252 W. M. Davis — Shaler Memorial Study of Coral Reefs. 



stones, have been submerged to a depth at least as great as the 

 present height of the limestone hills, leaving only the higher 

 volcanic summits visible in the resulting barrier-reef lagoon, as 

 in block 4. Submergence must then have been reversed to 

 emergence, as a result of which the island reached a greater 

 altitude above the sea than to-day, as in block 5 ; for only a 

 greater altitude could have permitted mature dissection deep 

 enough, as in block 6, to produce after a second submergence 

 the embayed shore line now seen around the dissected limestone 

 mass in block 7, as well as around the volcanic slopes. It 

 must have been during the second submergence that the 

 present barrier reef was formed by upward growth. 



It is interesting to recall in this connection that conclusions 

 very similar to those here announced for an elevated reef in 

 Fiji were reached over thirty years ago by W. O. Crosby in 

 his study of the elevated reefs of Cuba. His brief article con- 

 tains the folio wins: points: The lowest reef, "for hundreds of 

 miles, has a sensibly uniform altitude of about thirty feet, and 

 is unbroken, save where rivers have cut through it to reach the 

 sea." It " varies in width from a few rods to a mile or 

 more . . . The indescribably jagged and ragged rock is a lime- 

 stone, and largely made up of several kinds of modern-looking 

 corals . . . Near the landward side of the reef and especially 

 toward the bottom, as may be observed in the natural sections, 

 the coral limestone is interstratified with layers of sand and 

 gravel, materials washed from the hills while the reef was 

 growing. These beds are generally horizontal or slightly 

 inclined toward the sea. As we should naturally expect, this 

 fragmental material is most abundant near the tnouths of rivers, 

 where the reef is sometimes principally composed of it, showing 

 that the modern river valleys are older than the reef."* An 

 unconformable relation between the reef and its foundation is 

 not explicitly stated, but it is clearly implied. In view of all 

 this, Crosby concludes that this reef was formed during a sub- 

 mergence that preceded recent elevation. 



The movements of emergence and submergence recognized 

 on Yanua Mbalavu seem to have been local, for similar contem- 

 poraneous movements are not recorded on all the Fiji islands; 

 but as the islands are several miles apart, moderate warping 

 without pronounced faulting is sufficient to account for their 

 unlike behavior. Submergence being thus shown to be the 

 apparently essential condition for the formation of the heavy 

 limestone mass of Vanua Mbalavu, now uplifted and dissected, 

 it becomes highly probable that the original stand of the dis- 

 sected volcanic island was higher, as in block 2, than is shown 

 in block 3, in order that the entire thickness of the older reef 



*On the Elevated Reefs of Cuba, Proc. Boston Sbc. Nat. Hist., xxii, 124- 

 130, 1884 ; see pp. 124, 125. 



