Sect. II. 
ATOLLS. 
27 
as impure water and loss of food. For instance, Mr. 
Liesk informed me, 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 remem- 
bered, 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 of Nulliporce — Conglomerate — Depth of lagoons — 
Sediment — Beefs submerged wholly or in part — Breaches in the 
reef — Ledge-formed shores round certain lagoons — Conversion of 
lagoons into lai d. 
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 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 *tnay 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 
