48o 



NA TURE 



[April 15, 1922 



The Age of the Earth.^ 



By Prof. J. Joly, F.R.S. 



" 'T^HE Age of the Earth " is a somewhat ambiguous 

 J- 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 which 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 inter- 

 pretation refers the birth time to a still more remote 

 and indefinite epoch when the world became differen- 

 tiated as a planet by activities, of the nature of which 

 we are ignorant. Astronomical deductions and specula- 

 tions 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 our- 

 selves possessed of certain methods of estimating the 

 Age which are valid upon the assumption that denuda- 

 tion 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 proto- 

 plasmic existence and activity show that great 

 extremes of solar radiation cannot have affected 

 denudation for long periods in the past. Mere 

 climatal extremes do not sensibly affect solvent 

 denudation. Atmospheric circulation, being 

 largely conditioned by the earth's rotation and 

 the distribution of solar heat, cannot have varied 

 to any effective extent. 



(i) That a considerable percentage of the existing land 

 area being rainless, changes in continental area 

 cannot greatly affect the amount of denudation : 

 the belt undergoing denudation being merely 

 displaced outwards or inwards. The evidence 

 derived from palaeography and from the extent 

 of sedimentary deposits in all ages shows that 

 the present land area is not greatly different 

 from the past mean area. 



c) That the minor factors affecting solvent and 

 detrital denudation being very many and of very 

 different character are unlikely to combine at 

 any time, and for any long period, in one direc- 

 tion, so as to create a considerable departure 

 from the mean. 



Time will not permit a discussion of these statements. 



NO. 2737, VOL. 109] 



I shall refer but briefly to the methods by which the 

 statistics of solvent and detrital denudation are used 

 to afford the age of the ocean. 



(i) The chemistry of the ocean and of the rocks is 

 the key to our position. As the result of a comparative 

 study of the primary or igneous rocks and the secondary 

 or sedimentary rocks we find that, say, n grams of 

 sodium are shed into the ocean for each tonne of 

 igneous rock converted into sedimentary rock, and in 

 the ocean we find N grams of sodium. The total 

 denudation over geological time has, therefore, been 

 N/n expressed in tonnes of denuded igneous rock. Our 

 study also tells us the average total loss attending the 

 conversion of the primary rock into sediment, and so 

 we get the total of the secondary rocks in tonnes. We 

 now go to the principal rivers of the world and availing 

 ourselves of estimates which have been made of the 

 amounts of sediment — i.e. of secondary rock material 

 — which they transport from the land in a year, we 

 calculate the number of years which it would take to 

 lay down in the ocean the great mass of sediment 

 generated in the past ages. After certain allowances 

 this comes out as about 100 million years. 



(2) Again the total of oceanic sodium may give us 

 the Age in another and more direct way. We know 

 that by far the greater part of this sodium was carried 

 into it by the rivers during geological time. We turn 

 to the analyses of river water and estimate the total 

 annual supply of this element to the ocean. Dividing 

 the latter into the former and making certain allowances 

 we find an Age which is about 100 million years. 



(3) A third and more difficult method is" independent 

 of our knowledge of chemical denudation. We estimate 

 the maximum thickness of the integral sedimentary 

 deposits, and knowing the burthen of sediment con- 

 veyed per annum by the rivers, we estimate the maxi- 

 mum thickness of deposit annually derived from the 

 same ; we divide the latter into the former and find an 

 age which, again, is about 100 million years. 



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 of 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 SoUas, 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 -possible that 

 150 miUions 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. 



1 Discourse delivered at the Royal Institution on Friday, February 24. 



