1922] on The Age of the Earth 493 



Of these methods, that which involves the sodium modulus only 

 is the most direct. Of course the reason for selecting this particular 

 element as a modulus is because ol its great solubility, on account of 

 which it alone among the dissolved oceanic constituents has been 

 preserved from organic abstraction or chemical precipitation. This 

 method has been examined by many critics. Notably by Sollas, 

 who, in a presidential address to the Geological Society in 1909, 

 subjects it to searching examination. He concludes that a period of 

 175 millions of years may be reached upon certain assumptions, and 

 that this must be very nearly the maximum allowable. My own 

 examination of this method has led me to believe that it is passible that 

 15o millions of years may be indicated by it, and that 200 millions 

 of years would not be reconcilable with our present knowledge of the 

 factors involved. This would, as I have already stated, apply only 

 to the duration of sedimentation. It cannot be compared with data 

 which apply to an age dating back into the Archaean. 



There was, indeed, some scanty sedimentation in Archaean times. 

 AVe cannot form any estimate of its effects upon our numerator or 

 upon our denominator save that we seem entitled to conclude that 

 they were small. " The Archaean was essentially a period of world- 

 wide vulcanism, and in the relative proportion of rocks of igneous 

 and sedimentary origin represents a departure from the uniformity 

 of conditions of later geological time." I quote from the monograph 

 of Vane Hise and Leith. 



Before passing on to the results based upon radio-activity I must 

 refer to one point in particular which has been urged against accept- 

 ing present-day rates of denudation as a basis of time measurement. 

 It is said we live in a period of abnormal continental elevation 

 which, it is asserted, involves excessive solvent denudation. A little 

 attention to the nature and conditions of solvent denudation should 

 have sufficed to forestall the argument. But a ready method of 

 dealing with it is available. The continent of North America has a 

 mean elevation of 700 metres : it is being denuded at the rate of 

 79 tons per square mile per annum ; for South America the corre- 

 sponding figures are 650 metres and 50 tons. Now Europe has a 

 much lower mean elevation — 300 metres. Its rate of denudation is, 

 however, 100 tons per square mile per annum. The rate of solvent 

 denudation is, in fact, by measurement found to be less for the more 

 elevated land, as theoretically it should be. The argument then, if 

 it has any basis, would indicate that the age as found from solvent 

 denudation is excessive. 



Prior to the advent of those methods for investigating the earth's 

 age, which are based on radio-active changes in the elements, no 

 serious objections to the results reached by the geological methods 

 were raised, so far as I know. There were some, indeed, who 

 regarded the age as excessive. Thus Becker arrived at a lesser 

 figure by taking into account the progressive impoverishment of the 



