INTRODUCTION. ly 



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 Bales 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 forination, 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. 



[ Pal^ontoloot III.] 8 



