5 02 HISTORICAL GEOLOGY 



fresh and salt water marshes in which vegetation flourished. Similar 

 conditions are to be seen to-day in the Dismal Swamp of Virginia, 

 on the coast of New Jersey, the Carolinas, and Florida. 



Since less than five per cent, of the Coal Measures (Pennsylvanian) 

 consist of coal, the greater part of the system being composed of 

 sandstones, shales, clays, and in some localities limestones, it is evi- 

 dent that subsidence accompanied deposition. The submergence was 

 not continuous, however, but was interrupted by many halts, with oc- 

 casional slight elevations. When the sea bottom was built up suffi- 

 ciently, plants grew on it, and salt water marshes, which eventually 

 became fresh, appeared. In the course of years the trees fell, and 

 upon their fallen trunks others grew up. In the process of time their 

 remains made thick beds of peat. A too rapid subsidence inundated 

 the swamps, killing the vegetation, and the peat was then covered with 

 sediment. If the water was far from shore, beyond the reach of mud 

 and sand, limestones were deposited ; if close to shore, mud and sand 

 were laid down. When the downward movement ceased, the bottom 

 of the sea was built up until it again became shallow enough to permit 

 plants to grow on it. The order of deposition shown in Figure 449 

 (p. 473) is thus explained. As has been stated, an elevation some- 

 times occurred, as is shown by unconformities. Some of the uncon- 

 formities, however, were produced merely by the shifting of the stream 

 channels in the swamps. 



The number of coal beds in any vertical section varies greatly : 

 in Pennsylvania and Nova Scotia as many as 30 are known, while in 

 Illinois there are often less than 10. Some of these beds are workable, 

 but many are not. 



Extent and Structure of Coal Beds. — Individual coal swamps 

 of the Pennsylvanian were very extensive. The Pittsburgh coal bed, 

 one of the greatest in the world, extends over an area of at least 12,000 

 square miles in Pennsylvania, Ohio, and West Virginia. The extent 

 of some modern peat bogs compares favorably with those of the Penn- 

 sylvanian, but their thickness is much less. One extends across 

 Holland and Belgium into France, and the Alaskan tundra has a much 

 greater continuous area than the largest of those known in the past. 

 All coal beds are not of great extent, some corresponding to the small 

 peat bogs of to-day : one basin 200 yards in diameter was found to 

 have two coal beds, one two and the other 16 feet in thickness; and 

 another one 1 1 5 yards in diameter was found to have a coal seam 

 ei^ht feet in maximum thickness. A given thickness of coal repre- 



