284 GENERAL GEOLOGY. 



the region where it appears at the surface. The Marshall 

 coal is also remarkably free from admixture with foreign 

 mineral substances; but, at the same time, it is the most 

 variable of the persistent beds. Those beds which are termed 

 irregular in consequence of their interrupted continuity, also 

 afford a fair average quality of coal, but in consequence of 

 their less extensive distribution, they have not been wrought 

 to near the same extent as have some of the thinner persistent 

 beds. The aggregate thickness of the coals of this formation 

 is about eight feet. All these coals are light and brittle, 

 presenting a highly polished lustre upon fracture. 



Building Material. Stone suitable for building purposes is 

 not abundantly furnished by this formation. As a general 

 thing, the limestones occur in thin bands, and being of a 

 more or less fragmentary character and easily wasted from 

 exposure, their economic value is inconsiderable. However, 

 a three-foot bed in the lower part of the formation has been 

 opened on Mosquito creek, in Dallas county, affording a 

 very durable building stone. In the upper division of the 

 formation, well towards the top, another limestone deposit 

 furnishes a good quality of quicklime and answers the 

 purposes of ordinary masonry. This stratum appears at 

 numerous localities in Dallas and Warren counties. The 

 sandstones are usually too soft and easily destroyed to be 

 employed to any considerable extent in building. The bed 

 at the top of the middle division of the formation locally 

 affords a tolerable freestone. The best quarries in this bed 

 are those in the vicinity of Adel, in Dallas county, and to the 

 southward of Indianola, in Warren county. 



Ferruginous nodules occur in various beds, but no valuable 

 deposits of the ore have been as yet discovered in the forma- 

 tion. The under clays, associated with some of the coal- 

 beds, may possibly be found to answer for potter's use and 

 fire-brick, but as yet no experiments have been made upon 

 them with the view of determining their value for those 

 purposes. 



