INTRODUCTION. 9T 



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 



