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.