150 CORAL-REEFS. 



from what follows in the next chapter, when the area, in which they 

 were situated, commenced to rise, or at least ceased to subside. 

 During the enormous period necessary to effect an elevation of the 

 kind just alluded to, the surface would necessarily be denuded to a 

 great thickness ; hence it is highly improbable that any fringing-reef, 

 or even any barrier-reef, at least of those encircling small islands, 

 would be preserved. From this same cause, the strata which were 

 formed within the lagoons of atolls and lagoon-channels of barrier- 

 reefs, and which must consist in a large part of sedimentary matter, 

 would more often be preserved to future ages, than the exterior solid 

 reef, composed of massive corals in an upright position ; although it is 

 on this exterior part that the present existence and further growth of 

 atolls and barrier-reefs entirely depend. 



