THE BLACK SLATE AT THE FALLS OF 1HE 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 all 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 Cashaqua 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 
