Davis. — Significant Features of Reef-hordered Coasts. 23 



should be critically reviewed by the observer while he is still on the ground, 

 in order that he may give conscious attention to the details which are con- 

 firmatory of or contradictory to the different suppositions. The absence 

 of records regarding significant details in many accounts of reef-encircled 

 islands makes it impossible to use them in a settlement of the questions 

 at issue. 



SUBMEEGENCE BY OCEAN RiSE OR BY ISLAND SUBSIDENCE. 



The changes of level involved in producing coasts of submergence or of 

 emergence, and the changes in coral reefs therewith associated, may result 

 from various causes. Chief among these are, first, a local movement of the 

 earth's crust without significant alteration of ocean-level ; secondly, an 

 alteration of ocean-level due either to a distant movement of the earth's 

 crust or to the general transfer of detritus from continents to ocean basins ; 

 and, thirdly, an alteration of ocean-level due to climatic change, whereby 

 a considerable volume of water is withdrawn from or returned to the ocean 

 in connection with the making or melting of continental ice-sheets. 



As far as coral reefs alone are concerned, it is immaterial whether the 

 changes of level upon which their formation or their emergence depend are 

 caused by one of these processes or another ; but when it is sought to 

 assign coral reefs to their proper place in the history of the earth the causes 

 of the changes of level with which they are associated must be determined 

 as definitely as possible, and this is now the most difficult part of the 

 coral-reef problem. In order to solve it, search must be made for the 

 characteristics by which each kind of change of level may be recognized. 



Crustal subsidence operating over large areas was accepted by Darwin 

 and Dana as the whole cause of the subsidence with which coral reefs are 

 so generally associated. Local subsidence of volcanic islands, as a result 

 of their excessive weight, has been recently suggested by Molengraaff in 

 explanation of mid-Pacific atolls. It may seem at first sight that either 

 one of these processes would, if acting alone, cause a slight lowering of 

 ocean-level, whereby coasts of emergence would be produced around con- 

 tinental shores ; and in this case the resulting local submergence of the 

 reef-encircled islands would be a little less than the local subsidence. 



But a closer consideration leads to other conclusions : first, inasmuch 

 as general crustal subsidence is presumably associated with compensatory 

 uplifts of other areas, the changes in ocean - level thus caused may be 

 neglected, and with all the more reason when it is noted that if a sub- 

 siding island is only partly submerged while a compensatory uplift of equal 

 volume occurs on the ocean-floor without any emergence the result will be 

 a small rise of ocean-level ; secondly, if a large number of volcanic islands 

 are built up in succession by eruption from the ocean-floor in such intervals 

 of time that the earliest ones have subsided so far as to be crowned with 

 atoll reefs when the latest ones are formed, the total effect on ocean-level 

 will be not a fail, but a small rise (Davis, 1917b). 



In this connection let it be noted that modern investigation gives little 

 support to the old view that active volcanoes always occur in regions of 

 elevation. There is much evidence to show that the reverse is often true. 

 It is therefore desirable that the movements suffered by other islands in 

 the neighbourhood of young volcanic islands should be independently 

 worked out. It is certainly not legitimate to conclude, as has been done 

 by an observer in the Australasian region, that a certain atoll could not 

 have been formed by upgrowth during subsidence because an active volcano 



