104 POPULAR LECTURES AND ADDRESSES. 



make a few remarks on the practical bearing of 

 the limitations which I have adduced upon some 

 points of geological theory, which, when the 

 boundary between mineralogy and geology is once 

 passed, cannot be evaded even by those most 

 averse to speculation. 



23. Fourier's theory of the conduction of heat 

 renders it almost impossible to escape the conclu- 

 sion, that if the earth has been solid and habitable 

 continuously during the last fifty million years, 

 its rate of increase of underground temperature per 

 metre downwards must have been very sensibly 

 more rapid fifty million years ago than now. The 

 more recently discovered laws of thermodynamics 

 render it certain that the sun must have been 

 something very different fifty million years ago 

 from what he is now ; and almost certain that he 

 must have been then very much hotter. And we 

 find Sir Roderick Murchison l writing as follows, 

 on purely stratigraphical grounds : " I could here 

 " cite the works of many eminent writers for 

 " numerous evidences of the grander intensity of 



1 Silnria ; 1867 Edition, page 489. 



