CHAPTER II.

ON THE EVIDENCE OF EMERGENCE OR OF UPHEAVAL AT THE SEA-BORDERS.

ONE would have expected that in an island where submarine muds and tuffs are of such common occurrence at the surface, extending from the sea-border to elevations of 2,000 feet and over, upraised coral reefs would be also frequent and extensive. But it is remarkable that the uplifted masses of reef-limestone, so characteristic of the islands of the Lau Group, are here very scantily represented. It is certainly true that the fossiliferous volcanic muds that form the foundations of coral reefs are often exposed at and near the coast; but the elevated reefs that ought to be found reposing on them are rarely to be observed.

It is not to be inferred, however, that in a region so remarkable for the great development of reef-formations coral reefs did not then thrive in these localities, but rather that such a long period has elapsed since the emergence of the present sea-border that the upraised coral reefs at and near the coast have long since been in a great part stripped off by the denuding agencies. Notwithstanding this, it is evident that coral reefs could never have been very extensive at the sea-border during the last stages of the emergence; whilst they do not appear to have existed at all during the early periods of the history of the island.

In this connection it may be observed that hard compact limestones of any kind are rarely to be found, and only in a scanty fashion. The extensive development of dolomites and hard limestones, described by Mr. Andrews and others in the valley of the Singatoka in Viti Levu, is not a character of Vanua Levu. The foraminiferous and pteropod clays, which exist in the interior and often in the heart of the island, are not overlaid by ancient reef-limestones, but by great masses of volcanic agglomerate and coarse fossiliferous tuffs, the foraminiferous muds in their turn covering the core of massive volcanic rocks. There were no signs of coral-reef fragments in the volcanic agglomerates in any locality examined, notwithstanding that these agglomerates are so intimately associated with the fossiliferous tuffs and clays that their submarine origin could not be doubted.

The conditions for reef-formation evidently did not exist in that early stage of the island’s history, when the foraminiferous tuffs and clays, now occurring at elevations of 2,000 feet and over, were being deposited on the sea-bottom. At some time or other, however, these high mountain-slopes, previous to their emergence from the sea, must have been within the limits of the zone of reef-building corals. If reefs had been formed along those ancient coasts, or on the original shoals, they would have been in some cases preserved, as in the case of the foraminiferous tuffs and clays, by a covering of volcanic agglomerate. These soft submarine deposits have been in this manner saved from the destructive effects of denudation over a large part of the island whether on the higher slopes or at the lower levels; but no trace of reef-formation ever came under my notice in the higher regions of the interior. This is a puzzling point that will have to be considered in connection with the origin of the great mountain ridges, one of the most difficult problems in the history of the building-up of Vanua Levu.

I will now refer to the evidence of the latest stage of the upheaval of the island as indicated at and near the sea-border by the scantily occurring upraised reefs. The elevated reefs are mostly to be found on the south coast between Fawn Harbour and Na Viavia Islet off Harman’s or Savu-savu Point. Na Viavia Islet itself is 300 or 400 yards in length and is formed of much honeycombed reef-limestone, which is raised 10 or 12 feet above the high-water line. Proceeding eastward along the south coast of the Savu-savu promontory we next come upon uplifted reefs in a curiously isolated hill that rises on the coast between Naithekoro and Naindi Bay. This hill is about 250 feet in height and is composed in the mass of coral limestone. About 100 feet above the sea-level it exhibits an erosion-line, above which it rises precipitously to the summit. The west point of Naindi Bay is formed of reef-limestone reaching to a height of 40 to 50 feet and displaying in position massive corals, “Fungiæ,” and “Tridacna” shells. Near its base, four to five feet above the present high-water level, it shows an erosion line. This limestone overlies a rock in which blocks of volcanic rocks, five to six inches across, are imbedded in a calcareous matrix.

Raised coral limestone occurs at intervals on the coast between Naindi Bay and the mouth of the Salt Lake Passage, usually forming low islets, of which the smaller about 12 feet in height often assume, through the erosion of the sea at their base, that peculiar mushroom-shape, so characteristic of upheaval on reef-bound coasts. The passage into the Salt Lake lies in a slightly elevated reef-mass; and the islet which rises up in its centre to about a foot above the water-level is mainly formed of coral blocks, although I did not find any remains of coral on the low neck of land intervening between the Salt Lake and Natewa Bay. Eastward from the Salt Lake Passage to Nanutha in the vicinity of Fawn Harbour low cliffs of coral limestone, six to eight feet high and occasionally displaying massive corals in position, most frequently constitute the sea-border, rarely, however, extending more than a few paces inland or attaining there a greater elevation than 12 or 15 feet.

