656 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



This estimate obviously involves the assumption that the pre-Cambrian 

 rate of chemical denudation was no more rapid than the present rate. Since 

 the rate is controlled (apart from the influence of the terrane) principally by the 

 abundance of the organic acids attacking the bedrock, we may well suppose that 

 the well vegetated Ottawa river basin is witnessing solution at as rapid a rate 

 as in late pre-Cambrian time. It might be considered that a tropical tempera- 

 ture during the pre-Cambrian would have caused specially rapid solution of the 

 rocks at that time. This view is, however, hardly supported by an inspection 

 of the data relating to existing tropical and extra-tropical rivers. Furthermore, 

 the recent glaciation of the Ottawa basin has caused the removal of secularly 

 weathered 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 con- 

 trast between the late pre-Cambrian and present supply of calcium to the ocean 

 through weathering and river inflow. 



Variations in the Calcium Supply during and after the pre-Cambrian. 



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

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

 may have been less than one-twentieth as rapid, while the amount of animal 

 matter completely decaying on the sea floor, and therewith the likelihood of the 

 precipitation of calcium salts, may have been, respectively, thoiisands of times 

 greater than they are now. 



Immediately after the Huronian revolution and during the immensely 

 long period of baselevelling 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 con- 

 tinental enlargement. During the long erosion cycle the ocean was, for the first 

 rime, specially enriched in river-borne calcium salts. 



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 calcium content by preci- 

 pitation of the carbonate of calcium. Such contrast of surface and bottom water 

 would be due to the slowness of diffusion through a body of liquid so great as the 

 ocean. Under the conceived conditions the most favourable places for the 

 invention of calcareous hard parts would be, possibly, localized areas, such an 



