44 THEORY OP THE EARTH, 



feet certainty, although the periods which inter- 

 vened cannot be determined with any degree of 

 precision. These epochs form so many fixed 

 points, answering as rules for directing our in- 

 quiries respecting thip ancient chronology of the 

 earth. 



8. Examination of the Causes which act at present on 

 the Surface of our Globe. 



We now propose to examine those changes 

 which still take place on our globe, investigating 

 the causes which continue to operate on its sur- 

 face, and endeavouring to determine the extent 

 of those effects which they are capable of pro- 

 ducing. This portion of the history of the earth 

 is so much the more important, as it has been long 

 considered possible to explain the more ancient 

 revolutions on its surface by means of these still 

 existing causes; in the same manner as it is found 

 easy to explain past events in political history, by 

 an acquaintance with the passions and intrigues of 

 the present day. But we shall presently see that 

 unfortunately this is not the case in physical histo- 

 ry; the thread of operation is here broken, the 

 march of nature is changed, and none of the 

 agents that she now employs were sufficient for the 

 production of her ancient works. 



There still exist, however, four causes in full ac- 

 tivity, which contribute to make alterations in the 



