THE CHEMICAL EXAxMINATION OF THE MATERIALS FEOM FUNAFUTI. 383 



consequent accumulation of the magnesium carbonate, there cannot be any doubt, but 

 the exact nature of these conditions has not yet been determined. 



(d) Bearing of the Analyses made of the Funafuti Materials, upon the question of the 

 Causes of Dolomitisation in Coral- Reef Rocks. 



It has been conclusively shown by modern researches — as pointed out in the 

 preceding pages — that the free carbon dioxide in ocean waters exercises a solvent 

 action upon all carbonates carried down to the sea in suspension, as well as upon the 

 calcareous skeletons of plants and animals which have lived in the ocean and have 

 derived the materials of their skeletons from its waters. How far this solvent action 

 has been aided by the great pressure in the deeper parts of the ocean has not been 

 determined. Neither has the influence of temperature been exactly ascertained, 

 though the examples of altered Lithothamnion cited from Arctic regions (see p. 377) 

 prove that even in waters only a little above the freezing point solvent action is 

 taking place. 



This solvent action goes on more rapidly in the skeletons of those organisms which 

 are composed of the unstable aragonite than in those which consist of the more stable 

 calcite ; and where the skeleton consists in parts of different forms of calcium carbonate, 

 the aragonite portions are removed more rapidly than the calcite portions. 



It has been pointed out that in those organisms of loose texture, like corals and 

 calcareous algae, or those which contain much organic tissue combined with the 

 mineral matter of their skeletons, like fish and crustacean skeletons, the work of 

 solution appears to go on more rapidly than in the case of skeletons of compact 

 texture with little organic matter, like some of the foraminifera. As Sir John 

 Murray and Mr. Irvine remark : — 



" All those shells in which a considerable quantity of organic tissue is associated 

 with the carbonate of lime disappear in solution more rapidly than the shells of the 

 Foraminifera which contain little organic matter. During the whole of the 

 ' Challenger ' cruise only two bones of fishes, other than the otoliths and the teeth, 

 were dredged from the deposits, and all traces of the cetacean bones were removed, 

 except the dense ear bones, and the dense ziphioid beaks. The remains of crustacean 

 animals were almost wholly absent from deep-sea deposits, with the exception of 

 ostracod shells and the hard tips of some claws of crabs."* 



There can be little doubt that this result is due to the circumstance that the slow 

 decay and oxidation of the organic tissues must produce carbon dioxide in the most 

 intimate admixture with the carbonates, and that under these conditions solution 

 proceeds rapidly. 



There is reason to believe that this solvent action goes on with especial rapidity 

 in the case of the calcareous algae, Lithothamnion and Halimeda, which are now 



&" 



^Roy. Soc. Edin. Proc.,' vol. 17 (1889), p. 99. 



