156 R. A. DALY THE EVOLUTION OF THE LIMESTONES 



Since these various waters were working under like climatic (solu- 

 tional) conditions, the control of the terrane over the amounts of dis- 

 solved calcium and magnesium is manifest. 



After a detailed study of the question. Dubois estimates that on an 

 average, ceteris paribus, rivers flowing entirely over silicate rocks carry 

 only one-tenth as much calcium carbonate as rivers flowing entirely over 

 limestone, and remarks that even this fraction is almost certainly too 

 large. According to his estimates, only one-thirtieth of the calcium car- 

 bonate annually entering the sea has been newly formed through the 

 decomposition of silicates. The rest is derived from the direct solution 

 of limestone. He has further concluded that in early Archean time the 

 world's river system probably carried each year not more than one- 

 eighth as much carbonate to the ocean as the existing river system 

 carries. 4 



Effects of the post-Huroxiax Bevoltttiox 



From the lithological nature of the Huronian and pre-Huronian for- 

 mations as well as from other general considerations, we may believe 

 that the Huronian and pre-Huronian lands were chiefly composed of 

 acid, granitic and schistose rocks. The post-Huronian orogenic revolu- 

 tion lifted very thick and extensive ( Grenville and other) limestones, as 

 well as huge masses of basaltic rocks above baselevel. 5 From the quanti- 

 tative studies of Hanamann and Dubois we may believe with equal 

 readiness that the annual supply of calcium to the ocean after the revo- 

 lution was from two to five or more times that characteristic of Huronian 

 and pre-Huronian time. 



The revolution must have had another important effect — in decreasing 

 the sea-bottom area on which the precipitation of calcium carbonate took 

 place. The researches of the "Challenger' 5 chemists show that at depths 

 greater than 3,000 fathoms, little or no solid calcium carbonate can re- 

 main on the sea-floor. In fact, the tendency to the complete solution of 

 this salt is strong at all depths greater than 2,500, if not 2,000 fathoms. 

 This means that the permanent removal of calcium carbonate from the 

 present oceanic solution through the decay of animal carcasses at the 

 bottom seems to be possible only in about one-half of the existing ocean 

 basin — say 70,000,000 square miles. This area is partly neritic (depths 

 less than 200 fathoms) and partly bathyal (depths between 200 fathoms 

 and 2,000 fathoms ) . On account of the higher temperatures and lower 

 bottom pressures (pressure increasing the solubility of the carbonate) of 



4 E. Dubois : Proceedings of the Section of Sciences. Kon. Akad. van Wetenschappen. 

 Amsterdam, vol. 3. pp. 119-126. 



5 Cf. F. D. Adams : Journal of Geology, vol. 16. 190S, p. 617. 



