TllANSACTIONS OF SECTION C. 



719 



mean; tlius 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 sug- 

 gested, and to some extent shown, that these irregularities may be connected with 

 differences in conductivity of the rocks in whictt the observations were made, or 

 with 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 



Fig. 1. — Map of the Eritish Isles, showing the distribution of 

 rates of increase of temperature with descent. The 

 rates are taken from the ' British Association Report,' 

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



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 pro- 

 vince of Leijister, districts of relatively great stability, the remnants of an island 

 that have iu all probabilty stood above the sea ever since the close of the 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. 



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 as possible be eliminated ; but this will not be efl'ected by takino- 



