CORRELATION OF THE POTOMAC 

 FORMATIONS 



BY 



EDWARD W. BERRY 



The character of the evidence in general available for purposes of 

 correlation, has been admirably stated by Bernard/ Clark/ Ward/ and 

 others, and need not be dwelt upon at length in the present connection. 

 Some of the criteria, which may be given greater or less weight, may be 

 briefly enumerated as follows: 



1. Identical or closely allied (affiliated) genera -and species. This is the 

 old and always useful method of establishing correlations, though it prac- 

 tically ignores the principle of homotaxis. Too much reliance cannot 

 be placed on percentages, and the individual abundance of types is an 

 important factor which should always be carefully considered. 



2. Latest appearance of identical or allied types. This is of less value 

 than the next following. 



3. Earliest appearance of similar types whose ancestors presumably 

 originated elsewhere. 



4. Similarity in stage of development of geographically separated 

 faunas and floras. Facts of morphology, habit, and structure, and of 

 organs and tissues are considered as well as the systematic differentiation. 



5. Dominance of certain types or general character of the facies, in 

 part the reciprocal of Nos. 1-4. 



6. Diastrophism, or the relation of the broader changes of crustal his- 

 tory to biological history. 



^ Bernard, Elements de Paleontologie, Paris, 1895. 



^ Clark, Bull. U. S. Geol. Surv., No. 141, 1896, pp. 47-53. 



^Ward, Principes et m^tliodes d'etude de correlation geologique au moyen 

 des plantes fossiles. Compte-Rendu cinq. Sess. Congres Geol. Internat., Wash- 

 ington, 1891, pp. 97-109. In English in Amer. Geol., vol. ix, 1892, pp. 34-47. 



