CORALLINES. 
177 
six hundred ; Aituald to three hundred ; and Manonai to about fifty 
feet only. 
Around the Isle of Gambier the reef has a thickness of a thousand 
a nd sixty feet, at Tahiti of two hundred and thirty. Eound the Fiji 
Islands it is from two to three thousand. 
The fringing reefs immediately surrounding the island, or a portion 
°I it, might be confounded with the barrier reefs we have been de- 
scribing, if they only differed in their smaller breadth; but the 
circumstance that they abut immediately on the coast in place of being 
Se parated 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 
Wrier reefe. 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 
Va ries 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 
H°urbon ; all round this island the polypiers construct on the volcanic 
bottom of the sea detached mammalons, which rise from a fathom to a 
Whom 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 polypiers 
e Xist which may be ranked among the madreporic coasting reefs, in 
^sequence of the limited breadth of the gulf. Ehrenberg and 
(, ttprich examined a hundred and fifty stations in the Bed Sea, all of 
Wich had outlying fringing reefs of this description. 
It may be asked, With what rapidity are these coral and madreporic 
. a . s formed, so as to become atolls and fringing reefs ? To answer 
us question even approximately is very difficult. On the coast of 
16 Mauritius, according to M. d’ Archaic,* the learned professor of 
e lardin des Plantes, the edge of the reef is produced by Madrepora 
^f ynibosa, M. pocillifera, and two species of Astrea, which pursue 
6lr °perations at the depth of from eight to fifteen fathoms. At 
e base is a bank of Seriatopora, from fifteen to twenty fathoms in 
* “Cours de Paleontnlogie Stratigraphique.” 
N 
