The Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal.
The Mississippian is about twice as long as the Pennsylvanian, but due to the large carbon in the air the Carboniferous Period got its name. The two subperiods were long thought to have been more or less equal.