INTRODUCTION. 
29 
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. 
