674 JAMES PERRIN SMITH 



paleontologic data. But outcrops of strata are deceptive, and 

 often apparently continuous beds of the same character turn out 

 to contain a number of different formations. In the western 

 states such lithologic and stratigraphic correlations have been, 

 more often than not, erroneous, while in the Mississippi valley 

 region they have usually been at least approximately correct, 

 because the great geologic events that were the causes of the 

 stratigraphic changes were uniform over wide areas. Even today 

 the catastrophe doctrine of Cuvier makes itself felt, and we find 

 paleontologists and stratigraphers using unconformities as a basis 

 for the separation of Cretaceous from Jurassic, where the fossils 

 do not tell a definite story, as if the uplift and erosion would 

 necessarily come at the same time in Europe and America. 



Paleontologic correlation itself is not infallible ; it must be 

 used intelligently, its sources of error known and guarded against, 

 or else it is little more reliable than the lithologic method; these 

 errors lie chiefly in defective knowledge of the vertical and hori- 

 zontal range of species or genera chosen as criteria, and in erro- 

 neous identification of these forms. Careful collecting, accurate 

 field and laboratory discrimination, and wide knowledge of the 

 literature are the best safeguards. 



Two sorts of paleontologic correlation may be recognized, the 

 direct, and the indirect method. In a limited province, such as 

 existed in England and France during Cretaceous time, faunas 

 were distributed uniformly over the area and had the same range 

 in the two countries. Thus correlation of English and French 

 Cretaceous strata is simple and direct, for they represent sedi- 

 ments that were once continuous, that were laid down in the 

 same basins or along the same margins, under the same climatic 

 conditions, and contained the remains of a similar fauna. 



On a larger scale the problem of correlating the western 

 European Cretaceous beds with those of the Atlantic slope Cre- 

 taceous of North America is the same. These strata were all 

 deposited in the same faunal region, and although there are pro- 

 vincial differences the American and the western European faunas 

 are remarkably similar, with even many species in common to 



