102 



CARBONIFEROUS LIMESTONES 



being most prevalent in the West, through the region occupied by the carboniferous 

 and subcarboniferous limestones. It was not, however, until reaching the north- 

 west corner of Tama County, that rocks of the subcarboniferous era were seen 

 unequivocally in place. Here, on Section 7 or 8, Township 83 north, Eange 16 

 west, on the left bank of the Iowa, are bluffs of one hundred and twenty-five 

 to one hundred and fifty feet of elevation, on the slope of which both oolitic and 

 encrinital beds of carboniferous limestone protrude ; and where the river crosses the 

 corner of Marshall County, the characteristic fossil, Pentremites pyriformis, was 

 found, along with Terebratula plano-svlcata, Spirifer striatus, and Productus semi- 

 reticulatus. Here, too, a change can be perceived in the outline of the country ; in 

 the greater elevation of the hills, in the increased growth of timber, and in the 

 stiff, indurated, clayey subsoil. 



It is in this vicinity that those extraordinary mound-like forms of carboniferous 

 limestone, represented on page 101, appear as outliers, in view of the Iowa River. 



In the southeastern part of ToAvnship 84, Range 15 west, on land claimed by Mr. 

 Ballard, a thin seam of coal has been discovered, said to be about one foot thick, 

 overlaid by shale. In consequence of the sliding of the bank, no satisfactory 

 observations could be made, either to determine its true elevation above the river, 

 or its thickness. From the specimens found, it appears to be of inferior quality, 

 being much impregnated with sulphuret of iron. This is no doubt the mineral 

 that has given origin to the reports of the existence of copper ore, on this portion 

 of the Iowa ; for, on calling on Mr. Miller, who, I was informed, had some of the 

 copper ore in his possession, I was shown a sample of the same kind of pyrites. 



In the detritus of the river, just above the first fork of the Iowa, fragments of 

 coal are met with in the transported gravel, and below the entrance to the " Big 

 Woods," six or seven miles above the forks, bluffs of sandstone and a highly ferru- 

 ginous grit rise to the height of from one hundred and forty to one hundred and 

 fifty feet. The latter occupies about one-third of the upper portion of the exposure, 

 and contains from fifteen to twenty per cent, of oxide of iron, disseminated amongst 

 a fine siliceous gravel, which, by pounding and sifting, can be separated from each 

 other. Some portion of this ferruginous grit possesses a structure approaching to 

 oolite. The lower part of the section is formed of conglomerates or pebbly sand- 

 stones, together with grits, having strongly marked cross-lines of deposition. 

 Indeed, the rocks of this part of the Iowa River assume much the character of 

 the millstone grit of the north of England, which lies between the Yoredale Series, 

 and the coal-measures proper. 



The same formation extends for several miles along the river, rising into escarp- 

 ments of sixty or seventy feet, the summits of which are crowned with pines, 

 cedars, and other Coniferae. 



Near their termination, on the right bank, where the river makes two abrupt 

 bends, a seam of coal shows itself. It lies more after the manner of a metallic 

 vein than a bed of coal ; this arises, however, either from a dislocation, or tilting 

 of the strata, or, more probably, from a slide, by which it appears to have been 

 thrown nearly vertically. The confused position of the associate beds rather con- 

 firms this vieAV. 



