RULES FOR USE OF FOSSILS IX STRATIGRAPHY 189 



widely extended indication of definite point of time than is the last ap- 

 pearance of species of a fauna which has been a long time dominant. 

 To apply this principle practically it will be necessary to select, for 

 marking the transition from one formatian to the next, the horizon of 

 entry of the new fauna into the region rather than the total departure 

 of the old one. When we deal with homoeotopic species, it is evident 

 that nowhere will the lines be so sharp as where heterotopic faunas are 

 brought into contact at the division line. Such a fact, as has already 

 been said, cannot be relied on as sharply diagnostic of time. 



9. CHRONOLOGIC VALUE OF RELATIVE ABUNDANCE OF SPECIES IN A FAUNA 



The stage of life-history of a fauna is indicated with greater precision 

 by the relative proportion maintained by the species in relation to each 

 other than by the presence or absence of the species. A clear example 

 of this law is seen by the analysis of the faunules of the Portage zone of 

 the Chenango section, coming as it does in direct sequence above the 

 Hamilton fauna. Thirty-four of its 41 species were recurrent Hamilton 

 forms, but its dominant list of 11 species contains but one of the species 

 dominant in the Hamilton below, and 5 of this list are species not oc- 

 curring below, at least 2 of which are clearly mutants of Hamilton 

 species. 



Farther west in the Ithaca section only 33 out of 84 species in the 

 fauna are recurrent Hamilton species, and in the expression of the fauna 

 at Ithaca the dominant list of 12 species contains not a single Hamilton 

 dominant species, but all the species of the dominant Hamilton list are 

 present in the Ithaca fauna of Ithaca. This example illustrates the way 

 in which the shifting of the fauna affects it. The fauna is not utterly 

 destroyed, but its equilibrium is badly broken ; most of the species con- 

 tinue to live, but the dominant species lose their place ; rivals replace 

 them, and with the replacement considerable modification of several 

 species is at once evident. 



10. FORMATIONAL EQUIVALENCE NOT IDENTICAL WITH FAUNAL EQUIVALENCE 



There is a decided difference between formational equivalence and 

 faunal equivalence. When the term equivalent is meant to signify oc- 

 cupation of the same position in the time scale or series of strata, the 

 facts here presented show that formations may be equivalent (as the 

 Portage, Ithaca, and Oneonta) (or the Chemung of the Clean section and 

 the Catskill of the Catawissa or Monroe County sections), while the faunas 



