vertical section is given in Figure 2, in which the vertical scale has been
necessarily exaggerated. Its longest axis is ninety nautical miles, and
another line drawn at right angles to the first, across the broadest part,
is seventy. The central part consists of a level muddy flat, between forty
and fifty fathoms deep, which is surrounded on all sides, with the
exception of some breaches, by the steep edges of a set of banks, rudely
arranged in a circle. These banks consist of sand, with a very little live
coral; they vary in breadth from five to twelve miles, and on an average
lie about sixteen fathoms beneath the surface; they are bordered by the
steep edges of a third narrow and upper bank, which forms the rim to the
whole. This rim is about a mile in width, and with the exception of two or
three spots where islets have been formed, is submerged between five and
ten fathoms. It consists of smooth hard rock, covered with a thin layer of
sand, but with scarcely any live coral; it is steep on both sides, and
outwards slopes abruptly into unfathomable depths. At the distance of less
than half a mile from one part, no bottom was found with 190 fathoms; and
off another point, at a somewhat greater distance, there was none with 210
fathoms. Small steep-sided banks or knolls, covered with luxuriantly
growing coral, rise from the interior expanse to the same level with the
external rim, which, as we have seen, is formed only of dead rock. It is
impossible to look at the plan (Figure 1, Plate II.), although reduced to
so small a scale, without at once perceiving that the Great Chagos Bank is,
in the words of Captain Moresby (This officer has had the kindness to lend
me an excellent MS. account of the Chagos Islands; from this paper, from
the published charts, and from verbal information communicated to me by
Captain Moresby, the above account of the Great Chagos Bank is taken.),
"nothing more than a half-drowned atoll." But of what great dimensions,
and of how extraordinary an internal structure? We shall hereafter have to
consider both the cause of its submerged condition, a state common to other
banks in the group, and the origin of the singular submarine terraces,