described hy Mr Daricin. 41 



200 feet beneath the surface, they are checked in then- pro- 

 gress seaward, and therefore contmue tbeir work to the right 

 or left, keeping always within the requisite depth ; and thus 

 their instinct guides them to form the reef in the shape of a 

 girdle round the island, following the sinuosities of its shores, 

 keeping nearer them where the water deepens rapidly, and 

 farther off where it deepens slowly. Here we have a reason 

 why reefs may be circular, oblong, or of any other form which 

 islands assume. Mr Darwin's plates of Raiatea and Vanikoro 

 are good examples of the manner in which reefs adapt them- 

 selves to the outhne of the islands they encircle. 



The little architects carry up their fabric to the level of the 

 low water line, and there they stop. Suppose the island now 

 to subside 200 feet, either suddenly or slowly. They then 

 commence a new fabric on the top of the old, and again carry 

 it up to the low water level. But the island itself, besides 

 losing 200 feet of height, is contracted in breadth from its 

 low shores being covered with water ; the channel between it 

 and the reef becomes broader and deeper ; and the reef hav- 

 ing its basis at a depth beyond that whei-e living coral exists 

 becomes a " barrier reef.'' 



Suppose the island to subside other 200 feet. A third fa- 

 bric of coral now rises on the top of the second, till the reef 

 again reaches the low water level. But the island itself has 

 disappeared, and the lagoon which occupies its place, with the 

 encircling reef, now forms an " atoll.'' 



The subjoined figures illustrate what has been stated, and 

 shew the process by which a "Fringing reef "' passes into a 

 " Barrier reef,'' and a barrier reef into an " Atoll.'' 



; First Stage — The Fringing reef. 



a b a— A section of an island, roughly copied from one given by Mr 

 Davwin. 



S 1 — The surface of the sea. 



,. r A fringing reef formed within a small distance of its shores. 



