SUMMARY ON ORIGIN OF PRE-DEVONIAN LIMESTONES 167 



and post-Devonian limestones. Granting that the calcium and magne- 

 sium 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 accumulate 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 mag- 

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

 time. 



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

 cipitation of magnesium carbonate direct from sea-water has been possible 

 only under special conditions, so that the more recent time has seen the 

 minimum formation of magnesian deposits. 



Summary on the Origin of the pre-Devonian Limestones 



The close correspondence of the ratio Ca : Mg in the pre-Devonian 

 limestones with the ratio Ca : Mg in such type rivers as the Ottawa, Saint 

 Lawrence, and Mississippi, as well as with the average river, can hardly 

 be accidental. The readiest explanation of this correspondence seems to 

 be found in the view that all the pre-Devonian river-borne calcium and 

 magnesium were precipitated on the sea-floor. The ultimate products 

 are dolomites and magnesian limestones as well as more purely calcareous 

 limestones. The causes for the variability of their composition are 

 briefly discussed on pages 107-108 of the first paper. In cases where the 

 magnesian limestones are of pre-Cambrian age they are, in general, to be 

 regarded as precipitates on the floor of the open ocean and not as formed 

 in closed basins subject to intense evaporation. A study of the tables of 

 rock and river analyses has led the writer to ascribe a similar origin to 

 the staple pre-Devonian carbonate rocks as well as to many limestones 

 and dolomites of still later date. 



Testimony of the Grain of the pre-Ordovictan Limestones 



It may be added that a close study of the grain of unmetamorphosed 

 Cambrian and pre-Cambrian carbonate rocks has convinced the writer 

 that they are not of clastic origin nor of direct organic origin through 

 the accumulation of shells or skeletons. More than 7,000 feet of such 

 rocks are exposed in the Forty-ninth Parallel section of the Rocky Moun- 

 tain geosynclinal prism. Type specimens of these have been examined 

 microscopically. It was found that neither horizon nor distance from 

 the old shorelines affects the singularly monotonous grain of the rocks. 



