However, this movement of upheaval is so gradual that the utmost one can expect to do by the comparison of surveys made half a century apart is to show the lack of evidence of the destructive agency of erosion. As far as the comparison admits of judging, there seems to have been no important change on the coasts of the western end of the island during this period. The low neck of land connecting Naivaka with the main island, if we take the low-water line in the Admiralty chart as the limit, had much the same breadth at the time of both surveys. The depths in Mbua Bay remain about the same, with perhaps a shoaling of less than a fathom in places. There are two cays awash in the Admiralty plan of this bay which were described as sand-spits in the time of Wilkes. The promontory of Lekumbi could scarcely have been expected to show any extension during this time, since there are depths of 10 to 16 fathoms close to its extremity; and there is in fact no difference of critical importance indicated in the charts.
Some of the principal points of this chapter may be thus summed up:—
(1) Upraised reef-limestones are of very limited occurrence. They occur at and near the coast and do not extend higher than 300 feet. Their scarcity at the sea-border is to be attributed to the denuding agencies.
(2) Since foraminiferous muds and sedimentary tuffs with marine organic remains occur at all elevations up to over 2000 feet, it is assumed that the absence of reef-limestones in the elevated interior indicates the paucity or absence of reef-growths in the early stages of the history of the island. The overlying agglomerates have often preserved from destruction the soft sedimentary deposits beneath; but they seem to have never covered over a coral reef.
(3) The relation between the mangrove-belt and the reef-flat indicates a state of equilibrium which might have been established long ago. If the movement of emergence is still in progress, it must therefore be of a very gradual nature, since the normal relation between the mangrove-belt and reef-flat now prevails.
(4) From the circumstance that the submarine platform passes with a uniform slope into the low-lying plains, covered with submarine deposits, it may be inferred that a very gradual emergence is now in operation.