REPORT OF TEE CHIEF ASTRONOMER 655 



SESSIONAL PAPER No. 25a 



are five in number, including the Pigeon river of Minnesota and four rivers 

 in Sweden. The average content of calcium (and of magnesium) in these rivers, 

 together with the Ottawa at Ottawa city, is stated in the table. It will be seen 

 that the proportion of calcium is very close to that in the Ottawa alone. We 

 bave, therefore, corroboration for the view that the Ottawa is a good world 

 type of rivers draining late pre-Cambrian terranes. 



On the other hand, the Mississippi at New Orleans must be regarded as one 

 of the best types of rivers draining the average terranes of the present continents. 

 From Murray's average of nineteen rivers the present writer has calculated the 

 proportions of calcium (and magnesium) and has also (using Clarke's compila- 

 tion) calculated the contents of these elements in forty-four of the largest rivers 

 of the globe. In this second computation the individual analyses were roughly 

 weighted according to the areas of the respective river basins. The result is 

 believed to give a truer idea of the average content of calcium in the world's 

 rivers than does Murray's estimate. 



The results seem to show that the average world river, working on the 

 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 influence of climate subordinate, 

 in their respective control over the content 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 Mon- 

 treal. Even a few hundred square miles of Upper Cambrian and Ordovician 

 rocks' (largely limestones) below Ottawa city makes the calcium content 

 materially rise. 



Chemical Contrast of pre-Cambrian and later Eiver Systems. 



In spite of .the complexity of the whole problem, we may fairly conclude 

 that if, in the late pre-Cambrian time, the land areas were of their 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 characterizing 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. 



