266 



BARRIER-REEFS. 



there is not the smallest difference in general size, 

 outline, grouping, and even in quite trifling details 

 of structure between a barrier and an atoll. The 

 geographer Balbi has well remarked, that an en- 

 circled island is an atoll with high land rising out 

 of its lagoon ; remove the land from within, and 

 a perfect atoll is left. 



But what has caused these reefs to spring up at 

 such great distances from the shores of the inclu- 

 ded islands 1 It cannot be that the corals will not 

 grow close to the land, for the shores within the 

 lagoon-channel, when not suiTounded by alluvial 

 soil, are often fringed by living reefs ; and we shall 

 presently see that there is a whole class, which I 

 have called Fringing Reefs, from their close attach- 

 ment to the shores both of continents and of islands. 

 Again, on what have the reef-building corals, which 

 cannot live at great depths, based their encircling 

 structures ? This is a great 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, 



3032 f* 



1. Vanikoro. 2. Gambier Islands. 3. Ma 



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. 



