GEOLOGY OF THE AUBURN-GENOA QUADRANGLES 1 7 



veniently divided into two masses, from the presence of limestone 

 and fossils in the one and their absence in the other." 



On page 177 of the report on the Fourth District, Hall describes the 

 lower division and adds : '' This division terminates upward by a thin 

 band of limestone above which the shale is more fissile and gradually 

 passes from black to an olive or dark slate color." 



The limestone here referred to is now known as the Stafford lime- 

 stone. It is 8 to 10 feet thick in Erie county and diminishes gradually 

 to 4 inches on Flint creek in Ontario county. This hard limestone 

 layer is not found in exposures of this horizon east of Flint creek but 

 a band of gray calcareous shale containing many species belonging to 

 the fauna of the Stafford limestone, and that are absent from or very 

 rare in the adjacent shale, serves to mark the point of separation be- 

 tween the two divisions of these dark shales. 



The term Marcellus black shale as now used applies only to the 

 lower division, succeeding the Onondaga limestone and overlaid by 

 the Stafford limestone or in its absence, the lighter shales of the second 

 division now known as the Cardiff shales. Its thickness is 45 to 50 

 feet on the Auburn quadrangle. 



The change is abrupt from the blue Onondaga limestone to the 

 black Marcellus shales in Aladison and Onondaga counties but a few 

 thin calcareous layers are interstratified in the succeeding 13 feet and 

 a hard stratum 2 to 3 feet thick, known to geologists as the Agoniatite 

 limestone occurring at this horizon, is a persistent and easily recog- 

 nized feature of the Marcellus section from Schoharie county to the 

 town of Phelps, Ontario co., where it is finely exposed in the bed 

 of Flint creek. 



The shales that intervene between the Onondaga and the Agoniatite 

 layer in eastern central New York become more calcareous toward 

 the west and on this quadrangle this bed is composed principally of 

 impure limestones in layers a few inches thick with partings of black 

 shale, and a 5 inch stratum of shale at the base. At the exposure in 

 the bed of Flint creek the proportion of calcareous matter is still larger 

 and in the western part of the State the Agoniatite layer and the strata 

 below it have become so far assimilated to the Onondaga limestone as 

 not to be readily distinguished from it. 



The remaining upper part of this formation is a bed of black shale 

 the only notable feature of which is a row of spherical concretions 2 

 to 3 feet in diameter found in this horizon wherever exposed in the 

 central and western part of the State. 



