86 LIMESTONES OF RED CEDAR. 
of Iowa City, there is a good section of light-coloured, brownish-gray limestone, 
mostly of compact texture, forming a mural exposure of from thirty to forty-five 
feet. The lower beds lie in layers of from six to fifteen inches thick; the upper 
beds are in rugged, concretionary masses, very imperfectly stratified, and reticu- 
lated with a network of thin, siliceous, calcareous, and gypseous seams, and much 
lighter-coloured than the beds below them. 
These rocks, but particularly the upper beds, have an interlocking, suture-like 
structure of the joints. Towards the base of the exposure, from twenty to thirty 
feet above the Iowa River, is a bed of brownish limestone, mottled with gray, 
studded with fossil corals of the species Fuvosites Gothlandica,* Favosites polymorpha 
(varieties ramosa and tuberosa), Favosites fibrosa (?), Stromatopora concentrica, WS. poly- 
morpha, Lithostrotion pentagonum, L. ananas,+ Cyathophyllum flexwosum, C. turbi- 
natum (2), and others. This bed seems to be the representative of the upper coralline 
beds of the Falls of Ohio; the corresponding beds at Utica, Indiana; the coralline 
burrstone on the high ground between Madison and Vernon, in the same State, and 
the Onondaga limestone of New York. At this locality on the Iowa River, above 
these coralline beds, one hundred yards from the foot of the exposure, is a seam, 
three inches thick, of an earthy, carbonaceous substance, a kind of coal of humus, 
and adjoining it, a fissure or rent in the strata, running down nearly vertically, and 
having a southeast bearing; but no kind of metallic ore was detected among the 
crevice-earth. A similar substance runs between some of the strata, and in the 
joints of the rock. The “black stratum” included in the upper coralline beds of 
the Falls of Ohio, probably owes its colour to an impregnation with a substance 
analogous to that found on the Iowa, where it exists in a loose, earthy, friable 
condition, while in Kentucky it is more intimately blended with the rock. 
The rocks at this section on the Iowa have a local northerly dip of from two to 
three feet in a hundred yards, so that in the hollow at the head of the exposure, 
the coralline beds are at a higher level. There they can be seen to the depth of 
ten feet, composed throughout of a complete agglutination of the various species of 
coral above-mentioned, affording evidence that the whole mass must have been an 
ancient coral reef, of greater thickness and extent than is usually seen displayed 
in the strata of the palxozoic period, when these zoophytes did not rear such 
stupendous structures as at the present day; perhaps owing to interruptions from 
change of temperature of the ocean, as well as oscillations of its bed. Five feet 
above the coralline bed is a shell-bed, composed almost entirely of Gasteropoda, of 
the genera Huomphalus, Murchisonia, and Pleurotomaria, but being casts{ which 
do not weather out of the rocks, and which are only seen as sections on the frac- 
tured face of the bed, it is difficult to determine their specific characters. In the 
coneretionary and brecciated calcareous portion above, no well-defined fossils were 
discovered. 
A few rods higher up, a small ravine runs from the high ground towards the river, 
* The same specimen often shows both a double and single row of pores perforatin 
+ The specimens of Iowa City marble, often seen polished, are composed of 
{ The shelly part is sometimes converted into sulphate of lime. 
g the partition wall. 
this species. 
