INTRODUCTION. • 99 



the examinations confined to this great extent of outcrop and exposure, 

 the fauna would be regarded as very meagre indeed. 



The line of escarpment extending from the promontory of Porte de . 

 Morts, by Green Bay, Lake Winnebago, and Lake Horicon, indicates the 

 presence of lower formations; and it has been shown that a low axis • 

 extending from the northward has elevated the older rocks, while denuda- 

 tion has removed the higher beds over a wide area : and to this force of 

 denudation we owe this long escarpment of the Niagara limestone. This is 

 proved by the numerous outliers of the Niagara limestone between that 

 escarpment and the Mississippi river ; and finally before reaching the river 

 in the northern part of Illinois, the limestone becomes again a continuous 

 formation, with a trend northwest and southeast, and an escarpment facing 

 to the northeast. From the Mississippi river this escarpment extends 

 northwesterly through Iowa, till with greatly diminished thickness it 

 passes into Minnesota. 



I have heretofore shown the occurrence of this group in Northern 

 Kentucky, where it is marked by characteristic fossils; and I have myself 

 seen the limestone of this age, with numerous characteristic corals, near 

 Cape Girardeau on the Mississippi river in the State of Missouri. 



By reference to the map, it will be seen that the Niagara group has 

 been traced from east to west, almost continuously, over an extent in 

 a direct line of about twenty degrees, and along its line of outcrop at 

 least a distance of five hundred miles more ; while from its extreme 

 known northerly limits to its most southerly known point of exposure, 

 is more than twelve degrees of latitude. 



It should not be forgotten, moreover, that many of the fossils enclosed 

 in the drifted pebbles on the shores of Lake Superior are so nearly 

 identical, and others identical with Niagara species, that we must infer 

 the existence of an equivalent formation far to the northward, from 

 which these drifted fossils have been derived. Over this area the condi- 

 tion of the ocean at that period was of a depth and temperature so 

 nearly uniform, as to admit everywhere of the existence of corals of 

 the same genera, and, to a great extent, of the same species. 



