FACTS ON WHICH THE HYPOTHESIS RESTS 287 



of the body of a fauna entering the area in one order of succession and 

 departing in the reverse order. 



In the Ithaca section there occurs at tlie base of the fossiliferous zone 

 of the Ithaca member a bed containing al^undance of Spirifer (Reticu- 

 laria) Ice vis. The discovery of the same species at the top of the fos- 

 siliferous zone, as the normal Ithaca fauna become sparse, gave the first 

 suggestion that the faunas were moving or shifting, the Eeticularia zone 

 marking the first trace of the fauna to enter and the last to leave the 

 area. Confirmatory evidence was found in the order of succession of the 

 dominant species of the Ithaca fauna. These facts were reported in 

 1883.* 



The study of the mode of occurrence of Leiorhynchus still further 

 drew attention to the definite order in which a series of species came in 

 and went out of any given area. The species of the genus were generally 

 found abundantly at the base or at the top of fossiliferous zones rich in 

 brachiopods, in the midst of which Leiorhynchus was rare.^ 



RECURRENT HAMILTON FAUNA 



The occurrence in a single or few strata of several representatives of 

 an earlier fauna long after the formation to ivhich they are normal has 

 ceased. 



Slight traces of this fact were observed in the first survey of the De- 

 vonian section passing through Ithaca,^ and the fauna numbered 14 N 

 (page 15) was called a "recurrent Hamilton fauna" because of the ap- 

 pearance there of such species as Spirifer fimbriata, Sp. augusta, Pleuro- 

 tomariaxapillaria, and others; and higher up, in the midst of the Che- 

 mung section, at Chemung narrows, Tropidoleptus carinatus and Cypri- 

 cardella 'bellistriata, Phacops rana and Dalmanites calliteles were found. 



The discovery of such traces of an earlier fauna led to further search, 

 and as the evidence accumulated an elaboration and definite formulation 

 of the theory of recurrence of faunas was made, which has been set forth 

 in several papers, and is illustrated in detail in the folio of the "Watkins 

 Glen-Catatonk quadrangles, constituting folio number 169 of the U. S. 

 Geological Survey. 



ALTERNATE APPEARANCE OF DIVERSE FOSSILIFEROUS ZONES 



The facts there brought out are substantially as follows : 



There are exhibited in the sections mapped for the quadrangles two 



* Bulletin 3, U. S. Geological Survey, p. 20. and Proceedings of the American Associa- 

 tion for the Advancement of Science, vol. xxxlv. 1885, p. 222. etc. 

 5 See Bulletin 3. U. S. Geological Survey, 1883, pp. 16 and 17, 

 •Reported In 1883, Bulletin 3, U. S. Geological Survey, 



