COMPARISON OF THE OTTAWA AND OTHER RIVERS 16l 



average terrane and under average climatic conditions, carries about the 

 same proportion of dissolved calcium as the average water of the Saint 

 Lawrence at Ogdensburg and the Mississippi above Minneapolis. The 

 table indicates that the influence of the terrane is dominant and the in- 

 fluence of climate subordinate, in their respective controls over the con- 

 tent of calcium. 



The Mississippi above Memphis drains rock formations which together 

 make fairly good equivalents of the average Mesozoic and Cenozoic land 

 areas. So far as the influence of the average world terranes is concerned, 

 the Mesozoic and Cenozoic rivers were enriched in calcium about as 

 much as the existing world rivers. The early Paleozoic rivers were, on 

 the average, probably not much richer in calcium than the late pre- 

 Cambrian rivers. The control of the Paleozoic terranes on the calcium 

 content of the Ottawa itself is shown by the contrast between the Ottawa 

 city analyses and that at Sainte Anne near Montreal. Even a few hun- 

 dred square miles of upper Cambrian and Ordovician rocks (largely lime- 

 stones) below Ottawa city makes the calcium content materially rise. 



Chemical Contrast of pre-Cambrian and later River Systems 



In spite of the complexity of the whole problem, we may fairly con- 

 clude that if, in the late pre-Cambrian time, the land areas were of then- 

 present size, the ocean then received annually only a small proportion — 

 probably less than one-fifth — of the calcium supplied each year by the 

 present rivers. A contrast of the same order must have existed between 

 the calcium content of the late pre-Cambrian rivers and the rivers char- 

 acterizing most of Mesozoic and Cenozoic time. 



If the late pre-Cambrian lands had a total area but one-half as great 

 as the present total land area, the rivers may have carried annually to the 

 sea less than 10 per cent of the amount of calcium now carried to the sea 

 by the world's rivers. 



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 wit- 

 nessing solution at as rapid a rate as in late pre-Cambrian time. It might 

 be considered that a tropical temperature 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 



