212 LAY SERMONS, ESSAYS, AND REVIEWS. [xi. 



geology ; but, more or less vaguely, it is assuredly present in the 

 minds of most geologists. 



Such being the three phases of geological speculation, we are 

 now in position to inquire which of these it is that Sir William 

 Thomson calls upon us to reform in the passages which I have 

 cited. 



It is obviously Uniformitarianism which the distinguished 

 physicist takes to be the representative of geological speculation 

 in general. And thus a first issue is raised, inasmuch as many 

 persons (and those not the least thoughtful among the younger 

 geologists) do not accept strict Uniformitarianism as the final 

 form of geological speculation. We should say, if Hutton and 

 Playfair declare the course of the world to have been always the 

 same, point out the fallacy by all means ; but, in so doing, do 

 not imagine that you are proving modern geology to be in 

 opposition to natural philosophy. I do not suppose that, at the 

 present day any geologist would be found to maintain absolute 

 Uniformitarianism, to deny that the rapidity of the rotation of 

 the earth may be diminishing, that the sun may be waxing dim, 

 or that the earth itself may be cooling. Most of us, I suspect, 

 are Gallios, &quot; who care for none of these things,&quot; being of opinion 

 that, true or fictitious, they have made no practical difference to 

 the earth, during the period of which a record is preserved in 

 stratified deposits. 



The accusation that we have been running counter to the 

 principles of natural philosophy, therefore, is devoid of founda 

 tion. The only question which can arise is whether we have, or 

 have not, been tacitly making assumptions which are in opposi 

 tion to certain conclusions which may be drawn from those 

 principles. And this question subdivides itself into two : the 

 first, are we really contravening such conclusions ? the second, if 

 we are, are those conclusions so firmly based that we may not 

 contravene them ? I reply in the negative to both these 

 questions, and I will give you my reasons for so doing. Sir 

 William Thomson believes that he is able to prove, by physical 



