EVOLUTION OF LIMESTONE AND DOLOMITE 409 



The calcium content of the ocean is a variable controlled by 

 its solution and deposition in the ocean and its introduction from 

 the lands. It is therefore barely possible that calcium is now 

 accumulating in the sea, partly from direct chemical reasons, the 

 calcium content of the ocean being below the saturation point, 

 and partly because the shallow-water area, most conducive to the 

 biochemical deposition of calcium carbonate, is relatively limited 

 during the present epoch of continental expansion; and partly 

 because the shallow waters bordering the continents are now con- 

 siderably polluted by mud and other land debris which depreciates 

 the shore zone as a habitat for lime-secreting organisms. Present 

 climatic conditions also restrict the life zones favorable to shallow- 

 water limestone deposition. Murray and Irvine 1 have concluded 

 from experimental evidence that the calcium carbonate of the 

 sea is probably nearly constant in quantity, since the precipitating 

 agents of the sea probably maintain a balance between the intro- 

 duction and deposition of calcium carbonate, despite the fact 

 that the calcium carbonate content of the sea is below the satura- 

 tion point. Whether or not calcium carbonate is actually accumu- 

 lating in the sea seems uncertain, when the wide range of pre- 

 cipitating conditions controlled by the temperature, pressure, and 

 the relative abundance of living and decaying organisms is con- 

 sidered. Nor would annual analyses of sea water give any clue, 

 since it has been estimated that it would require about 680,000 

 years to accumulate the calcium carbonate now in the sea at the 

 present rate of contribution from the land, a fact which in itself 

 may be significant of the possibilities in this problem. Judging 

 from the selective solubility of calcium carbonate with respect to 

 depth, it seems that the widening of the epicontinental sea to 

 approximately 30,000,000 square miles during the Carboniferous, 

 as estimated by Chamberlin, must have given a tremendous 

 impetus to the deposition of calcium carbonates on these extensive 

 shallows. Here wave agitation might cause mechanical precipi- 

 tation, and would minimize the carbonic acid content of the waters 

 which might otherwise be influenced by organic decay. Evapora- 

 tion would tend to cause concentration. Thus in shallow waters, 



1 Proc. Roy. Soc. Edinburgh, XVII (1890). Si. 



