CHAPTER Y. 
THEORY OF THE FORMATION OF THE DIFFERENT CLASSES 
OF CORAL-REEFS. 
The atolls of the larger archipelagoes not formed on submerged craters, 
or on banks of sediment - Immense areas interspersed with atolls 
— Tlieir subsidence — The effects of storms and earthquakes on 
atolls — Recent changes in their state — The origin of barrier-reefs 
and of atolls — Their relative forms — The step-formed ledges and 
walls round the shores of some lagoons — The ring -formed reefs of 
the Maldiva atolls — The submerged condition of parts or of the 
whole of some annular reefs — The disseverment of large atolls — ■ 
The union of atolls by linear reefs — The great Chagos Bank — - 
Objections considered arising from the area and amount of sub- 
sidence required by the theory — The probable composition of the 
lower parts of atolls. 
The naturalists who have visited the Pacific, seem to 
have had their attention riveted by the lagoon-islands 
or atolls, — those singular rings of coral-land which 
rise abruptly out of the unfathomable ocean, — and 
have passed over, almost unnoticed, the scarcely less 
wonderful encircling barrier-reefs. The theory most 
generally received on the formation of atolls, is that 
they are based on submarine craters : but where can 
we find a crater of the shape of Bow atoll, which is five 
times as long as it is broad (Plate I., fig. 4) ; or like 
that of Menchicoff Island (Plate II., fig. 3), with its 
