STRUCTURE OF CORAL ISLANDS. 67 
The Chagos Bank lies about ten degrees south of the Maldives, and 
is ninety miles long and seventy broad. The rim is mostly submerged 
from five to ten fathoms. 
Mr. Darwin confirms the opinion of Captain Moresby, that this 
bank has the character of a lagoon reef, resembling one of the Mal- 
dives; and he states on the evidence of extensive soundings, that, if 
raised to the surface, it would actually become a coral island, with a 
lagoon forty fathoms deep. In the words of Captain Moresby, it is in 
truth nothing more than a half-drowned atoll.* 
Metia, and other elevated coral islands—In the Chagos Group we 
have an example of a sunken coral atoll. Metia affords an instance of 
one that has been elevated by some force; and several such are met 
with in the Pacific. Metia, or Aurora Island, is one of the western 
Paumotus. It is a small island, about four miles by two and a half 
in width, and two hundred and fifty feet in height; and it consists 
throughout of coral limestone. As we approached it from the northeast, 
its high vertical cliffs were supposed to be basaltic, and had much 
resemblance to the Palisades of the Hudson.t ‘This appearance of a 
vertical structure was afterwards traced to vertical furrowings by the 
waters dripping down its front, in connexion with stalagmitic incrusta- 
tions. Deep caverns were also seen. 
The cliff, though vertical in some parts, is roughly sloping in others, 
and on the west side the surface of the island gradually declines to 
the sea. 
The rock was found to be a white and solid limestone, seldom pre- 
senting any traces of its coral origin. In some layers there were 
disseminated corals, looking like imbedded fossils, along with some 
beautiful casts of shells; but for the most part it was as compact as 
any secondary marble, and as uniform in texture. Occasionally there 
were disseminated spots of crystallized calc spar. 
The caverns presented us with coarse stalactites, some of which 
were six feet in diameter; and handsome specimens were obtained, 
containing recent land shells, which had been enclosed while hiber- 
nating. { 
theory of their formation, as we shall hereafter see, is included in that of the parent 
atolls, of which they form the separate portions.” 
* Darwin, op. cit. p. 39. 
+ For a sketch of this island, see Narrative Exp. Exp., vol. i., p. 338, 
+ It is probable that more extensive caverns would have been found, had there been 
more than a few hours for the examination of the island. ‘The Rev. Mr. Williams, in 
