124 ON CORAL AND CORAL REEFS 



land is an island, the reef surrounds it like a low wall, 

 and the sea between the reef and the land is, as it were, 

 a moat inside this wall. Such reefs as these are called 

 "encircling" when they surround an island; and "bar- 

 jier" reefs, when they stretch parallel with the coast 

 of a continent. In both these cases there is ordinary 

 dry land inside the reef, and separated from it only by 

 a narrower or a wider, a shallower or a deeper, space of 

 sea, which is called a "lagoon," or "inner passage." 

 But there is a third kind of reef, of very common occur- 

 rence in the Pacific and Indian Oceans, which goes by 

 the name of " atoll." This is, to all intents and purposes, 

 an encircling reef, without anything to encircle; or, in 

 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 two 

 hundred or three hundred fathoms. The depth imme- 

 diately 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 one hundred 

 and twenty to one hundred and fifty feet. 



Thus, if the water of the ocean should 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 Xuovo within 

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

 a fringing reef would have the appearance of an ordi- 



