CHAPTER XVII 

 SUMMARY OF MESOZOIC HISTORY 



Although the Mesozoic was quite certainly shorter than the 

 Paleozoic, it must, nevertheless, have had a duration of at least 

 some millions of years. As the name indicates, the Mesozoic was 

 the era of transition between the Paleozoic and the Cenozoic. 

 Eastern North America had been to a large degree completed at 

 the time of the Appalachian Revolution, except for the addition of 

 the Atlantic and Gulf Coastal Plain belts. In western North 

 America, however, profound physical geography changes took 

 place, bringing that part of the continent almost to its present 

 condition, as regards relations of land and sea, only near the close 

 of the Mesozoic. The life of the Mesozoic, too, was distinctly 

 intermediate in character, those of the great groups of character- 

 istic Paleozoic organisms which did continue into the Mesozoic 

 having become extinct during the era, while many more modern 

 groups showed great development during the era. Certain other 

 important groups of organisms like the Cycads, Ammonites, and 

 Reptiles, were eminently characteristic of the Mesozoic and reached 

 their culmination during the era. 



Mesozoic Rocks 



The late Triassic stratified rocks of the Atlantic Coast are 

 sandstones, conglomerates, and shales, mostly of continental origin, 

 though in part at least probably of estuarine origin. Rocks of the 

 Triassic in the western interior are chiefly the Red Beds (shales, 

 sandstones, and limestones), with more or less salt and gypsum, of 

 terrestrial or lacustrine origin. On the Pacific Coast the strata are 

 of true marine origin and they consist of all sorts of typical sedi- 

 ments. 



Jurassic strata are wholly confined to the western interior and 

 Pacific borders, where they are all typical marine sediments, except 

 the earlier Jurassic beds of the western interior, which are of con- 

 tinental origin and probably also include some Red Beds. 



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