REPORT OF THE CHIEF ASTRONOMER 667 



SESSIONAL PAPER No. 25a 



rate, it was complete probably several million years ago seems evident from the 

 chemistry of the present ocean. According to Murray, the calcium sulphate 

 now dissolved in the ocean could be introduced by existing rivers in about 

 600,000 years. Since the sulphate is being rapidly decomposed by lime-secreting 

 organisms and converted into deposited carbonate, it is probable that much 

 more than 600,000 years have elapsed since the bathybial fishes and other scav- 

 engers colonized the general sea floor to depths of 2,500 fathoms. The test case 

 of the Black sea shows that the present content of calcium sulphate in ocean 

 water would be largely and rapidly diminished if the scavenging system were not 

 now at work in the ocean. 



The ratio of calcium to magnesium in the Ottawa river, the best available 

 type of rivers draining the average pre-Cambrian terrane, is 3.69 : 1. The ratio 

 for the Saint Lawrence, which is not far from representing a type of the rivers 

 which might drain the average late Paleozoic terrane, is 4.44 : 1. The ratio for 

 the Mississippi at Memphis, similarly a fair type of river draining the average 

 terranes of the Triassic, Jurassic, or Cretaceous, is 2.50:1. The ratio for the 

 Mississippi at New Orleans, a chemical world type for the present time is 3.9Q: 1. 

 The ratio for forty-four existing rivers is 4.18 : 1.* It appears, therefore, highly 

 probable that the ratio of calcium to magnesium for the world's entire river 

 system has been fairly constant from the pre-Cambrian to the present. We have 

 seen that this ratio is almost identical with that in the average pre-Devonian 

 limestone, but is much lower than the ratio for the Devonian and post-Devonian 

 limestones. Granting that the calcium and magnesium in sea water have been 

 introduced by the rivers, the sudden increase of the ratio Ca : Mg in the Devonian 

 limestones must mean that during the Devonian the magnesium began to accu- 

 mulate in the oceanic solution with special and unprecedented rapidity. On the 

 hypothesis that the ocean was nearly limeless in pre-Cambrian time and very 

 low in lime during early Paleozoic time, it follows that only a minute amount of 

 magnesium could have remained in the oceanic solution during pre-Devonian 

 time. 



Since the period of the general colonization of the sea-floor, the precipitation 

 of magnesium carbonate direct from sea water has been possible only under 

 special conditions, so that the more recent times have seen minimum formation 

 of magnesian deposits. The observations of Murray and Irvine on mud-waters 

 suggest that, at the present day, there may be a slow addition of magnesium 

 carbonate to the deposits of pure calcium carbonate shells or skeletons. A fairly 

 pure calcareous ooze or shell-bank or a porous coral reef may be charged with 

 decaying animal matter. "Within the myriad interstices of the deposit there is 

 sea water into which ammonium carbonate is being passed. This alkali preci- 

 pitates all the calcium salts in the quasi-imprisoned water. Thereafter will 

 follow a slow but steady precipitation of magnesium carbonate within the ooze 



* So far as this ratio is concerned, a single analysis of a river may have high 

 value in the discussion, since Dubois has shown that, no matter how much the 

 absolute amounts of solute in a river may vary throughout the year, the propor- 

 tions of the different salts remain nearly unchanged (E. Dubois, op. cit., p. 4S). 



