638 GEOLOGY 



morphic coal, produced from bituminous coal by processes similar 

 to some of those which metamorphose other sorts of rock. The 

 fact that metamorphic coal is usually found in regions where 

 erosion has exposed its beds (Fig. 437) has led to the conjecture 

 that exposure of the coal may be a factor in the problem, the ex- 

 posure favoring the escape of the volatile constituents, and so aiding 

 in the transformation of bituminous coal to anthracite. Beds of 

 bituminous coal are, however, often freely exposed. 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 recently advanced the idea that deep-seated, horizontal thrust 

 movements are the essential cause of de volatilization. 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 2 seem to indicate that the kind of vegetation enter- 

 ing into the make-up of the coal may have an important effect on 

 the product. Thus some coal seems to be made up largely of algae, 

 and such coal has rather distinctive qualities, if present interpre- 

 tations are correct. 



Other Products of Economic Value 



Iron ore. The iron ore of the Coal Measures occurs in layers 

 like the coal, or in the form of nodules which are often concentrate* I 

 at a given horizon, forming a nearly continuous layer. The iron of 

 the Coal Measures seems to have been largely deposited as a pre- 

 cipitate 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 either in 

 the form of the carbonate or ferric oxide. Subsequent oxidation 

 has changed some of the original carbonate into ferric oxide. The 



1 David White. Economic Geology, Vol. III. 



2 Especially those of Mr. White. 



