XX 
THEORY OF CORAL-REEFS 
503 
channel. This channel will be more or less deep, according to 
the rate of 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 resembles 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 
A'A', Outer edges of the barrier-reef at the level of the sea, with islets on it. B'B', The shores 
of the included island. CC, The lagoon-channel. 
A"A", Outer edges of the reef, now converted into an atoll. C', The lagoon of the new atoll. 
N.B .—According to the true scale, the depths of the lagoon-channel and lagoon are much 
exaggerated. 
SECTION OF CORAL-REEF. 
at which the effective corals can live:—the little architects 
having built up their great wall-like mass, as the whole sank 
down, upon a basis formed of other corals and their consolidated 
fragments. Thus the difficulty on this head, which appeared 
so great, disappears. 
If, instead of an island, we had taken the shore of a 
continent fringed with reefs, and had imagined it to have 
subsided, a great straight barrier, like that of Australia or 
New Caledonia, separated from the land by a wide and deep 
channel, would evidently have been the result. 
Let us take our new encircling barrier-reef, of which the 
section is now represented by unbroken lines, and which, as I 
have said, is a real section through Bolabola, and let it go on 
subsiding. As the barrier-reef slowly sinks down, the corals 
