COAL 453 



had already accumulated under sediment which the submergence 

 brought in its train. A second coal-bed in the same region points 

 lo the recurrence of swamp conditions, and means either (a) that 

 after submergence and burial of the organic matter, slight emergence 

 reproduced the conditions for bogs; or (6) that by sedimentation the 

 sea or lake bottom where the first bog had been was built up to 

 water-level, restoring swamp conditions. 



The number of coal-beds is, in many places, great. In some 

 parts of Pennsylvania it exceeds 20; in Alabama, 35 (not all 

 workable); in Nova Scotia (including some dirt-beds) about 80; 

 but in the Mississippi basin west of the Appalachians, the number 

 is in most places less than a dozen. In Illinois the workable beds 



Extent and relations of coal-beds. The widespread distribution 

 coal does not mean that any one marsh necessarily covered the 

 yhole of any one great coal-field. Some coal-beds, however, are of 

 reat extent. Thus the Pittsburgh bed is worked over an area of 

 >me 6,000 square miles l in western Pennsylvania, Ohio, and West 

 Virginia, and has at least an equal extent where too poor to be gen- 

 illy productive. Many coal-beds, on the other hand, are not 

 Uensive. From their thicker portions they thin out in all direc- 

 ions, grading into black shale in many places. Many facts sug- 

 that within the general area of a coal-swamp there may have 

 jn elevations (islands), interrupting the continuity of the swamps, 

 id therefore of the coal-beds. 



Varieties of coal. The ways in which the different varieties 

 coal arose have never been determined precisely. In general, 

 ithracite coal occurs in mountainous regions, where the coal and 

 jther layers of rock with which it is associated have been subject to 

 much dynamic action. Thus, in the mountains of eastern Penn- 

 sylvania (Fig. 384) the coal is mainly anthracite, while in other 

 coal -fields of the same age, where the strata are deformed much less, 

 the coal is bituminous. In Arkansas, where the strata have been 

 subject to some, but not to extreme dynamic action, the coal is semi- 

 unthracitic. 2 Where the dynamic metamorphism of the associated 

 rock has been great, as in Rhode Island, the coal has gone beyond 

 the anthracitic stage. Anthracite coal is found also in some places 

 (not in the Coal Measures of the United States) in contact with 



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

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



