I could not trace this for more than 2 m. or 3 m. in any direction. The fossils collected included fragments of the following molluscan shells: Trochus, Nassa, Arca decussata Sow. (?), Chione.
The north side of the Sunday crater ridge consists of pumice tuffs containing numerous fragments of lava rocks and hornblende-granite. Here in several places angular fragments of the granite may be seen projecting from the cliff-face, and those which were extracted were easily crumbled to their coarse constituent grains, whilst on the shore occur numerous water-worn boulders of various sizes up to about 50 cm. long. The pumice tuffs rest on hard andesitic tuffs, apparently belonging to the newer submarine series.
Within the crater on the western side of Blue Lake is a large flow of olivine-andesite (No. 34), dipping gently in a northerly direction, and which may be traced until its upper surface reaches the level of the lake and disappears under the pumice tuffs of the north side of the crater-ridge. Above this lava-stream to the summit of Mount Campbell, 285 m. above sea-level, there is nothing but pumice tuffs.
At the base of Moumoukai, on the eastern side of Blue Lake, a good section is exposed. The lower beds, about 40 m. in thickness, are evenly statified horizontal yellow andesitic tuffs (No. 14) of submarine origin and similar to those at D'Arcy Point, but differing in their composition to the larger series of newer submarine beds which enter so largely into the formation of the lower portion of Sunday Island. Above the submarine beds of Moumoukai is a stream of andesite (No. 9) identical in external appearance with the olivine-andesite on the opposite side of Blue Lake, and apparently only another part of the same flow. Above this are several other lava-flows, while the upper half of Moumoukai, in common with the whole upper part of the large Sunday crater rim, is composed entirely of pumice tuffs.
The inner crater surrounding Green Lake also consists of pumice tuffs. On the southern shore of the lake was found a fine-grained tuff (No. 32) containing rounded bombs.
From the foregoing description it will be gathered that Sunday Island is a tuff cone, with here and there near sea-level a few lava-flows and dykes. In three places only — Moumoukai, within the crater; Big Hill, in Denham Bay; and Scenery Bay—are there exposed to view cross-sections of a number of lava-streams. The floor of the crater is near sea-level, and cliffs exposing sections are the rule both within and without the crater, yet nowhere was hornblende-granite found in situ.
History of Sunday Island.
The Herald Islets may be looked upon as fragments of the wall of an ancient crater of which the greater portion has been demolished by the sea, and, though entirely detached from Sunday Island, I have no hesitation in considering this crater the first to appear in the vicinity of Sunday Island. There is no evidence to show that any land actually existed above sea-level in this locality when the disturbance which resulted in the Herald crater took place. The presence of littoral marine mollusca and corals, of which many fragments are entombed in the beds of Dayrell and Chanter Islets, however, indicates shallow-water conditions, and, even if a fragment of a once more extensive continental area did remain above the surface of the ocean, whatever living forms existed on it were entirely destroyed by subsequent volcanic eruptions, as no characteristic continental forms of life are without doubt indigenous to the Kermadec Islands.
The probable date of this eruption cannot, in a geological sense, be very remote. Those fossil molluscs which have been identified with tolerable certainty all belong to living species, most of them still existing in Sunday Island waters. One, Turbo argyrostomus, is not known to occur alive in the Kermadec Group, but is found in Queensland and the Pacific islands. Bearing in mind the fact that about 11 per cent, of the vascular plants of Sunday Island are endemic, the newer Pliocene would perhaps be sufficiently far back for the age of the Kermadec Islands.
Contemporaneous with the formation of the Herald volcano a small eruption occurred in Boat Cove, Sunday Island. This also was chiefly submarine, as what now remains of the crater-walls consists of andesite lava with many fragments of coral entangled (No. 22, 25).
Meantime some yellow andesite tuffs (Nos. 13, 14) and a little lava were being deposited under water further to the westward. These are now exposed at D'Arcy Point and on Moumoukai within the crater; the crumpled older submarine beds at Hutchison Bluff also were perhaps deposited about this time. The land, however, was rising, and a series of eruptions followed, resulting in the formation of a third volcano, whose crater was on the eastern side of Expedition Hill (the probable position of the inner wall of this crater, which may be called the Expedition crater, I have indicated by the broken oblique line in fig. 2). To this volcano I assign practically all the lava on Sunday Island—namely, most of the flows and dykes seen in section in the cliffs in Denham Bay, Scenery Bay, and within the crater (rocks Nos. 21, 37, 40, 41, 9, 34). Over all was laid a thick covering of andesite tuffs, from which evidently was derived the material for the newer submarine series.
