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 every where more or less the same species of fossils, 

 with constant accession of new forms. In some localities in the middle 



