154 R, A. DALY THE EVOLUTION OF THE LIMESTONES 



d, the inflow of salts-laden "juvenile" waters in the form of submarine 

 springs which are fed from the earth's interior. 



In tracing the history of the ocean water, the variations in the supply 

 of river-borne salts and in the rate of chemical precipitation of salts are 

 the factors which seem to permit of at least a crude analysis. They are 

 likewise the factors most important in their bearing on the origin of the 

 calcareous fossils in the rocks and on the origin of the limestones and 

 dolomites. These two problems have been attacked again and again, 

 but rarely, in either case, on the basis of a variable content of mineral 

 matter in the oceanic solution. 



The writer has discussed at some length the hypothesis that the secre- 

 tion of calcareous hard parts by marine organisms was first made possible 

 as a result of the increase of the land areas during the post-Huronian 

 orogenic revolution. 2 That enlargement of the continents caused a great 

 increase in the annual supply of river-borne salts to the ocean. The sup- 

 ply was specially enlarged by the upturning and erosion of the thick lime- 

 stones which had been deposited on the sea-floor of earlier pre-Cambrian 

 time. These limestones are regarded, on the hypothesis, as precipitates 

 of calcium and magnesium carbonates, thrown down when the river- 

 borne salts diffused to the ancient sea-bottom. The chief reagent for the 

 precipitation is considered to be the ammonium carbonate generated by 

 the decay of animal matter.* It is further postulated that in pre-Cam- 

 brian time the active scavenging system had not yet been evolved; 

 that therefore the amount of decaying animal matter on the pre- 

 Cambrian sea-floor was vastly greater than the amount now allowed to 

 decay on the bottom of the ocean. The smallness of the annual supply 

 of river-borne calcium salts, coupled with this specially rapid precipita- 

 tion of calcium carbonate, is supposed to have kept the pre-Huronian 

 ocean nearly limeless; only the minute traces of calcium salts contained 

 in the river waters as they diffused to the sea-bottom would be found in 

 the ocean of that time. At the bottom the water would be practically 

 limeless. 



The nearly limeless condition of the surface water was changed by the 

 extensive orogenic and epeirogenic movements of post-Huronian time. 

 In the Cambrian period the animal species had begun to armor them- 

 selves with the new material, henceforth present in the sea-water in suffi- 

 cient amount. The primitive chitinous shell now became strengthened 

 with phosphate and carbonate of calcium, and in the Ordovician many 

 species had adopted the armor or skeleton of pure calcium carbonate. 



2 American Journal of Science, vol. xxiii, 1907, p. 93. 



* The reaction is the same as that occurring within the living mollusc as it "secretes" 

 its shell. See R. Irvine and G. S. Woodhead. Proceedings Royal Society of Edin- 

 burgh, vol. 16, 1889, p. 352. 



