GEOLOGICAL USAGE OF TERM FAUNA 147 



While it is practicable to establish homotaxial equivalence between a 

 particular local fauna and the general fauna of some particular system 

 (and perhaps to locate the fauna in its lower, middle, or upper portion), 

 it does not follow that a closer degree of equivalence can be established 

 between two local faunas by the same criteria. In the first case the 

 general equivalence may be proven in a case in which few or none of 

 the species are identical species, but even in case the identity of species 

 in two formations is proven, the equivalence so established is only within 

 the limits of the known range of the species of the fauna. This range 

 in most known cases is at least as much as a third of the thickness of 

 the system in which it belongs. We are therefore forced to the convic- 

 tion that in the correlation of local formations the same species of fossils 

 alone (when so much as 50 miles of distance separates their stations) 

 can not be relied on for establishing more than a general homotaxial 

 relation of the formations compared. The limit of range of every species 

 is far greater, both above and below, than is indicated by any local 

 formation in which it occurs. Geologists have already recognized the 

 fact that uniform conditions of sedimentation are local as well as tem- 

 porary, and the same principle must be applied to fossils. The ver- 

 tical range of individual species, as well as that of their combination 

 into faunules, varies greatly with the local conditions that prevailed 

 during the life of the species, and thus their place in the vertical strati- 

 graphic column varies with geographic distribution. 



Summary and Conclusions 



What has been said refers to the geologic formation as a particular 

 mass of stratified rocks occupying a particular position or horizon in the 

 geological column and whose geographical extent may be determined. 

 What I am urging is that greater clearness of description and accuracy 

 of statement will be attained in describing such formations if all refer- 

 ence to time relations be dispensed with. Let us speak of them as 

 " homotaxial ; " but when their lithologic or paleontologic characters 

 differ, let us say so and call the formations by different names and indi- 

 cate their general relations to the standard scale simply by bracketing 

 them as Devonian, or, if the correlation be more definite, as Eodevonian 

 or Mesodevonian, as the case may be, but not as the equivalent of a for- 

 mation which we regard as formationally distinct. 



The next point I have to make is that in the definition, and particu- 

 larly in the mapping, of formations it is important to make the defini- 

 tion so that the mass not only can be located and recognized by the 

 terms of the definition, but so that its limits can be distinctly recognized 



