108 CORAL AND ATOLLS 



swelling lips about the plateau, and make cushion-like bosses 

 which tend to enclose a central flat depression in which sand 

 accumulates, and on which other and differently growing 

 species of coral may lodge and flourish. 



This process may be described as the normal accident of 

 the life-history of those species in which the equality of the 



Fig 



Aditlt Colony of Porltcs in which tpie Upper Zooms are 



KILLED BY SEDIMENT. 



zooids of every portion of the colony is a life-condition. It 

 furnishes a good example of the rule of Nature's utter dis- 

 regard for the life of the individual, for all those zooids on 

 the upper surface must certainly be killed by sedimentation 

 or by injury, when they have succeeded in making their 

 colony of sufficient size. 



The existence of these flat-topped masses of Porites has 

 been noted b}'- practically every observer who has visited reefs 

 of living corals, and various explanations have been given for 

 their origin. I have come to regard these rocks as beinof of 



o o o 



the utmost importance in the study of the question of the 

 development of atolls, and the deposition of sediment upon a 

 surface flat enough to allow of its resting I imagine to be the 

 active agent in their causation. 



I regard the finished colony of Porites as an atoll reef in 

 miniature^ — the central dead area representing the lagoon and 

 the raised margin representing the encircling reef; and the- 



