FORMATIONS AND PHYSICAL HISTORY 



505 



both are present, but the younger beds overlap the older system at 

 some points, and fall short of it at others. 



Tlu- system contains the common sorts of sedimentary rocks, 

 and some fragmental igneous rock. Jurassic formations also are 





Fig. 427. Section in the Sierras of California. The Jurassic (or Jura-Trias) 

 has Urn metamorphosed, and is associated with igneous rock, grdanddpt, igneous 

 rock, probably of Jurassic or Cretaceous age; si and slm, Jura-Trias (?) schist; Na, 

 A>, and J'l>, igneous rock, late Tertiary and Pleistocene. (Lindgrcn, U. S. (ic<>l. 

 Surv.) 



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, nearer the land whence sediment was derived, it attains 

 a thickness twice or thrice as great. In the western interior, it is thin. 



Surface distribution and position of beds. The Jurassic beds do 

 not now appear at the surface over large areas, being much con- 

 cealed by younger beds. In some areas they retain their original 

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

 metamorphosed (Fig. 427). This is especially the case in the 

 Sierra Mountains and in some ranges near the western coast. 



Close of the Period 



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, and the Cascade and Klamath mountains 

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

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

 that they have not had later periods of growth. It is probable that 

 the Coast Range of California began its history at the close of this 

 period, for deformed Jurassic beds (Golden Gate series) underlie 



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



