[ im ] 



SECTION XIV. 



THE MINEKALOGICAL CHANGES OBSERVED IN THE CORES OF THE 



FUNAFUTI BORINGS. 



Bi/ C. Gilbert Cullis, D.Sc, F.G.S. 



Contents. 



Page 



1. Introduction 392 



2. The Mineralogiciil Charactei's of tlie Kocks occurring between the Surface and 637 feet . . . 393 



(I. The Deposition of Secondary Calcite and Aragonite from Sohition 394 



/'. The Crystallisation of the fine Calcareous Detritus 398 



c. The Disappearance of Aragonite 400 



3. The Mineralogical Characters of the Kocks from G37 feet down to 1114 feet 404 



a. The Keplacement of Calcite and Aragonite by Dolomite, with the Formation of Casts . 405 



I). The Occurrence of FiI)rous Deposits in the Cavities 409 



c. The Characters of the Lowest Rocks reached in the Borins;s 414 



1. Introduction. 



The minerals which occur as the recognisable constituents of the Funafuti cores are 

 calcite, aragonite and dolomite. 



The small amount of calcium phosphate, which analysis shows to be present in all 

 l)arts of the boring, is probably included within these, as an invisible impurity ; it 

 has not been detected as a distinct mineral. The possi})ility of tlie occurrence of 

 magnesite, in certain of the rocks, has not been overlooked, but no evidence that 

 }nagnesium carbonate exists in them, otherwise than in combination with calcium 

 car})onate, has presented itself. 



The discrimination ])etween calcite, araji'onite and dolomite, in thin sections of the 

 rocks of modern coral reefs, which consist so largely of fresh calcareous organisms, 

 and in which sucli crystals as occur are generally of microscopic dimensions, is not 

 always a simple matter ; it is most readily effected by micro-chemicaJ methods. Of 

 these there are two which are of special value, and which have been used with the 

 most satisfactory results in the })resent investigation. The first is that of Meigen,* 



* ' Centialblatt fiir Mineialogie,' 1901, pp. 577, 578. 



