The Atoll of Funafuti, Ellice group : its zoology, botany, ethnology and general structure based on collections made by Charles Hedley of the Australian Museum, Sydney, N.S.W.
Physical Structure And Geology
Physical Structure And Geology.
The outline of Funafuti is that of a pear, the curved stem of which is directed southwards. On the east or windward side the outline is sketched in most firmly, the thread of reef and palm being here almost continuous; but on the leeward side so many and so wide are the gaps that the interspaces of surf far exceed those dots where the atoll rim emerges as dry land. The lagoon, a noble sheet of water about ten miles long and eight broad, thus bounded, is plentifully besprinkled with shoals, many of which rise to the surface and "break." Its maximum depth is thirty fathoms, the general level of the floor being about twenty, whence it steeply rises to the beach.
Beyond the atoll rim, I am informed by Captain Mervyn Field, R.N., of H.M.S. "Penguin," that his exhaustive series of soundings developed the interesting fact that Funafuti is not seated on any common ridge, or connected with the other members of the Ellice Group by any bank, but that it rises independently from the abyssal floor of the Pacific. The same was demonstrated to be the case with Nukulailai, and therefore the remainder of the Archipelago will probably prove "a range of deep sea cones," which Dana said† would be so "interesting a discovery." From the reef the atoll sloped steeply outwards to forty fathoms, whence to a hundred and fifty fathoms an almost precipitous cliff surrounded the island. Below this its lower slope, as was suggested to me by Prof. Sollas, compared with the contour of Mount Etna. The outlines of the atoll, as it appears on the surface, are. repeated with astonishing fidelity by the five hundred, thousand, and fifteen hundred fathom levels.
† Loc, cit., p. 372.
The largest islet of the atoll extends for seven miles, occupying about half the windward side. In shape it resembles a reversed capital L, or more nearly the Australian aboriginal club called "Liangle." The concave side is presented to the lagoon; against the centre of concavity sand has been banked up, so as to greatly increase the diameter of the islet, which here attains its maximum breath of seven hundred yards. Here is situated the principal or' permanent village, Fungafari; here also is the only supply of fresh water and the gardens. North and south of this area the islet rapidly narrows to a width of about a hundred yards, which is maintained for the greater part of its length. About a mile south of the village, at a spot called Luamanif, is a well beaten track, the porterage, where, to avoid the long pull by the passage, the natives haul their canoes overland across the islet, a distance of about seventy yards, and launch them on the other side. A considerable area of perhaps a dozen acres in the centre of the islet is occupied by a swamp, which from the fact of being ringed round with Rhizophora will be called the Mangrove Swamp. The native name of this locality is, I believe, Tisala. This swamp is somewhat the shape of a sagittate leaf of an aroid like the taro; the tip of the leaf answering to the south-east corner, while the lobes represent two branches, a broad western one stretching nearly across the island and penetrating almost to the village, and a narrow northern branch. Along its whole eastern border the swamp is walled in by a bank of shingle and rolled coral blocks, which rise twelve or fifteen feet above the flat, and on the further side of which the waves break at high tide. This shingle bank is narrowest and lowest in the centre, and carries a few scattered palms and pandanus. On its inland face a strip of Rhizophora luxuriates in soft, dark brown, rather deep mud. The chief expanse of the Mangrove Swamp is bare of vegetation, extremely level, of soft decomposing coral rock, whose interstices are filled with mud. At high tide it is covered ankle deep with water which drains away at half ebb. Following the retreating water northward, several large deep pools are encountered in the northern arm. On closer approach these are seen to be in such free communication with the ocean, that not the tides alone but every individual wave pulsates therein. Some have an easterly and westerly disposition, which suggests that they are breaks in the roofs of tunnels which extend under the shingle rampart, and open outside the reef a hundred yards away. A child, I was told, once disappeared into one of these pools, the dead body of which was afterwards recovered on the ocean beach. Striking as may be this natural siphon of the northern arm, by which the rising tide floods the swamp, yet the western limb surpasses it in interest. Here, at a spot a quarter of a mile east of the Mission Church, round flat-topped table-like bosses three to page 11four feet across rise a few inches above the general level. Just such masses occur as living coral in the reefs in the lagoon, and on flaking off a chip these prove to be a smallpored Porites. From these bosses of Porites extend in rays for several yards in every direction, thin flat stones on edge like tiles along a garden walk. A glance at a fragment serves to identify the latter as slabs of blue coral, Heliopora cærulea. On drawing Prof. Sollas' attention to this formation, he suggested that the Porites and its surrounding star of Heliopora evidently both lived in situ, and that they could not have existed at their present level where high tide alone bathes them. I am of opinion that the action of the tides is impeded in the Mangrove Swamp, but that the high tide, not the low one, must be the affected level; the height of coral growth is determined by the low tide not the high.
