I?. A. Daly — Limeless Ocean of Pre-Cambrian Time. 105 



other hand, we must conclude that the long-continued precipi- 

 tation of magnesium carbonate was effected by the action of a 

 strongly alkaline carbonate. We are thus naturally led to the 

 discussion of the possible precipitation of magnesium carbon- 

 ate also by the ammonium carbonate emanating from decaying 

 animals. 



The experiments on this subject are, at first sight, contradic- 

 tory. Linck has recently shown that when ammonium carbonate 

 is added to sea- water, aragonite is precipitated but no magnesium 

 carbonate was found by him in the precipitate.* On the other 

 hand, Pfaff, using an artificial sea-water similar in composition 

 to average sea-water, found that, after a certain interval of 

 time a part of the magnesium in the salts was thrown down 

 as the basic carbonate while there was an abundant precipita- 

 tion of calcium carbonate.f Pfaff's result accords with the 

 general experience of analytical chemists ; hydrous magnesium 

 carbonate will be precipitated by the alkaline carbonate if 

 time enough be allowed. 



The reactions for magnesium sulphate and chloride com- 

 monly assume the following forms : 



4MgS0 4 + 4(NH 4 ),C0 1 + H 9 = Mg 4 (C0 1 ) I (On) 1 + C0 9 



+ 4(NH 4 ) 2 S0 4 



and 



4MgCl 2 + 4(NH 4 ) 2 C0 3 + H 2 = Mg 4 (C0 3 ) 3 (OH) 2 + C0 2 + SNH 4 Cl 



The presence of ammonium chloride tends to prevent the pre- 

 cipitation of the carbonate from a solution of magnesium 

 chloride ; it is important, therefore, to note the fact that 

 ammonium chloride formed in this way at the sea-bottom dif- 

 fuses away to upper strata of the ocean -waters and would not 

 interfere with the final completion of the reaction. 



Besides the element of time and the undoubted presence of 

 an appreciable amount of magnesium salts in the pre-Cam- 

 brian sea, another principal factor must be considered. Hunt 

 has shown that the precipitation of magnesium carbonate from 

 sea-water by alkaline carbonates is facilitated if the calcium 

 salts be removed. Our hypothesis states that the latter were 

 absent from the bottom stratum through most of Eozoic time. 



Again we may turn to the noteworthy experiments of Mur- 

 ray and Irvine for a suggestion of the truth of the foregoing 

 statements. Their table No. YII is here reproduced. It 

 shows the composition of the precipitate thrown out of a mix- 

 ture of sea-water and urine after standing seven days, the 



*Nenes Jahrbuch fur Mineralogie, etc., Beilage Band xvi, p. 502, 1903. 

 f F. W. Pfaff, Neues Jahrbuch fur Mineralogie, etc., Beil. Bd., vol. ix, p. 

 504, 1894. 



