31 
ATOLLS. 
Ch. I. 
Flinders states tliat the Australian barrier has a 
similar inclination inwards, and I have no doubt it is 
of general occurrence, although, according to Ehren- 
berg, the reefs of the Eed Sea offer an exception. 
Cliamisso observes that ‘ the red colour of the reef (at 
the Marshall atolls) under the breakers is caused by a 
Nullipora, which covers the stone wherever the waves 
beat ; and, under favourable circumstances, assumes a 
stalactitical form,’ — a description perfectly applicable 
to the margin of Keeling atoll . 1 Although Cliamisso 
does not state that the masses of Nullipor® form 
points or a mound, higher than the flat, yet I believe 
that this is the case ; for Kotzebue , 2 in another part, 
speaks of the rocks on the edge of the reef ‘ as visible 
for about two feet at low-water,’ and these rocks we 
may feel certain are not formed of true coral . 3 
1 Kotzebue’s First Voyage, vol. iii. p. 142. Near Porto Praya, in 
the Cape de Verde Islands, some basaltic rocks, lashed by no incon- 
siderable surf, were completely enveloped with a layer of Nullipora. 
The entire surface over many square inches, was coloured of a peach- 
blossom red ; the layer, however, was of no greater thickness than 
paper. Another kind, in the form of projecting knobs, grew in the 
same situation. These Nulliporaa are closely related to those de- 
scribed on the coral-reefs, but I believe are of different species. 
2 Kotzebue’s First Voyage, vol. ii. p. 16. Lieut. Nelson, in' his 
excellent memoir in the Geological Transactions (vol. ii. p. 105), 
alludes to the rocky points mentioned by Kotzebue, and infers that 
they consist of Serpulas, which compose incrusting masses on the 
reefs of Bermudas, as they likewise do on a sandstone-bar off the 
coast of Brazil, as described by me in the London Phil. Journal, 
Oct. 1841. I have added my description as a short supplement to 
the present volume. These masses of Serpul* hold the same posi- 
tion, relatively to the action of the sea, with the Nulliporse on the 
coral-reefs in the Indian and Pacific oceans. 
3 Capt. Moresby, in his valuable paper ‘ On the Northern Atolls 
