220 



HISTORICAL GEOLOGY 



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part of the continent. 1 On map Fig. 119 the considerable areas 

 shown in California and western Nevada are mostly Jurassic rocks. 

 Some areas are also definitely known in 

 southern Alaska and in western Oregon. 

 In the western interior numerous small 

 areas of mostly late Jurassic rocks only are 

 known from northern Arizona and north- 

 western Colorado northward through Idaho, 

 Wyoming, western South Dakota, and 

 Montana. Rocks of late Jurassic age also 

 quite certainly occur in western British 

 Columbia east of the Cascade Mountains. 



As compared with all preceding systems 

 since the early Paleozoic, rocks of the Juras- 

 sic system are the least extensively devel- 

 oped on the continent. 



Character of the Rocks. — The Jurassic 

 rocks of California and Nevada, and the 

 southern coast of Alaska, are chiefly of 

 marine origin representing much or all of 

 the period. In the Sierras and Coast 

 Ranges the strata are usually metamor- 

 phosed and highly folded (e.g. Mariposa 

 slates of the Sierras, Fig. 132). 



In the western interior Triassic and 

 Jurassic strata have often not been satis- 

 factorily separated, and some "Red Beds" 

 (with gypsum) of continental origin, like 

 those of the Permian period, are probably 

 of Jurassic age. At any rate, the only 

 known true marine (Jurassic) strata in 

 that whole region are of late Jurassic age. 

 These rocks, which comprise all tj^pes of 

 ordinary sediments, especially limestones 

 and slates, are usually highly folded or 

 tilted in the Rockies, Wasatch Mountains, 

 Black Hills, etc., and hence are generally 

 exposed only in narrow belts. 



1 Certain non-marine deposits exposed along the Potomac River in Mary- 

 land have sometimes been called Jurassic, but the best evidence, as presented 

 by W. B. Clark, points to their Comanchean (Lower Cretaceous) age. 



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