[Chap. LII PLANTS OF THE PAST 725 



coal has resulted: bituminous in the plateaus, and anthracite in the 

 folded mountains of eastern Pennsylvania. The bituminous coals of the 

 eastern mountain region were converted into anthracite by the intense 

 heat of the folding rocks. Not as much metamorphism occurred in the 

 plateau and the plant debris was changed only to bituminous coal. 



Fig. 360. Group restoration of trees ol the CJarbomleruus penotl. Copyright by 

 Field Museum of Natural History. 



There were also long periods when the continent was far drier than 

 it is today and the desert areas were much larger, for example, during 

 the Silurian and Permian periods. When the Rockies were uplifted at 

 the close of the Cretaceous, the deciduous and broad-leaved evergreen 

 forests that existed in the interior of North America at that time were 

 graduallv killed by the increasing drought and became replaced by 

 grasslands. The eastern deciduous forest of today is the remnant of that 

 far more extensive forest of the early Tertiary. The grasslands of the 

 Central States are the continuation of the grasslands that became estab- 

 lished there during Tertiary times. 



Many changes in the area and shape of the land surface of North 

 America have occurred. During the Pliocene and prior to the Glacial 

 period (Pleistocene) the North American continent was elevated more 

 than 1000 feet above the present level. At that time more of the con- 



