THE JURASSIC PERIOD 707 



ern British Columbia, where the Lower and Middle Jurassic beds 

 have little representation, the sea extended farther east than during 

 the earlier part of the period. The deposits embrace all the usual 

 sorts of sedimentary rocks, as well as considerable beds of fragmental 

 igneous rock. Jurassic formations are also known at somewhat 

 widely separated points in Alaska. 1 On the shores of Cook Inlet, 

 10,000 feet of Middle and Upper Jurassic are reported. 



Thickness. The total thickness of the system in California does 

 not exceed 2,000 feet (in part tuff). Farther east, in western 

 Nevada, 2 nearer the land whence sediment was derived, it attains 

 a thickness twice or thrice as great, being made up of limestone 

 below, and slates above. In the western interior, its thickness is 

 relatively slight. 



Surface distribution and position of beds. The Jurassic beds do 

 not now appear at the surface over large areas, being concealed in 

 many places by younger beds. In some areas they retain their 

 original position, while in others they have been tilted, or even 

 folded or metamorphosed. This is especially the case in the Sierra 

 Mountains and in some other ranges near the western coast. 



CLOSE OF THE JURASSIC 



Orogenic movements. At the close of the Jurassic period, there 

 were considerable disturbances in the western part of North Amer- 

 ica. Great thicknesses of Triassic and Jurassic strata began to be 

 folded into the Sierras, 3 and the Cascade and Klamath 4 Mountains 

 farther north perhaps began their growth. It is not to be understood 

 that these mountains attained great height at this time, or that they 

 have not had later periods of growth. In the Klamath Mountains, 

 for example, there are deformed beds of late Tertiary age. After 

 this closing-Jurassic period of erogenic movement, the coast was 



1 See Alaskan Reports, U. S. Geol. Surv. 



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



3 Whitney, Geology of California, Vol. I, and Am. Jour. Sci., Vol. 

 XXXVIII, 1864; and Fairbanks, Am. Geol., Vol. IX, 1892, Vol. XI, 1893. 



4 Diller, Bull. Geol. Soc. Am., Vol. IV, p. 224, and 14th Ann. Kept., U. S. 

 Geol. Surv. 



