546 GEOLOGY. 



not shown in Fig. 241) were probably isolated from the beginning, 

 though at the outset the latter was considerably, and the former some- 

 what, larger than now. It is to be understood that, as in the case of 

 all other systems, the present margins of the Coal Measures are not 

 the original margins. At many points, as in Iowa, Tennessee, and 

 elsewhere, there are small outliers of Pennsylvanian formations which 

 show that the system once extended beyond its present borders. 1 



Productive Coal-fields. 



The productive coal areas of the Pennsylvanian (Carboniferous) 

 system in the United States are five in number. These are as follows : 2 



(1) The anthracite field, 3 which is confined to eastern Pennsyl- 

 vania, and contains an area of 484 square miles. This aggregate 

 area includes several elongate, nearly parallel, synclinal basins, the 

 longer axes of which have a northeast-southwest direction (Figs. 241 

 and 266). From the adjacent anticlines, and from the neighboring 

 shallower synclines, the coal-bearing beds have been removed by 

 erosion. The strata of this field may once have been continuous 

 with those of the next. 



(2) The Appalachian field, 4 which extends from the northern border 

 of Pennsylvania to central Alabama, a distance of about 850 miles 

 (Fig. 240), embraces an area of about 70,800 square miles, of which 

 about 75 per cent contains workable coal. Speaking in general terms, 

 the western edge of the sharply folded Appalachian belt forms the 

 eastern edge of the Appalachian coal-field. Barring a few outlying 

 synclines to the east, the strata of this field are gently undulating 

 or horizontal. The formations thicken to the east and thin to the 

 west. Few beds of coal are known to have great extent, but the Pitts- 

 burg bed in the Monongahela series, and the Sewanee bed in the Potts- 

 ville series, seem to be continuous over areas of several thousand square 

 miles. 



1 For details, see the geological reports of the States where the Carboniferous sys- 

 tem is present. Summaries of classifications for Kansas and Nebraska are given 

 (Prosser) in Jour. Geol., Vol. VII, pp. 342-56, and Vol. X, pp. 703-17. 



2 Hayes, 22d Ann. Rept., U. S. Geol. Surv. Pt. Ill, p. 15. 



3 Stock, idem, pp. 61-117. Also Geol. Surv. Repts. of Pennsylvania. 



4 Idem, White (D.), Campbell (M. R.), Hayes, and Haseltine, pp. 127-263. Also 

 State Geol. Surv. Repts. of Pennsylvania, Ohio, Maryland, West Virginia, Kentucky, 

 Tennessee, Alabama. 



