CORAL-REEFS, 



127 



thus been led, solve the curious problem, — what has given 

 to each class of reef its peculiar form ? 



Let us in imagination place within one of the subsiding 

 areas, an island surrounded by a " fringing-reef," — that 

 kind, which alone offers no difficulty in the explanation 

 of its origin. Let the unbroken lines and the oblique 

 shading in the woodcut (No. 4) represent a vertical section 

 through such an island; and the horizontal shading will 



AA — Outer edge of the reef at the level of the sea. 



BB — Shores of the island. 



AA' — Outer edge of the reef, after its upward growth during a period of 

 subsidence. 



CC — The lagoon-channel between the reef and the shores of the now encircled 

 land. 



B'B' — The shores of the encircled island. 



N.B. — In this, and the following woodcut, the subsidence of the land could 

 only be represented by an apparent rise in the level of the sea. 



represent the section of the reef. Now, as the island sinks 

 down, either a few feet at a time or quite insensibly, we 

 may safely infer from what we know 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 edge of the reef 

 and the beach proportionately 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 



