Sect. I. 
KEELING ATOLL. 
15 
lagoon through the main entrance. The most frequent 
coral in the hollows on the reef is Pocillopora ver - 
rucosa, which grows in short sinuous plates, or branches, 
and when alive is of a beautiful pale lake-red : a Mad- 
repora, closely allied or identical with M. pocillifera, is 
also common. As soon as an islet is formed, and the 
waves are prevented from breaking entirely over the 
reef, the channels and hollows become filled up with 
fragments cemented together by calcareous matter; and 
the surface of the reef is converted into a hard smooth 
floor ( C of wood-cut), like an artificial one of free- 
stone. This flat surface varies in width from 100 to 
200, or even 300 yards, and is strewed with a few large 
fragments of coral torn up during gales : it is uncovered 
only at low w T ater. I could with difficulty, and only 
by the aid of a chisel procure chips of rock from 
its surface, and therefore could not ascertain how 
much of it is formed by the aggregation of detritus, 
and how much by the outward growth of mounds of 
corals, similar to those now living on the margin. No- 
thing can be more singular than the appearance at low 
tide of this ‘ flat ’ of naked stone, especially where it is 
externally bounded by the smooth convex mound of 
Nulliporae, appearing like a breakwater built to resist 
the waves, which are constantly throwing over it sheets 
of foaming water. The characteristic appearance of 
this ‘ flat ’ is shown in the foregoing wood-cut of Whit- 
sunday Atoll. 
The islets on the reef are first formed between 200 
and 300 yards from its outer edge, through the accu- 
