INTRODUCTION. 
17 
This group of strata, elsewhere described as consisting of shales, shaly 
sandstones, sandstones and conglomerates, presents in its western extension 
a very different aspect. In tracing the direct continuation of the same 
rocks through Canada West, and by the northern side of Lakes Huron 
and Michigan, the arenaceous portions gradually disappear, the shales 
become lighter in color, and an accession of calcareous matter gradually 
takes place. In the Manitoulin islands of Lake Huron we find the higher 
parts of the group represented in great measure by beds of limestone 
separated by shaly seams, and abounding in that very characteristic coral 
Favistella favosa. The arenaceous portions of the group in this region 
consist of a few thin and very subordinate beds of argillaceous sandstone 
containing fucoids; the prevailing character being that of calcareous shales, 
with thin laminae of limestone. The lithological aspect of this group, as 
it is seen on these islands, at Point aux Baies on Lake Michigan, and on 
the shores of Green Bay, is the same as at Cincinnati, where the group is 
known as the “Blue limestone ”*. 
More recently, in 1855, I have had an opportunity of proving the 
existence of this group at numerous points along the Mississippi river, 
where its relations to the underlying and superior rocks leave no question 
as to its relative position. 
As already described, this group, at the Manitoulin islands of Lake 
Huron, on Green Bay, at Cincinnati and elsewhere, consists of calcareous 
strata with intercalated laminae or thin beds of limestone, and is highly 
fossiliferous; abounding in several species of Orthis, Leptana, etc. Until 
1855, its existence had been overlooked upon the Upper Mississippi; but 
I am now able to prove its occurrence in numerous localities in Northern 
Illinois, Wisconsin and Iowa. In these places, however, it consists almost 
entirely of calcareous soft shale or clay. The fossils are confined to two or 
* This formation, as it appears at Cincinnati, and at Madison (Indiana,), I first made known in 1841 
(American Journal of Science, vol. 42) as the equivalent or continuation of the Iludson-river group; 
but was subsequently induced to yield this opinion, and have admitted its fossils into the Trenton 
period, in the first volume of the Palaeontology of New-York. Later examinations, made in 1850 and 
1851, have shown conclusively the correctness of my first published opinion in regard to the age of 
these rocks. 
[ Palaeontology III.] 
3 
