CORAL REEFS 99 



in island groups where fringing and barrier reefs are 

 abundant. Again, as a fringing reef broadens, neither 

 the reef flat nor the boulder zone increase greatly 



Fig. 31. — Sectional Diagram to illustrate Formation or 

 an Atoll on a Mound of the Sea-Floor. 



, original mound ; B^-B 4 , building up of same by remains of deep- 

 sea animals, by pelagic deposits, and, lastly, by reef organisms 

 to the sea-level ; O-C 3 , outward extension by accumulation of 

 talus and other materials on the slopes and further growth of 

 reef organisms ; hollowing out of the surface by solution and 

 removal of mud in suspension. 



Fig. 32. — Sectional Diagram to illustrate Formation of an 

 Atoll on the Site of an Island of Loose Material which 

 has been partly washed away : such an island as is 

 formed by a submarine eruption. 



A, Original island ; B, bank after the top of the island has been 

 washed away to 20-50 fathoms ; C, atoll formed by the direct 

 and greater upgrowth of reef organisms on the edge of the bank. 



in breadth, the inner part (lagoon or shore flat) 

 broadening and taking up the extra breadth. 

 "jThe shore or lagoon flat is obviously formed where 

 once was boulder zone, and it should therefore be 

 strewn with boulders^ On a broad fringing reef such 



