Changes of Level in the Pacific Ocean. 243 



are Ualan, Banabe (Ascension or Pounypet), and Hogoleu, 

 all in the Caroline Archipelago. 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, marking 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 facts at hand which may give, at least, comparative 

 results. 



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

 dence of less subsidence than atoll reefs (xiii. 186), 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 little depression 

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

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



b. The subsidence producing an atoll, when continued, gra- 

 dually reduces its size, until finally it 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, live or ten degrees south 

 between thePaumotus and the Tarawan Islands, are the small- 

 est of the ocean : several of them are without lagoons, and 

 some not a mile in diameter. A.t the same time, in the Paumo- 

 tus, and among the Tarawan and Marshall Islands, there are 

 atolls twenty to fifty miles in length, and rarely one less than 

 three miles. It is probable, therefore, that the subsidence in- 

 dicated was greatest at some distance north of the boundary 

 line, over the region of small equatorial islands between the 

 meridian of 150° W. and 180°. 



Q 2 



