428 EDWARD STEIDTMANN 



A higher percentage of magnesium is retained in the residuals of 

 rock decay than calcium, but erosive processes are constantly 

 removing the residuals toward the margins of the lands, and 

 during periods of continental expansion a considerable proportion 

 is swept into the deep sea and permanently lost from the lands. In 

 consequence of a combination of organic and inorganic agencies, 

 the maximum deposition of limestones and dolomites is on the 

 submerged lands during periods of oceanic expansion. The per- 

 centage of calcium precipitated is higher than that of magnesium, 

 but the proportions of calcium to magnesium which are precipi- 

 tated bear some direct relation to their ratio in the rivers which 

 bring them to the sea. With the progressive elimination of 

 elastics and magnesium from the lands with geologic time, and in 

 their place the gradual accumulation of calcium in the form of 

 limestone, the proportion of calcium to magnesium contributed by 

 rivers to sea has increased with time. 



The writer is indebted to C. K. Leith for suggestions and 

 criticisms. 