After this copious outpour of lava the volcanic forces apparently were suspended for a while, a boulder beach was formed along the shore-line (which at one spot on the east coast and another in South Bay, coincides with the present shore-line), and the walls of the Expedition crater were considerably denuded by marine and subaerial agencies. Next followed a period of subsidence, denudation proceeding apace, resulting in the deposition of the newer submarine series. (Plate XXV.) Meantime some lava (Nos. 38, 39, 26) welled up and flowed in horizontal sheets in Denham Bay, and later some dykes penetrated the tuffs in other parts. At the base of the cliffs in Coral Bay I extracted a large Trochus shell, belonging to a species still living in Sunday Island, from a rough angular block of lava. This could only have come from a lava-flow some height up the
cliff. The amount of subsidence can be gauged by the thickness of submarine tuffs—namely, about 60 m. These submarine beds were covered with a considerable thickness of andesite tuffs with irregular bedding, probably ejected from the Denham Bay crater. Upheaval afterwards took place, and has continued till the present time, the island meanwhile being considerably reduced by the action of sea and rain.
An eruption resulting in the present large, or Sunday, crater was the last serious attempt of the island to add to its size. A violent outburst forced a way through the lava-flows forming the northern slopes of the Expedition volcano. No molten rock came to the surface in this new crater; only pumice tuffs containing numerous fragments of lava rocks and fewer of hornblende-granite and andesite-pitchstone (No. 2). The lava-streams of the northern slopes of the Expedition volcano were shattered to fragments, some of which, 2m. or 3 m. in diameter, were hurled up as far as Fleetwood Bluff. The pumice tuffs ejected from this crater form, as far as I can judge, the whole of the lower northern portion of the crater-rim, Mount Campbell and the upper part of the crater-rim resting on the extinct Expedition volcano. The Terraces, which originally must have joined the crater and extended for a considerable distance out to sea, were also the product of the Sunday crater. In describing their structure I have already pointed out that the largest boulders freed by marine denudation at Fleetwood Bluff are identical in external appearance with specimens (Nos. 9, 34) from the great ruptured lava-stream within the crater. It is in the ejectamenta of this crater alone that fragments of hornblende-granite were observed.
The struggles of the dying volcano of Sunday Island had not yet quite ceased, for an eruption on a small scale occurred within the very walls of the large Sunday crater, and resulted in the formation of a small pumice cone, of which Green Lake occupies the centre. The fine-grained tuff (No. 32) containing bombs is the product of the Green Lake crater.
Recent Volcanic Activity on Sunday Island.
Two eruptions have been recorded since the discovery of Sunday Island.* One occurred in 1814, and is recorded in the Sydney Gazette of the 17th September of that year (Smith, 1896); the other took place about the year 1872, and of this an account was printed in the Southern Cross (Sterndale, 1884; Smith, 1888, p. 337).
There are still many signs of volcanic activity on Sunday Island. Earthquakes are fairly frequent, and are usually followed by small landslips. Four rather severe shakes and many slight tremors were experienced on Sunday Island during the first ten months of the year 1908.
Fumaroles occur in several parts of Sunday Island.
Macauley Island, distant 109 km. from Sunday Island, is somewhat circular in outline, and entirely surrounded by cliffs, which can be scaled
[Footnote] * Mr. H. I. Jensen (Proc. Linn. Soc. N.S.W., vol. 31, p. 661) states that a violent eruption occurred in the Kermadec Islands in 1902. This may be a slip of the pen, as the vegetation in the crater has apparently never been disturbed since the new growth after the outbreak of 1872, and the inhabitants did not mention an occurrence, though I believe Sunday Island may have been deserted about the year 1902.
at one place only, the Lava Cascade. Mount Haszard, the highest point above sea-level, 230 m., is near the western side, and from here the land slopes away gradually to the eastward. Several deep ravines have been cut through the pumice tuffs by the action of rain-water.
The base of the island is composed of flows of basalt (No. 42), the upper surface of which, from being fairly high in the western cliffs, falls rapidly at first, and is then nearly horizontal. The basalt collected by Mr. Smith, and described by Professor Thomas (1888, p. 312), came from the Lava Cascade, which, according to Mr. Smith (1888, p. 341), belongs to a period later than that to which belongs the flows represented by rock No. 42. Above the basalt were beds of andesitic pumice (No. 43), averaging about 75 m. in thickness. From these beds Mr. Smith collected pitchstone, obsidian, and pumice, and from the surface of the island a basaltic scoria (Thomas, 1888, pp. 312, 314), which forms beds of considerable thickness overlying the pumice tuffs. (Plate XXIV, fig. 2.)
Haszard Islet is composed of lava, pumice, and scoria corresponding in the relative position to the beds of Macauley Island, but dipping in the opposite direction.
Two islets, the smaller of which is evidently but a detached portion of the outer edge of the crater-rim of the larger, are distant 35 km from Macauley Island, and separated from one another by a strait about 400 m. wide. The larger islet is a crater-rim, one side of which has been broken away, giving place to a small inlet (Macdonald Cove), in which a landing can be effected in easterly weather. The inner side of the crater-rim is mostly precipitous, and appears to be composed mainly of volcanic tuffs.
Over the crater-floor, about 4 m. above sea-level, are scattered many holes full of boiling water or mud, and much sulphur and siliceous sinter. The sea-water near the landing is kept quite warm by the hot water flowing into it. (Plate XXVI.)
