THE PENNSYLVANIAN PERIOD. 553 



and to the great erosion which followed. Beds of finer sediment suc- 

 ceed the conglomerate. On the basis of fossils, the older beds of the 

 Pennsylvania!! system in Colorado are thought to be older than the 

 basal beds of the same system in Nebraska and Kansas. 1 



West of the Great Plains the outcrops of the Carboniferous strata 

 often occur about the areas of older rock, where uplift and erosion 

 have exposed them. They appear as narrow borders about various 

 mountain ranges in South Dakota, Montana (mostly Mississippian), 

 Idaho, Wyoming 2 (where submergence was much more extensive in 

 the Pennsylvanian than in the Mississippian period), and Colorado. 

 Some of the fault-block and monoclinal 3 mountains of the west are 

 composed largely of Carboniferous limestone, and here the outcrops 

 cover large areas. The same formations appear at the surface in 

 large areas, in the southwest, especially Arizona, not in immediate 

 association with mountains (Fig. 240). In Nevada, the Carboniferous 

 (Mississippian or Pennsylvanian) formations rest on Cambrian or 

 Ordovician with apparent conformity. 4 



Figs. 246 to 251 show the positions and relations of the Mississippian 

 and Pennsylvanian systems at various points in the west. In most 

 cases the sections are from regions where the strata have been much 

 disturbed by folding, faulting, and the irruption of igneous rock. In 

 all cases the Carboniferous beds are indicated by C, or some com- 

 bination -of letters beginning with C. Fig. 246 shows the Carbon- 

 iferous system of the Black Hills, somewhat deformed, resting on the 

 Cambrian, and largely covered by younger formations. Fig. 247 

 shows the relations of the systems in the Yellowstone Park; Fig. 248 

 (Colo.) shows the system affected by igneous rock; Fig. 249 (Utah) 

 shows the system where it has been notably affected by intrusions of 

 igneous rock, the latter having, in its ascent, torn off and included 

 masses of the stratified rock; Fig. 250 (Ariz.) represents a section 

 where faulting has produced complicated stratigraphic relations; while 

 251 (Cal.) shows the system where it is notably deformed and meta- 

 morphosed. Figs. 244 and 245 show the system in still other relations. 



West of the Paleozoic land area, which, in the vicinity of the 40th 



1 Girty, Professional Paper, No. 16, pp. 258-267. 



2 Knight, Bull. 45, Wyoming Experiment Station, 1900. 

 3 Keyes, Am. Jour. Sci., Vol. XV, 1903, p. 207. 



* Weeks (cited by Spurr) and Spurr, Bull. 208, U. S. Geol. Surv., pp. 32 and 166-172. 



