INTRODUCTION. 
47 
shale succeeding the Upper Helderberg limestones. On the western side 
of the Cincinnati axis in Indiana, we know of the existence of this group, 
from the fossils which have come to hand from different localities, and 
the succession of beds corresponds to the same in New-York. At the 
Falls of the Ohio, we find the Marcellus shale with some succeeding 
calcareous bands, which are followed by greenish gray and olive shales 
and sandstones. In Illinois and in Iowa the group consists of a bluish 
gray calcareous shale, which is much more compact than in New-York ; 
and is succeeded by beds of limestone which in some places are of suffi¬ 
cient thickness to furnish quarries for building stone. 
The entire thickness in any of the exposures is not more than sixty 
feet, and though the mass is often crowded with fossils, the number of 
species is of course less than in the eastern localities of the group. In 
Missouri, Prof. Swallow estimates the thickness of the Hamilton group 
to be fifty feet. We thus see that from a mass of much more than one 
thousand feet in Eastern New-York, this group thins to one of fifty 
or sixty feet in its western extension. 
In studying this group on the western side of the Cincinnati axis, we 
find there precisely the same sequence as in New-York; and we are 
able to carry on continuous observations of some of the lower groups, 
both to the north and south, where the rocks are continuous. Together 
with the same sequence in Illinois and Iowa, we have numerous identi¬ 
cal fossils, and among them Strophodonta demissa, Atrypa reticularis, 
A. aspera, an Orthis similar to 0. tulliensis, Spirifer fimbriata, and other 
spirifers very similar to those of the Hamilton group in New-York. 
The Marcellus shale, as it is seen in New-York, is marked by bands 
of limestone containing abundance of large Goniatites; a‘nd in nearly 
the same horizon in Indiana we find a belt of shale containing great 
numbers of Goniatites, and some other fossils not unlike those in the 
same association in New-York. There can, therefore, be no doubt what¬ 
ever as to the true relations of the thin group of calcareous shales and 
limestones which we know in the Mississippi valley at Rock island, 