This limitation of the upraised reef-belt to the immediate vicinity of the coast is true of all this district. It is only when the sea-border is low and swampy that it is found 100 or 200 yards inland; and in any case as one follows it inland it soon gives place to the fossiliferous mud-rocks and tuffs of the interior. It should be noted that the upraised reefs of this region were rarely observed at greater heights than 20 feet above the sea, in fact usually at a much lower level. The exceptional occurrence in mass of reef-limestone at a height of 250 feet in a coast hill between Naithekoro and Naindi therefore lends colour to the idea that the elevated reefs formerly extended farther inland and that they have been stripped off by denudation.

On the north coast of the Natewa Peninsula elevated reefs are of very rare occurrence. I walked along the whole of that coast from the head of Natewa Bay to within four miles of Kumbulau Point and only found them in the locality, one to one and a half miles west of the mouth of the river Ndreke-ni-wai. Here there were two islets, 20 to 25 feet high and lying close to the shore, which were formed entirely of coral-rock, massive corals occurring in position in their lower part. Although, however, upraised reefs are so scantily to be found on this coast, other proofs of upheaval are to be observed in the fossiliferous tuffs exposed occasionally by the beach. On the east coast of this peninsula, between Ndevo and Loa, submarine tuffs and sandstones, at times fossiliferous, were alone noticed.

Upraised reefs are also very rare on the north coast of Natewa Bay. Here again I traversed the whole coast from the head of the bay to Undu Point, a distance as the crow flies of about 50 miles; but I find no record in my notes of any elevated reef-formations. However the calcareous nature of the volcanic tuffs exposed in places at the coast indicate emergence. The extreme rarity, if not the absence, of upraised reefs on this long stretch of coast, which is usually bordered by shore-reefs, is very remarkable, more especially since there is extensive evidence of upheaval in the plains of Kalikoso in the interior, as indicated in the succeeding paragraph.

On the other side of Undu Point, between that headland and Lambasa, elevated reefs did not come under my observation, although in the low-lying inland district of the Kalikoso lake silicified corals are scattered about in quantity at an elevation of 20 or 30 feet above the sea. But the emergence of the sea-border is shown in the occurrence of a “Globigerina” sedimentary tuff near Visongo at a height of 200 feet (see page 221), and by the occasionally calcareous character of the pumice-tuffs that mainly compose the coast cliffs. Near Nukundamu these tuffs of the shore cliffs inclose subangular fragments of massive corals of the size of a walnut; whilst in a cutting between Mbuthai-sau and Lambasa, about 50 feet above the sea, I observed bits of coral limestone in a basic tuff. Mr. Horne refers to seams or layers of coral limestone occurring in the volcanic agglomerate of the coast cliffs between Lambasa and Tutu Island.[6] Since his experience of this coast was mostly confined to a passage in a canoe along the shore, it is very probable that he only saw the beds of white pumice-tuffs that prevail in places on this coast. I found no beds of coral limestone in the shore-agglomerates of this coast, nor does Dana in his description of the pumiceous formation of the cliffs of Mali Point make any reference to them.[7]

Along the stretch of 50 miles of coast between Lambasa and Naivaka upraised reefs are of infrequent occurrence. However between Lambasa and Wailevu, coral limestone is extensively exposed in a low range of hills a mile or two inland but not over 100 feet above the sea. No elevated reefs came under my notice between the mouth of the Wailevu river and Nanduri Bay. That a small upheaval has been recently in progress in this part of the coast is indicated by two circumstances. In the first place an erosion-line about a couple of feet[8] above the high-water line, and a few paces removed from it, is displayed in the volcanic tuff of the point bordering the reef-flat on the east side of Nanduri Bay. In the next place there exist at different places in the midst of the mangrove-belt extensive bare mud-flats, sometimes several hundred yards across, which are only covered by the higher tides. These flats are quite bare of mangrove or any other vegetation and are often cracked on the surface and sun-dried and firm to walk upon.[9] These naked mud-flats in the midst of the mangrove tracts are peculiar to this part of the coast. Their general level must be between one and two feet above that of the mangrove belt in other parts of the island; and I infer that a slight upheaval or emergence has led to the death of the mangroves in these situations.

