DErTHS OF ATOLL LAGOONS 569 



premature at present to express a definite opinion as to the evidence for 

 or against subsidence that their lagoon depths may hereafter furnish ; but 

 in view of vi^hat has been intimated above as to the less disturbed history 

 of the central Pacific area than of its western part^ and in view of the 

 scarcity of submerged banks in the central area, it may be expected that 

 excessive depths will not be found, even in the lagoons of the largest 

 atolls, and that the lagoon depths w^ill, as a rule, vary in indirect propor- 

 tion to the diameter of the atoll, for reasons above noted. 



In other words, prevalent submergence resulting from intermittent 

 subsidence, compounded with Glacial changes of sealevel, as a result of 

 which the majority of central Pacific atolls seem to have been formed, has 

 apparently been, as Darwin assumed (115), not faster and frequently 

 slower than the upgrowth of coral reefs. Nevertheless, the large almost- 

 atoll of Truk, in the western Carolines, has lagoon depths of 57 fathoms, 

 and the almost-atoll of Mangareva, which includes the Gambler Islands, 

 south of the Paumotus, has depths up to 43 fathoms within the polygonal 

 space inclosed by its small volcanic islands, where abrasion could not act 

 effectively; both of these measures, but especially the first, must be diffi- 

 cult of explanation by abrasion, when the ocean was lowered less than 40 

 fathoms. 



REGIONAL OR LOCAL SUBSIDENCE 



The second consideration is that no compulsion inheres in the argu- 

 ment against Darwin's theory, based on the prevailing absence of coasts 

 of emergence backed by young coastal plains around continental borders; 

 for while it is true that a broad subsidence of the central Pacific floor by 

 some such measures as 1,000, 3,000, or 5,000 feet would cause the emer- 

 gence of continental margins by a quarter or less of those measures wher- 

 ever the continental borders themselves did not subside, and while it is 

 also true that Darwin inferred such measures of Pacific subsidence in 

 explanation of the many atolls in that ocean, it is not true that such sub- 

 sidence is essential to his theory. A local subsidence of reef foundations 

 will serve, as far as reef upgrowth is concerned, fully as well as a wide- 

 spread subsidence of the ocean floor. Not only so, if most coral reefs — 

 apart from those of the w^estern Pacific archipelagoes — are based on 

 oceanic volcanoes, as seems highly probable, and if the local subsidence 

 of such volcanoes on an otherwise stable ocean floor is the prime factor 

 in determining the upgrowth of their reefs and the transformation of 

 original fringing reefs into barrier reefs and atolls, then it may be shown 

 that, after a long period of oceanic eruptions, subsidences, and reef 

 growths, the ocean surface would be not lower, but somewhat higher, than 



