bounded by the dotted lines lie from fifteen to twenty fathoms beneath the
surface, and are formed of sand; the central space is of mud, and from
thirty to fifty fathoms deep.

FIGURE 2.--A vertical section, on the same scale, in an eastern and western
line across the Great Chagos Bank, given for the sake of exhibiting more
clearly its structure.

FIGURE 3.--MENCHIKOFF ATOLL (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern's "Atlas of the
Pacific;" originally surveyed by Captain Hagemeister; the depth within the
lagoons is unknown.

FIGURE 4.--MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
Powell; the white spaces in the middle of the separate small reefs, both on
the margin and in the middle part, are meant to represent little lagoons;
but it was found not possible to distinguish them clearly from the small
islets, which have been formed on these same small reefs; many of the
smaller reefs could not be introduced; the nautical mark (dot over a dash)
over the figures 250 and 200, between Mahlos Mahdoo and Horsburgh atoll and
Powell's island, signifies that soundings were not obtained at these
depths.

FIGURE 5.--NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern's "Atlas," compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the "Atlas of
the Voyage of the 'Astrolabe'." In Krusenstern's "Atlas," the reef is
represented by a single line with crosses; I have for the sake of
uniformity added an interior line.