572 HENRY SHALER WILLIAMS 



categories are used for formations of different relative size ; mak- 

 ing the list of names to be group, system, series, stage, as adopted 

 by the. international Congress. Each of these stratigraphic divi- 

 sions has its corresponding chronologic category, viz., era, 

 period, epoch, age. The European has no difficulty in express- 

 ing on the map, or in discussing, either the time or the structural 

 relations of the formation. On the map, the Walden, Lookout, 

 Bangor, and Fort Payne would be to him four series, together 

 constituting the Carboniferous system. By placing the Chat- 

 tanooga in the category of series he at once would indicate that 

 he does not regard the formation as necessarily representing the 

 whole Devonian system. On the other hand, when he speaks of 

 the Carboniferous period he is not discussing any local set of 

 formations but the total period of time in which lived a definite 

 set of plants and animals, only a few of which are discovered in 

 any one local formation. There can be no question that the 

 systems of the European geology can be recognized in this 

 country onlyhy the fossils — but that does not change them from 

 formational aggregates into time divisions. 



The implication in the Tenth Annual Report that the divisions 

 which are discriminated by fossils must be chronologic, and not 

 structural, suggests the way in which our usage may be improved ; 

 but the fallacy of the principle is seen by noticing that the 

 smaller formations (the series and etages of the international 

 nomenclature), are to be discriminated by their fossil contents 

 as well as the larger ones (the systems). If discrimination 

 "primarily by fossils" were to be the test as to whether the 

 division were structural or chronologic then formations would 

 become chronologic divisions in every newly surveyed area in 

 which actual lithologic continuity could not be traced to some 

 standard outcrop. 



These two sets of facts (structural and paleontological) both 

 have to do with the classification of formations on a time basis ; 

 and those who are accustomed to frame their conceptions of 

 geological time on the basis of one set of facts, find difficulty in 

 even conceiving that there is any other basis. 



