444 



PENNSYLVANIAN PERIOD 



Fig. 386. Map showing the known distribution of coal in the United States. 

 The black areas are the areas within which there is coal of the Pennsylvanian system 

 (anthracite and bituminous). The areas marked by dots in Virginia and North 

 Carolina represent Triassic (bituminous) coal. Those with vertical (lignite) and 

 horizontal (anthracite, bituminous, and lignitic-bituminous) lines represent coal of 

 the Cretaceous (Laramie) system, and those with diagonal (lignite) and crossed 

 (bituminous and lignitic-bituminous) lines represent coal fields of Tertiary age. 

 Some of the fields, as those of Washington and California, appear very small on this 

 map. The Cretaceous and Tertiary areas include only those where there is known 

 to be workable coal. (U. S. Geol. Surv.) 



which about 75 per cent contains workable coal. The western edge 

 of the sharply folded Appalachian belt is the eastern edge of the 

 Appalachian coal-field. With few exceptions, the strata of this 

 field are horizontal, or gently undulating. 



(3) The Northern Interior field, confined to the southern penin- 

 sula of Michigan, covers an area of about 11,000 square miles. The 

 strata of this field dip gently toward its center. 



(4) The Eastern Interior field, centering in Illinois, covers an 

 area of about 58,000 square miles (Fig. 386), and about 55 per 

 cent of it is productive. This field is set off from the Appalachian 

 field on the east, and from the Western Interior field on the west, by 

 broad low anticlines from which the Coal Measures, if ever present, 

 have been eroded. 



(5) The Western Interior and Southwestern fields (Iowa to Texas) 

 covers an area of about 94,000 square miles. On the west this field 



