. Report of a geological survey of Wisconsin, Iowa, and Minnesota : and incidentally of a portion of Nebraska Territory : made under instructions from the United States Treasury Department . ith the sulphuret of iron. After the iron and sulphur have undergoneoxidation, and are converted into the sulphate of the protoxide of iron, a mutualdouble decomposition is effected between that salt and the carbonate of lime, bywhich 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 be
. Report of a geological survey of Wisconsin, Iowa, and Minnesota : and incidentally of a portion of Nebraska Territory : made under instructions from the United States Treasury Department . ith the sulphuret of iron. After the iron and sulphur have undergoneoxidation, and are converted into the sulphate of the protoxide of iron, a mutualdouble decomposition is effected between that salt and the carbonate of lime, bywhich 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 theDes Moines, doubtless owe their origin to this kind of chemical reaction. Thewhole strata have a waved dip to the northwest, and some of the subordinate layersgradually thin out, and become blended with the enclosing matrix. This kind of 124 0 A 11 KUNIFEKUUS LI M E8TON E S wedge-shaped interpolation is well seen at the next section, a little higher upon the Des Moines, above Bald Point, where two thin seams of coal are seentapering away until they finally run out as they approach each other. (See SectionNo. 60, D.). MARL AND DRIFT RESTING ON CARBONIFEROUS STRATA, DES MOINES. At a westerly bend of the Des Moines, here represented, designated on the chartof the river, Bald Point, the hills are about two hundred feet high; laminatedsandstone forms their base, while the main body of the hill is composed of ash-coloured, marly earth and drift. One mile above Bald Point, ledges of reddishlimestone extend to the height of five or six feet above the water-level; and half amile further, or about eight miles above the Rapids, fragments of ironstone arescattered on the shore near the water-level, mixed with a conglomerate of graveland sand, cemented by oxide of iron. A few hundred yards above this place, onthe left bank of the Des Moines, below the mouth of Honey Creek, ledges of softsandstones project from under the dr
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