INTERNAL CONDITION OF THE EARTH. 301 



turc, due to the conduction inward of summer heat 

 and winter cold, are perceptible. This stratum is 

 on the average about 10 metres thick. Its thick- 

 ness, however, is different in different localities. It 

 depends on the thermal conductivity of the rocks 

 composing it. At greater depths than about 10 

 metres the effect of the variations of external 

 temperature with day and night and with the 

 seasons is not sensible. As we go down lower and 

 lower we find that the temperature of the strata 

 increases steadily. Speaking roughly, we find 

 that, on the average, the increase of under- 

 ground temperature in ordinary localities is about 

 1 C. per 30 metres of descent, or about i F. per 

 50 feet. 



There arc localities, such as Kreuznach in 

 Rhenish Prussia, in which the rate of increase of 

 underground temperature, as we proceed down- 

 wards, is much greater than this. These are 

 places, no doubt, where disturbances, in the way 

 of great outbursts of lava, have 'taken place in 

 comparatively recent times ; and I may suggest, 

 that careful observation of the rate of increase 



