322 CORALS AND CORAL ISLANDS. 



sional atoll ; but beyond this distance, there are none except- 

 ing the few in the Friendly Group, and one or two in the Fee- 

 jees. 



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 how much subsidence took place where it 

 stands; for they are actually so many registers placed over the 

 ocean, marking out, not only the sight 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 facts at hand, 

 which may give at least comparative results. 



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

 evidence of less subsidence than atoll reefs (p. 266). Conse- 

 quently, the great preponderance of the former just below the 

 southern boundary line of the coral island area, and farther 

 south the entire absence of atolls, while atolls prevail so uni- 

 versally north of this line, are evidence of some depression just 

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

 amount, north of the line or over the coral area. 



b. The subsidence produeing 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 subsi- 

 dence. 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 in length, and rarely one less than three 



