160 R. A. DALY THE EVOLUTION OF THE LIMESTONES 



Comparison of the Ottawa and other Eivers 



The Ottawa carries past Ottawa city only 23 per cent as much calcium 

 per volume as the Saint Lawrence river carries past Ogdensburg, and 

 less than 20 per cent as much calcium per volume as the Mississippi car- 

 ries past Minneapolis. About one-third of the Saint Lawrence basin is 

 occupied by the Great lakes, in which area probably very little solution 

 of calcium salts is taking place. Another large part of the basin is occu- 

 pied by the pre-Cambrian terranes where highly calcareous rocks are 

 relatively rare. The content of this river is therefore less than it would 

 be if the river basin were all occupied by the average rocks of the whole 

 continental area of the earth. The comparison of these three rivers is 

 specially instructive, since they are all working under essentially similar 

 climatic conditions, with nearly the same ratio of rainfall to run-off. 

 From the comparison it seems probable that, if the continents were all 

 of their present size and composed of rocks typical of the lands during 

 the late pre-Cambrian, the rivers would deliver to the sea annually not 

 more than one-fifth as much calcium as is carried by the existing rivers 

 of the continents. 



This conclusion becomes more convincing when the Ottawa water is 

 compared with the other rivers noted in table III. 



In Clarke's admirable compilation of river analyses, those referring to 

 rivers which drain pre-Cambrian terranes throughout their respective 

 basins are. five in number, including the Pigeon river of Minnesota and 

 four rivers in Sweden. The average content of calcium (and of mag- 

 nesium) 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 have, 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 pres- 

 ent writer has calculated the proportions of calcium (and magnesium) 

 and has also (using Clarke's compilation) 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 



