CHAPTER II. 



TIME CORRELATION OF MAMMAI.-BEARING HORIZONS. 



THE TWO GRAND PROBLEMS. 



American correlation. — The first problem is the chronologic correla- 

 tion of the purely fresh-water American horizons with one another, a 

 problem which in exact form has hitherto made slow progress owing 

 to the very loose methods of collecting fossils for purely anatomic and 

 descriptive purposes without closely recording geologic levels and 

 other geologic data. Now, thanks to the revival of the more exact 

 methods which characterized some of Hayden's and Leidy's work on 

 the Great Plains, there is promise of very rapid progress. Among 

 paleontologists we are indebted to Scott, Wortman, Matthew, Gidley, 

 Merriam, Sinclair, and others, but especially to Matthew's very accu- 

 rate and complete manuscript faunal lists." Accurate faunal leveling 

 began with Hatcher's explorations of the Chadron formation (Tita- 

 notherium zone), and has been the invariable rule of the American 

 Museum expeditions since 1901, 



American and Eurasiatic correlation. — The second problem, fol- 

 lowing especially Cope (1^79-1884), Marsh (1891), Filhol (1885), 

 Scott (1888-1894), and Osborn (1900), is the approximate chronologic 

 correlation of American horizons with Eurasiatic vertebrate horizons 

 and thus indirectly with European marine invertebrate horizons, 

 which is rendered possible by the well-known alternation of marine 

 and fresh-water horizons over large parts of central Europe. This 

 indirect method of correlation with the European marine stages is 

 facilitated by the partial alternation of marine and fresh-water forma- 

 tions in Florida, as studied by Dall,^ and will in time establish the 

 western American Tertiaries in the geologic world time scale. 



When these two grand problems of American correlation and of 

 American-Asiatic-European-African correlation are worked out we 

 shall be able (1) to establish a complete and very accurate geologic 



a The most thorough previous correlation of the American Tertiaries Is that of Matthew, A, pro visional 

 classification of the fresh-water Tertiary of the West: Bull. Am. Mus. Nat. Hist., vol. 12, 1899, pp. 19-75. 

 At this writing a second edition is in preparation, the partly completed manuscript of which has been 

 placed at my service hy Doctor Matthew. It will be printed herewith as an appendix.— H. F. O. ^See 

 pp. 91-120.) 



b Dall, W. H., Geological results of the study of the Tertiary fauna of Florida, 1886-1903: Trans. Wag- 

 ner Free Inst. Sci., Philadelphia, vol. 3, pt. 6, 1903, pp. 1541-1620. Also, A table of the North American 

 Tertiary horizons, correlated with one another and with those of western Europe, with annotations: 

 Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1898, pp. 323-348. 



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