I know little of the coast between Nanduri Bay and the mouth of the Ndreketi River. At two localities where I landed no elevated reef-formation was observed. Dana referring to the coast opposite Mathuata Island alludes only to the volcanic agglomerates. The low mangrove-bordered coast between the mouths of the Ndreketi and Lekutu rivers was not actually visited by me; but I traversed the region behind the broad mangrove-belt, and found occasionally in the tuffs and muds exposed in the river-banks marine-shells and foraminiferous tests, indicating an elevation of a few feet. I examined much of the coast between Lekutu and the extremity of the Naivaka peninsula, but came upon no upraised reef-rocks. In the low isthmus, 20 to 30 feet high, which connects this peninsula with the main island only volcanic rocks came under my notice. A palagonitic tufaceous sandstone exposed in the cliffs on the north coast of Naivaka contains a little carbonate of lime, and being probably a submarine deposit it implies an emergence of the sea-border.

Although I have been able to produce but scanty evidence of uplifted reefs on the north coast of Vanua Levu, it is probable, judging from the heights given in the Admiralty Sailing Directions, that such formations exist in a few of the numerous low islands and islets that front this coast. Some of these islands and islets, which are often not much more than reef-patches largely reclaimed by the mangroves, will be noticed below when considering the question of the extension of the mangrove belts since the survey of Commodore Wilkes in 1840.

Neither on the south coast of the peninsula of Naivaka nor on the west coast of the Sesaleka promontory did upraised reefs come under my observation; but my acquaintance with the last locality is very scanty. The emergence of the Sesaleka promontory is however indicated by the occurrence inland at heights of at least 700 feet of palagonitic tuffs, occasionally containing foraminifera.

With the long tract of coast between Naithombothombo Point and Solevu Bay, I am fairly well acquainted. However, with the doubtful exception of Lekumbi Point, no elevated reef-formations were observed. Evidence of an emergence of a few feet, and of a very extensive seaward advance of the land-surface in recent times, is afforded by a curious bed of marine shells exposed in the banks of the Mbua River, nearly two miles inland and in the vicinity of the Wesleyan Mission station. This is described on page 58. The submergence at some period of the watershed between the Mbua and Lekutu districts is indicated by the presence of microscopic foraminifera in the hyalomelan tuffs that are exposed in the dividing ridge.

Along the whole coast between the mouth of the Mbua River and Solevu Bay, there are but few if any traces of upheaval. Even volcanic tuffs are of rare occurrence, and there is only the case of the formation of Lekumbi Point to be here referred to. This singular low cape is described on page 60. Here it is sufficient to remark that it is monopolised by the mangroves except at the outer part where the swampy ground passes into the dry sandy soil of a reef-islet, occupied by the usual littoral vegetation, and raised only a foot or two above the high-water level. It exhibits on the beach the bedded sand-rock so often found on coral islets, but this in itself is no evidence of emergence.

Neither on the shores of Wainunu Bay nor in the Kumbulau peninsula were upraised reefs observed, although the presence in places of submarine tuffs inland and near the coast affords evidence of elevation. The same remark applies to the coasts of Savu-savu Bay.

I have little doubt that the absence of elevated reefs on the coasts of by far the greater part of the island is the result largely of denudation. In this case we have to explain why an island in a region of coral reefs exhibits on the surface of its interior submarine tuffs and clays in most localities, whilst uplifted reefs are very rarely to be found at the coast or in fact anywhere. This view receives support from the existence of traces of old elevated reefs in different parts of the island. These traces are afforded by the occurrence on the surface in different localities of silicified fragments of coral associated with concretions of chalcedony, bits of flints and hornstones, jasper, impure siliceous nodules, &c. The localities may be at the coast or a mile or two inland, and are not usually more than 100 or 200 feet above the sea. This subject is treated with some detail in Chapter XXV. Here I may say that such localities are confined mostly to the open, low, undulating districts on the north side of the island. Silicified corals are not always present with the fragments of chalcedony and other siliceous concretions that are found so frequently in these situations; but from their association in the plains of Kalikoso, where the silicification of corals may almost be observed in operation, the previous existence of corals may be more than suspected in localities where only the other siliceous materials are observed.

I pass on now to some general considerations regarding the relations of the mangrove-belt to the sea-border and the character of the slope of the land-surface as compared with that of the submarine platform. An accurate conception respecting these matters will help one to avoid some pitfalls in forming an estimate of the character of the movement of emergence which this region has experienced.

Beginning with the mangrove-belt, some curious preliminary reflections arise, when we endeavour to look back into the past stages of the history of a mangrove tract in an area of emergence. We might perhaps expect to find the remains of such a belt in the upraised sea-borders; or if no traces existed, we ought to find in some places an extension inland of the reef-flat on which the mangroves at one time flourished. If a rapid movement of emergence is now in progress, the mangroves ought to cover the whole or greater part of the reef-flat; and in the mangrove tract of an emerging area we might look for signs of central decay and marginal growth, the mangroves dying in the middle of the tract and flourishing at the advancing margins.

