242 POPULAR LECTURES AND ADDRESSES. 



though search in particular localities for a diminu- 

 tion of the rate of augmentation of underground 

 temperature in depths of less than a kilometre 

 may be of intense interest, as helping us to fix 

 the dates of extinct volcanic actions which have 

 taken place within ioo,OOO years or so, we know 

 enough from thoroughly sure geological evidence 

 not to expect to find it, except in particular 

 localities, and to feel quite sure that we shall not 

 find it under any considerable portion of the 



h's surface. If we admit as possible any such 

 di-.eontinuity within 900,000 years, we might be 

 prepared to find a sensible diminution of the rate 

 at three kilometres depth; but not at anything' 

 less than 30 kilometres if geologists validly claim 



mueh as 90,000,000 of years for the length of 

 ihe time with which their science is concerned. 

 X.\v this implies a temperature of I,OOO cent, at 

 the depth of 30 kilometres, allows something less 

 than 2,ooo n for the temperature at 60 kilometres, 

 and does not require much more than 4,000 cent. 

 at any depth however great, but does require at 

 the great depths a temperature of, at all events, 



