268 GENEEAL GEOLOGY. 



though sometimes forming beds of a few feet in thickness, 

 interbedded with calcareous shales. Although these beds do 

 not present as great an aggregate thickness as the sandstones 

 do, they are of far greater importance, both on account of 

 their economic value and their persistent character ; and, in 

 connection with the persistent coal horizons, they furnish the 

 most reliable guides in the study of this formation throughout 

 its extent in Iowa. The lithological peculiarities of the lime- 

 stone bands offer many contrasts to the limestones of the 

 Upper and Lower coal-measures. Usually they present an 

 earthy appearance, are seldom pyritiferous and carbona- 

 ceous, like some of the limestones found in the Lower 

 coal-measures, and are generally not as compact and pure as 

 those so prevalent in the Upper coal-measures. According to 

 the analysis of the State chemist, they have the composition 

 of ordinary limestone; but many of the beds are too frag- 

 mentary and easily disintegrated from exposure to be 

 valuable for building purposes. There are other beds, 

 however, which afford material for the manufacture of quick- 

 lime, and are employed in ordinary masonry. 



Three persistent coal horizons occur in this formation in 

 Iowa, besides other irregular beds and carbonaceous layers, 

 which sometimes display thin seams of coal. The strata 

 associated with the three principal coal-beds, give to this 

 member prominent lithological and palseontological peculi- 

 arities, in the main, quite distinct from those of any of the 

 horizons known in the Upper and Lower coal-measures. 



In the prosecution of the investigations in this formation, 

 we have been led almost intuitively to the recognition of 

 three sub-divisions, partly on the ground of convenience in 

 working out the details of the stratigraphical structure of the 

 formation, but no less so from the fact that each of these 

 divisions is everywhere defined by the same peculiarities. 

 Each of these members is limited above by a sandstone or 

 arenaceous bed, and just beneath this a thin bed of coal 

 occurs in each case. However, the arenaceous cap-rock of the 

 upper division is not always present as a dictinct sandstone, 



