CORAL REEFS. 25 



I should tell you that all coral structures, while the 

 Corals are building them, and before they are trans- 

 formed into land, are called Reefs. I have spoken of 

 the circular one which I have been describing as a 

 vail, because 1 thought you would understand my 

 'rvianing better ; and they are truly walls. But the 

 common name for them is Reef, and the coral animals 

 are called Reef-builders. 



Other kinds of islands are built by Corals ; sometimes 

 they build around an island which rises above the level 

 of the sea ; and then, of course, the centre is filled with 

 solid land, instead of being a lake inclosed by the coral 

 growth, as in the one I have been describing. The 

 circular ones we have been talking about, are Lagoon 

 Islands. There are many of them in the Pacific 

 Ocean. When people first made voyages in the Pacific 

 Ocean, they could not at all understand the meaning 

 of these islands, formed like rings, with calm water in 

 their centre. Usually, when the islands are large, they 

 do not close completely, but sometimes one or more 

 gaps are left in the ring, through which vessels can pass 

 in, and anchor in the quiet harbors formed within the 

 shelter of these coral banks with the trees that grow 

 upon them. You may imagine how surprised voyagers 

 must have been, when they first sailed through such an 

 opening in a circular coral island, and found themselves 

 in a quiet lake in mid-ocean. Sometimes these coral 

 structures are made into Lagoon Islands by the sinking of 

 the land around which they have begun to grow. Sup- 

 pose, for instance, that Corals establish themselves around 

 an island (see upper figure of diagram on page 28), and 

 the island gradually subsides below the level of the sea, 



