134 



UNITED STATES MINERAL RESOURCES 



trie power, 20 percent is used by the steel industry, 

 16 percent by the manufacturing industry, and 2 

 percent for all other purposes. Coal is also of great 

 future value and importance as a subsidiary source 

 of synthetic gas, liquid fuels, and lubricants. 



ENVIRONMENT OF COAL ACCUMULATION 



Coal is the compressed and altered residue of 

 plants that grew in ancient fresh- or brackish-water 

 swamps. As the plant remains accumulated they 

 were transformed into peat ; later they were altered 

 by diagenesis (chemical and physical changes occur- 

 ring before they bacame solidified), and still later 

 by metamorphism (chemical and physical changes 

 brought about by pressure and heat after they be- 

 came solidified). Coal contains widely varying 

 amounts of sand, silt, and mud that was washed 

 into the peat swamps, and this admixed sediment 

 forms the bulk of the ash of burned coal. The physi- 

 cal and chemical properties of coal and the coalifi- 

 cation process have been described in considerable 

 detail by Schopf (1948; 1956) and by Dapples and 

 Hopkins (1969). 



The accumulation of peat requires a humid cli- 

 mate to support a rich growth of vegetation, and 

 a high water table to permit prolonged accumula- 

 tion of plant material in a reducing environment 

 (See "Peat," this volume). Most of the large peat 

 deposits of Pennsylvanian age that were the pre- 

 cursors of coal mined extensively in the Eastern 

 and Central United States were formed near sea 

 level — some in estuaries or coastal lagoons, others 

 on large deltas or many coalescing deltas, others on 

 low-lying, broad coastal plains. These features form 

 characteristically in areas of gentle downwarping of 

 the sea floor marginal to the edges of an eroding 

 landmass. This topographically low position in an 

 area of gentle downwarping permitted periodic 

 transgressions of the sea. Some thick coal beds of 

 very wide areal extent required a very large and 

 wide coastal plain, a prolonged optimum rate of 

 plant growth and accumulation, a slow rate of sub- 

 sidence, and an equally slow encroachment of the 

 sea over periods measured in centuries. 



The transgressive sea ultimately covered the peat- 

 forming swamp and terminated plant growth. The 

 eroding landmass continued to supply sand, silt, 

 and mud to the sea, and this material settled in 

 layers over the submerged peat swamp. In time, 

 depending in length on the rate of sedimentation, 

 the depth of the transgressive sea, and the rate of 

 subsidence, this sedimentary material built up new 

 deltas, lagoons, and coastal plains conducive to the 

 development of new, younger peat-forming swamps. 



This sequence of deposition was repeated many 

 times by intermittent downwarping alone, but the 

 sequence might have been prolonged, shortened, or 

 terminated at any time by relatively minor move- 

 ments of land relative to the sea floor. In the very 

 delicate balance between sedimentation, subsidence, 

 and uplift of the land, the sea also regressed from 

 time to time. Peat swamps obviously formed during 

 the regressive phase of the cycle, but these were 

 subject to oxidation and are less commonly pre- 

 served. These cyclic repetitions of the conditions 

 allowing the formation of coal are documented in 

 many of the world's coal fields, but rarely as strik- 

 ingly as in a sequence of several thousands of feet 

 of sedimentary rock in West Virginia that contains 

 117 coal beds of sufficient geologic and economic 

 interest to have been described and named. 



Weight of the overlying sedimentary rock, heat 

 produced by depth of burial, structural deforma- 

 tion, and time all contribute to the progressive com- 

 paction and devolatilization of peat to form the 

 higher ranks of coal, which are discussed below. A 

 subsequent major uplift of the land relative to the 

 sea has raised the U.S. coal fields to their present 

 positions, exposing them to erosion and to view, 

 thereby permitting study and mine development. 



RANK OF COAL 



Coal is classified by rank according to the per- 

 centage of fixed carbon and heat content, calcu- 

 lated on a mineral-matter-free basis. As shown in 

 figure 15, the percentage of fixed carbon and the 

 heat content increase from lignite to low-volatile 

 bituminous coal as the percentages of volatile mat- 

 ter and moisture decrease. These changes are pri- 

 marily the result of depth and heat of burial, 

 compaction, time, and structural deformation. Rank 

 is thus a way of expressing the progressive meta- 

 morphism of coal. It is quite independent of grade, 

 which is a way of expressing quality. 



As coals of different rank are adapted to differ- 

 ent uses, rank is a major basis of difi'erentiation in 

 coal-resource calculations. In accompanying tables 

 and figures, the coal resources are expressed in 

 short tons. If arithmetic adjustments were made 

 for the contained heat values, the distribution pat- 

 terns would be changed somewhat because of the 

 lower heat values of lignite and subbituminous coal. 



GRADE OF COAL 



Coal is classified by grade largely according to the 

 content of ash, sulfur, and other deleterious con- 

 stituents. Thus far in work on coal resources, a 

 preliminary classification on the basis of sulfur 



