452 



BARRIER-REEFS. 



[CHAP. xx. 



they are 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 20 to 30 fathoms, and that the scale 

 is so Ismail that the plummets on the right hand show a depth of 



1. Vanikoro. 2. Gambler Islands. 3. Maurua. 



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

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

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



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 formerly 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 ; 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 accumu- 

 lation 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 150 miles beyond the northern point 



