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.