32 EVOLUTION OF THE EARTH 



part of the sodium chloride raises the percentage content of 

 bromine, magnesium chloride, and magnesium sulphate, so 

 that sodium becomes subordinate to magnesium and the ratio 

 of bromine to chlorine is increased. Subsequent dilution 

 would not change this ratio and the introduction of new salts 

 could never bring it back to the original composition. The 

 evidence from the sea itself is substantiated by the testimony 

 of the sedimentary rocks. The amount stored as impregna- 

 tions or as salt deposits in the sediments is quantitatively 

 negligible, either as compared to the volume of the sediments 

 or the mass of the oceanic salts. Salt deposits, furthermore, 

 so far as known, began to be present only in the Paleozoic, in 

 the later half of geologic time, the great masses of earlier 

 sedimentary strata being barren of them. 



It has been calculated that the total sodium in the ocean 

 would be derived from the weathering and erosion over all 

 the earth of a mantle of igneous rock of average composition 

 only 2,300 feet thick, corresponding to 6,500 feet as the 

 average thickness of erosion if restricted to the area of the con- 

 tinental platforms, including the lands and extending out to 

 a depth of 600 feet below sea-level. 



Daly has noted the significance of these facts upon the 

 hypotheses of earth-growth. 6 Chamberlin supposes an ocean 

 to have existed for long geologic ages upon the surface of 

 the earth during its growth from a body about half of its 

 present diameter and one-eighth of its present volume. The 

 planetesimal material, he holds, was weathered and sorted 

 into lighter and heavier portions, leading to the development 

 of lighter protuberant and heavy depressed areas. The 

 limited quantity of salt in the sea, however, is distinctly against 

 such a hypothesis of oceanic antiquity and continental build- 

 ing. The amount of erosion in evidence where the older 

 rocks are exposed as crystalline masses and from the great 



6 Daly, R. A., Igneous rocks and their origin, 1914, pp. 159, 163, 164. 



