64 GEOLOGY. 



Early and Middle 4 Jurassic beds in the western interior must be looked 

 on with question. 



The Upper Jurassic— During the Upper Jurassic (Upper and 

 Middle Oolitic of England) epoch, the areas of sedimentation were 

 greatly changed, indicating considerable changes in geography. On 

 the Pacific coast of the United States, in the latitude of California, the 

 sea appears not to have extended east of the Sierras. The ({olden 

 Gate series of the Coast Range perhaps belongs to this stage; ] but in 

 northern British Columbia, where the Lower and Middle Jurassic 1 eds 

 have little representation, the sea extended farther east than during 

 the earlier part of the period. South of the United States, Jurassic 

 beds of marine origin occur in western Mexico (Sonora), but it is not 

 known to what part of the system they belong. 



In addition to marine sedimentation along the Pacific coast, the 

 sea had access to a large area in the western interior, and covered 

 much of Wyoming, 2 Montana, 3 Utah, and Colorado, and parts of 

 several other states. 4 This is showm by the presence in these 

 States of sedimentary beds containing marine Upper Jurassic fossils. 

 The beds are chiefly exposed in the mountains (Wasatch, Uinta, Black 

 Hills, etc.) where the erosion which followed the uplift and deformation 

 of the strata has discovered their edges. 5 The general relations of land 

 and water in the west in the late Jurassic are shown in Fig. 348, but 

 it should be said that the distribution of the Jurassic in the west is 

 so imperfectly known that no map showing the relations of land and 

 water can lay claim to accuracy. 



The avenue through which the sea reached this region has not been 

 determined, but the fossils are so unlike those of the Calif ornian coast 

 as to have led to the inference that the waters of the interior did not 

 come in from the southwest. The absence of Jurassic strata over 



1 Fairbanks, Jour, of Geol., Vol. Ill, pp. 415-433. 



2 Logan, Jour, of Geol., Vol. VIII, p. 241; Knight, Bull. 45, Wyo. Exp. Station, 

 and Bull. Geol. Soc. of Am., Vol. XI, pp. 377-88, and the following folios, U. S. Geol. 

 Surv.: Hartville, Wyo., Yellowstone Park, Wyo., New Castle, W r yo.-S. D. 



3 See Little Belt Mountain, Fort Benton, Three Forks and Livingston folios, U. S. 

 Geol. Surv. 



4 For South Dakota, see Darton, 21st Ann. Rept. U. S. Geol. Surv., Pt. IV, and the 

 following folios, U. S. Geol. Surv.: Oelrichs and Edgemont,'S. D.-Neb. 



5 In addition to the above folios, U. S. Geol. Surv., see also the following: Anthra- 

 cite and Crested Butte, Ten Mile, and Telluride. 



