ISOSTATIC SUBSIDENCE OF VOLCANIC ISLANDS 571 



the disappearance of the . detritus that has been eroded from them. We 

 are thus led toward a favorable consideration of an idea that marks a new 

 advance in the discussion of the origin of coral reefs. 



Molengraaff's Yiev^s as to the local Subsidence of volcanic 



Islands 



Theoretical considerations in favor of a greater or less subsidence of 

 volcanic islands have been stated by Gerland (1894, 56) and Daly (1914, 

 186) ; but the most definite utterance on this subject is to be found in a 

 recent paper by Molengraaff (1916), from which several citations have 

 already been made above. This experienced author points out that oceanic 

 volcanic islands "are not isostatically compensated and, without excep- 

 tion, show a larger or smaller positive anomaly of gravity. ... On 

 account of isostasy itself, these volcanic islands, rising ... as cones 

 or groups of cones of considerable bulk, can not always remain in exist- 

 ence ; under the influence of gravity they will without exception yield and 

 sink down slowly, but gradually, and if this movement is not counteracted 

 by other forces they must disappear below the sea and finally approach 

 more and more the form of the ocean bed. ... It appears to me that 

 the yielding and slow sinking of the volcanic islands under the influence 

 of gravity must be regarded as the cause of the dow^nward movement of 

 large amount and long duration which must be assumed in order to ex- 

 plain the formation of barrier reefs and atolls in true oceanic regions, the 

 cause of which had as yet not been ascertained" (1916, 619, 620). 



It would not promote the solution of the coral-reef problem to insist 

 that this ingenious idea is wholly correct ; one may conceive that the sub- 

 sidence of volcanic islands may have more to do with the weakening of the 

 internal force that caused their eruptive growth than with their excessive 

 density. Be this as it may, the general idea here advanced is pertinent 

 and helpful. In so far as we may trust the argument given above, from 

 which it appears that the prevalence of shorelines of submergence on con- 

 tinental coasts is more consistent Avith the local subsidence of reef -bearing- 

 islands than with the broad subsidence of the Pacific Ocean bottom, Mo- 

 lengraaff's suggestion is supported. It is supported also by the contrast 

 between the long enduring subsidence of reef-formations that appears to 

 characterize the central Pacific area and the many irregular disturbances 

 of the western archipelagoes. But in thus favorably regarding the possi- 

 bility of relatively local subsidence of volcanic islands, deformation should 

 not be wholly excluded from the central area of the Pacific; the great 

 flexure of the Kermadec-Tonga region, the uplifted reefs of Oahu, and 



