66 



GEOLOGY. 



the most rational explanation of the Marine Jurassic beds in question 

 is that they were deposited in a great dependence of the north Pacific 



or the Arctic Ocean, 1 which covered 

 the area where the strata occur. 

 If this be correct, it must be sup- 

 posed that the northerly extension 

 of the marine Jurassic of the United 

 States has been concealed by later 

 beds, or destroyed by erosion, or 

 not discovered. 



The presence of fresh-water beds 

 of possible Upper Jurassic age (the 

 Morrison [Atlantosaurus, Como] beds 

 of Colorado, Montana, and Wyo- 

 ming) in some parts of the western 

 interior would, were their age estab- 

 lished, show that salt water was 

 not continuously present at all points 

 where deposition was taking place. 

 The Jurassic age of these beds 

 seems, however, to be doubtful (see 

 p. 119). 2 



The change in geographic con- 

 ditions in the western half of North 

 America, between the Middle and 

 Upper Jurassic, as shown by the 

 distribution of the corresponding formations, was as great as that 

 which sometimes separates one period from another. It was equally 

 great in other continents, but not in other parts of our own. 



Thickness. — The total thickness of the system in California does 

 not 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 the 

 later part of the Jurassic period. The 

 black areas represent known areas of 

 Upper Jurassic. The dotted line is 

 the conjectured outline of the bay. 

 (After W. N. Logan.) 



1 Neumayr suggested (Denkschr. K. Akad. Wiss. Wien, 1893, pp. 301-302) the 

 Arctic rather than the Pacific connections of the Jurassic deposits; but the similarity 

 of faunas of the north Pacific coast and the western interior, have commonly been 

 thought to point to the other conclusion. 



2 Lee, Jour, of Geol., Vols. IX and X, pp. 343-52 and 36-58 respectively; Darton 

 and Smith, Edgemont and New Castle, South Dakota, folios, TJ. S. Geol. Surv., and 

 Williston, Jour, of Geol., Vol. XIII, p. 338. 



1 King, Survey of the 40th Parallel, Vol. I. 



