was first led by it to observe the peculiarity in the question. At Peros
Banhos the submerged part is nine miles in length, and lies at an average
depth of about five fathoms; its surface is nearly level, and consists of
hard stone, with a thin covering of loose sand. There is scarcely any
living coral on it, even on the outer margin, as I have been particularly
assured by Captain Moresby; it is, in fact, a wall of dead coral-rock,
having the same width and transverse section with the reef in its ordinary
state, of which it is a continuous portion. The living and perfect parts
terminate abruptly, and abut on the submerged portions, in the same manner
as on the sides of an ordinary passage through the reef. The reef to
leeward in other cases is nearly or quite obliterated, and one side of the
lagoon is left open; for instance, at Oulleay (Caroline Archipelago), where
a crescent-formed reef is fronted by an irregular bank, on which the other
half of the annular reef probably once stood. At Namonouito, in the same
Archipelago, both these modifications of the reef concur; it consists of a
great flat bank, with from twenty to twenty-five fathoms water on it; for a
length of more than forty miles on its southern side it is open and without
any reef, whilst on the other sides it is bounded by a reef, in parts
rising to the surface and perfectly characterised, in parts lying some
fathoms submerged. In the Chagos group there are annular reefs, entirely
submerged, which have the same structure as the submerged and defined
portions just described. The Speaker's Bank offers an excellent example of
this structure; its central expanse, which is about twenty-two fathoms
deep, is twenty-four miles across; the external rim is of the usual width
of annular reefs, and is well-defined; it lies between six and eight
fathoms beneath the surface, and at the same depth there are scattered
knolls in the lagoon. Captain Moresby believes the rim consists of dead
rock, thinly covered with sand, and he is certain this is the case with the
external rim of the Great Chagos Bank, which is also essentially a
submerged atoll. In both these cases, as in the submerged portion of the