646 HISTORY OF GEOLOGY. 



logy employs as evidence of its statements. To 

 confound the two is the same errour as it would be 

 to treat philosophical history as identical with the 

 knowledge of medals. Geology procures evidence 

 of her conclusions wherever she can ; from minerals 

 or from seas; from inorganic or from organic bodies; 

 from the ground or from the skies. The geologist's 

 business is to learn the past history of the earth ; 

 and he is no more limited to one or a few kinds of 

 documents, as his sources of information, than is the 

 historian of man, in the execution of a similar task. 



Physical Geology, of which I now speak, may 

 not be always easily separable from Descriptive 

 Geology: in fact, they have generally been com- 

 bined, for few have been content to describe, with- 

 out attempting in some measure to explain. Indeed, 

 if they had done so, it is probable that their labours 

 would have been far less zealous, and their expo- 

 sitions far less impressive. We by no means regret, 

 therefore, the mixture of these two kinds of know- 

 ledge, which has so often occurred ; but still, it is 

 our business to separate them. The works of astro- 

 nomers, before the rise of sound physical astronomy, 

 were full of theories, but these were advantageous, 

 not prejudicial, to the progress of the science. 



Geological theories have been abundant and 

 various ; but yet our history of them must be brief. 

 For our object is, as must be borne in mind, to 

 exhibit these, only so far as they are steps discover- 

 ably tending to the true theory of the earth : and in 





