108 R. A. Daly — Limeless Ocean of Pre-Cambrian Time. 



more magnesian. Gradual changes in the rivers, . in the 

 marine currents, or in the configuration of the coast-line would 

 cause alternations in the composition of the precipitate, the 

 magnesium component rising or falling according to the 

 highly variable circumstances. The beds would further be 

 indefinitely varied according to the proportion and kinds of 

 mechanical detritus intermixed with carbonates. Eozoic sedi- 

 ment may be fetid to-day because of the residual animal 

 matter imprisoned in such detritus and chemical precipitate. 



When calcium salts, at or about the beginning of Cambrian 

 time, came into permanent excess in sea-water (i. e., excess 

 over the needs of lime-secreting organisms), the precipitation 

 of magnesium carbonate became more difficult, but this change 

 would be extremely slow. Even at the present day the pro- 

 portion of lime salts in sea-water is low. The existing rivers 

 are nearly the greatest rivers the world has known, so far at 

 least as drainage basins are concerned. Those rivers flow 

 through immense tracts of limestone and dolomitic formations 

 which evidently did not furnish carbonate to rivers of 

 Paleozoic or Mesozoic age. It is clear that Paleozoic aud 

 Mesozoic rivers may have sent even less sulphate and carbon- 

 ate of calcium into the sea than is now being poured into it. 

 We should, therefore, expect that, during the Paleozoic and 

 Mesozoic aeons, there was a less abundant precipitation of 

 magnesium carbonate than during the Eozoic, but a more 

 abundant precipitation than at the present time. 



Another factor has tended to operate in the same sense. 

 Not only have the growing rivers tended to increase the pro- 

 portion of calcium salts in the sea-water ; the establishment of a 

 scavenging system on the general sea-floor has rendered it 

 possible for the dissolved calcium sulphate to increase and so 

 lessen the precipitation of magnesium carbonate. 



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 constantly decom- 

 posed by lime-secreting organisms and converted into deposited 

 carbonate, it is probable that much more than 600,000 years 

 have elapsed since the abyssal fishes and other scavengers 

 colonized the general sea-floor. That the present content 

 of calcium sulphate in sea-water would be largely and rapidly 

 diminished if the scavenging system were not now at work in 

 the ocean, is to be inferred from the test case of the Black Sea. 

 The scavenging system was first developed probably several 

 millions of years ago, perhaps in Cretaceous time. Since 

 then the chemical precipitation of magnesium carbonate has 

 been possible only under special conditions ; the recent period 

 is a time of the minimum formation of magnesian deposits. 



