Prosser — Hamilton and Chemung Series. 



22] 



almost invariably the change from the Hamilton to the Sherburne formation 

 may be readily determined even in the eastern part of the area under con- 

 sideration. On the geologic map prepared for this report, the Tully limestone, 

 and its horizon where it could be recognized, was regarded as defining the upper 

 limit of the Hamilton formation. On the geologic map of New York, the 

 upper line of the Hamilton is represented as crossing the northwestern town 

 ships of Chenango county, Lincklaen, Otselic and Smyrna, several miles to 

 the south of the outcrops of the Tully limestone, which enters the county 

 near the northwestern corner of Smyrna township and was followed along the 

 southern side of Pleasant brook to Smyrna village. After the disappearance 

 of the Tully limestone the transition from the Hamilton to the Sherburne 

 formation was generally readily recognized, and this horizon as traced across 

 the eastern part of Chenango, Otsego, Schoharie, Albany and Greene counties, 

 is represented on the accompanying map as marking the upper limit of the 

 Hamilton formation. The above division of these rocks is confirmed palaeon- 

 tologically by the discovery of the characteristic Tully species, IthyTwhoneUa 

 vemtstula, Hall, at this horizon in the central part of Otsego county. 



Above the Sherburne formation is one, containing numerous fossils, that 

 may be followed westward from the Chenango valley across Chenango, Cort- 

 land and Tompkins counties to Ithaca, where it long ago received the name 

 "Ithaca group. 111 The Ithaca formation has a thickness of fully 500 feet in 

 the Chenango valley, and from there it may be traced eastward into Greene 

 county, which is apparently near the limit of its eastern extent. The upper 

 limit of the Ithaca formation is determined by the appearance of the coarse, 

 greenish-grey and red sandstones and shales of the overlying Oneonta forma- 

 tion. On the present map, the Sherburne and Ithaca formations are repre- 

 sented together, the line already defined as representing the top of the Hamil- 

 ton indicating the base, and the line separating the Ithaca and Oneonta 

 defining the top. On the geologic map of New York, from the Chenango 

 valley eastward, the base of the Oneonta sandstone w as regarded as forming 

 the top of the Hamilton formation, the Ithaca and Sherburne formations being 

 mapped as a part of the Hamilton formation. Consequently, with sonic 

 modifications, the line representing the top of the Hamilton on the State 

 map will be found to correspond to the line on the accompanying map repre- 

 senting the top of the Ithaca formation. From what has just been said it w ill 

 be understood that on the State map from the Chenango valley eastw ard there 

 is no line corresponding to what the author regards as the line of division 



1 Third Annual Report, Fourth Geolosiral District, New York (Assembly Document No. 1880), l>. 318. 



