CARBONIFEROUS LIMESTONES OF IOWA. 



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SECTION III. 



THEIR MINERAL CONTENTS. 



The lithological character of the lower series of subcarboniferous limestones is 

 not unfavourable to the retention of the metalliferous exhalations and segregations ; 

 these limestones being chiefly in continuous beds, of considerable thickness. And, 

 accordingly, beyond the range of the Geological Chart which accompanies this Report, 

 to wit, where this formation, in its southern range, encircles the southern boundary 

 of the Illinois coal-field, we find it affording some variety of minerals. 



In Iowa, however, no metallic veins, promising to be productive, have been dis- 

 covered traversing this formation. 



If, in this connexion, we contrast the state of things in the two districts, we find, 

 in Iowa, an absence of those conditions which, in Illinois, are accompanied with 

 mineral insinuations. 



Thus, the fluate of lime, traversing, in heavy veins, this formation in Southern 

 Illinois, and there forming the veinstone or gangue of the galena, does not show 

 itself in Iowa. Nor, though in this latter district abrupt faults frequently destroy 

 the continuity of the strata, do these usually extend further than from fifty to a 

 hundred feet ; whereas, in Southern Illinois, disturbances are not infrequent that 

 reach, in vertical dislocation, as many fathoms. 



My judgment is, then, that the Carboniferous Limestone of Iowa contains few 

 mineral veins ; no productive ones. 



In the way of building material, however, as of mineral manure, this series will 

 furnish to the future inhabitants much that is valuable. The best locations arc 

 indicated in the detailed description of the local geological sections of this forma- 

 tion. 



As to the middle or shaly division of this formation, already stated to be its 

 coal-bearing portion, the exact thickness, any more than the precise character of its 

 coal-beds, cannot be determined. In a country so recently settled, artificial sections 

 of strata are confined to a few quarries, opened on the immediate line of the public 

 works, and one or two shallow excavations for coal, on the Lower Des Moines. 

 Even the natural sections are often encumbered by slides of the clayey banks, or 

 by the detritus derived from the crumbling faces of easily-disintegrating shaly beds. 

 And thus the attempt to obtain accurate measurements of the succession and thick- 

 ness of strata is often frustrated. All the information at present accessible is 

 comprehended in the descriptive details hereinafter given. 



The workable seams of coal yet discovered in this formation do not exceed four 

 or five in number. Nor, as our measurements indicate, is it likely that the number 

 will be much increased, even when the coal-field comes to be fully known ; since, 

 in the British coal-fields, in the same depth of strata (less than a hundred feet), a 

 much greater number of seams than the above is rarely found.* 



* Of seams over one foot in thickness, there occur, in St. Anthony's Colliery, only six, in <i depth of 



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