406 CORAL AND CORAL REEFS 



where else. Imagine to yourself a heap of this material 

 more than one thousand miles long, and several miles wide. 

 That is a barrier reef; but a barrier reef is merely as it 

 were a fragment of an encircling reef running parallel to the 

 coast of a great continent. 



I told you that the polypes which built these reefs were 

 not able to live at a greater depth than 20 to 25 fathoms of 

 water ; and that is the reason why the fringing reef goes 

 no farther from the land than it does. And for the same 

 reason, if the Pacific could be laid bare we should have 

 a most singular spectacle. There would be a number of 

 mountains with truncated tops scattered over it, and those 

 mountains would have an appearance just the very reverse 

 of that presented by the mountains we see on shore. You 

 know that the mountains on shore are covered with vegeta- 

 tion at their bases, while their tops are barren or covered 

 with snow ; but these mountains would be perfectly bare 

 at their bases, and all round their tops they would be 

 covered with a beautiful vegetation of coral polypes. And 

 not only would this be the case, but we should find that for 

 a considerable distance down, all the material of these atoll 

 and encircling reefs was built up of precisely the same coral 

 rock as the fringing reef. That is to say, you have an 

 enormous mass of coral rock at a depth below the surface 

 of the water where we know perfectly well that the coral 

 animals could not have lived to form it. When those two 

 facts were first put together, naturalists were quite as much 

 puzzled as I daresay you are, at present, to understand 

 how these two seeming contradictions could be reconciled ; 

 and all sorts of odd hypotheses were resorted to. It was 

 supposed that the coral did not extend so far down, but 

 that there was a great chain of submarine mountains 

 stretching through the Pacific, and that the coral had grown 

 upon them. But only fancy what supposition that was, 

 for you would have to imagine that there was a chain of 

 mountains a thousand miles or more long, and that the 

 top of every mountain came within 20 fathoms of the surface 

 of the sea, and neither rose above nor sunk beneath that 

 level. That is highly improbable : such a chain of mountains 

 was never known. Then how can you possibly account 

 for the curious circular form of the atolls by any supposition 

 of this kind ? I believe there was some one who imagined 

 that all these mountains were volcanoes, and that the reefs 

 had grown round the tops of the craters, so we all stuck 



