300 EVOLUTIONAL GEOLOGY. 



supposed ''average rate of increase of temperature with descent below 

 the ground." It is doubtful whether we have yet discovered a rate 

 that in any useful sense can be spoken of as "'average.'" The widely 

 divergent views of different 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 association appointed to investigate this question 

 arrived at a rate of l*-" 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 reinvestigation of recorded measurements would, I believe, 

 lead to a rate of 1° F. in 8<) or 90 feet as more closely approaching the 

 mean. This would raise Lord Kelvin's estimate to nearly tifty 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° i^. in 130 

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

 irregularities may be connected with differences in conductivitv of the 

 rocks in which the observations were made, or to the circulation of 

 underground water; l)ut many cases exist which can not be explained 

 away in such a manner, ])ut are suggestive of some deep-seated cause, 

 such as the distribution of molten matter ])elow the ground. Inspec- 

 tion of the accompan3'ing 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, dis- 

 tricts of relatively great stabilit}", the renmants of an island that have 

 in all probability stood above the sea ever since the close of the Silu- 

 rian 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 subterra- 

 nean lakes of molten rock must as far as possible be eliminated; but 

 this will not l)e effected l)y taking the accepted mean of observed rates 

 of increase of temperature. Such an average is merely a compromise, 

 and a nearer approach to a correct result will possibly be attained by 

 selecting some low rate of increase, provided it be based on accurate 

 observations. 



It is extremely doubtful whether an area such as the British Isles, 

 which has so frequently been the theater of volcanic activity and other 

 subterranean disturbance, is the best fltted to afl'ord trustworthy re- 



