THE PENNSYLVANIAN PERIOD 637 



sippi basin west of the Appalachians, the number is often less than 

 a dozen. In Illinois the number of workable beds is nine. 1 



Extent and relations of coal-beds. The wide-spread distribution 

 of coal does not mean that any one marsh necessarily covered the 

 whole of any one great coal-field. A few of the coal-beds, how- 

 ever, are of great extent. Thus the Pittsburg coal-bed is worked 

 over an area of some 6,000 square miles 2 in western Pennsylvania, 

 Ohio, and West Virginia, and has at least an equal extent where 

 too poor to be generally worked. Many coal-beds, on the other 

 hand, do not occupy great areas. 'From their thicker portions 

 they thin out in all directions, often grading into black shale. 

 From these facts it is inferred that within the general area of a coal- 

 field there may have been elevations (islands) above the marsh 

 level, interrupting the continuity of the swamps, and therefore the 

 coal-beds. 



Varieties of coal. The ways in which the different varieties 

 of coal have arisen have never been satisfactorily determined. In 

 general it is true that the anthracite coal occurs in mountainous 

 regions, where the coal and other layers of rock with which it is 

 associated have been subject to more or less dynamic action. Thus, 

 in the mountains of eastern Pennsylvania (Fig. 436) the coal is 

 mainly anthracite, while in the other coal-fields of the same age, 

 where the strata are but slightly deformed, the coal is bituminous. 

 In Arkansas, where the strata have been subject to some, but not 

 to extreme dynamic action, coal is semi-ant hracitic. 3 Where the 

 metamorphism of the associated rock has been extensive, as in 

 Rhode Island, 4 the coal has gone beyond the anthracitic stage. 

 Anthracite coal is also found in some places (not in the Coal Meas- 

 ures of the United States) in contact with dikes, in situations like 

 those where other sorts of rock are metamorphosed. 



These phenomena long ago suggested that anthracite is meta- 



1 Reports on coal have been published by all states containing coal, where 

 there have been surveys. For local details see reports of Pennsylvania, Ohio, 

 Kentucky, Tennessee, Alabama, Indiana, Illinois, Iowa, Missouri, Arkansas, 

 Kansas, and Texas. 



2 White, West Virginia Geol. Surv., Vol.. II, p. 166. 



3 Ann. Kept. Ark. Geol. Surv., 1888, Vol. III. 



4 Geology of the Narragansett Basin, Mono. XXXIII, U. S. Geol. Surv. 



