AND <J 0 A L - M E A 8 U R E S OF 10 W A. 



123 



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 C. rariolata, 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 



