FORMATIONS AND PHYSICAL HISTORY 



355 



(Figs. 312 and 313) the strata still remain nearly horizontal, while 

 in some other regions they have been tilted, folded, and faulted 



Fig. 318. A section in the Menominee region of northern Michigan, showing 

 tin- Potsdam sandstone, -s, in unconformity with older formations. (Van Hise, 

 U. S. Geol. Surv.) 



Fig. 319. Section showing relations of the Cambrian in the Appalachian Moun- 

 tains. The strata are folded and faulted. -, Cambrian; O, Ordovician; S, Silurian. 

 Length, 13 miles. (Hayes, Cleveland [Tenn.] folio, U.S. Geol. Surv. Ordovician 

 and Silurian not separated in the original.) 



(Fig. 310). Where close folding has taken place, the rocks have 

 been more or less metamorphosed. In extreme cases the sandstones 



Fig. 320. Section showing the relations of Cambrian and other formations at 

 a point north of Leadville, Colorado. JR gn, Archean; -s, Cambrian ; ( ';wr, Carbon- 

 iferous; Jw, Jurassic; Ip and qp, igneous rocks. (Emmons, U. S. Geol. Surv.) 



have been converted into quartz schists, the shales into slates and 

 schists, and the limestones into marble. 



Close of the Period 



No physical changes of great importance seem to have marked 

 the close of the Cambrian period in America. Nowhere in our 

 continent, so far as now known, were mountains made at this time, 

 and nowhere were great areas of sea-bottom converted into land, 

 though local unconformities between this system and the next record 

 local changes in the sites of deposition. 



