THE BLACK SLATE AT THE FALLS OF THE OHIO. 151 



greenish shales, and alternating shales and flagstones, and finally heavy bedded 

 sandstones with intermediate, arenaceous, shaly partings. 



The Genesee slate has been regarded as constituting beds of passage to the 

 next formation, known as the Portage group ; and this, as well as the succeeding 

 shale, carries a few fossils, which are likewise known in the Hamilton group 

 below. This entire formation, consisting of shales and sandstones, and having 

 a thickness of a thousand feet or more in central New York, diminishes like 

 all the other strata in a westerly direction. On the shores of Lake Erie it has 

 become a succession of green and black shales and flagstones, with here and 

 there a lenticular mass of heavy-bedded sandstone, with abundant concretions, 

 and not unfrequently with lenticular masses of calcareous matter. Not only 

 this, but the lower beds of the Chemung group have become shaly and non- 

 fossiliferous; and the lines of demarkation between the formations can nowhere 

 be easily drawn. In this condition, these formations are exposed along the 

 Lake Erie shore, in New York, Pennsylvania and Ohio. 



The Portage group has been traced in the peninsula of Canada West, and 

 is finely exposed at Kettle Point on Lake Huron, where it carries ^,11 the 

 characteristics pertaining to its lower members in the Genesee valley or in 

 western New York, and holds, also, the same relative position to the Ham- 

 ilton group outcropping in close proximity. 



We have now to consider whether the Black slate in the vicinity of the Falls 

 of the Ohio is represented by the Genesee slate, or by any of the black slates 

 of the series of which it is the lowest member in New York. In this question 

 it should be remembered that some at least of the fossils of the Genesee slate 

 are known to pass upward into the green or olive Cashacma shale ; and that 

 so far as regards a grouping of the strata, the Genesee slate would more 

 properly form a member of the Portage group. Its relations to that formation 

 have always been recognized, having only been separated as a distinct member 

 to indicate the upward limitation of the Hamilton group. 



The strata below this Black slate, which have already been described, hold 

 the position and contain many of the characteristic fossils of the Hamilton 

 group, as already shown. The small Lingula of the Black slate differs little 



