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.