STRUCTURE OF CORAL REEFS. 7 



of bare reef; and through one or more of these intervals, a ship- 

 channel occasionally opens into the lagoon. The larger coral 

 islands are thus a string of islets along a line of reef. The king 

 of the Maldives bears the high-sounding title of " Ibrahim Sultan 

 King of the thirteen Atollons and Twelve Thousand Isles ;" 

 which Capt. W. F. W. Owen, R. N., remarks, is no exaggeration. 



A few small coral islands are simple reefs without lagoons. In 

 some cases they are bare banks of coral ; 'but generally, the usual 

 vegetation of the islands has obtained a foothold, and affords some 

 protection against the glare of the coral sand. 



With these general remarks we may enter upon the more par- 

 ticular consideration of the characters of reefs and islands. 



2. Characters of Fringing and Barrier Reefs. 



a. General features. — Fringing reefs have been described as 

 those that directly adjoin the shores of an island ; and the barrier, 

 as the exterior reefs, separated from the fringing reef, or from 

 the shores when there is no inner reef, by an open channel. 



While there are only narrow shore-reefs to many islands, around 

 others, a distant barrier extends like an artificial mole, sometimes 

 ten or even fifteen miles from the land, and enclosing not only 

 one, but at times several islands. Between the narrow fringing 

 platform and these remote barriers, there is every possible varia- 

 tion as to extent and relative position. The inner channel is 

 sometimes barely deep enough at low tide for canoes, or for long 

 distances may be wanting entirely. Then again, it is a narrow 

 intricate passage, obstructed by knolls or patches of coral, render- 

 ing the navigation dangerous. Again, it is for miles in length 

 an open sea, in which ships find room to beat against a head 

 wind with a depth of twenty, thirty, or even fifty fathoms. Yet 

 hidden reefs make caution necessary. Patches from a few square 

 feet to many square miles in extent are met with over the broad 

 area enclosed by these distant barriers. 



These varieties of form and position are well exemplified in a 

 single group of islands — the Feejees ; a chart of which Archi- 

 pelago by the Expedition is inserted at the close of this volume. 



Near the middle of the chart is the island Goro ; its shores, 

 excepting the western, are bordered by a fringing reef. The island 

 Angauj south of Goro, is encircled by a coral breakwater, which 

 on the southern and western sides runs far from the shores, and 

 is a proper barrier reef, while on the eastern side, the same reef is 

 attached to the coast and is a fringing reef. From these exam- 

 ples we perceive the close relation of barrier and fringing reefs. 

 While a reef is sometimes quite encircling, in other instances it 

 is interrupted or wholly wanting along certain shores ; and occa- 

 sionally it may be confined to a single point of an island. 



