GEOTHERMIC GRADIENT 



19 



Silurian period. To the north of this, as we enter a 

 region which was subject to volcanic disturbances during 

 the Tertiary period, the' rate increases. 



FIG. 1. Map of the British Isles, showing the distribution 

 of rates of increase of temperature with descent. The 

 rates are taken from the "British Association Eeport," 

 except in the case of those in the south of Ireland. 



It is obvious that in any attempt to estimate the rate 

 at which the earth is cooling as a solid body the disturbing 

 influence of subterranean lakes of molten rock must as far 



