86 



LIMESTONES OF RED CEDAE. 



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 Favosites GotMandica* Favosites polymorphs 

 (v arieties ramosa and t aberosa), Favosites fibrosa (?) , Stromatopora concentrica, S.poly- 

 morpha, Lithostrotion pentagonum, L. ananas,-\ Cyatliophyllum flexuoswn, C. tarbi- 

 natum (?), 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 palaeozoic 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 Euompludus, Murchisonia, and Pleurotomaria, but being casts J 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 

 concretionary 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 perforating the partition wall, 

 f The specimens of Iowa City marble, often seen polished, are composed of this species, 

 t The shelly part is sometimes converted into sulphate of lime. 



