120 ON CORAL REEFS AND ISLANDS. 



1. Subsidence indicated by atolls arid barrier reefs. 



In a survey of the ocean, the eye observing its numerous atolls, 

 sees in each, literally as well as poetically, a coral urn upon 

 a rocky island that lies buried beneath the waves. Through 

 the equatorial latitudes, such marks of subsidence abound, from 

 the Eastern Paumotu to the Western Carolines, a distance of 

 about six thousand geographical miles. In the Paumotu Archi- 

 pelago there are about eighty of these atolls. Going westward, 

 a little to the north of west, they are found to dot the ocean at 

 irregular intervals ; and at the Tarawan Group, the Carolines 

 commence, which consist of seventy or eighty atolls. 



If a line be drawn from Pitcairn's island, the southernmost of 

 the Paumotus, by the Gambier Group, the north of the Society 

 Group, Samoa, and the Salomon Islands to the Pelews, it will 

 form nearly a straight boundary trending N. 70° W., running be- 

 tween the atolls on one side and the high islands of the Pacific 

 on the other, the former lying to the North of the line, and the 

 latter to the South. 



Between this boundary line and the Hawaiian Islands, an area 

 nearly two thousand miles wide and six thousand long, there are 

 two hundred and four islands, of which only three are high ex- 

 clusive of the eight Marquesas. These three are Ualan, Banabe 

 (Ascension or Pounypet) and Hogoleu, all in the Caroline Archi- 

 pelago. South of the same line, within three degrees of it, there 

 is an occasional atoll ; but beyond this distance, there are none 

 excepting the few in the Friendly Group, and one or two in the 

 Feejees. 



If each coral island scattered over this wide area indicates a 

 subsidence of an island, we may believe that the subsidence was 

 general throughout the area. Moreover, each atoll, could we 

 measure the thickness of the coral constituting it, would inform 

 us nearly of the extent of the subsidence where it stands ; for 

 they are actually so many registers placed over the ocean, mark- 

 ing out not only the site of a buried island, but also the depth at 

 which it lies covered. We have not the means of applying the 

 evidence ; but there are fact* at hand, which may give at least 

 comparative results. 



a. We observe, first, that barrier reefs are, in general, evidence 

 of less subsidence than atoll reefs, (p. 89.) Consequently the 

 great preponderance of the former just below the southern boun- 

 dary line of the coral island area, and farther south the entire ab- 

 sence of atolls, while atolls prevail so universally north of this 

 line, are evidence of little depression just below the line; of less 

 farther south ; and of the greatest amount, north of the line or 

 over the coral area. 



