CHAPTER XXIV 

 THE JURASSIC PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



The eastern part of the continent. Jurassic formations have 

 not been identified certainly in the eastern half of the continent, but 

 there are beds outcropping along the western margin of the Coastal 

 Plain in Maryland that may belong to this system. In constitution, 

 structure, and all physical relations their alliance is with the Co- 

 rn anchean (Lower Cretaceous) of the same region, and since their 

 fossils are indecisive, this seems, at present, their best correlation. 



Erosion seems to have been the leading geologic process in the 

 eastern part of the continent during the period. Its effectiveness 

 may be judged by the fact that both the uplifted and deformed 

 Triassic system and the Appalachian mountain region farther west 

 were essentially base-leveled before the Comanchean period was 

 far advanced. The sediments worn away from these areas were 

 deposited somewhere, presumably east of the present coast. Ero- 

 sion seems to have been in progress also in the eastern interior over 

 most or all the area which emerged during the closing stages of the 

 Paleozoic. 



The western interior. In contrast with the eastern half of the 

 continent, deposition was in progress, probably, in some parts of 

 the western interior, though the possible early Jurassic beds of this 

 region have not been clearly differentiated from the Trias. There 

 is perhaps room for doubt whether the early and middle parts of 

 the system have much representation in this region. 



Late in the period, an arm of the sea extended itself over a large 

 tract in the western interior (Fig. 475), covering much of Wyoming, 1 



1 Logan, Jour. Geol., Vol. VIII, p. 241; Knight, Bull. 45, Wyo. l-ixp. 

 Station, and Bull. Geol. Soc. of Am., Vol. XI, pp. 377-388, and the Wyoming 

 folios of the U. S. Geol. Surv. 



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