454 PENNSYLVANIAN PERIOD 



intrusions of igneous rock. Other sorts of sedimentary rock are 

 metamorphosed in similar situations. 



These phenomena suggest that anthracite is metamorphic coal, 

 produced from bituminous coal by processes similar to some of those 

 which metamorphose other sorts of rock. The fact that most 

 metamorphic coal is found in regions where erosion has exposed its 

 beds (Fig. 385) led to the conjecture that exposure of the coal might 

 be a factor in the problem, the exposure favoring the escape of the 

 volatile constituents, and so aiding in the transformation of soft 

 coal into hard. Some beds of bituminous coal are, however, exposed 

 freely. Both dynamic action, involving pressure and heat, and 

 exposure would seem to be conditions favoring the development of 

 anthracite, but it does not follow that these are the only factors in 

 the problem, or that anthracite coal has never been produced in 

 other ways. White has advanced the idea that deep-seated, hori- 

 zontal thrust movements are the essential cause of devolatilization 1 . 



There are several varieties of bituminous (soft) coal, some of 

 which appear to depend on the nature and extent of the decay of 

 the vegetable matter before its burial, and some on the degree to 

 which the devolatilizing processes have been carried since burial. 

 Recent studies seem to indicate that the kind of vegetation enter- 

 ing into the coal may have an important effect on the product. 

 Some coal seems to be made up largely of algae, or of the spore- 

 cases of certain plants, and such coal has rather distinctive quali- 

 ties, if recent interpretations are correct. 



Other Products of Economic Value 



The iron ore of the Coal Measures occurs in layers, or in the 

 form of nodules concentrated at a given horizon, forming a nearly 

 continuous layer. The iron of the Coal Measures seems to have 

 been deposited largely as a precipitate from the waters of inland and 

 local basins while the other members of the system were being laid 

 down. Dissolved by the land waters from the soil and rocks, it 

 was brought to the marshes in some soluble form. In the marshes, 

 it was precipitated in the form of iron carbonate or iron oxide. Sub- 

 sequent oxidation has changed some of the original carbonate into 

 the oxide. The principal iron ores of the system occur in Penn- 

 sylvania and eastern Ohio. The system yields oil and gas in some 

 places, as in Oklahoma, Kansas, and Illinois. 



1 David White. Economic Geology, Vol. III. 



