670 HISTORY OF GEOLOGY. 



In reality, when we speak of the uniformity of 

 nature, are we not obliged to use the term in a very 

 large sense, in order to make the doctrine at all 

 tenable ? It includes catastrophes and convulsions 

 of a very extensive and intense kind ; what is the 

 limit to the violence which we must allow to these 

 changes? In order to enable ourselves to repre- 

 sent geological causes as operating with uniform 

 energy through all time, we must measure our 

 time by long cycles, in which repose and violence 

 alternate; how long may we extend this cycle of 

 change, the repetition of which we express by the 

 word uniformity? 



And why must we suppose that all our expe- 

 rience, geological as well as historical, includes 

 more than one such cycle? Why must we insist 

 upon it, that man has been long enough an observer 

 to obtain the average of forces which are changing 

 through immeasurable time ? 



The analogy of other sciences has been referred 

 to, as sanctioning this attempt to refer the whole 

 train of facts to known causes. To have done this, 

 it has been said, is the glory of astronomy: she 

 seeks no hidden virtues, but explains all by the 

 force of gravitation, which we witness operating at 

 every moment. But let us ask, whether it would 

 really have been a merit in the founders of physical 

 astronomy, to assume that the celestial revolutions 

 resulted from any selected class of known causes ? 

 When Newton first attempted to explain the mo- 

 tions of the moon by the force of gravity, and failed 