This most southern member of the Kermadec Group, distant 83 km. from Curtis Island, is a mere rock about 250 m. in length, rising to a height of about 50 m. above sea-level. It is composed of lava, including olivine-andesite (No. 46), much burnt and scoriaceous in places, and pierced by dykes.
Here I propose to summarize the evidence relating to the origin of the Kermadec Islands, referring firstly to that indicating a continental extension, and secondly to the several lines of evidence supporting the supposition that the islands have never exceeded much their present area.
According to Mr. Speight, the occurrence of many fragments of horn blende-granite in the pumice tuffs on the north coast of Sunday Island indicates the presence in close proximity of a granite foundation which at one time no doubt formed a land-surface (Speight, 1910, pp. 244, 249). Mr. Speight considers the claims of the islands to be classified as continental are worthy of serious consideration, and adduces certain lines of evidence chiefly biological, to support this view.
The only tuffs from which fragments of hornblende-granite were actually taken were those ejected by the last, or present, large crater of Sunday Island, and the presence of all the boulders noticed on the sea-shore and elsewhere can be explained by the release of fragments consequent on the removal of the exposed face of sea-cliffs and surface of the tuffs by marine and subaerial denundation. My observation, therefore, support the opinions expressed by both Mr. Smith (1888, p. 344) and Professor Thomas (1888, p. 315)—namely, the presence of boulders of plutonic rocks on Sunday Island could be explained by the supposition that they have been brought up from great depths by volcanic agency.
In the foregoing description of the geological structure of Sunday Island, and attempt to deduce therefrom the history of the island, I have endeavoured to make it clear that (except for the inclusion in some of the lavas and tuffs of coral and hornblende-granite respectively, and some calcite no doubt derived from coral in the Herald Islets) Sunday Island and the adjacent Herald Islets are built up entirely of volcanic materials ejected from five principal points of eruption. The first eruptions were submarine, but shallow-water conditions obtained. Elevation then took place, and much lava flowed above sea-level. The land-surface was afterwards considerably denuded, and during a temporary subsidence volcano tuffs were deposited under water. Lava-flows were meantime becoming less frequent, and finally ceased, but the ejection of tuffs above sea-level, the last of which were entirely pumiceous, continued for a long period, so that the main portion of the islands at the present time consists of tuffs which are rapidly disappearing into the ocean. The geological structure of Sunday Island, therefore, shows it to be built up in comparatively recent times on a submerged base, and that it never exceeded its present dimensions more than can be accounted for by marine denudation.*
The biological evidence which, with apparent exceptions, seems to support the supposition of an oceanic origin for the Kermadec Islands will now briefly be reviewed.
From a study of the flora, Mr. Cheeseman was convinced that the Kermadec Islands have received their plants by transoceanic migration (Trans. N.Z. Inst., vol. 20, p. 163). My further investigations, as far as I am able to judge, confirm his views. I see no plants in the flora whose presence in the group cannot reasonably be attributed to the agency of ocean-currents (drifting logs), wind, and birds.
As illustrating classes including land-animals the Gastropoda and Crustacea may be mentioned. About eighteen terrestrial species of the former and fewer of the latter were collected. They agree in being all small, and in this respect what ought to be expected of members of these classes capable of crossing wide stretches of ocean, where floating trees have probably been the means of transport.
If the presence of migratory birds in numbers at certain seasons of the year is an indication of the spot being on an old shore-line, then the Kermadec Islands afford little evidence of this character for such a connection.
[Footnote] * The island Eua, in the Tonga Group, has, according to Mr. J. J. Lister, had a history closely parallel to that of Sunday Island as sketched in this paper. Gabbro occurs as boulders on the shore, while garnet and tourmaline are found in the volcanic tuffs (Lister, Quart. Jour. Geol. Soc., vol. 47, p. 590). Mr. H. I. Jensen's views with regard to Eua, however, are similar to Mr. Speight's concerning Sunday Island quoted above (Jensen, Proc. Linn. Soc. N.S.W., vol. 31, p. 641).
Occasionally stragglers find their way there, but no migratory birds can be considered regular visitors to the group.
The presence in the Kermadec Islands of the Pacific rat (Mus exulans), the candlenut-tree (Aleurites moluccana), and the Polynesian ti (Cordyline terminalis) is perhaps suggestive of a continental connection; but I have given reasons elsewhere for supposing these to be introduced by Natives, of whose occupation on Sunday Island there is ample evidence (Oliver, 1910, p. 137; also see p. 539 of this volume).
List of Works referring to the Geology of the Kermadec Islands.
1884. Sterndale: “Sunday Island.” Appendix to Journals of the House of Representatives of New Zealand, A.—4, p. 64.
1887. Smith, S. Percy: “The Kermadec Islands; their Capabilities and Extent.” Wellington.
1888. Smith, S. Percy: “Geological Notes on the Kermadec Group.” Trans. N.Z. Inst., vol. 20, p. 333.
1888. Thomas, A. P. W.: “Notes on the Rocks of the Kermadec Islands.” Trans. N.Z. Inst., vol. 20, p. 311.
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