FERNS, FOSSILS AND FUEL 
Toward the end of the Tertiary, great mountain ranges 
were raised by a shifting in the earth’s crust. The yearly 
temperature, which had been slowly cooling, sank more 
rapidly until, during the glacial epoch, an enormous 
ice shield accumulated, covering a large part of North 
America, Northern Europe, and Northern Asia. The 
glacial epoch consisted of several advances and several 
retreats of the great ice masses; at the times of retreat the 
climate became warmer, only to grow cold again when the 
ice came back. These warm periods we call inter-glacial 
epochs and the cold ones glacial epochs. 
We must conclude, however, that it was warmer dur¬ 
ing some of the inter-glacial epochs than it is now. For 
fossil wood of the Osage orange, which does not now 
grow wild north of Tennessee, has been found near 
Toronto in an inter-glacial formation. It has been ob¬ 
served in Finland that certain plants are retreating south 
again, which would indicate a cooling of the present 
climate. This gradual decrease in yearly average tem¬ 
perature has not been measured by scientific observation, 
but it must exist there. Perhaps we are not in a post¬ 
glacial epoch but in an inter-glacial one. Ten thousand 
years from now ice may cover Chicago, and much of 
the existing life of North America and northern Europe 
may again be driven south. 
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