FIRST CALCAREOUS FOSSILS 163 



ing system may already have had some effect in the late pre-Cambrian, 

 thus increasing the chances that some calcium could remain in the oceanic 

 solution. 



Since lower Cambrian time the continents have in part undergone sub- 

 mergence and emergence, but they have doubtless never resumed their 

 small total area characteristic of the pre-Huronian period. In any case 

 we have obvious proofs that the ocean has, since the Cambrian, contained 

 enough calcium for the needs of lime-secreting organisms, and the natural 

 explanation is to be found in river inflow. 



Origin of the pre-Cambrian and early Paleozoic Limestones and 



Dolomites 



The hypothesis that the pre-Cambrian sea was nearly limeless involves 

 the corollary that magnesium as well as calcium must have been precipi- 

 tated from the sea-water. The precipitated salt may have been the 

 hydrous carbonate of magnesium, which then united with the calcium 

 carbonate to form dolomite ; or crystals of dolomite may have grown at or 

 near the surface of the bottom mud in much the same way as they are 

 growing today in the buried (porous) strata of the Funafuti atoll. 8 The 

 chemical grounds for this belief were partially discussed in the writer's 

 first paper. 9 It was there pointed out that ammonium carbonate is, 

 under the conditions of the open-sea floor, almost or quite powerless to 

 precipitate magnesium carbonate from the oceanic solution unless all 

 calcium salts have been removed from the solution. In the absence of 

 calcium salts, magnesium can slowly but surely be precipitated by the 

 alkali. We should therefore expect that the formation of magnesium 

 limestones would continue in the ocean until the general scavenging sys- 

 tem was established, thus largely inhibiting the action of the powerful 

 organic alkali. On this view the average pre-Cambrian limestone should 

 show a ratio of calcium to magnesium which is close to their ratio in the 

 average pre-Cambrian river. A similar ratio should characterize those 

 Paleozoic limestones that were formed before the establishment of the 

 general scavenging system. After that system was established, magnesium 

 would begin to accumulate in the ocean. 



Average Eatio of Calcium to Magnesium in the Limestones of 

 the different periods 



The writer has attempted to test these conclusions quantitatively. For 

 this purpose nearly 900 analyses of types of pre-Cambrian, Paleozoic, 



8 The atoll of Funafuti: London, 1904, pp. 392, 413, etc. 



9 American Journal of Science, vol. 23, 1907, p. 104. 



