234 The American Naturalist. [April, 
This division represents an old shore deposit, and carries, in its 
fauna and flora, evidence both of its terrestial and marine origin ; 
and marks, as well, the dawn and culmination of terrestial vegeta- 
tion of the old Devonian time, in Iowa. 
The thickness of this division is probably thirty feet or more, 
although only about twenty-five feet have actually been observed. 
These shales, which represent only a local sub-division of the 
Hamilton, were first recognized by Mr. D. S. Deering, of Indepen- 
dence ; and subsequently described by Prof. S. Calvin, as " Some 
Dark Shales Below the Devonian Limestone at Independence, Iowa" 
(Bulletin of U. S. Geological Survey, Vol. IV., No. 3, 1878.) 
In this publication, the statement was made (p. 726) ''That the 
shale in question is not a mere local deposit, but is distributed all 
along the outcrop of the Devonian Rocks of Iowa." 
An extended study of all the Devonian rocks of this State, and 
the record of numerous borings along its Eastern outcrop, and 
at other points, has failed to adduce any evidence of the existence 
of this formation at other localities. 
One of the highest members of the Hamilton, in its Eastern ex- 
tension, is a soft, friable, brownish-yellow sandstone, which is well 
shown as it out-crops on Pine Creek, some distance above " Pine 
Creek Mill." This stratum of sandstone here forms a bold escarp- 
ment or cliff, about forty feet in height, is obliquely and discor- 
dantly stratified throughout, dips rapidly in a southerly direction, 
and is, so far as observed, devoid of fossils. 
At Independence, the blue shales (equal Marcellus Shales) are 
succeeded upward by heavy bedded, sometimes indistinctly strati- 
fied dove-colored and buff limestone, and intrusive beds of shale, 
with a thickness of twenty-one feet. The lower portion of the lime- 
stone here is indistinctly stratified, but is often crossed diagonally 
and irregularly by seams which cause it to split into uneven slabs 
and fragments. 
As we recede to the West and Northwest from the attenuated 
Eastern outcrop of the Hamilton, the rocks overlying the blue shales 
are seen to rapidly increase in thickness, until, on the Wapsipine- 
can River, only one and one-half miles from the exposure of blue 
shales they are seen to attain an estimated thickness of sixty-five 
feet ; while on the same stream at Littleton, ten miles to the North- 
west, the same rocks are observed to attain a slightly greater thick- 
