BIGSBY PALEOZOIC ROCKS OF NEW YORK. 385 



This is a remarkable approximation for localities so very distant, and 

 in so insignificant a stratum. 



Fossils recurrent in New York. — These are only three or four. 

 A. affinis and A. reticularis are both found in the Clinton group of 

 Silurian date, while A. prisca is Devonian strictly, as we find it only 

 in the Hamilton and Chemung groups. 



Fossils typical. —They are three or four : — Orthis resupinata (and 

 perhaps Rhynchonella. cuboides), Avicula reticulata y and A. signata 

 (Hall), with Calymene marginalis. 



Genesee Slate. 



Mineral Character. — In New York (Hall, Rep. p. 218), it is a 

 great development of argillaceous fissile black slate, in colour and 

 general character much resembling Marcellus Shale. It contains 

 concretions or septaria in one or more layers. Mineralogically and 

 palseontologically, this rock is everywhere the same, on Lake 

 Cayuga or Lake Erie. From the fineness of its particles and its 

 mineral uniformity, this slate must have been deposited at a qui- 

 escent period. Near Ludlow Village, it contains two narrow veins, 

 composed of a mixture of serpentine and limestone, somewhat like a 

 trap-rock : there are two similar veins near the Second Falls at this 

 village (Vanuxem). 



Prof. Rogers finds this set of slates to be brownish or bluish-black, 

 and very fissile in Pennsylvania (Johnston's Atlas). 



Transition. — This is not mentioned in the New York Reports, as 

 far as I can find. 



Position. — Conformable with the surrounding sedimentary strata. 



Place. — It is not seen to the east of Smyrna, Chenango County ; 

 but it extends westwards through the counties of Seneca, Yates, 

 Ontario, &c, to Lake Erie, in the neighbourhood of which it thins 

 off and disappears. 



It rests on Tully Limestone at Kidder's Ferry, Lake Cayuga, and 

 at Crooked Lake, and on the Moscow Shales of the Hamilton group 

 (Hall). 



Thickness. — On Seneca Lake and in the county of Ontario, this is 

 150 feet ; but on Lake Erie it is only 27 feet. It is 300 feet thick in 

 Pennsylvania according to Prof. Rogers (Johnston's Atlas). 



Fossils. — The fossils found in this State are only ten, according to 

 published statements ; and all these occur about the summit. They 

 are, — 



Tentaculites fissurella. *Orthis quadricostata. 

 *Lingula concentrica. Atrypa quadricostata. 



* spatulata. reticularis. 



*Orbicula Lodensis. didyma? 



Leptsena (Stroph.) setigera. Avicula fragilis. 



The fossils marked with an * occur on Lake Cayuga, and are 

 typical. 



This stratum has considerable fossil-relations with Marcellus Shale, 

 as may be seen on referring to Tables VIII. and IX. 



