124 CRITIQUES AND ADDRESSES. [vi. 



summit, with coral polypes busily engaged in fabricating 

 coral ; while, below this comparatively narrow belt, its 

 surface is a bare and smooth expanse of coral sand, 

 supported upon and within a core of coral limestone. 

 Thus, if the bed of the Pacific were suddenly laid bare, 

 as was just now supposed, the appearance of the reef- 

 mountains would be exactly the reverse of that presented 

 by many high mountains on land. For these are white 

 with snow at the top, while their bases are clothed with 

 an abundant and gaudily-coloured vegetation. But the 

 coral cones would look grey and barren below, while 

 their summits 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 

 polypes cannot live at a greater depth than 100 or 150 

 feet, how can they have built up the base of the reef- 

 cone, which may be 2,000 feet, or more, below the 

 surface of the sea I 



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 1,000 miles or more, and so nearly of the 

 same height, that none should rise above the level of the 

 sea, nor fall 150 feet below that level? 



