276 CORALS AND CORAL LSLANDS. 



universally north of this line, are evidence of some depression 

 just below the line ; of less farther south ; and of the greatest 

 ■amount noTth of the line or over the coral area. 



b. The subsidence producing an atoll, when continued, 

 gradually reduces its size until it finally becomes so small that 

 the lagoon is obliterated ; and, consequently, a prevalence of 



■ these small islands is presumptive evidence of the greater sub- 

 sidence. We observe, in application of this principle, that the 

 coral islands about the equator, five or ten degrees south, be- 

 tween the Paumotus and the Gilbert Islands, are the smallest of 

 the ocean ; several of them are without lagoons, and some not 

 a mile in diameter. At the same time, in the Paumotus, and 

 among the Gilbert and Marshall Islands, there are atolls twenty 

 to fifty miles iu length, and rarely one less than three miles. " 

 It is probable, therefore, that the subsidence indicated was 

 greater at some distance north of the boundary line, over the 

 region of small equatorial islands, between' the meridians of 

 150° W. and 180°. 



c. When, after thus reducing the size of the atoll, the sub- 

 sidence continues its progress, or when it is too rapid for the 

 growing reef, it finally sinks the coral island, which there- 

 fore disappears from the ocean. Now it is a remarkable fact 

 that while the islands about the equator, above alluded to, 

 indicate greater subsidence than those farther south, there is 

 over a region north of these islands, that is, between them 

 and the Hawaian Group, a wide blank of ocean without an 

 island which is nearly twenty degrees in breadth. This area 

 lies between the Hawaian, the Fanning, and the Marshall 

 Islands, and stretches off between the first and last of these 

 groups, far to the north-west. 



Is it not then a legitimate conclusion that the subsidence 

 which was least to the south beyond the boundary line, and 

 increased northward, was still greater or more rapid over this 

 open area ; that the subsidence which reduced the size of the 

 islands about the equator to mere patches of reef was further 

 continued, and caused the total disappearance of islands that 

 once existed over this part of the ocean ? 



