Nature of the Coral Bank. 227 



fathoms, and as each h'nc of soundings reached the central de- 

 pressed area of the bank, a depth of about thirty fathoms. The 

 islands were for the most part situated near the margin of the 

 bank, and were in every case surrounded by a zone of shallow 

 water. Hence it is obvious that if the entire structure were 

 suddenly to undergo an elevation of about fourteen fathoms, or 

 eighty-four feet, it would present the appearance of an atoll 

 studded with comparatively lofty islets, and enclosing a lagoon 

 of still water sixteen fathoms in depth. 



The outer edge of the bank was exceedingly abrupt, for within 

 a ship's length the soundings changed from ten or fourteen fathoms 

 on the margin, to no bottom with one hundred fathoms of line 

 immediately outside the edge. This precipitous character of the 

 reef-edge was found to be the same throughout its entire extent. 

 At various points over this area growing corals were obtained at 

 depths ranging from twelve to twenty-tv^^o fathoms, the latter 

 being somewhat greater than the limit in depth at which it is 

 generally agreed that reef-forming corals can live. It therefore 

 follows, that if the entire bank were now to subside bodily at a 

 faster rate than the corals can by their growth raise the surface 

 upwards, these organisms would soon be immersed below their 

 natural limit, and would consequently die. But we have no 

 evidence of a subsidence having occurred, beyond the fact that 

 the bank, as a whole, bears a resemblance to a submerged atoll, 

 while on the other hand there are some positive evidences of 

 elevation to be seen in the overlying islands. At Eagle Island, 

 the general surface — that is to say, all the land within the inner, 

 or coral-drift beach — is level, and consists of dead coral in situ ; 

 so that if denuded of its present covering of low vegetable growth, 

 it would present much the same appearance that a broad plat- 

 form of iringing reef might, if elevated above high-water mark 

 and allowed to remain exposed to the weather for a io-w years. 

 The same is the case as regards the greater part of one of the 

 African Islands which we visited ; while its northern extremity 



