408 CORAL AND CORAL REEFS 



that height everywhere else since the reefs began to be 

 formed, the geography of the world in general must have 

 been very different indeed, at that time, from what it is 

 now. And we have very good means of knowing that 

 any such rise as this certainly has not taken place in the 

 level of the sea since the time that the corals have been 

 building their houses. And so the only other alternative 

 was to suppose that the land had gone down, and at so slow 

 a rate that the corals were able to grow upward as fast 

 as it went downward. You will see at once that this is 

 the solution of the mystery, and nothing can be simpler 

 or more obvious when you come to think about it. Suppose 

 we start with a coral sea and put in the middle of it an 

 island such as the Mauritius. Now let the coral polypes 

 come and perch on the shore and build a fringing reef, which 

 will stop when they come to 20 or 25 fathoms, and you 

 will have a fringing reef like that round the island in the 

 illustration. So long as the land remains stationary, so 

 long as it does not descend so long will that reef be unable 

 to get any further out, because the moment the polype 

 embryos try to get below they die. But now suppose that 

 the land sinks very gradually indeed. Let it subside by 

 slow degrees, until the mountain peak, which we have 

 in the middle of it, alone projects beyond the sea level. 

 The fringing reef would be carried down also ; but we 

 suppose that the sinking is so slow that the coral polypes 

 are able to grow up as fast as the land is carried down ; 

 consequently they will add layer upon layer until they form 

 a deep cup, because the inner part of the reef grows much 

 more slowly than the outer part. Thus you have the 

 reef forming a bed thicker upon the flanks of the island ; 

 but the edge of the reef will be very much further out from 

 the land, and the lagoon will be many times deeper ; in 

 short, your fringing reef will be converted into an encircling 

 reef. And if, instead of this being an island, it were a 

 great continent like Australia, then you will have the 

 phenomenon of a barrier reef which I have described. The 

 barrier reef of Australia was originally a fringing reef ; 

 the land has gone slowly down ; the consequence is the 

 lagoon has deepened until its depth is now 25 fathoms 

 and the corals have grown up at the outer edge until you 

 have that prodigious accumulation which forms the barrier 

 reef at present. Now let this process go on further still ; 

 let us take the land a further step down, so as to submerge 



