Sect. II. 
ATOLLS. 
35 
Whether a smooth convex mound of Nulliporse, like 
that which appears as if artificially constructed to 
protect the margin of Keeling Island, is of frequent 
occurrence round atolls, I know not ; but we shall 
presently meet with it under precisely the same form, 
on the outer edge of the ‘ barrier reefs ’ which en- 
circle the Society Islands. 
There appears to be scarcely a feature in the 
structure of Keeling reef, which is not of common, if 
not of universal occurrence, in other atolls. Thus 
Chamisso describes 1 a layer of coarse conglomerate, 
outside the islets round the Marshall atolls, which 
‘ appears on its upper surface uneven and eaten 
away.’ From drawings with appended remarks, of 
Diego Garcia in the Chagos group and of several of 
the Mai diva atolls, shown me by Captain Moresby, 2 it 
is evident that their outer coasts are subject to the 
same round of decay and renovation as those of 
Keeling atoll. From the description of the atolls in 
the Low Archipelago, given in Captain Beechoy's 
Voyage, it is not apparent that any conglomerate 
coral-rock was there observed. 
The lagoon in Keeling atoll is shallow : in the 
atolls of the Low Archipelago the depth varies from 20 
to 38 fathoms, and in the Marshall Group, according 
to Chamisso, from 30 to 35 : in the Caroline atolls it 
of Maldivas ’ (Geographical Journal, vol. v.), says that the edges of 
the reefs there stand above water at low spring tides. 
1 Kotzebue’s First Voyage, vol. iii. p. 144. 
2 See also Moresby on the Northern Atolls of the Maldivas, Geo- 
graphical Journal, vol. v. p. 400. 
