DISCRIMINATION OF TIME-VALUES IN GEOLOGY 58 1 



Since the thickness, as well as the kind of sediment, varies 

 with the geographical locality, the association of a geographical 

 name with the lithologic character of the formation becomes a 

 definite and precise mode of identifying a formation ; so that 

 the name "Medina sandstone" becomes a definite formation 

 name, the characteristics of which may be observed, and their 

 definition fully elaborated at Medina, where the formation 

 appears with its typical characters. Its place in a geological col- 

 umn, its thickness, and its composition, are all made definite by 

 the name Medina; any sandstone outcropping elsewhere can be 

 classed as " Medina sandstone " only by possessing the charac- 

 ters considered to be essential to formational continuity and 

 integrity. Of course one of those characters may be theoreti- 

 cally defined as the geologic time of its deposition ; this may 

 be indicated by the contained fossils. But formational continuity 

 and identity may be established in the case of non-identity of 

 fossils, and in one section the fossils may rise to a higher strati- 

 graphic plane than in another; so that ^.zX.wdX formational coiitiiiuity 

 may disagree with the evidence of diiration presented by the fossil spe- 

 cies. This fact is illustrated in the case of the Catskill formation, 

 which is known to occupy a stratigraphic position in eastern 

 New York and Pennsylvania continuous with rocks further west, 

 possessing both different lithologic characters and containing 

 fossils regarded as characteristic of the Chemung formation. 

 Such facts find easy expression when the nomenclature is fur- 

 nished us by which to separate a geochron from a biochron. 

 The Spirifer disjnnctns biochron is a different thing from the 

 Chemung geochron. In defining the Chemung formation it has 

 become necessary to distinguish it from the formation following 

 it. This following formation in many sections in New York and 

 Pennsylvania is a red sandstone and shale lacking the marine 

 fossils, sometimes holding fish of possibly marine, but probably 

 brackish-water habitat. For purposes of mapping and classifica- 

 cation, the Chemung formation is succeeded by the Catskill 

 formation ; but the evidence is conclusive that the time of depo- 

 sition of the red sediments of the Catskill formation of eastern 



