GEOLOGIC SECTION 299 



evidence of organisms has l^een obtained. In time, according to various 

 estimates, this represents from 2 T 0,000 to over 1,000,000 3- ears. 



Geologic Section 

 the zones and intervals 



The samples of the genus come from five distinct zones in the follow- 

 ing order: 



r Swartwood zone : 



Chemung J Interval of 400 feet. 



I Owego zone : 

 [ Interval of 400 feet, 



r Van Etten zone : 

 J Interval of 900 feet. 



^ j Ithaca zone : 



[ Interval of 600 feet. 



Hamilton Hamilton zone. 



HAMILTON ZONE 



In the Hamilton formation of Cayuga Lake the genus Ehipidomella 

 ranges through the upper 300 feet of the rocks (according to Professor 

 Cleland's careful elaboration of the facts, given in Bulletin 206 of the 

 United States Geological Survey), and for the upper 200 feet it is a 

 common species in almost every fossiliferous zone. The samples sub- 

 jected to exact measurement were obtained from the middle of this zone, 

 perhaps 200 feet below the Tully. The whole of the Tully limestone, 

 the Genesee black shale, and the Sherburne member of the Portage, each 

 with a distinctive fauna, intervened between the first and second zone 

 from which our specimens were obtained. 



ITHACA ZONE 



The second set of samples of the recurrence of Ehipidomella were 

 obtained in a single layer in Buttermilk Creek south of Ithaca by Dr. 

 E. M. Kindle. They lie below the main fossiliferous zone of the Ithaca 

 member of the Portage formation, fully GOO feet above the first horizon 

 of the Ehipidomellas of the Hamilton. About 900 feet of strata, holding 

 the typical Ithaca fauna of the upper or Enfield member of the Portage, 

 intervene between the Ithaca and Van Etten zones. 



VAN ETTEN ZONE 



The zone in which the next sample of the genus Ehipidomella appears 

 is about 900 feet above the Ithaca zone in (what has been called and 



