664 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



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. Notwithstanding 

 the slowness of the precipitation of magnesium carbonate at ordinary tempera- 

 tures, some excess of magnesium salts in solution in that sea might easily 

 permit the basic magnesium carbonate to be thrown down in very high proportion 

 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. 

 Ear from shores the chemical deposit would be 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 circum- 

 stances. The beds would further be indefinitely varied according to the propor- 

 tion and kinds of mechanical detritus intermixed with carbonates. Eozoic 

 sediment 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 con- 

 cerned. 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 and Mesozoic rivers may have sent even 

 less sulphate and carbonate of calcium into the sea than is now being poured 

 into it. We should, therefore, expect that, during the Paleozoic and Mesozoic 

 aaons, there was a less abundant precipitation of magnesium carbonate than 

 during the Eozoic, but a more abundant precipitation than at the present time. 



Average Ratio of Calcium to Magnesium in the Limestones of the Different 

 Periods. — The writer has attempted to test these conclusions quantitatively. 

 Eor this purpose nearly 900 analyses of types of pre-Cambrian, Paleozoic, Cre- 

 taceous. Tertiary, and Quaternary-Recent limestones have been calculated, so as 

 to show the average ratio of calcium to magnesium throughout the series. The 

 analyses were taken from the government survey reports of Canada and the 

 United States ; from Logan's ' Geology of Canada ' ; from the state survey 

 reports of Arkansas, Indiana, Iowa, Kentucky, Minnesota, Ohio, Pennsylvania, 

 West Virginia, and Wisconsin; from the reports of the Ontario Bureau of 

 Mines; from Firket's elaborate paper on the limestones of Belgium,* and from 

 the list of analyses supplied for this report. 



The selection is far from being as complete as it might be made, but it 

 is believed that enough analyses are represented to give a fairly accurate idea 

 of the variation of the ratio through geologic time. The number of pre-Cam- 



* A. Firket : Annales Societe Geologique de Belgique, Vol. 11, 1883, p. 221. 



