M. A. Daly — Limeless Ocean of Pre- Cambrian Time. 97 



precipitated carbonate of calcium. The usual equations for 

 the reactions may be noted : 



CaS0 4 + (NH 4 ) 2 C0 3 = CaC0 3 + (NH 4 )„S0 4 

 CaCl 2 +(NH 4 ) s CO s = CaCO 3 + 2NH 4 0l 



Both of these reactions are reversible,* so that new calcium 

 carbonate introduced by rivers into sea-water after the original 

 sulphate and chloride had been converted, would be first 

 changed to the sulphate or chloride and then finally pre- 

 cipitated. 



The precipitation occurs, of course, only in the bottom 

 stratum of the sea-water. Diffusion and the vertical inter- 

 change of water must tend, in a long period, to remove all the 

 calcium salts from the ocean. At length there would remain 

 in solution only a minute quantity of calcium salts brought 

 into the ocean by the short pre-Cambrian rivers and not yet 

 diffused to the bottom stratum. 



Experiment shows that the pure magnesium salts of sea- 

 water from which calcium salts have been eliminated are 

 unavailable for the elaboration of carbonate shells and skele- 

 tons by organisms, although the organisms live and thrive in 

 such water. Granting that the essential protoplasmic require- 

 ments were, in pre-Cambrian time, the same as now, experi- 

 ments thus show the complete possibility of abundant pre-Cam- 

 brian marine life in the form of soft-bodied, highly divershed 

 animal types. 



The Eozoic seon was, then, divided into two parts, a long 

 period during which the calcium salts inherited from the 

 Azoic sea were being precipitated, and a much longer period 

 during which the steady evolution of animal types took place 

 in an essentially limeless sea. 



Duration of the limeless sea. — The conditions suitable for 

 the development of lime-secreting organisms might have been 

 established in three different ways. 



Putrefaction on the sea-floor has, among its other effects, 

 the generation of much sulphuretted hydrogen by the decom- 

 position of sulphates. The bottom of the Eozoic ocean may 

 have thus been poisoned by the gas in a manner similar to that 

 observed in the world's largest perfect desert, the basin of the 

 Black Sea. The evolution of bottom scavengers or at least 

 the colonization of the general sea-bottom, may have been long 

 delayed. Nevertheless, it is possible that the emanation of 

 sulphuretted hydrogen from sea-water in which calcium sul- 

 phate was almost entirely removed (leaving magnesium sul- 



* Like hydrochloric acid, most chlorides are practically completely dis- 

 sociated in dilute aqueous solutions. Analytical Chemistry, by F. P. Tread- 

 well, trans, by W. T. Hall, New York, p. 249, 1905. 



