AGASSIZ : FIJI ISLANDS AND CORAL REEFS. 8 



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2,500 feet from the shore line, and perhaps seven feet above hio-h water 

 mark. Down to a depth of 80 feet nothing but recent reef coral rock 

 was encountered, but from that point to a depth of over 300 feet the 

 limestone passed through was of a very different character. It contained 

 but few corals, being composed almost entirely of the shells of moUusks, 

 maiuly bivalves. The rock was white, chalky, and resembled in every 

 way the rocks of the Vicksburg age of Florida and of Yucatan ; but its 

 age has not yet been accurately determined. Enough, however, is clear 

 to show that the limestones which form the substratum upon which 

 rests the recent fringing reef of Honolulu do not belong to the present 

 period. Mr. ^IcCandless assured me that limestones like those I had 

 the opportunity of examining while the boring was going on are identi- 

 cal 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 mollusks he had only paid attention to the occurrence 

 of occasional corals, and supposed that the lower limestone formed 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 

 mainly 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 conclu- 

 sions both as to the age of the limestones and regarding the position of 

 the line of demarcation between the modern coral reef and the under- 

 lying older limestone substratum. 



If my conclusion is correct, that such atolls in the Fiji Islands as Wai- 

 langilala, Xgele Levu, and many others to which I have referred in this 

 report, are foi'med upon platforms of submarine erosion of elevated ter- 

 tiary limestones, and if, further, in similar atolls in the Paumotus, the 

 Gilbert, Ellice, and other groups, the substratum underlying the mod- 

 ern coral reef is likewise composed of tertiary limestones, it will become 

 apparent that such borings as those 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 underlying volcanic 

 substratum, will only give us the thickness of the tertiary coralliferous 

 limestone beds forming the substratum upon which the modern coral reef 

 has grown, — a thickness which in the Ellice group can only be ascer- 

 tained by boring, while in Fiji it can be ascertained approximately from 

 the height of the islands composed of elevated tertiary limestones. 



