THE JURASSIC PERIOD. 67 



nearer the land whence sediment was derived, the system attains 

 a thickness of 5000 to 6000 feet, being made up of 1500-2000 feet of 

 limestone below, and 4000 feet of slates above. In its western interior, 

 it is far less. 



Surface distribution and position of beds. — In spite of the fact 

 that the Jurassic beds are somewhat widely distributed, they do not 

 now appear at the surface over large areas. In many places they 

 are covered by younger beds, and from some areas where they 

 once existed they have been removed by erosion. In some areas 

 they retain their original position, while in others they have been 

 tilted, or even folded and metamorphosed. This is especially true 

 in California, where the slates of the system contain many of the 

 gold-bearing quartz veins of the region. 



With the sedimentary beds of the Pacific coast (California) there 

 are considerable beds of fragmental igneous rock, showing that 

 volcanic forces were here active on a somewhat extensive scale during 

 the Jurassic period. 



Jurassic in Alaska. — Jurassic formations are known at somewhat 

 widely separated points in Alaska, but their horizon within the sys- 

 tem has not been established. 1 



Close of the Jurassic. 



Orogenic movements. — At the close of the Jurassic period consider- 

 able disturbances occurred in the western part of North America. 

 Nearly 25,000 feet of strata, 7000 feet of which belong to the Triassic 

 and Jurassic, began to be folded into the Sierras, 2 and the Cascade 

 and Klamath 3 Mountains farther north perhaps began their growth 

 at the same time. In northern California and southern Oregon, in 

 the latitude of the Klamath Mountains, the coast was somewhat far- 

 ther west than now, after this period of orogenic movement. 3 There 

 is some reason to think that the axes of these mountain ranges were the 

 scenes of earlier disturbances (Vol. II, p. 584), but of these earlier move- 

 ments the record is meager. Their existence is inferred from the 

 greater complexity and metamorphism of the pre-Triassic beds. It 



'Spurr, 20th Ann. Rept. U. S. Geol. Surv., Pt. VII, pp. 235-6. 



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

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



3 Diller, Bull. Geol. Soc. Am., Vol. IV, p. 224, and 14th Ann. Rept. U. S. Geol. Surv. 



