iM-l BARRIER-REEFS. &J 



laid down, both vertically and horizontally, on the same scale of a 

 quarter of an inch to a mile. 



It should be observed that the sections might have been taken in any 

 direction through these islands, or through many other encircled 

 islands, and the general features would have been the same. Now 

 bearing in mind that reef-building coral cannot live at a greater depth 

 than from twenty to thirty fathoms, and that the scale is so small that the 

 plummets on the right hand show a depth of 200 fathoms, on what are 

 these barrier-reefs based ? Are we to suppose that each island is 

 surrounded by a collar-like submarine ledge of rock, or by a great bank 

 of sediment, ending abruptly where the reef ends ? If the sea had for- 

 merly eaten deeply into the islands, before they were protected by the 

 reefs, thus having left a shallow ledge round them under water, the 

 present shores would have been invariably bounded by great precipices ; 



i. Vanikoro. a. Gambier Islands. 3. Maurua. 



The horizontal shading shows the barrier-reefs and lapoon-channels. The inclined 

 shading above the level of the sea (AA), shows the actual form of the land ; the inclined 

 hading below this line, shows its probable prolongation under water. 



but this is most rarely the case. Moreover, on this notion, it is not 

 possible to explain why the corals should have sprung up, like a wall, 

 from the extreme outer margin of the ledge, often leaving a broad space 

 of water within, too deep for the growth of corals. The accumulation 

 of a wide bank of sediment all round these islands, and generally 

 widest where the included islands are smallest, is highly improbable, 

 considering their exposed positions in the central and deepest parts of 

 the ocean. In the case of the barrier- reef of New Caledonia, which 

 extends for one hundred and fifty miles beyond the northern point of the 

 island, ir the same straight line with which it fronts the west coast, it 

 is hardly possible to believe, that a: bank of sediment could thus have 

 been straightly deposited in front of a lofty island, and so far beyond 

 its termination in the cnen sea. Finally, if we look to other oceanic 



