CENOZOIC MAMMAL HORIZONS OF 

 WESTERN NORTH AMERICA. 



By Henry Fairfield Osborn. 

 INTRODUCTION. 



FORMATIONS AND ZONES. 



The main purpose of this paper is faunistic rather than geologic. 

 Many of the geologic ''groups" and "formations" referred to are 

 still imperfectly defined and known, either as to geographic extent or 

 as to lithologic content. Many of the geologic terms used are there- 

 fore not to be regarded as final. It should be clearly understood 

 also that the geologic sections are largely diagrammatic and in most 

 cases are not to be interpreted as giving a clue to the lithologic content. 



LIFE ZONES. 



It is proposed, according to the ruling of the International Geo- 

 logical Congress and the old practice of invertebrate paleontologists, 

 to use the word ''zone" for the faunistic levels of such geologic forma- 

 tions or groups as may be synchronized by the presence of certain 

 distinctive animals. Thus we may speak of the Vintatherium zone 

 of the upper Bridger formation or of the lower "Washakie." The 

 word "beds," previously used in the same sense, is liable to cause 

 confusion because it is used also for formations. 



GEOLOGIC FORMATIONS. 



A "formation" has been defmed as follows by the United States 

 Geological Survey : 



In all classes of rocks the cartographic units shall be called formations. 



The discrimination of sedimentary formations shall be based upon the local 

 sequence of the rocks ^- ^ * and the geologist must select for the limitation of 

 formations such horizons of change as will best express the geologic development 

 and structure of the region and will give to the formations the greatest practical unity 

 of constitution. In determining this unity of constitution all available lines of evi- 

 dence, including paleontology, shall be considered. Each formation shall contain 

 between its upper and lower limits either rocks of uniform ch^iracter or rocks more 

 or less uniformly varied in character as, for example, a rapid alternation of shale and 

 limestone. * * The definition of a formation - * * should include a 



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