DUAL NOMENCLATURE. 151 



mung formation, except that it does not express all the truth or 

 the exact truth, for when the Catskill deposits are found in rela- 

 tion to the Chemung deposits in a continuous section, some 

 Catskill facies of sedimentation do succeed the marine Chemung, 

 but it may not all succeed all the Chemung, as the testimony of 

 numerous observers in Pennsylvania and further south shows. 



When, however, we are talking of the time-scale the confu- 

 sion is more apparent ; for it is clear that one period cannot 

 both precede and follow a second period. Either the Catskill 

 Period must follow the Chemung, or the Chemung must follow 

 the Catskill, or else they are synonymous terms, and one is 

 superfluous. When Mr. Darton proposes that Catskill be used 

 in place of Chemung, he is reasonable on the old basis of the 

 old method of classification, but the facts are not thus eluci- 

 dated. The facts are that the Chemung period is synchronous with 

 a part of the Catskill period, but that the Catskill formation is 

 distinct from the Chemung formation, and when they occupy the 

 same section the Catskill formation succeeds the Chemung. 



Those who have watched the discussion of the classification 

 of the Upper Devonian, will remember that one of the greatest 

 difficulties presented in the progress of field studies has been the 

 fact that collectors have so frequently reported fossils where 

 they ought not to be. Formations, classified as Catskill in the 

 books, have yielded Chemung fossils, or, above so-called Catskill 

 rocks, Chemung fossils have appeared, or above Hamilton rocks, 

 in rocks regarded as Portage, have been found Hamilton fossils 

 again. Geologists on the Pennsylvania Survey have met with 

 serious rebuke for proposing Chemung-Catskill and similar names 

 which have thrown discredit upon the integrity of the formations. 

 These discrepancies have been interpreted to be evidence that 

 the observers could not tell the two formations apart, or had 

 been mistaken in their observations ; but the true interpretation 

 is that the criteria for determining the divisions of the time-scale 

 have disagreed with the criteria of the formation-scale. The 

 latest phase of the discussion has appeared in the papers of 

 Stevenson, Darton and Prosser, before cited. 



