394 T^he American Geologist. December, looi 
Island, opposite Davenport, in the Le Claire limestone. "In the midst 
of this was the impression of a large Euomphalus distinct from any- 
known in the surrounding rocks and very similar to a Carboniferous 
form, X X. The conclusion seems therefore irresistible that subse- 
quent to the uplifting of these rocks and their denudation, the mater- 
ials of the coal measures were deposited upon the surface of the older 
rocks."* He cites several other instances. At Clinton these pockets 
of Carboniferous clay are frequent in the quarries at the bluff and in: 
pits found in every direction where the rock is uncovered on the 
town plot. The crevices and cavities in the quarries were filled from 
the top about fifty feet above the present river. In one observed there 
was a chimney opening down to a large cavity, extending across the 
quarry, a hundred feet or more. There were also numerous smaller 
pits in the vicinity, in which the clay looked like cannel coal and had 
fragments of carbonized wood in it. The larger cavity was exca- 
vated for pottery clay. There were no fossils found except some 
pieces of pryritized wood. The darkest material put into the fire 
turned very red from the large portion of oxide of iron. A large 
deposit of this clay was found at the foot of another quarry. 
My attention has been called several times to indications of sup- 
posed coal mines in the deep channels of streams that run into the 
river. Pits were filled with coal-like clay similar to that above 
described. 
The openings of these crevices in the blufif are fifty or sixty feet 
above the river, 575 feet above sea level, say 635 feet. 
This shale and a Carboniferous sandstone with traces of Car- 
boniferous fossils have been traced into Jones county, twenty or 
thirty miles north. In fact it has been foimd in counties farther 
north. The flood-plain of the Mississippi at Clinton is 200 feet be- 
low the prairie level. A subsidence of a hundred feet would cover 
these deposits. At Rapids City, thirty miles below Clinton, coal is 
mined, its under-clay resting on the Niagara rock on a level with the 
flood-plain of the river. The subsidence of a hundred feet allowed 
the Carboniferous sea to flow up the river valley and cause the de- 
posit of the Carboniferous material of that period. It is therefore 
obvious that the valley is pre-Glacial by a long period. 
The record is much more ancient. It has been demonstrated that 
the original surface of Iowa was furrowed by deep channels and great 
denudation, probably in the Devonian age. Deep wells 300 feet or 
more in the drift, and mounds capped with the Niagara limestone 
near Dubuque and Galena, are evidences of the great erosion. The 
singular region called the driftless area in the corner of Iowa and 
Wisconsin, is probably a fair representation of the land surface of 
this middle west in Paleozoic times. This driftless portion is cut into 
conical hills 200, 300 and even 500 feet high, divided by ravines and 
canyons, the streams of which have cut channels down to the level 
'Hall's Iowa Reports, vol. i, p. 130 ^^^^^ 
