162 R. A. DALY THE EVOLUTION OF THE LIMESTONES 



glaciation of the Ottawa basin has caused the removal of secularly weath- 

 ered rock, so that the formations now exposed to erosion contain nearly 

 their original amount of soluble matter. For this reason the calcium 

 content of the existing river may be near its possible maximum for a 

 region of average rainfall. 



Without further entering upon this confessedly obscure subject, we 

 may retain the foregoing estimate as indicating the order of magnitude 

 in the contrast between the late pre-Cambrian and present supply of cal- 

 cium to the ocean through weathering and river inflow. 



Variations in the Calcium Supply during and after the pre- 

 Cambrian 



Before the post-Huronian revolution the supply of river-borne calcium 

 to the ocean was almost certainly less than one-fifth as rapid as it is 

 today, and it may have been less than one-twentieth as rapid, while the 

 amount of animal matter completely decaying each year on the sea-floor, 

 and therewith the likelihood of the precipitation of calcium salts, may 

 have been, respectively, thousands of times greater than they are now. 



Immediately after the post-Huronian revolution and during the im- 

 mensely long period of baseleveling which followed it, the annual supply 

 of calcium to the ocean may have approached rivalry with the present 

 annual supply. The supply doubtless diminished somewhat as more and 

 more of the Huronian and pre-Huronian limestone and basaltic areas 

 were lessened by erosion and as the Laurentian granite batholiths were 

 uncovered and exposed to solution; but this change must have been very 

 slow, and it did not annul the critical effect of continental enlargement. 

 During the long erosion cycle the ocean was, for the first time, specially 

 enriched in river-borne calcium salts. 



The first calcareous Fossils 



This special influx of calcium salts may be conceived as keeping the 

 surface layers of the sea-water sufficiently supplied with calcium for the 

 needs of lime-secreting organisms, while the bottom layers lost their cal- 

 cium content by precipitation of the carbonate of calcium. Such con- 

 trast of surface and bottom water would be due to the slowness of dif- 

 fusion through a body of liquid so great as the ocean. Under the con- 

 ceived conditions the most favorable places for the invention of calcareous 

 hard parts would be, possibly, localized areas, such as the open sea oppo- 

 site the greater river deltas, or such as the epicontinental seas more or less 

 isolated during the orogenic revolution. The slow spread of the scaveng- 



