18 THE AGE OF THE EARTH 



authorities as to the presumed value of this rate may well 

 lead to reflection. The late Professor Prestwich thought 

 a rise of 1 F. for every 45 feet of descent below the zone 

 of constant temperature best represented the average ; 

 Lord Kelvin in his earlier estimates has adopted a value 

 of 1 F. for every 51 feet ; the Committee of this Asso- 

 ciation appointed to investigate this question arrived at 

 a rate of 1 F. for every 60 feet of descent ; Mr. Clarence 

 King has made calculations in which a rate of 1 F. for 

 72 feet is adopted ; a re-investigation of recorded measure- 

 ments would, I believe, lead to a rate of 1 F. in 80 or 90 

 feet as more closely approaching the mean. This would 

 raise Lord Kelvin's estimate to nearly fifty millions of 

 years. 



When from these various averages we turn to the 

 observations on which they are based, we encounter a 

 surprising divergence of extremes from the mean ; thus 

 in the British Isles alone the rate varies from 1 F. in 

 34 feet to 1 F. in 92 feet, or in one case to 1 F. in 130 

 feet.* It has been suggested, and to some extent shown, 

 that these irregularities may be connected with differences 

 in conductivity of the rocks in which the observations 

 were made, or to the circulation of underground water ; 

 but many cases exist which cannot be explained away in 

 such a manner, but are suggestive of some deep-seated 

 cause, such as the distribution of molten matter below 

 the ground. Inspection of the accompanying map of the 

 British Isles, on which the rates of increase in different 

 localities have been plotted, will afford some evidence 

 of the truth of this view. Comparatively low rates of 

 increase are found over Wales and in the province of 

 Leinster, districts of relatively great stability, the rem- 

 nants of an island that have in all probability stood for 

 the most part above the sea ever since the close of the 

 * See note on the Geothermic Gradient, p. 61. 



