REVISED PRINCIPLES OF CORRELATION 481 



The aggregate range of a few broadly conceived species of the latter kind 

 finally became world-Avide, but their geographically separated fossil oc- 

 currences commonly are easily distinguishable and seldom, if ever, so 

 nearly contemporaneous as to be of practical value in exact stratigraphic 

 correlation. 



Correlation of Paleozoic formations in distinct provinces seems espe- 

 cially difficult, and the difficulties seem no greater when the provinces 

 are on different continents as when they are on the same continent. In- 

 deed, I have found it easier to establish time relations between Paleozoic 

 formations in Europe and parts of North America than between those of 

 the Ohioan and the Cordilleran provinces in the United States. Fortu- 

 nately a few of the widely transgressing faunal horizons are recognizable 

 in the two regions. The task, therefore, is not hopeless, for with these 

 datum planes the relations of the intervening parts may be satisfactorily 

 determined by means of the physical criteria of cliastrophism. 



The difficulties encountered in correlating by invertebrate faunas are 

 less formidable when it comes to t*he Mesozoic and Cenozoic formations, 

 for here we have less and less to do with diversely originating marine 

 deposits in far inland areas. Besides, though the matching of the younger 

 Atlantic and Pacific faunas is perhaps no less unsatisfactory, we need 

 not at once call on the physical criteria. This final recourse is deferred 

 until the evidence of the land animals and plants, which became promi- 

 nent in the meantime, has been exhausted. The land organisms bridge 

 the land gaps between the marine invasions and should be accepted as 

 serving the same purpose in correlating deposits of distinct marine prov- 

 inces as that performed by the widely transgressing marine faunas. 



EXAMPLES SHOWING TENTATIVE NATURE OF CORRELATIONS BASED ON 

 SUPPOSEDLY CHARACTERISTIC GENERA 



Now permit me to mention a few of many instances tending to show 

 the arbitrariness — consequently the probable tentativeness — of age rela- 

 tions generally credited to supposedly characteristic fossils, particularly 

 of the grade of genera. 



Formerly mention of the Olenellus fauna was only another way of re- 

 ferring to Lower Cambrian remains. Now, though the fact has not yet 

 appeared in the textbooks, it is recognized that an excellent expression of 

 this fauna persisted into Middle Cambrian time. 



Similarly the fauna containing trilobites of the genus Crepicephalus 

 was thought to be confined to the Middle Cambrian, but this also proved 



