676 BR. F. WOOD JONES ON A [June 15, 



is entirely a product of the lagoon, and so is tlie lagoon con- 

 glomerate ; and both of these substances depend for their construc- 

 tion on the deposition of calcium carbonate around particles held in 

 close contact. Again, fragments of dead colonies that are trundled 

 up and down the lagoon shores by every tide, are commonly made 

 more hard and more heavy by a rich deposit of calcium carbonate 

 in the interstices of their structure. Finally, if solution of 

 calcium carbonate is taking place within the confines of the lagoon, 

 its action must be a feeble one, for in the Cocos lagoon are wide 

 areas covered by dead coral colonies, killed, as we definitely know, 

 in 1876 ; and these dead masses have resisted solution during the 

 past 30 years. 



(iii.) Suggestions 2^ut forward hy the Author to explain the 

 development of Coral structures. 



As an outcome of observations made on the Cocos-Keeling atoll, 

 it is suggested that the process of " Sedimentation " takes the 

 largest share in the production of most of the stages of an atoll's 

 history. The bed of the open ocean is composed of matter that 

 has fallen from the sui'face ; sedimentation is always taking place 

 all over the ocean. In certain places, sometimes owing perhaps to 

 the influence of oceanic cu~rrents, 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, 

 we do not know. Whatever their original nature they become 

 essentially " Sedimentation " 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 con- 

 fidently expected, for waves vary enormously in their size and in 

 their power to stir the underlying water. Yet we 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 below 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 sediment- 

 ation that banks foi-med by sediment may be raised. A bank 

 so raised would rise to such a plane, but could not go beyond it, 

 for the v/ave 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 



