462 



SECTIONS OF BARRIER-REEFS. [chapTxx: 



attachment to the shores both of conthients and of islands. 

 Again, on what have the reef-building corals, which cannot 

 live at great depths, based their encircling structure? This 

 is a gieat apparent difficulty, analogous to that in the case 

 of atolls, which has generally been overlooked. It will be 

 perceived more clearly by inspecting the following sections, 

 which are real ones, taken in north and south lines, through 

 the islands with their barrier-reefs, of Vanikoro, Gambler, 

 and Maurua ; and they are laid down, both vertically and 

 horizontally, on the same scale of a quarter of an inch to a 

 mile. 



303Z/* 





I. Vanikoro. 2. Gambler Islands, 3. Maurua. 



The horizontal shading- shows the barrier-reets and lagoon-channels. The 

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

 land ; the inclined shading below [this line, shows its probable prolongation 

 under water. 



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 

 10 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 



