1889.] Description of the Devonian Rocks of Iowa. 233 
Xo well-marked lithological or biological sub-division of these 
rocks has been observed. ^ 
In the eastern extension of the Corniferous rocks, in Iowa, they 
are seen to be succeeded upward by gray, brown and buff, calcare- 
ous and argillaceous shales, limestone, and coarse and fine-grained 
sandstones of the Hamilton group. 
While at some localities the two divisions are sharply defined, both 
lithologically and biologically, still at other points these charac- 
ters of the two formations so gradually blend as to make it a mat- 
ter of great difficulty, if not an impossibility, to designate just where 
the line of separation between the two groups should be drawn. 
As might be naturally expected, throughout the area occupied by 
these divisions, the mingling of their faunas is much more strongly 
marked at their junction with each other. 
In their interior area, the line of division between the two groups 
IS nowhere distinctly shown, either by lithological or biological evi- 
dence. According to the record of the boring of the artesian well 
at Davenport, kindly furnished me by Mr. A. S. Tiffany, and which 
may be considered as approximate, the thickness of the Cornifer- 
ous rocks, in that vicinity, is shown to be one hundred and eighty 
feet. 
At one locality. Independence, the Corniferous limestones are 
succeeded upward by a blue shale, which here forms the base of 
the Hamilton, and which, from its order of sequence, we would con- 
sider to be the equivalent of the '' Marcellus Shales" of eastern sec- 
tions, although differing in some respects, in its lithological and bi- 
ological characters, from them. 
The beds of this serial are somewhat variable, lithologically, con- 
sisting of thin bands of concretionary limestone, and dark blue, 
argillaceous, fine-grained shales, which are highly charged with bi- 
tuminous matter, and interlaminated by seams of coal, from one- 
eighth to one-fourth of an inch in thickness. This shale weathers, 
on exposure, to a light blue clay, and contains an abundance of 
fossil shells, a few species of corals and cerinoid remains ; while some 
of the beds hold numerous remains of land plants {Lepidodendron 
and Sigillaria). 
' This Division has been referred, by Rev. Dr. Barris, to the Upper Helderberg, 
and its thickness estimated at nearly one hundred feet, ("Local Geology of Daven- 
port and Vicinity.") Proceedings of Davenport Academy of Science, Vol. II, 1S80. 
This formation has also been referred to the Corniferous, by Mr. A. S Tiffany, 
(Geology of Scott County, Iowa, and Rock Island County, Illinois, etc., 18S5.) 
