508 



AV. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



The surface of unconformable contact has about the same slope as the 

 uncovered spurs ; it dips under sealevel to an unknown depth, x, as shown 

 in the section within sector E : hence the volcanic island must have for- 

 merly stood, while suffering erosion before the limestones were formed, x 

 feet higher than now, as in sector A. The limestones are much eroded 

 and their upper surface must once have been ij feet higher than now. 

 Hence after the island had been maturely eroded it must have subsided 

 {x -\- 600 -\- y) feet while receiving the unconformable limestone cover, 

 which presumably assumed the form of an almost-atoU, sector B — that 

 is, a barrier-reef mass in which only a very small central island 930 — 

 (600 -\- y) feet high survived. As the present limestone remnants are 

 greatly reduced from their initial form, whatever that form was, it is 



Figure 6. — Evolution of Vanua Mhalavu, Fiji 



manifestly only as an inference that the initial form is here described as 

 a "barrier-reef mass" ; but that inference appears to me a very reasonable 

 one, in view of all the elements of the case. Had the original limestone 

 mass accumulated as a shoal or submarine platform, 30 or 40 fathoms 

 deep, bearing patches of coral here and there but not inclosed by a mar- 

 ginal coral reef, the volcanic island that rose from the shoal would have 

 been clift; but the volcanic island, where the limestone remnant lies 

 unconformably on it, as in sector E, has a moderate slope: hence the 

 limestones were probably accumulated as part of a great barrier-reef and 

 lagoon mass, as above stated. Had the central island of such a shoal been 

 protected by fringing reefs, it would not have been clift, but the altitude 

 of the fringing reefs above the general surface of the shoal would demand 

 a greater subsidence than is here assumed. 



