120 CRITIQUES AND ADDRESSES. [vi. 



other words, without an island in the middle of its 

 lagoon. The atoll has exactly the appearance of a vast, 

 irregularly oval, or circular, breakwater, enclosing smooth 

 water in its midst. The depth of the water in the lagoon 

 rarely exceeds twenty or thirty fathoms, but, outside 

 the reef, it deepens with great rapidity to 200 or 300 

 fathoms. The depth immediately outside the barrier, or 

 encircling, reefs, may also be very considerable ; but, at 

 the outer edge of a fringing reef, it does not amount 

 usually to more than twenty or twenty-five fathoms ; in 

 other words, from 120 to 150 feet. 



Thus, if the water of the ocean could be suddenly 

 drained away, we should see the atolls rising from the 

 sea-bed like vast truncated cones, and resembling so 

 many volcanic craters, except that their sides would be 

 steeper than those of an ordinary volcano. In the case 

 of the encircling reefs, the cone, with the enclosed island, 

 would look like Vesuvius with Monte Nuovo within the 

 old crater of Somma ; while, finally, the island with a 

 fringing reef would have the appearance of an ordinary 

 hill, or mountain, girded by a vast parapet, within which 

 would lie a shallow moat. And -the dry bed of the 

 Pacific might afford grounds for an inhabitant of the 

 moon to speculate upon the extraordinary subterranean 

 activity to which these vast and numerous "craters" 

 bore witness! 



When the structure of a fringing reef is investigated, 

 the bottom of the lagoon is found to be covered with fine 

 whitish mud, which results from the breaking up of the 

 dead corals. Upon this muddy floor there lie, here and 

 there, growing corals, or occasionally great blocks of dead 

 coral, which have been torn by storms from the outer 

 edge of the reef, and washed into the lagoon. Shell-fish 

 and worms of various kinds abound ; and fish, some of 

 which prey upon the coral, sport in the deeper pools. 



