430 HISTORICAL GEOLOGY 



THE MESOZOIC ERA IN NORTH AMERICA 



Changes in the form of the continent. At the close of 

 the Paleozoic era the continental platform of North America 

 had been left largely above the sea. Only on the Pacific 

 coast did the ocean come farther inland than now. On the 

 east side of this extensive continent stood the rugged moun- 

 tains of the Appalachian system, perhaps not unlike the 

 Andes of to-day. In the West lay broad, arid plains with 

 occasional salt lakes, but it is improbable that high mountains 

 stood there at that time. 



As the era continued, the sea tended more and more to 

 overspread the land. Late in the Jurassic period a long 

 gulf came in across the depressed lowland which is now 

 occupied by the great mountains of western Canada. A 

 little later the Atlantic Ocean began to encroach upon the 

 eastern and southern border of the continent, and along its 

 shores were deposited the earliest sediments of the present 

 coastal plain. Finally, in the Cretaceous period, the depres- 

 sion of the central western part of the continent allowed the 

 sea to submerge a broad strip extending from the Arctic 

 Ocean to the Gulf of Mexico, thus leaving North America 

 divided into two smaller land masses. 



The scene of earth movements, like that of sedimentation, 

 was shifted to the West in the Mesozoic era. Not being 

 resurrected by further warping, the Appalachian Mountains 

 in the East had been gradually worn down to low hills with 

 broad valleys between. They remained in this condition 

 through the later part of the era. The crumpling of the 

 Pacific coast strip had doubtless produced a series of great 

 mountain ranges, among the descendants of which are the 

 Sierra Nevada, Cascade, and Alaskan ranges of to-day. After 

 a long period of comparative quiet during the Comanchean 

 and Cretaceous periods the level strata of the western in- 

 terior were arched and locally complexly folded, thus estab- 

 lishing the third great North American mountain system, - 



