THE VOYAGE OF THE BEAGLE 499 



and let this island with its reefs, 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 

 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 becoming lower and smaller, and the 

 space between the inner edge of the reef and the beach pro- 

 portionately 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 accumu- 

 lated 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 ha-ve been feet of 

 subsidence, that small limit of depth 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 con- 

 tinent fringed with reefs, and had imagined it to have sub- 

 sided, a great straight barrier, like that of Australia or New 

 Caledonia, separated from the land by a wide and deep chan- 

 nel, 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 



