GENEKAL CONCLUSION 169 



of the pre- Cambrian and that of the present time. The comparison 

 strengthens the hypothesis that the ocean during an immense part of pre- 

 Cambrian time was chemically unfit for the secretion of calcareous tests 

 and skeletons. The pre-Cambrian fauna is thus regarded as largely a 

 "jellyfish" fauna, although siliceous and chitinous fossils may be looked 

 for in pre-Cambrian rocks. 



The ratio of calcium to magnesium in the rivers draining the pre- 

 Cambrian terrane is almost identical with the ratio of calcium to mag- 

 nesium in the average pre-Cambrian and pre-Devonian limestones. 

 Nearly all of these limestones are credited to chemical precipitation, 

 which steadily removed both calcium and magnesium from the pre- 

 Cambrian ocean as fast as those elements were introduced by the rivers. 

 The chemical reaction, which was largely or wholly responsible for the 

 precipitation of the carbonate rocks, is also the reaction considered as 

 responsible for the nearly limeless state of the pre-Cambrian ocean. Be- 

 cause the ancient dolomites and limestones were deposited, that ocean was 

 nearly limeless; in this conception there is neither paradox nor incon- 

 sistency. The actual content of calcium in the pre-Cambrian ocean was 

 at any moment extremely small, as the dilute solution of river-borne salts 

 diffused to the bottom; to the pre-Cambrian organisms the ocean was 

 practically limeless. 19 



In late pre-Cambrian time the deposition of the carbonates may have 

 been no more than one-tenth as rapid as the deposition of the carbonates 

 now forming, through all causes, beneath the sea. Immediately after the 

 orogenic revolution of the somewhat earlier pre-Cambrian (post-Huro- 

 nian), the deposition must have been at a maximum rate, though that 

 rate may not have reached the one now prevailing. In any case, estimates 

 of the earth's age, when derived from the rate and amount of past sedi- 

 mentation, should take account of secular variations in the supply of 

 river-borne salts to the ocean. 



It is suggested from the facts noted in this paper that the magnesium 

 now contained in the sea in amount greater than a mere trace began to 



19 As a result of his remarkable researches on the waters found in the deeper mines 

 of the Lake Superior region, Lane has concluded that these are "connate" waters — that 

 is, waters which were trapped and buried in the sediments and lava-flows formed on 

 the pre-Cambrian sea-floor (A. C. Lane's paper, read at the thirteenth annual meeting 

 of the Lake Superior Mining Institute, June, 1908). It may be a good working hypoth- 

 esis to consider the extraordinarily high content of chlorine in the many analyses as of 

 connate origin, but the likewise abundant calcium present can be explained as due to 

 solution along the walls of the ancient pores and fissures. To put it briefly, some ele- 

 ments of the mine waters may be connate and of marine derivation, but such original 

 water must have been chemically changed by metasomatic interchange with the in- 

 closing (always lime-bearing) rocks during the post-Cambrian period. Mine waters 

 from pre-Cambrian terranes can not, therefore, in the writer's view, afford safe indica- 

 tions as to the calcium content of the pre-Cambrian ocean. 



XIII — Bull. Geol. Soc, Am,, Vol, 20, 1908 



