RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 323 



sive, are recognizable in North America with as much, certainty as they 

 are in Europe, although some of them, the Triassic and Jurassic, for 

 example, have not been so completely recognized. 



On the other hand the recognition of the substagcs, or division of 

 systems, which are more or less clearly definable in Europe, has in no 

 case been made with rational satisfaction in North America, although 

 many geologists have attempted their full recognition on this conti- 

 nent, and some have even attempted that of secondary divisions of 

 systems not only in North America but in other parts of the world, by 

 means of their European characteristics. 



I do not mean to say that at least some of the more general divisions 

 of systems are not distinguishable in North America as well as in 

 Europe, such for example as the Upper, Middle and Lower Devonian, 

 Upper and Lower Cretaceous, etc. I also do not mean to say that cer- 

 tain of the faunal and floral types which characterize divisions or sub- 

 stages, such as those of the European Cretaceous from the Neocoiniau 

 to the Danian, inclusive, as well as similar divisions of other systems, 

 have not beeu discovered in North American strata and in those of 

 other parts of the world.* 



I claim, however, that while the systems are satisfactorily recogniz- 

 able as already stated, their upper and lower limits are often illy de- 

 finable, and that they often do not accord with the recognized limits in 

 Europe, and that the same is also the case with the large general divi- 

 sions of systems referred to. Furthermore, I claim that in case of the 

 presence in North American strata of types which characterize any of 

 those divisions of the European Cretaceous and other systems just 

 referred to they are so often commingled with certain of those types 

 which characterize one or more other divisions of the same system 

 there that they can not have the same chronological significance on the 

 two continents. That is, types which are characteristic of different 

 divisions of a system in Europe, and which occur there in a certain 

 order of succession, are known to occur in American divisions of the 

 same system in a different order of succession. It is therefore evident 

 that the presence in a group of American strata of any one, or even 

 more, of the types which characterize a given division of a system in 

 Europe does not prove the absolute identity of that division in Amer- 

 ica. 



Although, as before stated, all the systems of the European scale have 

 been satisfactorily recognized in North America, their upper and lower 

 limits are not only often illy definable and sometimes discordant with 

 those of corresponding systems in Europe, but those limits have been 

 designated as occurring ai different horizons by those geologists re- 

 spectively who rely upon different kinds of fossil remains. For exam- 



* Although I more particularly compare North American strata with those of Eu- 

 rope, and oftener refer to the Cretaceous system than to others', I assume that tht 

 facts and principles involved are of world-wide application. 



