INTRODUCTION. 
37 
recognized in its marked characteristics far to the westward. The Niagara 
group, in its pre-eminently distinctive features, has been shown to follow 
this great northwestern curve, and to stretch far beyond the Mississippi 
river. The Onondaga-salt group follows a parallel line of outcrop, and 
is traced into Iowa in thin strata ; its continuity, interrupted in some 
slight measure by the axis which occurs in Wisconsin, can be followed 
over this great extent of country, and still to the westward. 
In the Lower Helderberg group, however, the line of outcrop and of 
principal accumulation has been from northeast to southwest. A great 
change in the condition of the ancient ocean did supervene after the 
deposition of the strata charged with the peculiar crustaceans already 
noticed. Instead of finding the outcrops of the Lower Helderberg strata 
in lines parallel with those of the preceding rocks, the relative direc¬ 
tion of the main accumulation and the principal line of exposures is 
diagonally across the others. 
The investigations of the Canadian Geological Survey have shown 
the occurrence of these rocks in great force far to the northeast, in 
Gaspe on the Gulf of St. Lawrence, and the strata are traced thence 
southwesterly : they are seen near Montreal, lying unconformably upon 
the Utica slate. 
In New-York the strata of the Lower Helderberg group are exposed 
on both sides of the Hudson river. Upon the east they form an outlier 
known as Becraft’s mountain, and on the west they are seen in the 
Helderberg mountains, where, in the absence of the intermediate 
formation, they often succeed the Hudson-river shales, or, in other 
places, with the intervention of a few feet of other rocks. Westward 
we find these strata gradually thinning out, and we have scarcely any 
evidence of their existence in New-York west of Oneida county. On 
the other hand, when we follow the same beds in a southwesterly 
direction from the Helderberg mountains through Pennsylvania, Mary¬ 
land, Virginia, and Tennessee, we everywhere find the same group of 
strata, and bearing everywhere more or less the same species of fossils, 
with constant accession of new forms. In some localities in the middle 
