1836.] THEORY OF CORAL-REKFS, 473 
subside. Now as the island sinks down, either a few feet at 
a time or quite insensibly, we may safely infer, from what 
AA. Outer edges of the fringing-reef, at the level of the sea. BB, The shores of the 
fringed island. 
A’ 
‘, Outer edges of the reef, after ita upward poh during a period of subsidence, 
now converted into a barrier, with islets on it. B/B’. The shores of the now encircled 
island. CC. Lagoon-channel. 
N.B. In this and the following woodcut, the subsidence of the 1and could be repre 
sented only by an apparent rise in the level of the sea. 
is known of the conditions favourable to the growth of coral, 
that the living masses, bathed by the surf on the margin of 
the reef, will soon regain the surface. The water, however, 
will encroach little’ by little on the shore, the island becom- 
ing lower and smaller, and the space between the inner edge 
of the reef and the beach proportionally broader. A section 
of the reef and island in this state, after a subsidence of several 
hundred feet, is given by the dotted lines. Coral islets are sup- 
posed to have been formed on the reef; and a ship is anchored 
in the lagoon-channel. This channel will be more or less deep, 
according to the rate ef subsidence, to the amount of sediment 
accumulated in it, and to the growth of the delicately branched 
corals which can live there. The section in this state resem- 
bles in every respect one drawn through an encircled island: in 
fact, it is a real section (on the scale of 517 of an inch to a mile) 
through Bolabola in the Pacific. "We can now at once see why 
encircling barrier-reefs stand so far from the shores which they 
front. We can also perceive, that a line drawn perpendicularly 
down from the outer edge of the new reef, to the foundation of 
solid rock beneath the old fringing-reef, will exceed by as many 
feet as there have been feet of subsidence, that small limit of 
depth at which the effective corals can live:—the little archi- 
tects having built up their great wall-like mass, as:the whole 
sank down, upon a basis formed of other corals and their. conso- 
