142 



LECTURE Vn. 



above their element, are compelled to carry on their operations more 

 and more remote from the former theatres of their constructive 

 energies, but cannot extend deeper than their allotted thii-ty fathoms ; 

 the direction of their submarine masonry is centrifugal and descend- 

 ing. Where the land that supports them is, on the contrary, 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 relation 

 of the coral-builders to the unstable 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 



illustrated by these dia- 

 68 



grams from Mr. Dar- 

 win's work.* Figure 68. 

 represents the section of 

 an island («, b), sur- 



Barrier Reef. 



Fringing Reef. 



rounded by a fringing reef, r, rising to the surface of the sea, s. 1. 

 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 69. 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 /, now the 

 margin of a barrier reef, separated by the lagoon channel n, from the 

 remnant of the land h. 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 



remains a perfect atoll, 

 of which figure 70. re- 

 presents a vertical sec- 

 tion. In this diagram 

 r" is the living and 

 growing outer margin of 

 the encircling reef, and the lagoon channel is now converted into the 

 calm central lake », of the atoll. Thus by the process of subsidence 



Atoll. 



CXXXIL 



