20 THE AGE OF THE EARTH 



as possible be eliminated ; but this will not be effected 

 by taking the accepted mean of observed rates of increase 

 of temperature : such an average is merely a compromise, 

 and a nearer approach to a correct result will possibly be 

 attained by selecting some low rate of increase, provided 

 it be based on accurate observations. 



It is extremely doubtful whether an area such as the 

 British Isles, which has so frequently been the theatre 

 of volcanic activity and other subterranean disturbance, 

 is the best fitted to afford trustworthy results ; the 

 Archaean nucleus of a continent might be expected to 

 afford surer indications. Unfortunately the hidden 

 treasures of the earth are seldom buried in these regions, 

 and bore-holes in consequence have rarely been made in 

 them. One exception is afforded by the copper-bearing 

 district of Lake Superior, and in one case, that of the 

 Calumet and Hecla mine, which is 4,580 feet in depth, 

 the rate of increase, as determined by Professor A. 

 Agassiz, was 1 F. for every 223'7 feet.* The Bohemian 

 "horst" is a somewhat ancient part of Europe, and in 

 the Przibram mines, which are sunk in it, the rate was 

 1 F. for every 126 feet of descent. In the light of these 

 facts it would seem that geologists are by no means com- 

 pelled to accept the supposed mean rate of increase of 

 temperature with descent into the crust as affording a 

 safe guide to the rate of cooling of a solid globe : and if 

 the much slower rate of increase observed in the more 

 ancient and more stable regions of the earth has the 

 importance which is suggested for it, then Lord Kelvin's 

 estimate of the date of the " consistentior status " may be 

 pushed back into a remoter past. 



If, as we have reason to hope, Lord Kelvin's somewhat 



* Subsequent investigation has shown that this result is erroneous. 

 Mr. Lane considers that the rate of increase in the Calumet district 

 is 1 ' F. for 107 feet. 



