Geology . be sup-posed that the northerly extensionof the marine Jurassic of the UnitedStates has been concealed by laterbeds, or destroyed by erosion, ornot discovered. The presence of fresh-water bedsof possible Upper Jurassic age (theMorrison [Atlantosaurus, Como] bedsof Colorado, Montana, and Wyo-ming) in some parts of the westerninterior would, were their age estab-lished, show that salt water wasnot continuously present at all pointsw7here deposition was taking Jurassic age of these bedsseems, however, to be doubtful (seep. 119).2 The change in geographic con-ditions in the wes
Geology . be sup-posed that the northerly extensionof the marine Jurassic of the UnitedStates has been concealed by laterbeds, or destroyed by erosion, ornot discovered. The presence of fresh-water bedsof possible Upper Jurassic age (theMorrison [Atlantosaurus, Como] bedsof Colorado, Montana, and Wyo-ming) in some parts of the westerninterior would, were their age estab-lished, show that salt water wasnot continuously present at all pointsw7here deposition was taking Jurassic age of these bedsseems, however, to be doubtful (seep. 119).2 The change in geographic con-ditions in the western half of NorthAmerica, between the Middle andUpper Jurassic, as shown by thedistribution of the corresponding formations, was as great as thatwhich sometimes separates one period from another. It was equallygreat in other continents, but not in other parts of our own. Thickness.—The total thickness of the system in California doesnot exceed 2000 feet (in part tufa). Farther east, in western Nevada,3. Fig. 348.—Map showing the general re-lations of land and water in the west-ern part of North America during thelater part of the Jurassic period. Theblack areas represent known areas ofUpper Jurassic. The dotted line isthe conjectured outline of the bay.(After W. N. Logan.) 1 Neumayr suggested (Denkschr. K. Akad. Wiss. Wien, 1893, pp. 301-302) theArctic rather than the Pacific connections of the Jurassic deposits; but the similarityof faunas of the north Pacific coast and the western interior, have commonly beenthought to point to the other conclusion. 2 Lee, Jour, of Geol., Vols. IX and X, pp. 343-52 and 36-58 respectively; Dartonand Smith, Edgemont and New Castle, South Dakota, folios, Geol. Surv., andWilliston, Jour, of Geol., Vol. XIII, p. 338. 1 King, Survey of the 40th Parallel, Vol. I. THE JURASSIC PERIOD. 67 nearer the land whence sediment was derived, the system attainsa thickness of 5000 to 6000 feet, being made up of 1500-2000 feet oflimestone below, and 400
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