STRUCTURE OF CORAL ISLANDS. 63 
The dagoons of the smaller islands are usually very shallow; and 
in some, merely a dry bed remains, indicating the former existence of 
water. Instances of the latter kind are met with only in islands less 
than three miles in diameter; and those with shallow lagoons are 
seldom much larger. These shallow waters, when direct communica- 
tion with the sea is cut off, become, in some instances, very salt by 
evaporation, and contain no growing coral, with few signs of life of 
any kind: and in other cases, they are made too fresh for marine 
life, through the rains. At Enderby’s Island the water was not only 
extremely saline, but the shores of the lagoon were in some places 
incrusted with salt. But when there is an open channel, or the tides 
gain access over a bare reef, corals continue to grow, and a considera- 
ble portion of the lagoon may be obstructed by them. At Henuake, 
the sea is shut out except at high water, and there were consequently 
but few species of corals, and those of small size. At Ali (Peacock’s 
Island) there was a small entrance to the lagoon, and though com- 
paratively shallow, corals were growing over a large part of it. 
In the larger islands, the lagoons contain but small reefs compared 
with their whole extent; the greater part is an open sea, with deep 
waters and a sandy bottom. ‘There are instances, as at the southern 
Maldives, of a depth of 50 and 60 fathoms. Twenty to thirty-five 
fathoms is the usual depth in the Paumotus. This was the result 
of Captain Beechey’s investigations; and those of the Expedition, 
though few, correspond. It is however probable that deeper soundings 
would be found in the large island of Nairsa (Dean’s). In the 
Tarawan Group, southeast of the Carolines, the depth, where exa- 
mined by the Expedition, varied from 2 to 35 fathoms. Mr. Darwin 
found the latter depth at Keeling’s Island. Chamisso found 25 to 35 
fathoms at the Marshall Islands. 
The bottom of these large lagoons is very nearly uniform, varying 
but little, except from the occasional abrupt shallowings produced 
by growing patches of reef. Soundings bring up sand, pebbles, shells, 
and coral mud; and the last-mentioned material appears to be quite 
common, even in lagoons of considerable size. It has the same cha- 
racter as above described. ‘The bluish clay-like mud of the harbour 
of 'Tongatabu may be classed with these deposits.* It appears, there- 
fore, that the finer coral material of the shores prevails throughout 
* Darwin describes this mud as occurring at the Maldives, and at Keeling Island, (op. 
cit, p. 26;) Kotzebue mentions it as common at the Marshall Atolls, and Lieutenant 
Nelson observed it at the Bermudas. 
