ELEVATED REEFS OF VANUA MBALAVU, FIJI 509 



The present remnant of the limestone mass, where it has not been 

 wholly stripped from its volcanic core, has an embayed shoreline, indicat- 

 ing a recent submergence of z feet ; hence, after the formation of the lime- 

 stones, the compound island must have been elevated by {ij + 600 ■\- z) 

 feet, as in sector C, and long exposed to erosion, as in sector D. The ele- 

 vation as well as the preceding subsidence need not have been precisely 

 uniform; both may have included a moderate tilting such as v^ill be 

 shown below to have accompanied a follov^ing subrecent subsidence. It 

 was during this subrecent subsidence, after the island had stood z feet 

 higher than today, that the present barrier reef shown in sector E must 

 have grown up. 



The present lagoon, the floor of which is more or less aggraded, is only 

 20 fathoms deep on its western side, but slants down to a depth of 80 or 

 90 fathoms on its eastern side, as Agassiz pointed out ; hence the measure, 

 z, of recent submergence is a variable quantity, and the submergence 

 could not have been due only to the postglacial rise of ocean level, but 

 must have involved a slanting subsidence which, where greatest, appears 

 to have been about 600 feet. Moreover, the subsidence must have been 

 relatively local, for 30 miles to the southwest the undissected and there- 

 fore recently elevated atoll of Vatu Vara rises to a height of 1,030 feet. 

 I have employed the evidence of local elevation and subsidence furnished 

 by Vanua Mbalavu to determine the conditions of origin of several small 

 near-by atolls (1916, 5), and believe that thus, for the first time, fairly 

 direct proof has been provided for the upgrowth of atolls during subsi- 

 dence. 



Although good evidence of repeated changes in level and of reef-lime- 

 stone formation during times of subsidence seems to be furnished by 

 Vanua Mbalavu, there is no reason for believing that this interesting 

 island is an exceptionally favorable illustration of Darv^in's theory. My 

 reason for visiting it was not that its history was thought to be more sig- 

 nificant than that of other islands in Fiji, but simply that it was on the 

 route of a small trading steamer : a hurricane happened along as we 

 arrived there, and refuge was taken for two nights and a day in one of 

 the limestone embayments; the volcanic part of the island was seen on 

 the following day of fine weather, after which another limestone embay- 

 ment was entered for a night. In the first embayment the limestones 

 were seen to be horizontally stratified, as shov^n in section in the middle 

 of sector E, figure 6. 



The geological history above inferred for the island has been confirmed 

 by Foye's more detailed observations (1917). Other islands may well 

 have an equally suggestive story to tell; thus the volcanic island of 



