ESTIMATES OF TIME 837 



brought down as rain. As it is the contribution only of new sodium 

 which should enter into the calculation on the age of the ocean, Becker 

 makes up a balance sheet of this annual new sodium as follows : 



3.0 X 10" = sodium chloridized by chlorine of igneous rocks, 

 32.5 X 10" ^sodium chloridized by atmospheric chlorine. 

 62.5 X 10" = unchloridized sodium. 



98.0 X 10":^ annual contribution of new sodium. 



It is seen that his assumption increases the contribution of now sodium 

 by one-half. 



Becker's second assumption is that the annual rate of sup|)ly of sodium 

 from igneous rocks to the sea has decreased through geological time ac- 

 cording to a logarithmic curve. This is made quantitative by various 

 secondary assumptions. He considers that tlie continents at the beginning 

 had the same area as at present, and then consisted entirely of igneous 

 rocks. The mean area of land throughout past time he takes as 0.8 of 

 the present area and the present exposure of igneous rocks to lie between 

 0.25 and 0.3 of the mean land area. Assume that the annual contribution 

 of sodium has diminished directly with the area of igneous rocks v^hich 

 has lessened in the following manner: In a certain time interval T, it is 

 half the original area, in 2 T it is one-fourth, in 3 T it is one-eighth, in 4 T 

 it is one-sixteenth, and so on, following the same law of decay as the loss 

 of velocity of a body moving in a resisting medium and the decay of 

 radioactivity. On this basis Becker derives an age of the ocean between 

 70,000,000 and 50,000,000 years. Of these, he prefers the liigher figure. 

 These estimates are very close to half the lengths of time obtained by 

 using Joly's assumption of uniform rate of supply. Looking forward to 

 a continuation of Becker^s falling rate following a logarithmic- curve, 

 there will come a time, he conceives, when the erosion of new igneous rock 

 will be negligible and the sea can never come to have more than 1.43 to 

 1.33 of its present salt. 



In comment on Becker's method it may be said that his assumption 

 regarding decreasing rate seems as unsafe as his other assumption regard- 

 ing the constancy of composition of oceanic salts. Both Joly and Becker 

 note the present high average elevation of the continents, but regard it as 

 a secondary factor in the matter of rate of supply and one for whicb 

 allowance need not be made. Eeasons have been discussed, however, for 

 holding that it is a major factor, which has multiplied the rate of erosion 

 many times above what it has been during long periods of quiet marked 

 by wide-spread shallow seas. Schuchert's paleogeographic maps show, 



