230 The American NaUtralist. [April, 
in most of the details of stratigraphical, lithological and palaeon- 
tological composition, differs greatly from the others; although 
all at the same time, by various links of evidence, demonstrate that 
they represent the same geological age, and usually show, more or 
less distinctly, a similar order of sequence. 
In this report it is our aim to deal, more particularly, with the 
typical section (Iowa) of the Interior Continental Area. 
The area of surface occupied by the rocks of Devonian age in 
Iowa comprises a wide strip of country, the general trend of which 
is Northwestward and Southeastward. 
It is about two hundred miles in length and fifty miles in width ; 
the general details of its outlines may be seen upon the geological 
map of the State ; which, however, demands some important modi- 
fications. 
The rocks of this age, in Iowa, have been referred by geologists 
to different epochs ; for instance, the shales and sandstone, which 
occupy the upper portion of the Devonian stratum near the mouth 
of Pine Creek, and at other points on the Mississippi, to the Che- 
mung group ; and the limestone and shales, occupying a " lower "? 
horizon, at Davenport, Iowa City, Independence, &c., and the shales 
at Rockford and Hackberpy, to the Hamilton Group (Hall's Geol- 
ogy of Iowa, Vol. I. Part i and 2^ 1850). 
The rocks also at Cedar Falls, have been referred by Professor 
A. H. Worthen, to the Chemung group {Loc. cit.) 
Some years later, in 1873, a reexamination of some of the rocks 
of this age was made by Hall and Whitfield, and the limestone 
at Waterloo, and the shales at Rockford, were declared to be the 
equivalents of the New York Corniferous and Chemung Groups, 
respectively (23d Report on State Cabinet of New York, pp. ,223-226) 
Again, Prof. H. S. Williams, in 1883 (American Journal of Science, 
February, 1883), referred the shales at the top of the Devonian, 
at Rockford and Hackberry, to the base of the Chemung of the 
New York Geologists, and, more recently, to the upper part of the 
Hamilton of the New York Section (American Geologist, Special 
Number, 1888, pp, 240, 242, &c.). 
On the other hand, Dr. C. A. White (Geology of Iowa, 1870, Vol. 
I., p. 178) is of opinion that all the Devonian strata of Iowa, belong 
to a single epoch, the Hamilton. 
By various other writers, the rocks of this age have been referred 
to each of the several divisions of the New York section. 
