Sect. II. 27 



SECTION SECOND. 



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

 Zone of Nidhporce — 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, compar- 

 ing them with Keeling atoll. The Maldiva atolls and 

 the Grreat 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. 4, Plate I.), 

 and Clermont Tonnere has nearly the same proportions. 

 In the Marshall Archipelago (the Ealick and Radack 

 group of Kotzebue) several of the atolls are more than 

 thirty miles in length, and Kimsky Korsacoff is fifty- 

 four long, and twenty wide at the broadest part of its 

 irregular outline. Most of the atolls in the Maldiva 

 Archipelago are of great size, one of them (which, how- 



