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.