166 HISTORICAL GEOLOGY 



luxuriant vegetation. It should be borne in mind that workable 

 coal seams constitute only about 2 per cent of the containing 

 strata which are sandstones, shales, clays, and, in some localities, 

 limestones. 



I Perhaps no single coal seam in the world underlies such a large 

 area (12,000 to 15,000 square miles) as the famous Pittsburg coal 

 bed. It is worked over an area of about 6000 square miles, and for 

 2000 square miles it averages 7 feet in thickness. Most of the 

 swamps or bogs of Pennsylvanian time were much smaller than 

 this. 



In the anthracite coal district of eastern Pennsylvania, the 

 famous " Mammoth" coal bed is remarkable for its great thickness 

 up to 50 or more feet. 



It may be of interest to consider the length of time necessaiy 

 for the accumulation of so many coal beds one above the other. 

 A vigorous growth of vegetable matter on an acre has been esti- 

 mated to produce the equivalent of 100 tons of dried organic 

 matter per century. This amount compressed to the specific 

 gravity (1.4) of coal would cover an acre less than two-thirds of an 

 inch deep. Considering that four-fifths of the organic matter 

 escapes as gases in the process of coal making, we find that it would 

 take nearly 10,000 years to make one foot of coal. Now, since the 

 total thickness of coal beds in the Pennsylvanian system is often 

 from 100 to 250 feet, it is readily seen, on this basis, that the time 

 necessary for the accumulation of the coal deposits was from 

 1,000,000 to 2,500,000 years. On a conservative basis, the time 

 necessary for the deposition of the sediments was fully as long, 

 so that the Pennsylvanian period appears to have had a duration of 

 no less than 2,000,000 to 5,000,000 A^ears. 



Close of the Pennsylvanian. — At the end of the period the 

 remarkable, near sea-level, coal-swamp geographic conditions in 

 the eastern United States were largely brought to a close by emer- 

 gence of the lands distinctly above sea level, except in a narrow 

 trough lying along the western side of the Appalachian Moun- 

 tain axis and in which Permian deposits accumulated. From the 

 Great Plains westward also the marine waters were notably re- 

 stricted. In the east, at least, the emergence was probably due to 

 a beginning of the great orogenic movements which culminated 

 in the Appalachian Mountain Revolution at the close of the 

 Paleozoic era. 



