92 



LECTURE VII. 



and descending. Where the land that supports them is, on the con- 

 trary, in progress of submergence, they are compelled to build their 

 edifices progressively higher and in a narrower circuit ; in other words 

 the direction of their growth is centripetal and ascending. The 

 terms ascending and descending of course only here apply to the re- 

 lation of the coral-builders to the land, not to the level of the un- 

 changing sea. 



The formation of an atoll by the upward growth of the corals during 

 a gradual sinking of the land forming their supporting base is illus- 

 trated by these diagrams 

 48 from Mr. Darwin's work. 

 '-■'^j^iurit,' Figure 48. represents the 

 section of an island (a,b), 

 surrounded by a fringing 

 reef, r, rising to the surface of the sea, s. ]. As the land sinks 

 down, the living coral, bathed by the surf on the margin of the reef, 

 builds upwards to regain the surface. But the island becomes lower 

 and smaller, and the space between the edge of the reef, r, and the 

 beach proportionately broader. A section of the reef and island, after 



a subsidence of several 

 hundred feet, is given in 

 figure 49. The former 

 living margin of the reef, 

 r, is now dead coral, 

 dragged down to depths at which the polypes cease to exist ; but their 

 progeny continue in active life at r', now the margin of a barrier-reef, 

 separated by the lagoon channel n, from the remnant of the land b. 

 Let the island go on subsiding, and the coral reef will continue 

 growing up on its own foundation, whilst the water gains on the land, 



until the highest point is 

 covered, and there re- 

 mains a perfect atoll, of 

 which figure 50. repre- 

 sents a vertical section. 

 In this diagram r" is the 

 living and growing outer margin of the encircling reef, and the la- 

 goon channel is now converted into the calm central lake n, of the 

 atoll. Thus by the process of subsidence the fringing reef (fff. 48.) 

 is converted into the barrier reef (Jiff. 49.), and this into the atoll 

 (Jiff. 50.). 



If the movement of the land should now be reversed, and the 

 level of the sea be again brought back by elevation of the island, to 

 the line (s. 1, ^ff. 50.), an island apparently composed exclusively 

 of coral rock, like Elizabeth Island, would be the result. 



