250 CORAL AND ATOLLS 



waves, greatly hastens the progress of this change ; for com- 

 paratively small rocks may here become flat-topped ; but even 

 when injury starts the process, the destructive action of 

 sedimentation completes it. Some importance is attached to 

 this method of the formation of flat-topped rocks, because 

 it is believed that the process helps in the understanding 

 of some other points in coral bionomics. 



In the course of a walk over the barrier flats, it may 

 easily be seen that flat-topped colonies do not assume an ab- 

 solutely uniform plane, as they should do if exposure caused 

 the phenomenon, and that dome-shaped colonies may very 

 often be found to be raised higher than flat colonies. Cases 

 that are particularly instructive are met with in the sharp- 

 edged pools common in some parts of the barrier flats. In 

 these the breccia layer will be found to cease suddenly, and 

 an overhanging ledge forms the margin of the pool, into which 

 the particles that are swept across the flats are always pouring. 

 They may be only three feet deep at low tide, and fifty yards 

 or more in length. Numerous colonies of corals will be found 

 in them, and all the colonies of Poritcs that are of any size 

 will be flat-topped, whilst bordering the edge of the breccia 

 rock, three feet above them, there is a flourishing growth of 

 Pavonia of the type P. repens, Brug. The Pavonia lives upon 

 the under side of the overhanging edge, and is free of the 

 sediment which sweeps over the flats to be deposited over the 

 lower area occupied by the Porites. 



Not only does the deposition of the sediment produce the 

 flattening of the tops of these Porites colonies, but it also 

 affects, in various ways, the growth-forms of other species 

 living in the pools ; but these other modifications do not con- 

 cern the present inquiry. 



Now a dome-shaped rock of Porites is a colony of a myriad 

 individuals, and it is not an illegitimate stretch of reasoning 

 to compare it to a reef that is composed of a myriad colonies. 



The individual colony, under the action of sedimentation, 

 has come to consist of a flat, sand-strewn area ringed around 

 by a raised vigorous margin of growing coral zooids ; it is 



