252 CORAL AND ATOLLS 



the water. Eight reefs on this edge have already reached the 

 surface, and three bear islands upon them. 



Macclesfield Bank lies farther to the north ; its depth is 

 from 40 to 60 fathoms, and its edge is from 10 to 19 fathoms 

 below the waves. Prince Consort shoal, the Great Seychelles 

 Bank, and a great many others present a like condition, and are 

 in fact atolls below the waves, representing, on a grand scale, 

 the condition of the flat-topped Porites colonies. Other reefs 

 are representatives of the other stages which we have noted in 

 the development of these colonies ; some are dome -shaped, and 

 others are flat, the activity of the central colonies not having 

 diminished, and the rim being as yet undeveloped. 



It would seem to be possible that some dome-shaped reefs 

 might rise near to the surface of the sea as domes, just as we 

 have seen the Porites colonies do on the barrier ; their history 

 then becomes similar to that of these colonies, and when they 

 reach the waves their central portions will be less vigorous 

 than their outer margins. 



W"e have now arrived at the stage at which we may picture 

 a coral reef risen near to the surface of the sea ; its central 

 platform is depressed ; its colonies are less vigorous in their 

 growth, and sediment is being deposited upon its floor. The 

 changes have so far been brought about by the effects which 

 sedimentation produced naturally upon coral growth. No 

 other agency is involved in this explanation than the easily 

 verified reaction of the living coral colonies to the harmful 

 effects of particles deposited upon them. 



The development of the great edged reefs goes on unseen 

 below the surface of the sea, but the process may be realised 

 in watching the evolution of the flat-topped Porites colonies, 

 and the changes in the many smaller reefs to be found in the 

 lagoon. There are a hundred diminutive submerged atolls to 

 be seen any day in the coral- covered sedimentation banks of 

 the eastern side of the Cocos lagoon ; for every little mound 

 that rises near to the surface shows the condition that I have 

 imagined and described. 



