FERNS, FOSSILS AND FUEL 
the period. Though we know nothing of the terrestrial or 
land flora, whatever it may have been, it was undoubtedly 
present, because plants appear in a high stage of develop¬ 
ment in the Devonian period with which the younger 
Paleozoic opens. 
Geologists divide the younger Paleozoic age into the 
Devonian, the Mississippian, the Pennsylvanian, and the 
Permian periods. From all of them we have large deposits 
of fresh-water sediments; in consequence we are acquainted 
with fossil remains of many land plants that flourished 
throughout this epoch. 
During the Pennsylvanian period, plant life reached a 
climax which resulted in the deposition of the material 
represented today by the bituminous and anthracite coal 
beds which form a circle around the northern hemisphere, 
in the United States, England, northern France, Belgium, 
Germany, Russia, Siberia, and China. 
At the end of the Pennsylvanian age and throughout 
the Permian, there was a great deal of mountain raising 
and a resultant cooling of the climate. Whenever the 
earth’s crust was disturbed to any great extent, there was 
an inevitable interruption of the usual warm temperatures 
and a temporary cooling. In some cases, as in the Per¬ 
mian period, this took place mostly in the southern hem¬ 
isphere, in contrast to the ice age which immediately pre- 
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