AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EAKTH. 287 



We have, as we have seen, evidence that this may not l)e much. Pos- 

 sibly the half million years would more than cover the entire solvent 

 effects of the ocean. 



We have to consider, indeed, in this matter that the ocean was not 

 always charged with its present dissolved salts. The primeval ocean, 

 most probably after the free acids were satisfied in the solution of the 

 silicates, carried chiefly chlorides indeed, but chlorides of lime, mag- 

 nesia, and other metals. The subsequent changes were those of 

 replacement for the greater part. We have no reason, however, to 

 suppose that these salts could act substantially differently from the 

 chlorides of sodium now constituting the larger part of the chlorides.^ 



We can only, from what we know, gather some idea of the order of 

 magnitude of the correction for oceanic solvent denudation. It 

 appears almost certain " that this can not exceed a very few million 

 years. 



The allowances we felt justified in making in the earlier part of this 

 paper left our estimate at eighty-nine millions of years. The least 

 speculative part of our knowledge inclines us to believe that this is 

 probably a major limit. "^ Taking into account our uncertainty in 

 many particulars attending these corrections, and as to the constancy 

 throughout the past of solvent denudation, and bearing in mind that 

 any approximation to a correction for marine denudation must be 

 attended with this same uncertainty, but that the latter correction will 

 undoubtedly be subtractive, we think that it is at least justifiable to 

 claim that our present knowledge of solvent denudation of the earth's 

 surface points to a period of between eighty and ninety millions of years 

 having elapsed since water condensed upon the earth, and rain and 

 rivers and the actions continually progressing in the soils began to 

 supply the ocean with materials dissolved from the rocks. 



Appendix I. 



To facilitate review of the niunerical quantities adopted in the 

 calculations involving the age of the earth, the chief data are here 

 collected: 



Area of land ( Murray and Wagner) 55,814,000 square miles. 



Ratio of oceanic to land area 2.54 : 1. 



Hence, oceanic area 141,767 X 10^ square miles. 



Mean depth of ocean ( Murray) 2,076 fathoms = 2.393 miles. 



Bulk of ocean 339,248 X 10^ miles. 



Mass of a cubic mile of sea water 43 X 10* tons. 



^A. Agassiz thinks the solvent power of the ocean during some of the earlier 

 geological deposits was far less than during later times. See Three Cruises of the 

 Blake, I, p. 147. 



^See the summary of positive and negative errors contained in Appendix II, and, 

 more especially, set off 1 and 2 of the errors going to make the estimate a maximum 

 against 1 among those tending to render it a minimum. 