We are therefore here facing unequivocal evidence of elevation in Funafuti to the extent at least of the range of the tide, since low water springs is the highest level to which the Porites and Heliopora could have reached. They probably also grew in smooth and sheltered water. The cone in which the island rises from the abyss suggests the proximity of volcanic force to give an upward thrust. In Honden Island and Osnaburgh Island Dana* has given striking instances of slightly upheaved atolls,
Around the western edge of the Mangrove Swamp, and most noticeable in the north arm, is an old beach where a breccia of coral fragments in a platform two or three feet above the swamp has been eaten back by wave action. That this breccia formerly extended as a sheet over what is now the surface of the swamp, is indicated by a few isolated and worn cakes of it, outliers in other words, near the centre of the flat; but whether or not it overlaid the Heliopora I possess no evidence to show, although I incline to the opinion that it did.†
The beach outside the Mangrove Swamp is furthest to windward of any land in the atoll; reverting to my comparison of the islet to a Liangle, this spot corresponds to the blade of the weapon. In other words it is the most exposed corner of Funafuti.
* Loc. cit., pp. 333 and 335. Darwin declined (Structure and Distribution of Coral Reefs, 1874, p. 169) to accept these evidences of slight elevation, and endeavoured to otherwise explain an apparent instance of it which he observed (op. cit., p. 21) at Keeling Island.
† A too brief note (Qt. Journ. Geol. Soc, 1872, xxviii., p. 381) by S. J. Whitnell (? Rev. S. J. Whitmee) upon raised coral rock in situ at Funafuti, may refer to the place I have here described, but I rather suppose that the subfossil coral exposed by the beach section of breccia was mistaken for coral in the position of growth.
The shingle embankment referred to continues along the whole windward face of the atoll, being highest at the eastern angle and diminishing north and south where the trade winds strike the beach obliquely. On the leeward side it is entirely absent. Six feet above the usual level of the ocean waves it represents the greatest altitude, the culminating peak, of the atoll. Great blocks of coral packed high and toppled over by gales of past years, all weathered and discoloured, compose the inland face of the bank, their appearance recalling a heap of blackened lava and scoriæ from some volcanic hill side. A similar scene reminded Dana of "a vast field of ruins. Angular masses of coral rock, varying in dimensions from one to a hundred cubic feet, lie piled together in the utmost confusion; and they are so blackened by exposure, or from incrusting lichens, as to resemble the clinkers of Mauna Loa; moreover, they ring like metal under the hammer. Such regions may be traversed by leaping from block to block, with the risk of falling into the many recesses among the huge masses. On breaking an edge from the black masses, the usual white colour of coral is at once apparent."* On the seaward face the blocks of coral are smoothed, rounded, and beach worn, till all semblance of their Actinozoan origin has been ground away.
* Loc. cit., p. 178.
It would appear, therefore, that these rocks were here consolidated under the conditions which still prevail. A little excavation with a crowbar shows the surface to be usually harder than the underlying strata. Often an apparently solid crust when overturned exhibited a lower surface bristling with pebbles that adhered to the mass by one end only. The process of consolidation, whether solution by sea water and deposition or not, having operated apparently on the upper surface and to a slight depth only.
* A formation apparently similar to this breccia is described by Darwin from Keeling Island, and by Chamisso from the Marshall Group.—? Structure and Distribution of Coral Reefs, 1874, pp. 16 & 34.
† Moresby—New Guinea, 1876, p. 79.
‡ In article "Polynesia," Encyc. Britt., (9), xix., 1885, p. 420.
Passing inland from the coast anywhere on the windward islets a descent is gradually made on a surface of loose blocks, from a yard in diameter downwards, of broken and decaying coral. The weather has etched the upper faces deeply, and exhibits beautifully the structure particularly of the astrean species. The hardest kinds, as Montipora, Heliopora, and Millepora, had suffered little, but softer species crumbled readily under the blows of a hammer. Most of the surface of the eastern islets was of this inhospitable description, and very cruel to a traveller's limbs and raiment was it. Now and then among the loose, broken blocks, a ridge of breccia running parallel to the islet's length could be detected. Though of so barren an aspect, this country supports a vegetation of Ngia, Ngashu, Fau, Fala, Boua, and palms, sufficiently dense to everywhere shade the ground, Nowhere is this description of country more than a foot or two above highwater mark, and little depressions commonly occur even in places remotest from the sea, where, when high, the tide leaks in and spreads in shallow pools, such are always densely enclosed by a thicket of Ngia and Ngashu.
* Loc. cit., p. 186.
