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CARBONIFEROUS LIMESTONES 



From half a mile to one mile higher, sandstones overlie the shaly, indurated 

 argillaceous beds, thus : 



Sandstone, 

 Argillaceous shale, 

 Shaly sandstone. 



Here vertical seams of indurated clay traverse the shaly sandstones, much in the 

 same manner as on the Cut-Off of the Wabash River, in Posey County, Indiana. 



At Elbow Rapids, two or three miles further up, beds of ripple-marked sand- 

 stone lie two feet above the river-level, underlaid by laminated sandstone, and 

 covered by shaly rock, and beds of brown limestone, containing the pygidium of 

 Griffithides. At the base of a ridge near the mouth of Moore's Branch, is an out- 

 crop of sandstone, and an impure limestone ; in the former, some imperfect fluted 

 impressions were found, analogous to those which characterize the Sigillaria, and 

 about one mile below the First Forks, impure gray limestone, sandstone, shale, and 

 shaly sandstones, protrude beneath the drift. These constitute the only exposures 

 of rock witnessed on this branch of the Des Moines, and although partial, they are 

 yet sufficient to show the extension of the carboniferous rocks westwardly beyond 

 the Raccoon Fork of the Des Moines. 



On our return to Fort Des Moines, I sent the canoe and men down the river 

 with the collection, while I crossed the interior of Iowa, by way of Middle River 

 and Pisgah, to Council Bluffs, on the Missouri River, with a view to trace the ex- 

 tension of the carboniferous rocks further westward, through the interior of Iowa. 



From the nature of the country, this became a difficult task. In the northern 

 part of Iowa, deep alluvial and drift deposits so effectually conceal the rocks, except 

 in the immediate cuts of the main streams, that I only succeeded in obtaining the 

 desired information at a few points on the principal water-courses. 



Below the crossing of North River, ten miles west of Fort Des Moines, at a 

 settlement known as Linn Grove, dark, schistose, productal limestone was found, 

 along with a hard, brittle, ponderous, ferruginous limestone, which seems to origi- 

 nate in shaly beds, only very partially exposed in a cut for a mill site. This lime- 

 stone has been used by the neighbouring settlers in the construction of fire-places. 

 It is said that, at a low stage of water, an imperfect seam of coal can be seen below 

 the ford ; and the blacksmith of the settlement has obtained coal in small quantities 

 a few miles above the mouth of North River, as well as at Black Oak Grove, 

 between the two forks of Middle River. At the latter place, lime has been burnt 

 for the use of Fort Des Moines. 



The nucleus of the more elevated grounds, under the drift on North River, 

 seems to be composed of soft freestones, which have been employed, along with the 

 ferruginous limestone, in building chimneys. On the west side of Middle River, 

 loose slabs of reddish-buff limestone lie strewn on the slope of a hill ; but no sec- 

 tion is exposed by which to determine its proper relative position. 



In the bed of a branch which flows into the Clanton Fork of Middle River, light- 

 gray limestone is in place, containing Fusulina cylindrica, Chonetes variolata, and 



