THE COAL MEASURES. 



239 



Mississippi flows, and which include the Illinois prai- 

 ries, it appears like a continuous floor, forming an 

 almost continuous flat ; for although the superficial 

 level is irregular, that of the calcareous formation, 

 lying beneath the arable soil, seldom seems to change 

 its elevation materially." 



Immediately resting upon it, though sometimes 

 alternating with it, we find 



The Coal Measures. 

 The Coal Measures, so called, consist of several 

 regular strata, the most important of which are va- 

 rious kinds of sandstones, differing considerably in 

 structure and in appearance ; shale, or bituminous 

 slate ; slaty clay ; coal, bituminous and anthracite ; 

 ironstone ; to which may be added, the millstone grit 

 above, and the carboniferous limestone beneath, which 

 alternate occasionally with the coal measures. As 

 the coal measures thus sometimes overlie the mill- 

 stone grit and shale, and sometimes alternate with 

 the carboniferous limestone, it is evident they may 

 occupy any place, or, in geological language, be of 

 any date between the new and old red sandstone. 

 Coal has never been found in either of these last 

 formations in quantities sufficient to pay the ex- 

 pense of working. 



Anthracite Coal Measures. 



These comprise a very miscellaneous group of 

 materials, consisting of beds of coal, and extensive 

 series of shales, sandstones, and conglomerates, in 

 frequently repeated alternations. 



Professor Rogers, state geologist of Pennsylva- 

 nia, gives the following account of them in that 

 state: "Among the coal-seams at the base of the 

 series we often find a conglomerate of the very 

 coarsest sort, identical in all particulars with the 

 rock which characterizes the upper portion of the 

 next underlying formation. This rock marks the 



