DEVELOPMENT OF THE REEF 247 



continental land to a depth of 150 fathoms by currents in 

 mid-ocean (Gardiner) requires a greater measure of proof than 

 has been adduced. These may be the sites of coral reefs, 

 but in mid-ocean it would seem probable that the banks upon 

 which reefs and atolls are formed are essentially sedimentation 

 structures ; and those which do not actually start as purely 

 sedimentation banks ultimately develop the characters of such 

 foundations. 



The Aftee- Development of the Reef.— We have there- 

 fore arrived at the stage in which a bank has become clothed 

 with a growth of the true reef-building corals. The whole 

 patch has risen towards the surface, and consists, in all 

 probability, of a scattered series of colonies, lying in irregular 

 confusion about the top of the bank. This is of course a well- 

 known stage, and great numbers of such reefs are present in 

 seas that are favourable to coral growth. 



It remains now to follow the progress of this coral reef 

 between the Limiting Line of Sedimentation and the surface 

 of the sea; and to see what factors will bring about the 

 changes that are known to be developed in this process. It 

 is here maintained that in this stage again the action of 

 sedimentation plays the leading part in the history of the reef. 

 In order to study the action of sedimentation upon reefs, 

 some idea of its workings may be gained by following the 

 process as it affects colonies. This question will first be 

 dealt with. 



There is a very common condition of certain coral 

 colonies which has received special attention in the works 

 of both Darwin and Semper, namely, the state of the 

 flat- topped rocks formed by the old colonies of Pontes. 

 As actually seen upon the reef, the colonies consist of 

 great boulders of rock — for it is the large colonies that best 

 exhibit this form — the tops of which are flat, or even some- 

 what hollowed ; the flat surface is destitute of living zooids, 

 but it is ringed about by a raised, rounded edge of healthy 

 members of the colony. The dead flat surface of the 

 rock is generally occupied by a layer of sand, and, in the 



