to investigate the Structure of a Coral Reef by Boring. .511 



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 HeUopora 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 air ; on Funafuti they 

 require constant submergence, and we are thus led to regard their 

 upper surface as marking what 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 admi- 

 rably 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 sea- 

 wards. 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 neces- 

 sary 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 accord- 

 ingly 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 

 ot' the day, the reef woulcb 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 occur- 

 rence of an interrupted line of low cliffs, sometimes passing into a 

 series of pinnacles, generally about 4 feet in 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 consoli- 

 dated breccia of coral fragments, and are now in process of denuda- 



