GEOTHERMIC GRADIENT 61 



branches of mathematics will discuss these results in 

 relation to the level of no strain. 



In this connexion a word may be added in reference to 

 the geothermic gradient. A statement made on page 17 

 called forth a protest from the late Professor Everett, to 

 whose unwearied zeal as Secretary of the Committee 

 appointed by the British Association to investigate the 

 rate of increase of underground temperature geologists 

 owe a great debt of gratitude. In the twenty-second 

 report of this Committee we read: * " . . . In view of the 

 fact that the President of Section C last year characterised 

 the variation in the British Isles ' from 1 in 34 feet to 1 

 in 92 feet ' as ' a surprising divergence from the mean,' it 

 is well to emphasise the connexion between gradient and 

 conductivity. If there is anything like uniformity in the 

 annual escape of heat from the earth at different places 

 there must necessarily be large differences in geothermic 

 gradients, since the rate of escape is jointly proportional 

 to the gradient and the conductivity." 



The notion that " there is anything like uniformity in 

 the annual escape of heat from the earth at different 

 places " is hardly likely to commend itself to the geologist. 

 Such indications as there are point altogether to the con- 

 trary. The mere existence of volcanos obviously in- 

 validates the statement as an absolute affirmative, and 

 ancient laccolites show that in past time concealed sources 

 of heat lay not very far from the surface. 



If, then, there is not uniformity in the annual escape of 

 heat at different places, it might be thought unnecessary 

 to labour the question further ; yet in view of the im- 

 portance of the subject to geological inquiry it may be 

 worth while to point out that the connexion between 

 gradient and conductivity does not account for the 

 surprising difference of extremes from the mean ; an 

 instance will make this clear. Of the various rocks 

 * "Beport Brit. Assoc.," 1901, p. 66. 



