70 



GEOLOGY. 



Jurassic gulf of the northwestern part of the continent disappeared 

 at the close of the period. 



It should perhaps be added that until very recently no part of 



rf^^^g! 



Fig. 352. — A section in southern Montana. ^? = Archean ; €, Cambrian (Flat- 

 head and Gallatin formations) ; D, Devonian (Jefferson and Three Forks for- 

 mations) ; Mm, Mississippian (Madison formation) ; Pq, Pennsylvanian (Quad- 

 rant formation) ; Je, Jurassic (Ellis formation) ; Kd, Kmc, and Kl, Cretaceous 

 (Dakota. Colorado, and Montana, and Laramie formations); bbr, igneous rock. 

 (Peale, U. S. Geol. Surv.) 



the geology of the United States has received less careful study, and 

 is less well understood, than that of the Permian, Triassic, and Jurassic 



Fig. 353. — Section showing the relation of the Jurassic beds in the West Humboldt 

 range of Nevada. M, Archean; T, Red beds; Jst, Triassic (Star Peak); «/, 

 Jurassic; Nh, Pliocene; P, Pleistocene. (King, U. S. Geol. Surv.) 



systems of the western half of the continent. The reason is twofold: 

 (1) The systems are in regions where relatively little detailed work 



Fig. 354. — Section in the Sierras of California, showing the Jurassic (or Jura-Trias) 

 system where it has been metamorphosed, and where it is associated with igneous 

 rock, grd and dpt, igneous rock, probably of Jurassic or Cretaceous age; si and 

 * slm, Jura-Trias (?) schist; Na, Nr, and Pb, igneous rock of late Tertiary and Pleis- 

 tocene age. (Lindgren, U. S. Geol. Surv.) 



has been done, and (2) the non-marine character of most of the beds 

 and their paucity of fossils, makes their interpretation difficult. 



Foreign Jurassic. 



Europe. — Jurassic strata are exposed in many and widely separated 

 localities in Europe, though for the most part in relatively small areas 

 only. They appear at the surface in a wide belt across England, from 



