474 



HISTORICAL GEOLOGY 



(2) Appalachian Field. — The great coal field (Fig. 450) of the 

 world is that which underlies an area of about 50,000 square miles in 

 central and western Pennsylvania, western Maryland and Virginia, 

 West Virginia, and eastern Ohio, Kentucky, and Tennessee. In 



Fig. 450. — Map showing the distribution and extent of the Carboniferous coal 

 fields (black), and more recent coal fields (lines). 



this should be included the anthracite field, confined to an area of 484 

 square miles in eastern Pennsylvania. 



(3) Michigan Coal Field. — This field covers an area of only 1 1,000 

 square miles and was probably formed in an isolated basin. It is not 

 of great value as compared with the Appalachian field, since it is deeply 

 buried and the coal beds are usually comparatively thin. 



(4) The Indiana- Illinois Field covers an area of about 58,000 square 

 miles, of which 30,000 square miles are underlain by workable coal. 



(5) The I owa-Missouri-Texas Field extends from northern Iowa to 

 central Texas and covers an area of about 94,000 square miles, being 

 about 800 miles from north to south. The Indiana-Illinois and Iowa- 

 Texas fields were probably once continuous, but are now separated 

 by the Mississippi Valley. 



The Pennsylvanian strata dip beneath much younger strata when 

 traced westward. In the mountains of the Great Basin region, they 

 are found to consist of marine deposits and contain no coal. The 

 coal fie Ids of Wyoming and Colorado are of a later date. 



