6 PETKIFACTIONS AND THEIR TEACHINGS. 



The subdivisions of the strata are chiefly founded on the 

 differences observable in the faunas and floras that is to say, 

 in the assemblages of animals and plants which, according to 

 the present state of our knowledge, characterise the respective 

 series of deposits. A few localities are inserted because they 

 will be referred to hereafter. I will only remark that many 

 of the details in the above classification must be considered as 

 arbitrary and provisional ; but " hard lines are admissible in 

 Science, whose object is not to imitate Nature, but to interpret 

 her works." ' 



1 Mr. Greenough. 



