401 



perhaps to the influence of oceanic currents, sometimes to the presence 

 of an already existing elevation upon the ocean bottom, this sediment 

 will tend to make ridges or banks. Many such banks are known to exist 

 in the depths of the sea. 



What may be the nature of the original elevation that has become 

 covered by this deposit of Globigerina and Pteropod ooze, Ave do not 

 know. Whatever their original nature they become essentially "Sedimen- 

 tation" banks. 



The question then arises as to where beneath the surface of the sea 

 will the building of banks by sedimentation become arrested. The answer 

 may be partly given by determining where wave action ceases to be felt 

 below the surface of the sea, and the data to be derived from published 

 observations on this point show the level to be somewhat inconstant. 

 Its variability would be confidently expected, for waves vary enormously 

 in their size and in their power to stir the underlying water. Yet Ave 

 know that there is some point between the surface of the ocean and the 

 bottom, above which the action of waves is felt and sediment will not 

 come to rest in open ocean , and beloAV which there is no wave stirring 

 and sediment may rest and build banks and raise the ocean bottom. 

 This point is considered important; and the plane in which this line of 

 stasis occurs is named the limiting line of sedimentation. It is 

 therefore to the limiting line of sedimentation that banks formed 

 by sediment may be raised. A bank so raised would rise to such a 

 plane, but could not go beyond it, for the waA^e motion would keep the 

 particles moving, and thus level out the top of the bank and flatten it, 

 so that it formed a plateau at the level of the limiting line of 

 sedimentation. It is claimed that the bathymetrical limit of the reef- 

 building corals is intimately associated, if not coincident, Avith the 

 limiting line of sedimentation, and that it is therefore a variable 

 plane depending on the local conditions of the sea. The reasons for 

 this coincidence are to be found in the study of the living corals them- 

 selves; and I have come to the conclusion that the presence of matter 

 suspended in the water is the most potent factor in determining the 

 unsuitability of an environment for coral life. Where sediment is at all 

 times liable to fall upon the living zooids, reef-corals will not flourish: 

 we Avould therefore not look for their luxuriant presence below the 

 limiting line of sedimentation. In the wave-stirred area above this 

 line, however, they can and do flourish. We therefore arrive at the 

 presumption that sediment can build banks up to this hypothetical line, 

 and reef-corals can build banks from this line up to the surface of the 

 sea. There is therefore no reason why coral colonies should not settle 

 upon the bank and start the development of a reef. As a matter of fact 



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