Page:Proceedings of the Royal Society of London Vol 60.djvu/547

Rh they extend. Towards the seaward side this broad corner is occupied by a mangrove swamp, the floor of which is formed by a dead coral reef, constituted almost wholly of two species, one a massive Porites, and the other Heliopora ccerulea. For a great] part of the day this floor lies bare and dry, the frayed ends of the Heliopora standing like broken reeds, 6 inches above its surface, and the great clumps of Porites forming a series of stepping stones of equal height. Neither of these corals stands long exposure to the a ir; on Funafuti they require constant submergence, and we are thus led to regard their upper surface as marking Avhat was at one time the level of low tide in the swamp; but since the present level of low tide is below the level thus indicated, some change must have occurred in the level of low tides. Not necessarily an elevation of the reef: Darwin has admirably discussed this explanation, and it is quite conceivable that some change in local conditions, such as the exclusion of the sea by the growth of the hurricane beach, may have produced a local alteration in the height of the tides. The swamp communicates with the sea by pits in its floor, which enter subterranean channels running seawax*ds. These passages are so narrow that the tide rises and falls in the swamp much more slowly than in the open sea. To determine whether any change of level has taken place, it thus becomes necessary to compare the highest and lowest water level of the swamp with that of the sea or of the lagoon. I accordingly levelled across the island from the lagoon to the sea, crossing the swamp on the way, and found that the high-water level at spring tides is 1 foot 10 inches below high water (spring tides) of the lagoon, so that given free access of the sea, the Heliopora reef would be covered 1 foot 10 inches deeper than at present, but it is now submerged from 10 inches to 2 feet 2 inches at high-water springs, and would accordingly be submerged from 2 feet 8 inches to 4 feet, with free access of the sea, The range of spring tides is at least 6 feet, as I learn from Lieutenant Dawson, but I am not quite sure that an extreme range of 9 feet 8 inches has not been observed. Taking, however, the smaller number, it becomes clear that for a considerable part of the day, the reef would be exposed to the air. It is not likely that under these conditions the corals would continue to live, and I think, therefore, that the reef must have undergone some slight elevation, to the amount, perhaps, of 4 feet. This conclusion is in accordance with Dana’s view, and is supported by observations on some other features of the island, such, for example, as the occurrence of an interrupted line of low cliffs, sometimes passing into a series of pinnacles, generally about 4 feetiu height, as measured from low water level. In the annexed section the cliffs are farther from the land than is usually the case. These cliffs consist of a consolidated breccia of coral fragments, and are now in process of denuda­