12 



inois Natural History Survey Circular 49 



most of the coalfields of the state. This forest, very different from 

 our present one, consisted chiefly of tree ferns, scale trees, and rush 

 trees. Tree ferns no longer occur in Illinois, but some kinds still 

 persist in the tropical areas of the world. The scale trees are repre- 



lATF CRETACEOUS 



EOCENE 



Fig. 9. — Maps of North America at various times in the post, showing the differences in the 

 exposed land areas. In all four maps the stippled areas represent oceans; the white and striped 

 parts represent land. Although these maps do not show all the known details, they present 

 moderately accurate pictures of the geography of the various times. They do not by any means 

 show all the changes that took place in the physiography. They ore simply a set of samples 

 showing the physiography at the following times: late Cambrian, about 500 million years ago; 

 middle Pennsylvanian, about 290 million years ago (the striped area represents forests, many of 

 which formed coal beds); late Cretaceous, about 70 million years ago; Eocene, about 50 million 

 years ago. (Adapted from Dunbar 1960.) 