North and south of the Mangrove Swamp the region of decayed coral blocks does not immediately occur, but a considerable area of sandy soil intervenes. To the south a large tract of this is under cultivation, and more was so used when the atoll carried a larger population. Here also are the wells and bathing pools. To this area Dana's remarks† are quite applicable: "There is but little depth of coral soil, although the land may appear buried in the richest foliage. In fact, the soil is scarcely anything but coral sand. It is seldom discoloured beyond four or five inches, and but little of it to this extent; there is no proper vegetable mould, but only a mixture of darker particles with the white grains of coral sand. It is often rather a coral gravel, and below a foot or two it is usually cemented together into a more or less compact coral sand-rock."
The northernmost islet of the Funafuti atoll stands out of water higher by several feet than does any other. It occurred to me that the whole atoll had indeed a slight tilt from north to south, but I had no opportunity to decide whether it were so. On this particular islet there was richer red soil, plants grow here unseen elsewhere, there is also the best garden with flourishing bananas, not cultivated in a swamp in the usual Ellice Island fashion but on dry ground.
* Whitmee—A Missionary Cruise in the South Pacific, 1871, p. 35.
† Loc. cit. p. 179.
Everywhere small peebles§ of pumice the size of a walnut might be collected on the beaches. The natives say that a few years ago much pumice came ashore, coincident with which the fish from without the lagoon became unfit for food. A further account of this pumice will be found in the accompanying Report by my colleague, Dr. T. Cooksey.
* Loc. tit., p. 179, figs. 1 and 2.
† Jukes—Voyage of the "Fly," 1847, i., p. 16.
§ These peebles of pumice are of very frequent occurrence on the shores of the inlets of the east coast of Australia. This subject has been discussed at length by Messrs. David and Etheridge in Rec. Geol. Surv. N.S.W., 1890, ii., 2, p. 27. And for Polynesia see Guppy—The Solomon Islands, their Geology, &c, 1887, Chap. x.
‖ Loc. cit. p. 608.
¶ Hernandia peltata, Meissn.—See Vegetation post.
An exact survey of the islets of the atoll was executed by Captain Mervyn Field and his officers during the visit of H.M.S. "Penguin," and for further details their work in the forthcoming Admiralty chart may be consulted.
The lagoon at Funafuti appears to be in course of filling up, though the agencies at work must take long to make a perceptible advance in so huge a task. In Vaitupu this has been partly, and in Nurakita wholly accomplished. The land gains upon the water at many points. A small cay in the heart of the lake presents a permanently dry surface, while low tide shows many patches of sand and gravel above water. Scattered over the whole lagoon are numerous small reefs of upwards of an acre in extent, for all of which (being good fishing grounds) the natives have distinguishing names as Fasua Takau, the Clam Shell Reef. These reefs are in a thriving condition and evidently growing vigorously. Those near enough to the surface to permit wading at low water, offered to the naturalists of the Expedition their best collecting grounds. Other reefs lying deeper seen through a water telescope, called to fancy a "rockery "in some botanical garden, if for boulders be taken round masses of Porites or Goniastræa, tufts of soft Alcyonaria for ferns, and branching Gorgonia for shrubs.
* Scævola kænigii, Vahl. See Vegetation.
† The version I heard on Funafuti was that the ship's chronometer was taken through a port of the captain's cabin,—a much more serious offence.
North and south of Funafuti islet are shallow passages* a few hundred yards in width, interruptions in the thread of land which encloses the lagoon but not in the reef rim upon which the islets stand. At low water these are nearly dry, to windward the surf breaks upon the outer edge of the reef, which continues from islet to islet without reference to the passage, and to which my previous description of low mounds, crevasses, and inner platform applies. Within these the passage offers a broad, almost level floor of shingle and rolled blocks. This area is nearly destitute of life, the great rush of water sweeping all before it and the unstable floor giving little holdfast. A few of the hardiest Gasteropods and odd scraps of living coral contrive however to withstand these adversities. Coming to the lagoon shore the passage floor is seen to extend into it in a fan, identical in shape and structure with the fan a mountain torrent spreads on entering a lake. Below and beyond the steep delta slope a coral garden stocked with fish, shells, sea anemones, and many other pretty things, flourishes exceedingly. A collector remembers with what cupidity he, floating over them in a canoe, gazed at treasures so near in the clear water and yet so far from sketch book or microscope. As well as I could ascertain the water, driven by the surf, pours from without to within across the passage, during ebb tide as well as flood. "Whether or not these passages are growing into islets there was nothing to show, if so the shingle floor might represent the breccia in course of formation; but certainly the filling in of the lagoon proceeds at the passage delta.
* These "passages" are not to be confounded with, the deep and navigable channels through which warships may enter the lagoon.