ON CORAL AND CORAL REEFS 129 



would be gay with a richly-coloured parterre of flower- 

 like coral polypes. 



The practical difficulties of sounding upon, and of 

 bringing up portions of, the seaward face of an atoll or 

 of an encircling reef, are so great, in consequence of the 

 constant and dangerous swell which sets towards it, 

 that no exact information concerning the depth to 

 which the reefs are composed of coral has yet been 

 obtained. There is no reason to doubt, however, that 

 the reef-cone has the same structure from its summit 

 to its base, and that its sea-wall is throughout mainly 

 composed of dead coral. 



And now arises a serious difficulty. If the coral pol- 

 ypes cannot live at a greater depth than one hundred or 

 one hundred and fifty feet, how can they have built up 

 the base of the reef- cone, which may be two thousand 

 feet, or more, below the surface of the sea ? 



In order to get over this objection, it was at one time 

 supposed that the reef-building polypes had settled 

 upon the summits of a chain of submarine mountains. 

 But what is there in physical geography to justify the 

 assumption of the existence of a chain of mountains 

 stretching for one thousand miles or more, and so 

 nearly of the same height, that none should rise above 

 the level of the sea, nor fall one hundred and fifty feet 

 below that level ? 



How, again, on this hypothesis, are atolls to be ac- 

 counted for, unless, as some have done, we take refuge 

 in the wild supposition that every atoll corresponds 

 with the crater of a submarine volcano ? And what ex- 

 planation does it afford of the fact that, in some parts 

 of the ocean, only atolls and encircling reefs occur, 

 while others present none but fringing reefs ? 



These and other puzzling facts remained insoluble 



