794 A TREATISE ON METAMORPHISM. 



connected seas are not saturated with calcium, or even approximately so. 

 According to Mendele'en 7 ," 1 kg. of water saturated with carbon dioxide will 

 dissolve 3 grams of calcium carbonate. The amount of calcium carbonate 

 in the ocean at the present time, per kilogram of water, is .11869 gram, or 

 3.956 per cent of the above amount;'' but since sea water is not saturated 

 with C0 2 , the amount of calcium carbonate which could be held is less 

 than 3 grams per kilogram. It is certain that under present conditions the 

 precipitation of calcium carbonate from the ocean is mainly accomplished 

 by organisms, as explained on pages 792-793. But in early geological 

 periods, before life was abundant and great quantities of calcium carbonate 

 were extracted from the sea by animals, it is barely possible that super- 

 saturation occurred and chemical precipitation resulted. Supersaturation 

 could have been brought about by two factors: First, steady additions of 

 calcium carbonate from the land in connection with evaporation from the 

 sea produced concentration. Second, it is possible that the amount of 

 carbon dioxide in the atmosphere, and therefore in the ocean, was greater 

 then than at present. As the carbon dioxide in the atmosphere was used 

 up by the processes of carbonation, carbon dioxide would pass from the 

 ocean to the atmosphere, and a decrease in the amount of this compound 

 in solution would result in chemical precipitation of the calcium carbonate. 

 The two causes for precipitation considered apply to the entire body 

 of the ocean. Precipitation caused by them alone would be very wide- 

 spread, if not universal. The theory of chemical precipitation in the ocean 

 has greater plausibility if there be combined with the two previous causes 

 the existence of partly inclosed tropical mediterranean seas, which received 

 great contributions of materials from large rivers and lost great quantities of 

 water by evaporation. Under these circumstances chemical precipitation 

 might occur without premising saturation of the entire ocean. It is therefore 

 theoretically possible that extensive limestone deposits were produced by 

 chemical precipitation in early geological ages. But if the ocean as a whole 

 did at any time contain enough calcium carbonate to saturate it, or even 

 enough to nearly saturate it, the subsequent process of limestone building by 



a Mendeleeff, D., The principles of chemistry, trans, by Geo. Kamensky, Longmans, Green & Co., 

 London, 1897, vol. 1, p. 592. 



'Dittrnar, William, The composition of ocean water: Report of the Scientific Results of the 

 Exploring Voyage of H. M. S. Challenger, 1873-76; vol. r, Physics and Chemistry, 1884, pp. 2, 204. 



<■ Mendeleeff, cit, vol. 1, p. 592. 



