146 H. S. WILLIAMS — NOMENCLATURE AND CLASSIFICATION 



graphic scale. This conception is fully elaborated in the early reports 

 of the International Geological Congress, and the divisions of the time 

 scale — " era," " period," " epoch," " age " — are there regarded as strict 

 equivalents of " group," " system," " series," "stage," so far as their ap- 

 plication to the facts of stratigraphy is concerned. 



Most geologists, having been accustomed to use these terms as inter- 

 changeable, may find difficulty in recognizing the bondage to old ideas 

 which this usage enforces. 



Although the American geologists have adopted another system of 

 nomenclature, the influence of this implication is still apparent in the 

 confusion of chronologic language with physical facts. To avoid this 

 confusion the writer began several years ago to classify fossils into 

 faunas, irrespective of the formational limits to which they were sup- 

 posed to be restricted, and the fact has clearly developed that forma- 

 tional limits do not by any means mark the range of either the fossils of 

 a formation or of the integrity of associations of species into groups of 

 faunas. Thus the fact has developed that the local formation which in 

 a local section contains a diagnostic fauna is limited below and above, 

 not by the beginning and ending of the life history of the particular 

 fauna, but only by the beginning and ending of the fauna of the particu- 

 lar locality where the sedimentation took place. In another locality, it 

 may be not far distant, the same fauna may appear at a lower or higher 

 stratigraphic horizon and in its integrity. It may also reappear in the 

 same locality after having been entirely absent from the sediments for a 

 period of time represented by hundreds or thousands of feet of sedi- 

 ments, and in such cases the fauna is more apt to show disturbance of 

 its contents than when the whole fauna has become shifted. This expla- 

 nation will make it clear why the presence of a species of the Hamilton 

 fauna in the Romney of Maryland, Virginia, Kentucky, or Indiana does 

 not furnish proof of the Hamilton period, epoch, or even formation, as 

 those terms are used in current literature. 



The fossils of the Hamilton formations of New York undoubtedly have 

 a definite stratigraphic range, which we may hope to determine in the 

 future, but that will not change the stratigraphic limits of that forma- 

 tion. It will, however, enlarge the chronologic limits expressed by the 

 fauna of the Hamilton formation. The facts already in hand show that 

 the New York Marcellus below and the Nunda (" Portage ") and part of 

 the Chemung formations above are included in the time period through 

 which the Hamilton fauna ranges. In order to distinguish the names 

 of these two units the fauna of the Hamilton formation of New York is 

 called the Tropidoleptus fauna. 



