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 uttderlying formation. This rock marks the 
