24 



NEW YORK STATE MUSEUM 



It is 65 feet thick at the mouth of the Genesee river gorge at Mt 

 Morris and 90 feet in Ontario county. There are large exposures of 

 these beds at Mt Morris, on the shores of Conesus and Honeoye 

 lakes, in the Bristol valley, at the head of Canandaigua lake and in 

 the West River valley in Yates county. 



Pterochaenia fragilis is abundant in thin layers of this 

 shale at the exposure on Smoke's creek south of Windom. Other 

 fossils are exceedingly rare. 



Portage beds 



Middlesex black shale 



As in the case of the Hamilton beds and other larger divisions it 

 has also become necessary to recognize the distinct component 

 units of the Portage division by separate terms. The Middlesex 

 black shale is the lowest of the subdivisions of the former Portage 

 group. It is abundantly exposed in the Middlesex valley in Yates 

 county, whence the name [see N. Y. State Mus. Mem. 6 and Hand- 

 book 19]. It appears as a bed of densely black slaty shale 6 feet 

 thick superjacent to the West River shale at Windom; is found on 

 Eighteen Mile creek and the south shore cliffs and well exposed on 

 Pike creek. 



It may be easily distinguished at nearly all of the outcrops from 

 the West River shale by its darker color and more bituminous char- 

 acter. Like all the preceding divisions, it increases slowly in thick- 

 ness toward the east and is 35 feet thick in Ontario county, where 

 a few thin sandstone slabs and hard blue shales are interstratified 

 at the bottom and near the top. 



The Middlesex shale contains very few fossils, but L i n g u 1 a 

 1 i g e a is common in this horizon on Pike creek, North Evans ; 

 and it occasionally carries in the central part of the State Sand- 

 bergeroceras syngonum, Pterochaenia fragi- 

 lis and Spathiocaris emersoni, together with plant 

 and fish remains. 



Cashaqua shale 



The Cashaqua shale is another well characterized subdivision of 

 the Portage beds, for which the original name given by the early 

 geologists has been revived. The type locality of this formation 

 is along Cashaqua creek, a confluent of the Genesee river. The 

 formation is composed of light blue-gray or olive soft shales 

 in which are interbedded at frequent intervals concretionary 

 calcareous layers 2 to 4 inches thick. Layers of dark or black 



