CARBONIFEROUS SYSTEM. 267 



hand, or upon both, by wide shelving slopes; as, for example, 

 along some portions of the South Raccoon river in Dallas 

 county, and on North river in Warren county. There are 

 certain indefinable topographical features associated with 

 this formation, with which the careful observer cannot fail 

 to be impressed, however difficult it may be to convey a clear 

 description of their peculiarities and extent. 



2. GEOLOGICAL STRUCTURE. 



The Middle coal formation is composed of alternating beds 

 of clay, sandstone, and limestone. The clays, or shales, 

 greatly predominate, constituting indeed the bulk of the 

 formation. These beds possess all the lithological characters 

 common to the shales in both the Upper and Lower coal- 

 measures, which considered separately, present somewhat 

 striking contrasts; but in this intermediate formation these 

 features are so blended or associated as to produce persistent 

 and easily distinguished lithological peculiarities. They 

 present all the variegated appearances commonly observed 

 in the shales of the Upper coal-measures; and also the 

 arenaceous shales prevalent in the Lower coal-measures, are 

 again represented in this formation, though to a less consid- 

 erable extent. These strata are subject to more or less 

 variability in thickness, thickening and thinning locally, but 

 always constituting the most prominent feature in the strati- 

 graphy of the formation. 



The sandstones are the least important members of the 

 formation, there being few persistent horizons, and these are 

 generally represented by thin beds. However, as was 

 observed in reference to the shales, the sandstones are also 

 quite variable in thickness, sometimes being so exaggerated 

 as to replace other beds, locally changing the normal aspect 

 of the strata with which they are associated, and giving rise 

 to many difficulties in their identification. 



Thin bands of limestone are found in all parts of the 

 formation, usually occurring in single layers of a few inches, 



