1922] The Age of the Earth 491 



WEEKLY EVENING MEETING, 



Friday, February 24, 1922. 



Sir James Crichton-Brotoe, J.P. M.D. LL.D. F.R.S., 



Treasurer and Vice-President, in the Chair. 



John Joly, D.Sc. F.R.S. 



The Age of the Earth. 



" The Age of the Earth " is a somewhat ambiguous phrase. From 

 the geological point of view it is generally understood to mean the 

 age of the ocean : in other words, the age of the earth since the 

 beginning of those geological surface changes w T hich are due to 

 denudation. But another meaning may be ascribed to the term. 

 We may assume the beginning to date from the cooling of a highly 

 heated surface to the point of solidification. In this case we include 

 in the age those long periods of Archaean time during which the 

 activity of water played a subordinate part and volcanic commotion 

 prevailed among the semi-fluid, rocky constituents of the globe. Yet 

 a third interpretation refers the birth time to a still more remote 

 and indefinite epoch when the world became differentiated as a planet 

 by activities of the nature of which we are ignorant. Astronomical 

 deductions and speculations regarding the age are mainly concerned 

 with the last period. 



What I have to say will be restricted, almost entirely, to the first 

 interpretation of the term. I mean by the age of the earth the 

 period which has elapsed since its surface became the scene of world- 

 wide denudative forces and the foundations of organic evolution were 

 laid. 



In virtue of these denudative forces we find ourselves possessed 

 of certain methods of estimating the age which are valid upon the 

 assumption that denudation proceeds in our time at a rate not greatly 

 differing from its mean rate over geological time. 



The bases of this assumption are as follows : — 



(a) That the chief factor in denudative activity being the rain 

 supply falling on the land, solar heat and atmospheric circulation are 

 primary causes. The life on the globe since very early times and 

 the narrow temperature limits conditioning protoplasmic existence 

 and activity show that great extremes of solar radiation cannot have 

 affected denudation for long periods in the past. Mere climatal 



