] 836.] 



THEORY OF CORAL-REEFS. 



473 



subside. Now as the island sinks down, either a few feet at 

 a time or quite insensibly, we may safely infer, from what 



^#^**'""" 



A A. Outer edges of the fringing-reef, at the level of the sea. BB. The shores of the 

 frinjjed island. 



.\'A'. Outer edges of the reef, after its upward growth during a period of subsidence, 

 now converted into a barrier, with islets on it. B'B'. The shores of the no.v encircled 

 island. CC. liagoon-cliannel. 



NB. In this and the following woodcut, the subsidence of the land 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 lov/er 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 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 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- 



