Agassiz — Tertiary Elevated Limestone Reefs of Fiji. 167 



ing reef of Honolulu do not belong to the present period. 

 Mr. McCandless assured me that limestones like those I had 

 the opportunity of examining while the boring was going on, 

 are identical with those to which he called Mr. Dana's and my 

 attention in 1888, and that until I pointed out to him that the 

 white limestone was almost wholly made up of Mollusca he had 

 only paid attention to the occurrence of occasional corals and 

 supposed the lower limestone to form the continuation of the 

 recent modern reef. But as I have stated, this lower limestone 

 differs from reef rock both lithologically and in its being 

 made up of fossil mollusks. 



It is very clear that when boring in a coral reef district in 

 which it is difficult or impossible from other data to determine 

 what geological changes may have taken place or the probable 

 age of any limestone we may pass through in boring, it may 

 be very easy to draw wrong conclusions both as to the age of 

 the limestones and regarding the position of the line of 

 demarcation between the modern coral reef and the underly- 

 ing older limestone substratum. 



If my conclusion that such atolls in the Fiji as Wailangilala, 

 Ngele Levu and many others to which I shall refer in my final 

 report, are formed upon platforms of marine erosion of elevated 

 Tertiary limestones is correct, and if further in similar atolls in 

 the Paumotus, the Gilbert, Ellice and other groups, the sub- 

 stratum underlying the modern coral reef is likewise composed 

 of Tertiary limestones, it will become apparent that such bor- 

 ing as that carried on at Funafuti will not help us in any 

 way to solve the problem of the formation of atolls by modern 

 coral reefs. Such a boring, even should it reach the underly- 

 ing volcanic substratum, will only give us the thickness of the 

 Tertiary limestone beds forming the substratum upon which 

 the modern coral reef has grown, a thickness which in the 

 Ellice group can only be ascertained by boring, while in Fiji it 

 can be ascertained approximately from the height of the 

 islands composed of elevated Tertiary limestones. 



Under what conditions these Tertiary coralliferous lime- 

 stones of great thickness have been deposited is a distinct 

 question from that of the formation of atolls through subsi- 

 dence by the upward growth of corals during the present 

 geological period. Neither the borings through a coral reef 

 growing upon a substratum of Tertiary limestone, nor the exam- 

 ination of the outer edge of a coral reef formed upon a substra- 

 tum of volcanic rocks, has given us (in Fiji) any evidence of 

 the great thickness of a modern coral reef. On the contrary, 

 all the evidence I have gathered in Fiji tends to prove that a 

 coral reef forms only a comparatively thin crust upon the plat- 

 form of submarine erosion (whatever be its geological struc- 

 ture) upon which it may have found a footing. 



