INTRODUCTION. 
tl 
. In regard to the preceding formations, constituting the Potsdam and 
Calciferous sandstones in their various phases, we have already sufficient 
knowledge of the characteristic fossils to show that the fauna of this lower 
division of our palaeozoic series corresponds in many respects with that 
designated by Barrande as the “Fauna Primordiale” in Bohemia. 
The rocks and groups of the Second Great Period, as determined by 
the relations of its fauna, consist of the following : 
7. Hudson-river group. 
6. Utica slate. 
5. Galena limestone. 
4. Trenton limestone. 
3. Black-river limestone. 
2. Birdseye limestone. 
1. Chazy limestone. 
The first member, as already stated, is absent over a large part of the 
western and northwestern country, while the second and third members 
are likewise frequently very thin or altogether absent. We have learned, 
moreover, that the fossils characteristic of the Trenton limestone do 
sometimes appear at a lower horizon than had been supposed, while some 
of the fossils of the lower members occur in the Trenton limestone. In 
that part of the country we occasionally find fossils of the Black-river and 
Trenton limestones mingled together, with one or two forms of lower 
species, in a rock possessing the characters of the Black-river limestone. 
The investigations in the Canada Geological Survey show that certain 
parts of the limestone formation contain many of the Black-river and 
Trenton species, intermixed in the greatest profusion. 
The lines of demarcation, therefore, between these different rocks, are 
not everywhere so well defined as in certain parts of New-York. Never¬ 
theless the value of these subdivisions remains, since we find no difficulty 
in recognizing the Black-river and Birdseye limestones at numerous points, 
through New-York and Canada, by the northern margins of Lakes Ontario, 
Huron and Michigan, and thence across Wisconsin to the Mississippi river, 
