i8«9-] Description of the Devonian Rocks of Iowa. 235 
The following is a partial list of the species occurring atthishor- 
The rocks of the lower portion of the Hamilton are generally- 
heavier bedded, more compact, and uniform in texture, and usually 
a more pure limestone than those of the upper portion The pre- 
vailing color of the strata of this horizon, is blue, and bluish-gray. 
In the northern portion of Johnson County, (for instance, at the 
"State Quarry," Robert's Ferry, Solon, etc.,) occurs a bed 
of peculiar grayish-white limestone, nothing like it being known to 
exist in other portions of the State. 
This bed has a thickness of from six inches to six feet, or more^ 
is very crystalline throughout, and is made up, to a considerable ex- 
tent, of broken shells of different species of Brachiopoda, some of 
which are not known to occur elsewhere in Iowa. 
For convenience in subsequent allusion, this bed is here designa- 
ted the Shell Bed. 
Underlying this shell bed is a stratum of very hard, fine-grained, 
blue brecciated limestone. 
This limestone is observed at various localities in this portion of 
the State, and is known to extend as far North as Raymond 
Station, in Black Hawk County. 
The upper portion of this division is made up, for the most part, 
of thin bedded magnesian and common limestone, and soft, impure^ 
calcareous, argillaceous and silicious, shales and sandstones, of a 
prevailing grayish-buff color. 
In the Eastern portion of Floyd County, some beds of shale, oc- 
cupying a considerable area, are extensively sun-cracked ; this indi- 
cating an elevation of the sea-bottom here, and the exposing of it 
for some time to etheral conditions and the burning rays of the sun. 
The extreme upper portion of this division is almost everywhere, 
a hard, fine-grained, and brittle, grayish or dove-colored limestone, 
and singularly devoid of organic remains. 
Immediately succeeding the limestone, in portions of Floyd, Cerro 
Gordo and Worth Counties, and constituting the highest member 
of the Hamilton group in the State, is a stratum of stiff blue clay, 
varying from twenty to twenty- five feet in thickness. 
This formation, which is entirely devoid of organic remains, may 
be best seen as it outcrops on Lime Creek and Willow Creek, in 
Floyd and Cerro Gordo Counties, particularly at Rockford, Hack- 
