254 PHYSIOGRAPHY. [chap. 



of about 1,200 miles, and standing at an average distance 

 of twenty or thirty miles from the coast. The channel 

 between this barrier-reef and the land is termed " the inner 

 passage," and has a depth of about twenty or five-and-twenty 

 fathoms ; while, outside the barrier-reef, the depth of the sea 

 suddenly increases to many hundred fathoms. 



In addition to the fringing and barrier-reefs, there is yet 

 another kind, differing from those principally in being 

 quite isolated from other land. The coral-rock thus forms 

 a true island, rising from the sea usually as a low strip of 

 land, more or less ring-shaped, but generally of irregular 

 outline. In places, the strip of coral-land may bear a rich 

 growth of cocoa-nut palms and other tropical forms of 

 vegetation ; while, inside the rim of land, there is a shallow 

 lake, or lagoon, of clear green water, which strikingly con- 

 trasts with the dazzling white coral-rock of the beach. 

 Access to the lagoon is gained by a gap in the shore, and 

 thus the island generally presents a horse-shoe shape. 

 Several openings may occur in the belt of land, and the 

 island consequently becomes broken up into a chain 'of 

 islets. These coral islands are plentifully scattered through 

 the Pacific and Indian Oceans, and are often known under 

 the Maldive name of atolls. 



In explaining the formation of coral-land, it should be 

 remembered that the corals themselves are powerless to 

 raise the land above low-water mark, for the polypes perish 

 when exposed above water. Dry land is, however, formed 

 mechanically ; blocks of dead coral being broken off by 

 waves from one part of the rock, and piled up upon 

 another. The loose blocks are cemented into compact 

 masses by means of coral-sand and coral-mud, produced by 

 the tear and wear of the coral-rock. In the case of fringing 

 reefs, the seaward, and, in that of atolls, the windward, side 



