FORMATIONS AND PHYSICAL HISTORY 



443 



of them have great thickness (40 to 50 feet); yet the proportion of 

 coal in the Coal Measures is rarely so much as i :4o, and that of iron 



re is much less. The classification of the Pennsylvanian system of 

 e east now in common use is as follows: 1 

 f 4. Monongahela 

 3. Conemaugh 

 Pennsylvania^ 2 Allegheny 

 [ i. Pottsville 

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

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

 ourian. 



Productive coal-fields. The Pennsylvanian system does not 

 ntain coal in workable quantity everywhere, though coal is widely 

 istributed as far west as the 

 th or 97th meridian in Okla- 

 oma, and nearly to the icoth 

 eridian in Texas. The pro- 

 uctive coal areas of the system 

 n North America are six in 

 number, as follows: 2 



(i) The anthracite field, of J 



Fig. 384. Map showing the areas of 

 anthracite coal in Pennsylvania. 



istern Pennsylvania, with an 

 irea of 484 square miles. It 

 icludes several elongate, nearly parallel, synclinal basins (Figs. 384 

 id 385). From the associated anticlines, and from the neigh- 



K- 385. Section across Panther Creek basin in the anthracite region of 

 5 ennsylvania, showing the structure and the coal beds (black). (Stock, U. S. 

 jeol. Surv.) 



)oring shallower synclines, the coal beds have been worn away, 

 "he strata of this field may once have been continuous with those 

 the next. 



(2) The Appalachian field, which extends from Pennsylvania to 

 Alabama (Fig. 386), has an area oif about 70,000 square miles, of 



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



2 22d Ann. Kept., U. S. Geol. Surv., Ptl. Ill, p. 15. 



