238 M. Brongniart on the importance 



But this character of succession in the formation of 

 the beds of the earth, is frequently associated with other 

 remarkable differences, such as the nature of the rocks, 

 their structure on the great scale, their acknowledged order 

 of superposition, the minerals that accompany them, &c. : 

 now these mineralogical differences are almost always in 

 accordance with the characters derived from the general 

 resemblance of the organic remains in those deposities, 

 which are regarded as of the same formation from their geo- 

 logical characters, and they are found very generally in 

 accordance with their differences in the inrerse'^.-ase. 



Yet there are cases in which these two classes of charac- 

 ters, without being in manifest opposition, no longer accom- 

 pany each other. 



These cases occur in the two formations I am about fo 

 refer to the chalk. The question then is, to which of the ivfo 

 characters we should give the preference, in order to deter- 

 mine the epoch of formation of the rock, which no longer 

 presents them associated together, that is to say, to answer 

 the following question : 



" When in two rocks, far distant from each other, the 

 rocks themselves are of a different nature, whilst the organic 

 remains are analogous, should we, from this difference, re- 

 gard these rocks as of a different formation, or should we, 

 from the general and properly determined resemblance of 

 the organic remains, consider them of the same epoch of 

 formation, when the order of superposition does not evi- 

 dently oppose itself to this conclusion ? " 



We must not forget that one of the principal objects of 

 geology, is to distinguish the different epochs which have 

 succeeded each other in the formation of the globe, and to 

 determine which are the rocks that have been formed at 

 nearly the same epoch. Now, it will be acknowledged 

 that rocks of very different natures may be formed at the 

 same time, almost at the same moment, not only in different 

 parts of the globe, but also in the same place. 



We cannot but admit a conclusion drawn from facts which 

 •we have under our eyes ; for all that now occurs on the 