When, however, we look at the mangrove-belt, as it at present exists around much of the coast of this island, we find that, except in the vicinity of the mouths of rivers, there extends beyond it a considerable extent of bare reef-flat, varying usually between 200 and 1,000 yards in width, and covered by the rising tide. There is no evidence of recent emergence in this condition of things. This relation between the mangrove-belt and the reef-flat indicates a state of equilibrium which might have been established long ago. It is the normal relation that exists between reef and mangrove growth; and it excludes all but very gradual movements of upheaval or emergence of the sea-border. It is not always easy to see why there should be this fine adjustment between the rapidly-growing mangrove and the slowly-growing reef. Under normal conditions, however, that is to say, when the land is stationary or when the change of level is of a very gradual nature, the reclaiming agency of the mangrove receives a check, and this relation between the mangrove-belt and the outer reef-flat is maintained.

Actual acquaintance with such localities soon forced me to the conclusion that whilst a gradual emergence or upheaval of 3 or 4 feet in a century would not materially affect the relation between the mangrove-belt and the reef-flat, a sudden or rapid change of level of that amount would destroy the mangroves around the whole island. There is some evidence, however, of there having been a rapid upheaval of this kind in different parts of the coast: and it follows, therefore, if this movement was general, that the present mangrove-belts date only from the last upheaval. But this elevation may have occurred ages ago; and the equilibrium between mangrove-belt and reef-flat may have been long since established. Accordingly, the breadth of the mangrove-belt can afford no indication of the period that has since elapsed. From data referred to below, it is evident that the mangrove-belt, taking its average width, away from the estuaries, at about 500 yards, might have been formed in two or three centuries, whilst a thousand years or more may have passed since it assumed its present relation to the reef-flat. If, therefore, upheaval is in progress, it must be of a very gradual character, since the normal relation of mangrove-belt to reef-flat now prevails.

There are indeed signs of such a gradual movement of emergence or of elevation being in operation on the north coast of Vanua Levu at the present time. I have before referred (page 11) to the extensive bare mud-flats in the midst of the mangrove-belt between Nanduri and Lambasa, which are well represented on the Tambia coast and in Nanduri Bay. They are only covered by the higher tides, and in the intervals their surfaces are dried and cracked by exposure to the sun. Here we have the central decay and the marginal growth which would be expected in a mangrove tract situated in a gradually rising area.

An indirect indication of such a slow upheaval on the north coast is to be found in the circumstance that the great submarine platform, which reaches seaward to the line of barrier-reefs, 15 to 20 miles away, passes gradually, as it extends landward, into the low-lying plains that constitute the sea-border between Lekutu and Ravi-ravi Point. As shown in the profile-section on p. 62, these low coast districts are prolonged inland, with an average rise of between 20 and 30 feet in a mile, to the heart of the island; and we have here an extension inland of the slope of the submarine platform. These broad inland plains, and I may here include those behind Lambasa, are covered over much of their surface with submarine tuffs and clays in such a manner that we may almost trace their continuity at the coast with similar deposits now in actual formation beyond the low-water level on the surface of the submarine platform.

A glance at the map of the island, where these inland plains are indicated by the 300 feet of the contour-line, will make this point more clear. These plains are traversed by the Sarawanga, Ndreketi, Wailevu, and Lambasa rivers; and so slight is the fall that cutters usually ascend the rivers for several miles, whilst the tide extends for a considerable distance up their courses. That the emergence of the inland plains of Kalikoso in the eastern part of the island is comparatively recent there can be but little doubt. In that locality as described on page 224, the low marshy land, surrounding the fresh-water lake of Vakalalatha, although five miles inland, is only elevated 20 to 30 feet or less above the sea, and silicified corals are scattered over its surface.

There is one other method of ascertaining the character and amount of elevation that may be still in progress in this island namely the comparison of the results of surveys of the coasts at different periods. In this manner data may be obtained as regards the growth of the mangrove belt, changes in size of the low reef-islets and islands, and alterations in depth. For this purpose I have employed the charts of the north and west coasts of the island made by Commodore Wilkes in 1840[10] and the Admiralty charts 379 and 382 as completed from the survey of these coasts by Commander Combe in 1895-96.

