DUAL NOMENCLATURE. 



155 



midst of it ; that is, the Ithaca fauna is both below and above it. 

 Again, upon going further west, the same interval is filled by a 

 non-brachiopod marine fauna, i. e., that of the typical Portage 

 group of Genesee valley, and the Ithaca group of Ithaca is in the 

 midst of the corresponding rocks of the Portage, as the Oneonta 

 is in the midst of the Ithaca. This I interpreted, in the paper 

 referred to, as indicating that the succession of faunas was con- 

 trolled by oscillation of levels during the accumulation of the 

 deposits, the eastern shores rising during the first half of the 

 Portage (of the Genesee valley series) so that, during the par- 

 ticular stage represented by the fossiliferous zone of the Ithaca 

 group. Portage fossils were being accumulated in the Genesee 

 valley sediments, Ithaca fauna at Cayuga lake meridian, and, from 

 Chenango eastward to Oneonta, sands with fresh water Amnigenia 

 and Holoptychius characteristic of the Catskill formation. 



THE FOKMATIONS OF THE DEVONIAN SYSTEM IN THE APPALACHIAN 

 REGION AND THEIR RELATIONS TO THE TIME-SCALE. 



