THE PENNSYLVANIAN PERIOD 621 



western Pennsylvania. The unequal thicknesses, even where the 

 formation has not suffered from erosion, are due partly to the un- 

 evenness of the eroded surface on which it rests, partly to unequal 

 rates of ' sedimentation, and partly to unequal duration of the time 

 of sedimentation in different regions. The formation is usually 

 so firmly indurated that the outcrops of its tilted beds have be- 

 come ridges. 



THE COAL MEASURES 



Above the Pottsville conglomerate and its equivalents in the 

 central and eastern parts of the continent, lie the formations 

 known collectively as the Coal Measures. They consist of a suc- 

 cession of alternating beds of shale, sandstone, conglomerate, lime- 

 stone, coal, and iron ore. The succession differs greatly in different 

 regions, but shale and sandstone perhaps recur more frequently 

 than any other members of the series, and in thicker beds. Both 

 the coal and some of the iron ore are in layers interstratified with 

 the other members of the series, and are to be looked upon as strata 

 of rock. Important as the coal and iron ore are from an economic 

 point of view, they make up but a small part of the Coal Measures. 

 Although there are many beds of coal in some regions, and although 

 some of them have great thickness (40 to 50 feet), the proportion of 

 coal in the Coal Measures is rarely so much as 1 : 40, and that of iron 

 ore is much less. The classification of the Pennsylvanian and 

 Permian systems of the east now in common use is as follows: 1 



Present Old 



Permian Dunkard formation (or series) = Upper Barren Coal 



Pleasures 



C4. Monongahela " " = Upper Productive 



Coal Measures 



I 3. Conemaugh " " " = Lower Barren Coal 

 Pennsylvania^ Measures 



I 2. Allegheny " = Lower Productive 



Coal Measures 

 [L Pottsville 



A twofold division is common farther west. Thus in Iowa the 

 lower division is called the Des Moines, and the upper, the Missouri- 



1 Prosser, Am. Jour. Sci., 4th series, Vol. XI, p. 191, 1901. For recent 

 review of the Pennsylvanian system of the Appalachian region, see Stevenson, 



