544 W. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



formed deltas, well shown on charts in Agassiz' Fiji report (1899, plates 

 5, 6), that have not only filled the embayments in which the rivers must 

 have mouthed when the last submergence took place, but have advanced 

 so as to narrow and shoal the lagoon. Many fringing reefs must have 

 been smothered under the widening front of the advancing deltas. The 

 Eewa, the largest river of the island, discharges a great volume of water 

 when in flood; its delta has already covered certain parts of the barrier 

 reef, and has converted the narrow lagoon, ordinarily 15 or 20 fathoms 

 deep, into shallow mud flats for a long-shore distance of 15 miles. A 

 continuance of delta growth, undiminished by future subsidence, may be 

 expected to kill the corals on the outer face of the barrier reef as far as 

 the river muds are spread, and the progradation or retrogradation of the 

 delta front will then depend on the relative strength of the constructive 

 river and the destructive waves. 



It is stated above that the "soapstone" strata of the dissected and partly 

 submerged coastal plain of mid-southern Viti Levu rest unconformably 

 on an eroded foundation of resistant volcanic rocks, and this implies that 

 subsidence took place during the deposition of the coastal-plain strata. 

 A coral reef should have been formed during that subsidence, first as a 

 fringe, later as a barrier, and it is highly probably that a reef of this 

 kind was formed during the earlier stages of soapstone deposition ; but no 

 such reef is now visible in the soapstone district, although an elevated 

 reef, apparently of "soapstone" age, occurs farther west, vv^here the soap- 

 stone is of small thickness or wanting. The elevated reef at Suva, in the 

 soapstone district, is a small affair that was formed and buried as a con- 

 formable member of the soapstone series. It is therefore permissible to 

 suppose that the incipient barrier reef, which should have been formed in 

 the earlier stages of soapstone deposition, was smothered and buried in 

 the later stages. This supposition is made the more reasonable when it is 

 recognized that the delta deposits now accumulated within and on the 

 present barrier reef are small in volume compared to that of the soapstone 

 series, and yet they are almost sufficient, near the rivers, to smother the 

 barrier reef of today. 



As far as information is at hand, the northwestern side of Viti Levu 

 is not bordered by soapstone strata; and the barrier reef there incloses a 

 vast lagoon, which deepens offshore to nearly 60 fathoms, as has been 

 adverted to above and as will be further noted below. Thus here, as at 

 Vanua Mbalavu, a tilting of the land-mass seems to have accompanied 

 the uplift by which the soapstones were emerged, and the subsidence in 

 response to which the present barrier reef grew up. Changes of ocean 

 level could not determine unequal changes of this kind. 



