1898.] Funafuti, Rotuma and Fiji. 469 



of the dolomitization having taken place under water with the 

 reef in situ. Abrasion from climatic causes has certainly gone on 

 to a very large extent. The rain water takes up a certain amount 

 of carbonic acid gas, when exposed to the air, and by means 

 of this acts on the limestone, dissolving its carbonates of calcium 

 and magnesium. There can be no doubt that this is due to a 

 chemical compound being formed, and I would suggest that a 

 certain amount of pure calcium magnesium carbonate \MgCa(C0 3 ) 2 } 

 is always formed at the same time and deposited on the evapora- 

 tion of the water. One litre of water saturated with carbonic 

 acid gas under atmospheric pressure dissolves '31 gram of calcium 

 magnesium carbonate at 18° C, '88 gram of calcium carbonate at 

 10° C, and 1"31 gram of magnesium carbonate at 10° C 1 . Pure 

 dolomite hence is far less soluble than either calcium or mag- 

 nesium carbonates, and thus its percentage in the rock should 

 slowly be increased, especially to the detriment of the carbonate 

 of lime, which in the first place formed the greater part of the 

 rock. The rock should then practically be a mixture of pure 

 calcium magnesium carbonate with pure carbonate of lime and a 

 small quantity of earthy ingredients. It should vary greatly in 

 the relative amounts of the carbonates of calcium and magnesium, 

 and the analyses, before mentioned, indicate that it does so ; 

 Dana also from similar ] rock from the island of Metia noted 38'07 

 per cent, and 5*29 per cent, of carbonate of magnesium 2 . 



An additional factor in the formation of the dolomite might be 

 the large amount of magnesium salts raised in the sea-water of 

 the lagoons and pools in the reef at the time that these islands 

 were elevated. Dittmar in the Challenger Reports has shown 

 that magnesium chloride forms 10878 per cent, of the salts in sea 

 water, and that other magnesium salts form nearly 5 per cent., 

 while lime salts do not form quite 4 per cent. On the external 

 cliffs the spray from the sea might have some effect with a slow 

 elevation, but the specimens analysed from Namuka were both 

 taken from the inner side of the ridge ; a series of analyses of 

 specimens taken at intervals from a short horizontal core obtained 

 from one of the outer cliffs would, however, at once settle the 

 extent to which this has been a factor of importance. 



There is no method of estimating directly the amount of time 

 which would be required for this metamorphosis to a hard crystal- 

 line state. If it has taken place in the Lau Group as I suppose, 

 it should in course of time take place also in the raised reefs of 

 Viti Levu. It may be argued that traces of such a metamorphosis 

 are not seen in other parts of the world, and that at any rate they 

 should be seen in the exposed limestone cliffs of Viti Levu. They 



1 Handbuch der Anorganischen Chemie. Daminer n. 2. 



2 Loc. cit. p. 393. 



