380 HISTORICAL GEOLOGY 



lost. Peat is merely a compressed but spongy mass of car- 

 bonized plants, such as we may now find beneath swamps. 

 Soft or bituminous coal has lost far more of the gases and 

 liquids and is a firm rock. Anthracite or hard coal is nearly 

 all (91 to 95 per cent) carbon a hard rock bearing little trace 

 of its origin. 



The loss of the volatile parts of the coal is a very slow pro- 

 cess. Thus we find that the marsh deposits of more recent 

 periods are but partly converted into coal, while, at the 

 other extreme, the most ancient beds are reduced to impure 

 carbon alone, in the form of graphite. It is not, however, 

 altogether a matter of age. In some places, as in Colorado, 

 igneous intrusions have baked the soft coal J into anthracite, 

 or even coke. Wherever the coal-bearing strata have been 

 strongly folded, the coal is found to be much harder than in 

 strata of the same age where they have not been folded. Thus 

 the Coal Measures of eastern Pennsylvania contain anthracite 

 because the strata were crumpled, while in the western part 

 of the state they are flat and afford only soft (or bituminous) 

 coal, the age of the rocks being the same in both localities. 



Coal resources of the United States. The wonderful 

 development of manufacturing in the United States is due in 

 no small degree to the presence of great coal beds in the popu- 

 lous eastern states. No other country, except perhaps China, 

 is so well provided with this essential resource (Fig. 401). 



Marshy plain in the East. Returning to our picture of 

 the United States in Pennsylvanian times, we may think of 

 the eastern part of the country as generally low and monoto- 

 nously flat. Vast swamps probably bordered the sea which 

 lay to the west, as they now fringe the coast of New Jersey, 

 the Carolinas, and Florida. On the east they were flanked 

 by the land mass of Appalachia. Inland, along the sluggish 

 rivers, fresh-water marshes, like those of the Mississippi and 

 the Yukon, probably covered large areas. Gradual but halting 

 submergence of the region seems to have been in progress. 



1 These particular coal deposits are of much later age. 



