AND COAL-MEASURES OF IOWA. 523 
On Section 16, same township and range, we discovered, at an elevation of eighteen 
feet above the water-level, a band of translucent calcareous spar, having the 
prismatic structure of arragonite; the fibrous prisms collected, however, into perfect 
conical bundles, with the apex of the cone turned either directly downwards, or 
sometimes upwards, the external surface of the cones crimped, possessing, in fact, 
at the same time, the most perfect tutenmergel structure, as if produced by some 
combined process of stalactitic infiltration and simultaneous crystallization. The 
calcareous tutenmergel band is one and a half to two inches in thickness, thinning 
out, and slightly dipping towards the east. It is enclosed in dark argillaceous 
shale, with some layers of finely laminated carbonaceous limestone, charged with 
the spines and fragments of Productus Flemingii (?), and containing, amongst other 
fossils, a beautiful, delicate, small species of Chonetes, allied to CG. variolata, but 
distinguishable from it, in being ornamented by a greater number of ribs, and given 
in the Appendix under the name of Chonetes semiovalis. In the shales, over the 
tutenmergel bed, is an imperfect seam of coal; the whole being crowned with sand- 
stones, which are laminated where they rest on the shale, but pass afterwards into 
thicker layers. (See Sections No. 51 and 52, D.) At this locality, as well as at 
several other points, both above and below the productal limestone, the septaria 
and calcareous blocks embedded in the shale have the external aspect of hydraulic 
cement. 
A mile and a half above this, the brown productal limestone lies within two feet 
of the water, associated with gray clay, and covered with drift. 
About two miles above our encampment of 31st August, and four to five miles 
above the Rapids, three beds of coal are exposed, but they are all of an inferior, 
slaty character, much impregnated with sulphuret of iron. The lowest lies at an 
elevation of eight feet above the water-level, and is three inches thick; the middle 
bed is twenty-five feet up the bank, and varies from two to three feet in thickness ; 
the upper bed is thirty-eight feet above the water-level, and is about two feet thick, 
intersected with a band of sulphuret of iron. Argillaceous and shaly beds, including 
septaria and ironstones, constitute the intervening strata; at the upper end of the 
exposure, laminated sandstones overlie the whole, as shown in Section 59, D. Some 
layers, both in the lower and upper part of the exposure possess the tutenmergel 
structure. The ironstone is not very pure, nor in great abundance ; some sulphuret of 
zinc is disseminated with the iron. The position of the selenite at this locality shows 
its origin ; agglutinated crystallizations are seen shooting forth from calcareous masses 
charged with the sulphuret of iron. After the iron and sulphur have undergone 
oxidation, and are converted into the sulphate of the protoxide of iron, a mutual 
double decomposition is effected between that salt and the carbonate of lime, by 
which there result sulphate of lime, and either carbonate of the protoxide of iron, 
or, if a further oxidation of the iron ensues, a hydrated peroxide of iron, the car- 
bonic acid being set free. Much of the ironstone disseminated in the shales of the 
Des Moines, doubtless owe their origin to this kind of chemical reaction. The 
whole strata have a waved dip to the northwest, and some of the subordinate layers 
gradually thin out, and become blended with the enclosing matrix. This kind of 
