THE PENNSYLVANIAN PERIOD 625 



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

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

 miles (Fig. 438), embraces an area of about 70,000 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. With 

 few exceptions, the strata of this field are gently undulating or 



Fig. 438. Map showing the known distribution of coal in the United States. 

 The black areas are the areas within which coal of the Pennsylvania 

 system (anthracite and bituminous) occurs. 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.) 



horizontal. Few beds of coal are known to have great extent, but 

 the Pittsburg bed in the Monongahela series, and the Sewanee bed 

 in the Pottsville, seem to be continuous over areas of several thou- 

 sand square miles. 



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

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



