422 EDWARD STEIDTMANN 



Conclusion: Increase of the ratio of calcium to magnesium in 

 rivers with geologic time. — Evidence has been presented to show 

 that the ratio of calcium to magnesium of stream water is influenced 

 primarily by the ratio of calcium to magnesium of the terranes 

 which they drain, being generally higher than that of the terranes. 



Climate exerts a modifying influence. Aridity lowers the 

 ratio of calcium to magnesium of the stream waters, causing a 

 concentration of calcium in the soils, while humidity has the 

 opposite effect. The interaction of the salts carried in solution 

 by streams and ground waters with the land materials, particu- 

 larly those high in clay, results in a greater loss of magnesium from 

 the waters than of calcium, thus tending to increase the ratio of 

 calcium to magnesium of the streams and ground waters. 



Regardless of any theory of the origin of the earth, geologic 

 evidence points to the igneous rocks as the primitive source of the 

 sedimentary rocks. The streams of the primitive lithosphere, 

 therefore, probably approached in their chemical character the 

 present streams, flowing over crystalline rocks, subject to climatic 

 and other modifications. Such streams have a ratio of calcium 

 to magnesium approximating 3 to 1. The best figure given for 

 the average calcium to magnesium ratio of the streams of the 

 world is approximately 6 to 1. The latter figure probably should 

 be greater, considering the abundance of limestone on the conti- 

 nents. Little doubt therefore remains that the proportion of 

 calcium to magnesium in the streams is now higher than in earlier 

 stages of the earth's history. It is also highly probable that the 

 increase in the ratio of calcium to magnesium in the rivers has 

 been continuous with geologic time, because of the progressive 

 increase in this ratio in the limestones deposited during geologic 

 time, and because of the selective deposition of limestones on the 

 submerged continents. 



STATEMENT OF HYPOTHESIS 



The conclusion has been reached that dolomites develop pre- 

 dominantly in the sea rather than by the metamorphism of lime- 

 stones after their emergence from the sea. Hence the decline in 

 the percentage of dolomite in going up the geologic column seems 



