INTRODUCTION. 
27 
of that assemblage; and the original name proposed by them, of the 
“ Protean group,” well expresses its heterogeneous and varying character. 
Taking, therefore, for the base of the Niagara group a line which was found 
to be constant and could admit of no question, they have probably left out 
some beds which, with farther knowledge, might with equal or greater 
propriety be included in the Niagara group. 
In the present state of our knowledge of these formations, and the known 
expansion of the lower one in the northeast, together with the fact that in 
this direction we are to look for the source of our sedimentary materials, 
we must regard the lower shaly member of the Niagara group as only a 
modification of the shale formations preceding it in the Clinton epoch. 
Moreover the Niagara shale, in its eastern extension, is less fossiliferous, 
and assimilates more in character to the shales in the Clinton group. 
These points of enquiry can only be determined by careful comparison of 
the different members as they occur in New-York and in the northeast; 
a work which is rendered difficult, by the wide hiatus existing in these 
formations between Central New-York and the nearest points at which 
they are known in Canada East. 
Tracing the Niagara group from New-York westward, we find the shaly 
member gradually disappearing, and the entire group becomes calcareous. 
The limestone of this formation, which in Western New-York forms a bold 
escarpment known as the Mountain Ridge, maintains this character in 
Canada West, and its last locality is the promontory of Cabot’s Head. 
Although broken and denuded in its extension across the northern part of 
Lake Huron, it is nevertheless clearly seen in the islands marking the 
great curve from Cabot’s Head on Lake Huron to the promontory of Porte 
de Morts on Lake Michigan at the entrance of Green Bay. From the 
latter point, with some undulations and breaks in its outline, the bold 
escarpment is traced by the eastern shore of Green Bay to the head of 
those waters, and thence by the Fox river and Lake Winnebago to Mil- 
waukie. In this vicinity and southward, it gradually disappears beneath 
accumulations of drift; but it is nevertheless traced in distant exposures 
