CORALLINES. 175 



with reefs, it varies considerably. The Isle of Tahiti rises six thousand 

 eight hundred feet above the level of the sea ; the Isle of Maurua to 

 six hundred ; Aituaki to three hundred ; and Manonai to about fifty 

 feet only. 



Around the Isle of Gambier the reef has a thickness of a thousand 

 and sixty feet, at Tahiti of two hundred and thirty. Bound the Fiji 

 Islands it is from two to three thousand. 



The fringing reefs immediately surrounding the island, or a 

 portion of it, might be confounded with the barrier reefs we have been 

 describing, if they only differed in their smaller breadth ; but the 

 circumstance that they abut immediately on the coast in place of being 

 separated by a channel or lagoon more or less deep and continuous, 

 proves that they are in direct communication with the slope of the 

 submarine soil, and permits of their being distinguished from the 

 barrier reefs. The dangerous breakers which surround the Mauritius 

 are a striking example of the fringing reef. This island is almost 

 entirely surrounded by a barrier of these rocks, the breadth of which 

 varies from a hundred and fifty to three hundred and thirty feet ; 

 their rugged and abrupt surface is worn almost smooth, and is rarely 

 uncovered at low water. Analogous reefs surround the Isle of 

 Bourbon ; all round this island the polyps construct on the volcanic 

 bottom of the sea detached mammalons, which rise from a fathom to 

 a fathom and a half above the water. 



Madreporic coasting reefs present themselves also on the eastern 

 coast of Africa and of Brazil. In the Bed Sea, reefs of corals exist 

 which may be ranked among the madreporic coasting reefs, in conse- 

 quence of the limited breadth of the gulf. Ehrenberg and Hemprich 

 examined a hundred and fifty stations in the Ked Sea, all of which 

 had outlying fringing reefs of this description. 



It may be asked, With what rapidity are these coral and madreporic 

 banks formed, so as to become atolls and fringing reef si To answer 

 this question even approximately is very difficult. On the coast of 

 the Mauritius, according to M. d' Archaic,* the learned professor of the 

 Jardin des Plantes, the edge of the reef is produced by Madrepora 

 corymbosa, M. pocillifera, and two species of Astrea, which pursue 



* " Cours de Paleontologie Stratigraphique." 



