EVOLUTION OF LIMESTONE AND DOLOMITE 425 



precipitated would depend upon conditions of equilibrium con- 

 trolled mainly by temperature, concentration, the amount of 

 carbonic acid in the air and ocean, and organic processes. As a 

 result of the preceding selective influences, the calcium to magne- 

 sium ratio of the limestones would tend to be higher than that of 

 the solutions contributed to the sea. However, contemporaneous 

 with continental expansion, as at the present time, limestone deposi- 

 tion would be at a minimum, which might involve a concentration 

 of calcium in the sea until more favorable conditions of precipi- 

 tation arise. Limited areas of shallow water, vigorous erosion, 

 continental climates, and other supplementary conditions make 

 periods of continental expansion more favorable to the deposition 

 of elastics than limestones. 



Gradually the lands waste away, the ocean advances over the 

 continents, partly in consequence of fill from the land, in part, 

 perhaps, as a result of secular earth movements which cause a 

 shallowing of the ocean basins. The rivers carry less and less 

 debris to the sea, and deposit it farther and farther inland from 

 the margin. On the submerged continental areas, covered by 

 shallow seas, which now may be three or more times as extensive 

 as they were during the preceding period of continental expansion, 

 chemical and biochemical processes combine in making this an 

 era of limestone building. From experimental and field evidence, 

 the inference is drawn that the ratio of calcium to magnesium in 

 the deposited limestones is influenced primarily by their respective 

 rate of contribution from the land, and modified by selective organic 

 and inorganic agencies working to a common end. 



As postulated by Chamberlin, with the expansion of the seas, 

 the zonal, diversified continental climates tending toward aridity 

 and refrigeration yield to more uniform, mild atmospheric con- 

 ditions. A widening of the life zones favorable to limestone 

 deposition follows. Thus in the Devonian, corals thrived in the 

 now ungenial climate of Hudson Bay. With world-wide climatic 

 moderation, a new condition of equilibrium is established between 

 the carbon dioxide of the sea and air. Warm water absorbs less 

 carbon dioxide than cold. The sea begins to contribute its excess 

 of carbon dioxide to the air, in consequence of which the calcium 



