It. A. Daly — Limeless Ocean of Pre-Cambrian Time. 107 



In the mud-water calcium sulphate is absent, magnesium 

 sulphate is deficient when compared with average sea- water; 

 calcium carbonate is increased, and magnesium carbonate and 

 ammonium sulphate are both present. The high chlorides 

 show that the carbonates are not in excess because of fresh- 

 water inflow. The ratio of magnesium carbonate to calcium 

 carbonate is 1:3. When the clear water filtered from the 

 mud was boiled for a short time, a crystalline precipitate was 

 thrown down, consisting of 73 - 3 per cent calcium carbonate 

 and 26*7 per cent magnesium carbonate. The formation of 

 both carbonates is ascribed by Murray and Irvine to the 

 reaction of ammonium carbonate chiefly on the sulphates, a 

 conclusion which cannot be doubted, especially in view of the 

 presence of ammonium sulphate in the mud-water. The 

 alkaline carbonate was, of course, derived from decaying 

 animal matter contained in the muds. 



These different experiments teach that hydrous carbonate of 

 magnesium can be precipitated by ammonium carbonate 

 emitted from decaying animal remains ; that the precipitation 

 is much slower than in the case of calcium carbonate and is 

 retarded by the presence of calcium salts in the solution. We 

 thus see how, in the nearly limeless sea-water of pre-Cam- 

 brian time, the proportion of precipitated magnesium car- 

 bonate would be high, even, possibly, approaching the ratio in 

 true dolomite. Indeed, it is quite possible that precipitates of 

 pure basic carbonate of magnesium later changed to magnesite, 

 were formed in those places in the sea-basin where the calcium 

 salts were completely absent from the oceanic composition. 



Clearly our ideas must not be too rigid regarding the 

 detailed history of these early magnesium deposits. We can- 

 not say how far the sea-waters in which animal life first began 

 were charged with magnesium salts. We cannot say how far 

 these and the other salts brought in by the early rivers con- 

 tributed to the formation of the extensive dolomites and 

 magnesium limestones known to occur in pre-Cambrian 

 terranes. Our hypothesis holds that the calcium carbonate of 

 the dolomites and of the pure calcium-limestones was, for most 

 of the Eozoic seon, introduced to the sea by the rivers. Not- 

 withstanding the slowness of the precipitation of magnesium 

 carbonate at ordinary temperatures, some excess of magnesium 

 salts in solution in that sea might easily permit the basic 

 magnesium carbonate to be thrown down in very high pro- 

 portion when compared with the precipitate of the other 

 carbonate. What determined the actual composition of any 

 one bed cannot be declared. Opposite the mouth of a large 

 river we might expect beds of practically pure calcium 

 carbonate. Far from shores the chemical deposit would be 



