94 CORAL AND ATOLLS 



The type of vegetative growth is affected mostly by the 

 physical conditions of the water in which it lives, but the actual 

 structure of the coral depends greatly upon the presence, or 

 absence, of sediment. Some very strange results are pro- 

 duced by Avaters in which sediment is held suspended, and 

 from which it is being deposited. Sediment will alter the 

 appearance of a coral more strikingly than any other influence. 

 The deposition of sediment is the greatest agent in causing 

 coral death : corals are very easily killed by even compara- 

 tively little sediment, and are profoundly altered by it, if they 

 are to successfully resist its influence. 



The extent of silt formation at the surface of an atoll in 

 mid-ocean is hard to imagine, and it has certainly not been 

 appreciated by those experimenters who have attempted to 

 estimate the age of an atoll by catching the silt in a net as it 

 passes into the lagoon. Adown the submarine slopes of the 

 atoll, for a hundred miles east and west, the bottom was found 

 by the cable soundings to be strewn with fragments of coral 

 and finely triturated coral- sand, and it is only an uncertain and 

 inconstant fraction which passes into the lagoon. Silt is one of 

 the shifting influences of the atoll, and so may visit the coral 

 colonies for only a brief portion of their lives, and then partial 

 death and strange repair-growths result. In the lagoon, and 

 in some portions of the barrier pools, silt is a constant factor, 

 and here shows to the greatest advantage the modifications 

 which it is capable of producing in coral growths. Speaking 

 generally, silt alters the vegetative growth-form of colonies 

 only in as far as it produces flat-topped rock masses by killing 

 the uppermost zooids, and causes amorphous and irregular 

 growths by partly killing the uppermost growing zooids of the 

 growing points. But in the surface structure of the coral it 

 produces great and wonderful changes. Its eff'ects are best 

 studied by comparing the upper and lower surfaces of partial 

 plates. In these plates, the upper flat surface is alone exposed 

 to the action of the deposition of sediment, and here the corallites 

 tend to be small, and to be raised from the general surface, 

 and the intervening spaces themselves tend to be sculptured 



