126 . POPULAR ILLUSTRATIONS OF 



singular sea, while fringing reefs, as before mentioned, are found at 

 each end. 



But in some instances, though rare, these "barrier reefs " appear 

 as circles of land covered with trees and vegetation. Mr. Darwin 

 explains this by the fact that floating masses of vegetable matter are 

 caught by, and entangled in, the uppermost ridge of the coral wall. 

 The Coral thus killed at the surface breaks up, and, mingling with 

 the extraneous matter, forms the basis of a soil in which the seeds 

 of the cocoa or other trees take root and grow, forming a wall of 

 vegetation round the island, as seen in that of Bolabola, of which 

 Mr. Darwin gives a sketch. 



It is very rare, however, that the whole' of the upper part of the 

 reef, as in this instance, is converted into land. More frequently, as 

 would be supposed, and in accordance with the theory of their for- 



Fig. 176. 



mation, the land appears as a series of small islets, with smooth 

 water channels between them, through which ships pass from the 

 stormy sea into the quiet and lovely oasis within the reef. 



Now let us imagine the process of sinking to go on as before, 

 until the top of the mountain has gone below the surface of the sea ; 

 and at the same time let us suppose the top of the coral wall to have 

 been, as in the case of the " barrier reef," partially converted into 

 land. We shall then have an "atoll," or "lagoon island," as shown 

 in the diagram 176, and actually seen in the Low and Caroline 

 Archipelagos, and in the majority of islands in the Pacific ; in the 

 Saya de Maeha, Cosmoledo, Chagos, Maldivas, Laccadives, and 

 Keeling Island in the Indian Ocean, and the Paraceles, &c., in the 

 China Sea. 



