AGE OF EARTH AND OCEAN — KNOPF 195 



700,000 years (1 foot in 140 years) . With this as a basis, he estimated 

 that the Ordovician and Silurian together required 13,000,000 years 

 for their deposition. Although the assumptions made in reaching 

 this final result are admittedly large, yet we can agree with the 

 author that " extended UFe of this method may lead to some approxi- 

 mation to the order of magnitude of the geological periods." Ap- 

 parentl}'' we are on the threshold of obtaining some reliable measures 

 of the duration of geologic time from the evidence of the strata 

 themselves. 



Schuchert has summarized the biologic evidence bearing on the 

 question at issue and concludes that from the rate of organic evolu- 

 tion there can be no way of determining the length of geologic time. 



Another way of measuring geologic time is based on the amount of 

 sodium in the ocean. Given the amount of sodium washed each year 

 into the ocean and dividing this quantity into the total amount in the 

 ocean, there is obtained " the age of the ocean." The figure thus 

 obtained, by Joly and by Clarke, in round numbers 100,000,000 years, 

 has an apparent high accuracy, enhanced moreover by the fact that in 

 reaching this figure corrections amounting to a few per cent were 

 applied. Two large assumptions underlie the whole method; 

 namely, that the rate at which sodium is supplied to the ocean has 

 been constant throughout geologic time, and that the sodium has 

 steadily accumulated in the ocean. Both assumptions are known to 

 be untrue. The rate of erosion, or more specifically the rate of 

 solvent denudation, has varied widely throughout time owing to 

 varying size and height of the continents and to changing climatic 

 conditions. The present rate of solvent denudation is probably 

 much higher than the average rate during geologic time, but how 

 much higher is beyond the present power of science to evaluate. 

 Furtliermore, the sodium washed into the ocean, it is beginning to 

 appear, does not all remain dissolved but is in part removed by ad- 

 sorption and base exchange with the newly deposited sediments. 

 As we now see it, the problem of the age of the ocean based on the 

 accumulation of sodium is not whether we can apply corrections 

 of a few per cent to the estimate of 100,000,000 years, but whether 

 this estimate is of the right order of magnitude. 



The methods of age determination based on radioactive disin- 

 tegration involve the least nvimber of assumptions. They are ex- 

 plained by Kovarik and Holmes. The methods are based ultimately 

 on the fact that the radioactive elements uranium and thorium dis- 

 integrate spontaneously at constant determinable rates and yield 

 a stable product, lead, whose atomic weight varies according to the 

 proportion contributed by its radioactive parents. The disintegra- 

 tion of uranium (and thorium) proceeds according to the laws of 

 149571—33 14 



