THE LESSONS OF A QUARRY. 213 



The calling into play of two important factors in 

 thought explains the whole mystery. First of all, our 

 geologist reasons inductively, from the present back- 

 wards to the past he argues about things he does 

 not know, from his knowledge of present-day things. 

 " The present is the key to the past ; " this is the motto 

 of the geologist of to-day. If I place a poker in the 

 fire for a given time, it becomes red-hot ; and from 

 this piece of present-day knowledge, I feel I am com- 

 petent to argue similarly about the behaviour of all 

 other bits of iron, whether pokers or not, and whether 

 they existed ages ago, or exist now. 



It is clear, however, that in my argument, I am 

 tacitly leaning upon a very sound and stable principle, 

 which forms the second factor in geological logic. If 

 the behaviour of pokers in the present is the key to 

 the knowledge of pokers in the past, I must be well 

 assured of one feature about bits of iron, and fire, and, 

 indeed, all other phenomena of Nature namely, that 

 they are all governed by defined laws, which act uni- 

 formly, and which are not subject to erratic change. 

 I am leaning in my argument, in short, upon that 

 principle we call " the uniformity of Nature," and 

 this is, in truth, a very stable support. For, from 

 astronomy to the ways of living things, we do not find 

 the universe to be dominated by erratic behaviour. On 

 the contrary, its affairs are governed by stern immu- 

 table laws, the very rigidity and regularity of which 

 form the glory of astronomy, and the great pride of 

 mathematical science. 



With the affairs of the world, viewed as a mass of 

 rocks, the case is the same ; and the geologist, there- 

 fore, is on safe ground when he reads the past in the 

 light of the present. The forces now operating in and 



