706 



GEOLOGY 



here is concealed beneath younger formations, so that its original 

 extent is not known. In the latitude of Nevada and Utah, the 

 earlier formations of the system extended, east to longitude 117. 



Fig. 478. Section in southwestern South Dakota. Pr, Proterozoic; -?, 

 Cambrian (Deadwood sandstone); Mp, Mississippian (Pahasapa and 

 Englewood limestones); Pm, Pennsylvanian (Minnelusa sandstone); 

 Pm, Permain (?) (Minnekahta limestone and Opeche shale); Ts, Trias- 

 sic (Spearfish shale); Ju, Jurassic (Unkpapa sandstone); Cm, Coman- 

 chean (?) (Morrison formation); Kdl, Comanchean below and Creta- 

 ceous above (Lakota and Dakota sandstones). (Edgemont, S. D. 

 Folio, U. S. Geol. Surv. Vertical scale X 2. Darton and Smith.) 



Fig, 479. A section in southern Montana. JR = Archean; -, Cambrian 

 (Flathead and Gallatin formations) ; D, Devonian (Jefferson and Three 

 Forks formations); Mm, Mississippian (Madison formation); Pq, 

 Pennsylvanian (Quadrant formation); Je, Jurassic (Ellis formation); 

 Kd, Kmc, and Kl, Cretaceous (Dakota, Colorado, Montana, and Laramie 

 formations) ; bbr, igneous rock. (Peale, U. S. Geol. Surv.) 



Fig. 480. Section in the Sierras of California, showing the Jurassic (or 

 Jura-Trias) system where it has been metamorphosed, and when- it 

 is associated with igneous rock, grd and dpt, igneous rock, pn>l>:il>ly 

 of Jurassic or Cretaceous age; si and slm, Jura-Trias (?) schist ; \>i. \r, 

 and Pb, igneous rock of late Tertiary and Pleistocene age. (Liml- 

 gren, U. S. Geol. Surv.) 



The Lower Jurassic beds generally rest on the Trias conformably. 

 where both are present, but the younger beds overlap the older 

 systems at some points, and fall short of it at others. 



In the later part of the period, the sea appears not t lia\ 

 tended east of the Sierras in the latitude of Culifornia. but in north- 



