36 CORAL-REEFS. 



that some years before our visit unusually heavy rain killed 

 nearly all the fish in the lagoon, and probably the same 

 cause would likewise injure the corals. The reefs also, it 

 must be remembered, cannot possibly rise above the level 

 of the lowest spring-tide, so that the final conversion of the 

 lagoon into land must be due to the accumulation of 

 sediment ; and in the midst of the clear water of the ocean, 

 and with no surrounding high land, this process must be 

 exceedingly slow. 



Section Second. 



General form and size of atolls, their reefs and islets. — External slope. — 

 Zone ef Nullipara. — Conglomerate. — Depth of lagoons. — Sediment. 

 — Reefs submerged wholly or in part. — Breaches in the reef. — 

 Ledge-formed shores round certain lagoons. — Conversion of lagoons 

 into land. 



I will here give a sketch of the general form and structure 

 of the many atolls and atoll-formed reefs which occur in the 

 Pacific and Indian Oceans, comparing them with Keeling 

 atoll. The Maldiva atoll and the Great Chagos Bank 

 differ in so many respects, that I shall devote to them, 

 besides occasional references, a third section of this 

 chapter. Keeling atoll may be considered as of moderate 

 dimensions and of regular form. Of the thirty-two islands 

 surveyed by Capt. Beechey in the Low Archipelago, the 

 longest was found to be thirty miles, and the shortest less 

 than a mile ; but Vliegen atoll, situated in another part of 

 the same group, appears to be sixty miles long and twenty 

 broad. Most of the atolls in this group are of an elongated 

 form ; thus Bow Island is thirty miles in length, and on an 

 average only six in width (see Fig. 5, Plate III.), and 

 Clermont Tonnere has nearly the same proportions. In 

 the Marshall Archipelago (the Ralick and Radack group of 



