218 
GEOLOGY OF THE FOURTH DISTRICT. 
signaling the group. Farther east the green shale (Cashaqua shale) is often darker in color, 
and more sandy, becoming largely interstralified with flagstones which have much the character 
of those which lie above it on the Genesee, except in the absence of a peculiar species of 
Fucoides. At the same time, the small Avicula, which is so abundant in the Genesee slate, 
is almost equally abundant in the green shale and flagstones at Penn-Yan. 
Again, when we go westward as far as Lake Erie, we find, that instead of the flagstones 
succeeding the green shale as on the Genesee, a black shale is the first mass above. This 
soon gives place to green shale ; and we have alternations of green and black shale for many 
hundred feet, before coming to the thin-bedded sandstones. 
26 . GENESEE SLATE. 
Upper Black slate , and Black shale and slate, of the Annual Reports. 
(Part of No. 8? Pennsylvania Survey.) 
[Lower part of P. woodcut, page 27. See also Sections, Plates V. and VII., and County Sections.] 
Superimposed upon the Tally limestone, or, in its absence, resting upon the Moscow shale, 
we find a great development of argillaceous, fissile, black slate. Where its edges only are 
exposed, it withstands the weather for a great length of time, and often presents mural banks 
in the ravines, river courses, and upon the shores of lakes. Where the surface of the strata 
are exposed, it rapidly exfoliates in thin even laminag. On disintegration, it is often stained 
with iron, owing to decomposition of pyrites ; but in many instances, and the greater number 
of localities, it retains its deep black color. In this it is distinguished from some beds of 
black slate in higher situations, which always become stained with hydrate of iron on their 
edges, and upon the surface of the laminae. 
In color and general characters it greatly resembles the Marcellus shale; and aside from 
position, it would be difficult to distinguish the two in the absence of fossils. 
Constituting a part of the great series of shales, with flagstones and thin-bedded sandstones, 
which occupy all the southern range of counties, it forms no conspicuous feature in the scenery 
or topography upon the general surface. In ravines and river banks, it is usually seen in 
connection with the rocks below or above, or with both, and forms only an element in the deep 
escarpments. Its greatest development in the district, and a point where it appears more 
prominently alone, is at the opening of the gorge of the Genesee river at Mount Morris.* 
* From the circumstance that other shales above this appear in the same gorge, it would have been desirable to give an¬ 
other name; but no locality is known, which is not more objectionable than this. Since it is very probable, also, that this 
