274 REPORT OF NATIONAL MUSEUM, 1892. 



siliferous and that the great series was once made up of formations 

 similar to those which have been defined on preceding pages, but it 

 does not necessarily follow that the divisions which are now recogniz 

 able by physical characteristics correspond to those formations. It is 

 probable that they more nearly correspond to systems or to the larger 

 divisions of systems as they are recognized in the great scale of the 

 fossiliferous rocks of the earth. Therefore the discussion of formations 

 in this essay does not necessarily apply to the pre-Cambrian stratified 

 rocks. 



The following conclusions concerning formations are deducible from 

 the facts which are stated in the foregoing paragraphs: 



While they are physical objects and have only a physical existence 

 their proper characterization is chiefly biological. 



They are charaoterizable mainly by the fossil remains of aquatic 

 faunas. 



Neither their physical nor biological limits are sharply defined except 

 as a result of accidental causes. 



Their geographical limitations are indefinite except those which 

 were occasioned by shore lines. 



They do not necessarily bear any close relation to one another as to 

 geographical area, thickness, or the duration of tiihe in their accumu- 

 lation. 



Although they are thus unequal to one another they constitute the 

 only available physical units for local or regional stratigraphic classi- 

 fication. 



Because of their limited geographical extent they can not be used as 

 units of the universal classification of the stratified rocks. 



