422 



HISTORICAL GEOLOGY 



fill it up, but the chief cause of its withdrawal is probably 

 to be found in gentle changes of level, this time the re- 



FIG. 441. Probable geography of North America near the middle of the 

 Cretaceous period. The dotted pattern represents terrestrial deposits. 



verse of those which had caused it to overspread the land. 

 As the sea receded, the zones of sedimentation (ooze, mud, 

 sand, and river deposits) began a slow retreat. This migra- 



