454 



THEORY OF CORAL-REEFS. 



[CHAP. xx. 



vast areas, interspersed with low islands, of which not one rises 

 above the height to which the wind and waves can throw up 

 matter and yet are constructed by animals requiring a foundation, 

 and that foundation to lie at no great depth. Let us then take an 

 island surrounded by fringing-reefs, which offer no difficulty in 

 their structure; and let this island with its reef, represented by 

 the unbroken lines in the woodcut, slowly subside. Now, as the 

 island sinks down, either a few feet at a time or quite insensibly, 

 we may safely infer, from what is known of the conditions favour- 



' 



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

 fringed island. 



A'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 now encircled 

 island. CO. Lagoon-channel. 



N.B. 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. 



able 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 becoming 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 supposed 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 

 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 perpendicu- 

 larly 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 



