176 
GEOLOGY OF THE FOURTH DISTRICT. 
most part, unlike those of the limestones. Over the whole area known to be occupied by the 
Corniferous limestone, there is an abrupt change from that rock to a black fissile argillaceous 
slate ; while fossils, which have been abundant in the limestone, either cease entirely or are 
succeeded by others of a totally different character. 
The Marcellus shale is the commencement of what may be considered as one great period 
of the New-York system, the materials of which are very unlike in different parts, or isolated 
portions ; but viewed as a whole, present a general similarity in the products and their causes. 
The variation in lithological character is attended by a greater or less change in the character 
of the fossils ; and though every successive division is strongly marked by its peculiar organic 
types, still some species of the lower divisions may always be found. The line of demarka- 
tion between these divisions is sometimes, and in some places, well defined, while at others 
there seems a gradual merging not only of the lithological products, but also of the fossils. 
The divisions heretofore enumerated in the Fourth District are sufficiently well marked 
throughout its extent, while at the east they are gradually less distinct, until at last in many 
places there is no distinguishable line of separation. 
This arises both from the greater similarity of lithological products throughout, which the 
whole presents at the east, and also from the gradual ascent of some of the fossils which at 
the west are confined to lower divisions. This will be made more apparent under the descrip¬ 
tion of the several subdivisions. 
The whole series, from this point as far as the base of the Old Red Sandstone, seems to have 
been coeval and coextensive. All the members gradually diminish in thickness in a west and 
southwest direction, and the fossils of all seem to keep pace with this diminution, until at last 
the mass is nearly or quite non-fossiliferous. At their eastern commencement the lithological 
character is similar throughout the greater part of the mass, but as we progress westward we 
find a gradual separation, the more shaly portions with some calcareous matter taking the 
lower position, while the sandy and the alternations of shaly and sandy deposits hold a higher 
place. This character is maintained nearly the whole distance to the Mississippi river. 
