16 THE AGE OF THE EARTH 



Jefore proceeding to the discussion of estimates of 

 ; drawn from a study of stratified rocks let us first 

 consider those which have been already suggested by 

 other data. These are as follows: (1) Time which has V 

 elapsed since the separation of the earth and moon, fifty- 

 six millions of years, minimum estimate by Professor 

 G-. H. Darwin. (2) Since the " consistentior status," 

 twenty to forty millions (Lord Kelvin). (3) Since the*" 

 condensation of the oceans, eighty to ninety millions, 

 maximum estimate by Professor J. Joly. 



It may be at once observed that these estimates, 

 although independent, are all of the same order of 

 magnitude, and so far confirmatory of each other. Nor 

 are they opposed to conclusions drawn from a study of 

 stratified rocks ; thus Sir Archibald Geikie, in his 

 Address to this Section last year, affirmed that, so far as 

 these were concerned, 100 millions of years might suffice 

 for their formation. There is, then, very little to quarrel 

 about, and our task is reduced to an attempt, by a little 

 stretching and a little paring, to bring these various 

 estimates into closer harmony. 



Professor Darwin's estimate is admittedly a minimum ; 

 the actual time, as he himself expressly states, " may have 

 been much longer." Lord Kelvin's estimate, which he 

 would make nearer twenty than forty millions, is founded 

 on the assumption that since the period of the "consis- 

 tentior status " the earth has cooled simply as a solid 

 body, the transference of heat from within outwards 

 having been accomplished solely by conduction. 



It may be at once admitted that there is a large amount 

 of truth in this assumption ; there can be no possible 

 doubt that the earth reacts towards forces applied for a 

 short time as a solid body. Under the influence of the 

 tides it behaves as though it possessed a rigidity approach- 

 ing that of steel, and under sudden blows,, such as those 



