102 J. D. Dana — Origin of Coral Reefs and Islands. 



The great reef-grounds along the northwest sides of the two 

 large Feejee islands, Vanua Lebu and Viti Lebu, do not indicate 

 a subsidence proportional to their width. 



Each of these islands is over ninety miles long, and together 

 the trend is northeastward.* The northwestern reef-grounds 

 are 10 to 25 miles wide ; while on much of the southeast side of 

 each island there is (according to the Wilkes Chart), only a 

 fringing reef. The true explanation of the great width is found, 

 not in the amount of subsidence alone, but largely in the ex- 

 istence there of a broad area of submerged land at relatively 

 small depths. This inference is sustained by the fact that the 

 outer barrier reef, after being simply a barrier reef for 125 

 miles with but two rocky islets in its course, becomes in the 

 same line westward for 70 miles, a range of high narrow reef- 

 bordered islands (called the Asaua Range), and then bends 

 around southward through other rocky islands to meet the west 

 end of Viti Lebu. The reef-grounds have thus a chain of 

 islands as their boundary for a length of 100 miles, and simply 

 a barrier reef with two rocky islets for the rest of the line (125 

 miles). 



The following figure illustrates in a general way the above 

 condition. It is a section across the reef-grounds, i &, and the 



outer barrier reef i, with a fringing reef at I ; and supposing it 

 to have a rocky island at i, it represents a section (farther to 

 the southwest) across the reef-grounds and the outer range of 

 islands. The reason for the existence of only fringing reefs 

 for much of the southeast side has not been particularly inves- 

 tigated. 



16. Local elevations within the sinking area are not evidence 

 against the Darwinian theory of subsidence. Local disturb- 

 ances and faults, as both theory and the rocks of the continents 

 show, are almost necessary concomitants of a slowly progressing 

 change of level. Besides this, igneous conditions beneath a 

 region are a common source of local displacements. Such dis- 

 placements are therefore to be looked for in the tropical oceans, 

 since the various high islands are volcanic, and the coral islands 

 probably have a volcanic basement ; and, moreover, the islands 

 are not unfrequently shaken by earthquakes. The causes of 



* A map of the Feejee group is contained in my Corals and Coral Islands, and 

 of larger size in Wilkes's Narrative of the Expedition. 



