DEATH-PROCESSES OF CORAL COLONY 129 



ever keeping the coral colonies within the limit of upward 

 growth ; but their action is not altogether detrimental to the 

 corals, for though, where island beaches are formed, many 

 fragments of living coral are cast ashore only to perish, still 

 many more, where no such dry land exists, are safely lodged 

 in a new resting-place. Broken fragments are swept across 

 the flats, they lodge in pools, they become stranded under the 

 lee of boulders, or are washed into the lagoon ; and each of 

 these fragments, if not too badly damaged, will form the 

 nucleus of a new colony in a suitable habitat. If a large 

 colony of Madrepora be broken up in the rock pool where it 

 flourishes, the great majority of its fragments will continue to 

 grow and branch out into new colonies ; and if some of these 

 fragments are swept onwards by the waves, they will form 

 pioneers for the species when lodged in a suitable environment. 



Freshening of the water from the excessive tropical rains 

 has been said to cause wholesale death among the lagoon 

 corals ; and in high islands where rivers flow into the sea, the 

 fresh water is well known to be a great cause of the absence 

 of coral growth. Before Darwin's visit to the atoll it is said 

 that an abnormal rainfall killed many corals, and again in 

 1866 the fresh water is said by the Governor to have stood 

 for a height of several inches on the surface of the lagoon, so 

 heavy and continuous was the rain. Again, in May 1896, the 

 rains were abnormal, and the freshening of the water destroyed 

 the lagoon algae and fish ; and this in such quantities that 

 when Mr. Arthur Keyser visited the islands in July, the dead 

 fish were still being cleared from the lagoon. There is no 

 doubt that the rain would have to be long continued, for all 

 those corals that live in rock pools are immune to the 

 influence of the fresh water accumulated during a heavy 

 downpour at low tide. When the tide is low and the rainfall 

 is heavy, the rock pools undergo a remarkable degree of 

 freshening, and so too does the lagoon if the weather is calm 

 and the rainfall is a sudden one. 



In the lagoon the surface specific gravity may fall to 1021, 

 but I have never found it lower, and the perpetual churning 



