THE ERA OF THE FERNS 
with the formations of Eurasia: 
North America 
1. Upper 
Pennsylvanian 
(Monongahela) 
(Conemaugh) 
Western and 
Central Europe 
1. Stephanian 
Russian and 
Siberia 
1. Ouralian 
2. Middle 
Pennsylvanian 
(Allegheny) 
3. Lower 
Pennsylvanian 
(Pottsville) 
2. Westphalian 
2. Moscovian 
The greatest coal deposits of the world are found in 
the Pennsylvanian formation, but there is coal also in 
a number of other formations like the Mississippian in 
America and the Kulm in Europe, and in nearly every 
period after the Pennsylvanian. The coal fields of the 
southern hemisphere belong primarily to the period im¬ 
mediately following the Pennsylvanian, the Permian. 
Twice in the history of the earth an enormous accumu¬ 
lation of plant material has occurred, and it was then 
that the greatest coal beds were laid down. These two 
periods are the Pennsylvanian and the middle of the 
Tertiary. There must have been a coincidence of cli- 
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