THEORY OP THE EARTH. 69 



23. Of the Importance of Extraneous Fossils, or 

 Petrifactions, in Geology. 



The importance of investigating the relations 

 of extraneous fossils with the strata in which they 

 are contained, is quite obvious. It is to them alone 

 that we owe the commencement even of the Theo- 

 ry of the Earth ; as, but for them, we could never 

 have even suspected that there had existed any 

 successive epochs in the formation of our earth, 

 and a series of different and consecutive operations 

 in reducing it to its present state. By them alone 

 we are enabled to ascertain, with the utmost cer- 

 tainty, that our earth has not always been cover- 

 ed over by the same external crust ; because we 

 are thoroughly assured that the organized bodies 

 to which these fossil remains belong, must have 

 lived upon the surface, before they came to be bu- 

 ried, as they now are, at a great depth. It is only 

 by means of analogy, that we have been enabled 

 to extend to the primitive formations, the same 

 conclusions which are furnished directly for the 

 secondary formations by the extraneous fossils 

 and if there had only existed formations or strata 

 in which there were no extraneous fossils, it could 

 never have been asserted that these several for- 

 mations had not been simultaneous. 



It is also owing to these extraneous fossils, slight 

 as is the knowledge we have hitherto acquired 



