THE ERA OF THE FERNS 
different way than they are now, the regions which are 
now Arctic could have been in much more southern 
latitudes, where luxuriant plant life was possible. While 
there are great difficulties in explaining the mechanism 
of continental movement, nevertheless it seems to be 
gaining ground steadily. 
During the Pennsylvanian period, great bays reached 
into the interior of North America from the Gulf of 
Mexico. These bays were frequently above the level of 
the sea and as often fell below it. In Illinois, for instance, 
they rose above the sea level at least sixteen times and 
were sixteen times covered by inundations. Similar bays 
extended throughout Pennsylvania, West Virginia, Ohio, 
eastern Kentucky, Tennessee, and Alabama. Another 
bay stretched through Texas, Arkansas, Oklahoma, Mis¬ 
souri, and Iowa. One of these bays filled practically all 
of Illinois, western Indiana, and western Kentucky. How 
the various bays of North America were connected with 
each other and with the Gulf of Mexico is not yet quite 
clear. A bay also extended into northern Michigan but 
probably was early detached from the rest. 
When these great inland bays rose above the surface 
of the sea, or, in other words, when the sea level receded 
a few feet, opportunity was given for the development 
of extensive shore swamps. When the sea advanced again, 
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