It was not easy to make many good comparisons in the case of the advance of the mangrove-belt of the main coast. There certainly has been no great advance seaward of the margin of the mangroves in this half century. The average amount probably lies between the estimate obtained for the coast opposite Mathuata Island, where there has either been no change or an advance of only 100 yards or so, and that for the advance seaward of the mangrove promontory of Lekutu which amounts to 500 or 600 yards. In this last case, however, much of the extension may be due to the advance of the mangroves on the mud brought down by the Lekutu river, so that, as far as these data show, the average advance of the belt of mangroves on this coast between 1840 and 1895 would appear to be slight.[11]

On the other hand, the mangrove-borders of the several low islands and islets, mainly formed of reef-_débris_, that lie off the coast, have often extended themselves during this period in a marked degree. The results of my comparisons are given below, the rate of advance being obtained by halving the increase in length or breadth as measured between the mangrove-borders, the breadth being used in the long islands.

_Advance of the Mangrove-Borders of Low Islands on the North Coast of Vanua Levu between 1840 and 1895._

Thukini, or Gibson Island of Wilkes 700 to 800 yards Nangano, or Piner’s Island of Wilkes 300 to 400 " Nandongo, or Nuvera of Wilkes 500 " Talailau (two new islands) 400 to 900 " Nukunuku or Clark’s Island of Wilkes } Not much change. Thakavi, or Day’s Island of Wilkes }

It will be noticed that the islands of the Talailau Reef are not marked in the chart of 1840; they are both low mangrove islands, the largest being slightly under a mile long and the smallest a little under half a mile. In Nukuira Island, the Vatou of Wilkes, there has been a decrease of about two-thirds of a mile during this period. The difference between Thukini in 1840 and in 1895 is very noticeable. In the time of Wilkes the mangroves only occupied about one-third of the reef-patch. Now they occupy about two-thirds, the area of the reef-patch remaining much about the same. Taking the minus and plus values of all the islands here measured, the average rate of the advance of the mangrove-margins during this half-century may be placed at about 250 yards in the case of these reef-islands, which would amount to a mile in 400 years.

It is probable that a long island like Ndongo, which is about four miles in length, has been formed by the union of smaller mangrove islands. Therefore, taking half its maximum breadth of a mile as a guide, it would at this average rate of growth require two centuries for its formation. But since the extension of the mangroves depends on the growth of the reef-patch, which takes place on the average at a much slower rate, it follows that this can only be a minimum limit for the age of this island. We can only assume that if the reef-patch had suddenly appeared 200 years ago, Ndongo Island could by this time have acquired its present dimensions. It does not follow that the mangrove border has been continuously advancing. A hundred years ago there may have been a state of equilibrium between the growth of the mangrove and the reef-patch, which does not now exist. All we can say of some of these low islands is that the mangroves have been rapidly extending their margins during the last half century, and that the normal adjustment between reef-growth and mangrove-growth, which must have once existed, does not now prevail.

There is evidence of the shoaling of the ship channel amongst these islands to the extent of about a fathom during this period.[12] The usual depth immediately around the patches, on which the islands have been formed, is 8 to 10 fathoms. If, therefore, the shoaling is a general process, it is to be inferred that although the outward growth of the reef-patches would be usually very slow, probably not over fifty yards in a century, there must be times when, in shallowing depths, the growth of the reef-patch would be comparatively rapid; and it is at such times that the adjustment between the relations of mangrove and reef-patch would be upset so that the advance of the mangroves would be for a time unrestricted.

It is, therefore, apparent that the rate of growth of one of these low islands is not to be determined by the rate of growth of the mangrove-tract occupying the surface. The subject is a complicated one; but I think enough has been said to show that the destructive agencies do not prevail on this great submarine platform on the north coast of Vanua Levu.

If the data here adduced of the increase of the low islands, of the shoaling of the channels, and of the advance of the delta of the Lekutu river,[13] are well founded, all the islands, islets, and reef-patches that lie along this north coast will be united to each other and to the main island within a thousand years.

The facts here produced do not directly indicate a movement of upheaval but they are quite consistent with the conclusion that the great movement of elevation which has built up Vanua Levu by the union of several smaller islands is still in operation at its coasts. To assume that there is now in progress at the sea-border the same process of island-building which has produced Vanua Levu, as we now see it, is to assume a uniformity in nature’s methods which is disregarded by the hypothesis that the great submarine platform, from which the large islands of Viti Levu and Vanua Levu now arise, represents the work of marine erosion into the flanks of the upheaved islands since the last elevation. The origin of this submarine platform is dealt with in