158 



Dr. J. D. Dana on the Origin of 



reef-grounds are 10 to 25 miles wide ; while on much of 

 the south-east 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 the subsidence 

 alone, but largely in the existence 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 seventy 

 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 k, and 



the outer barrier-reef i, with a fringing reef at I ; and sup- 

 posing it to have a rocky island at i, it represents a section 

 (further to the south-west) across the reef-grounds and the 

 outer range of islands. The reason for the existence of only 

 fringing-reefs for much of the south-east side has not been 

 particularly investigated. 



16. Local elevations within the sinking area are not evi- 

 dence against the Darwinian theory of subsidence. Local 

 disturbances 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 displacements 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 base- 

 ment ; and, moreover, the islands are not unfrequently shaken 

 by earthquakes. The causes of local displacement by either 

 method would not necessarily interfere with any secular 

 change of level in progress. 



17. The shore-platform of an atoll, or the "flat," as called 

 by Darwin, situated just above low-tide level, consists usually 



