ACTINOZOA. 



173 



more than 30 miles wide, and 10-25 fathoms deep. A similar reef, though of 

 less extent, borders the western shore of New Caledonia. In many cases, as 

 in the Society Islands and elsewhere, a similar barrier or, as it is here called, an 

 encircling reef surrounds an island or group of islands, having at intervals Dreaks 

 in it, which lead into the deep lagoon channels and are often situated opposite 

 the valleys on the land. Low islands (coral islands] formed of coral rock and 

 sand thrown up by the sea, and supporting- a littoral vegetation, are often 

 situated on the reef. 



A' * 



FIG. 144 d. Diagram illustrating Darwin's view of the formation of a barrier reef from a 

 fringing reef (from Darwin). AA outer edge of the fringing reef at the level of the sea ; 

 BB shores of the island ; A' A' outer edge of reef (now a barrier reef) after its upward growth 

 during a period of subsidence ; CO the lagoon channel between the reef and the island ; 

 B'B' the shore of the island. 



The lagoon islands or atolls consist of a similar ring-shaped reef, enclosing 

 an open lagoon in which there is no island (Fig. 1446). There may be gaps 

 leading into the lagoon, or the ring may be so complete, that, as in the case of 

 Fakaofu in the Union Islands, it is left at low tide brim full of water standing 

 some feet above sea level. The floor of the lagoon is generally nearly flat, 

 shelving slightly towards the reef, and is often from 20 to 35 fathoms below 



0' 



FIG. 144e. Diagram illustrating Darwin's view of the formation of an atoll from a barrier reef 

 by subsidence. A' A' outer edges of the barrier reef at the level of the sea. The cocoanut 

 trees represent coral islets formed on the reef. CO the lagoon channel ; B'B' the shores of 

 the island generally formed of low alluvial land and of coral detritus from the lagoon channel ; 

 A" A" the lagoon of the newly formed atoll. According to the scale the depth of the lagoon 

 and of the lagoon channel is exaggerated. 



the surface whereas the sea outside the reef rapidly deepens, so that it is not 

 uncommon to find a depth of more than 1000 fathoms within a mile of the 

 reef. The slope of the Seaward side of the reef below the region of growing 

 coral in many cases exceeds an angle of 45. Low islands (coral islands) with 

 glittering white sand and characteristic vegetation are often situated on the 

 reef, scattered or united into a continuous belt of land. 



The mode of the formation of the atoll has been much discussed. Whatever 

 the foundation of an atoll may be, it is requisite (1) that it should at one time, 



