Page:The Journal of geology (IA journalofgeology21894univers).pdf/165
mung formation, except that it does not express all the truth or the exact truth, for when the Catskill deposits are found in relation 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 confusion 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 elucidated. 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.