INTRODUCTION. 
43 
affecting the conditions of a vast area, and with a corresponding change 
in the fauna ; and if we regard this evidence as sufficient for the dawn 
of a new period, we would limit the commencement of the Devonian 
to this horizon. 
The Upper Helderberg group, in its fullest development, consists of 
four members, the Cauda-galli grit, the Schoharie grit, the Onondaga 
and Corniferous limestones. The first, when characteristic, is a dark, 
gritty slate, which, even in its unaltered condition, has a cleavage 
vertical to the line of deposition, and is generally destitute of fossils ; 
but with surfaces covered with curved, fucoid-like markings which 
have given it its name. This rock constitutes beds of passage from the 
Oriskany sandstone, and graduates above into the Schoharie grit, which 
is an arenaceous limestone, weathering to a brownish color, and suc¬ 
ceeded by the gray subcrystalline coralline formation which is known 
in New-York as the Onondaga limestone, while the Corniferous lime-* 
stone consists of the higher dark-colored cherty beds of the group. 
In tracing this formation westward through New-York, the lower 
members gradually thin out, and neither the Cauda-galli nor the Scho¬ 
harie grit are known so far west as the centre of the State. It is in the 
Schoharie grit in the eastern counties of New-York, Albany, Greene, 
and Schoharie, that we first find those bony plates belonging to the 
early fishes. These plates or scales, with some fragments of bones, are 
all that we yet know of the occurrence of that class of animals in this 
period of our geological history. In the succeeding limestones at the 
west, there is an increasing number and variety of these ichthyolites. 
This limestone formation is of great extent. Tracing it through New- 
York and Canada West, we find it reappearing in Michigan, extending 
to the northern extremity of the southern peninsula., and forming the 
summit of Mackinac island; thence trending southward, it appears in 
Indiana and Illinois, and is traced into Iowa, where it is almost non- 
fossiliferous. Corresponding to the general contour of the country, 
this limestone lies on the two sides of the low Cincinnati axis. On the 
eastern side, it appears in Sandusky, Ohio, and thence is traced south- 
