CORAL REEFS, AND ISLANDS. 



27* 



an encircled island, in fact, it is a real section, 

 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 architects having 



FIG. 55. 



Lower horizontal line showing sea level with islets on it. 

 Upper horizontal line, the sea level after further subsidence, 

 with the reef converted into an atoll. 



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. 



" Let us take our now 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 (fig. 55). As the barrier 

 reef slowly sinks down, the corals will go on vigorously 



T 2 



