FORMATIONS AND PHYSICAL HISTORY 429 



In the Great Plains and West of Them 



The Mississippian system is known in Oklahoma and South 

 akota, where deformation and erosion have brought the strata to 

 the surface (Fig. 374). Farther west the distribution of the system 

 shows that the present mountain region, as far west as the nyth 

 eridian, was mostly submerged, though there were perhaps numer- 

 als islands. North of the United States, also, marine conditions 

 prevailed widely. Much of the system in the west is limestone, 

 ough clastic formations are not wanting. The system is exposed 

 out many of the mountains, and over considerable areas in 

 izona and perhaps in New Mexico. It rests on the Ordovician 

 many places, and locally overlaps all earlier Paleozoic systems. 

 ing on the Proterozoic. In parts of Colorado (Leadville) the Mis- 

 si>Mppian limestone and dolomite constitute one of the richest ore 

 horizons of the state. In many parts of the west the Mississippian 

 system is unconformable beneath the Pennsylvanian. 2 



Igneous activity. According to present interpretations, there 

 was great igneous activity in the west during this period. The 

 ea affected by vulcanism at this time, or soon after, extended 

 m Alaska on the north to California on the south. 3 Dikes 

 ect the system of Southern Illinois and adjacent parts of Ken- 

 cky, but the date of their intrusion is not known. 



1 Bain, Econ. Geol., Vol. Ill, and Bull. 2, 111. Geol. Surv. 



2 The Mississippian is not differentiated from the Pennsylvanian on the maps 

 most of the western folios of the U. S. Geol. Surv., though the two are differ- 

 tiated in the texts especially in the later folios. 



3 Dawson, Can. Geol. Surv., 1886, p. 85. 



