THE MISSISSIPPIAN PERIOD 601 



were perhaps numerous islands, 1 some of which occupied the position 

 of existing mountain cores. North of the United States, also, 

 marine conditions prevailed widely. The several stages of the 

 period, as defined in the Mississippi basin, have not been separately 

 recognized in the west. Much of the system in the west is lime- 

 stone, though clastic formations are not wanting. 2 The system 

 is exposed about many of the mountains of the west, and over con- 

 siderable areas in Arizona and perhaps in New Mexico. It rests on 

 the Ordovician in many places, and it sometimes overlaps all 

 earlier Paleozoic systems, lying upon the Proterozoic. It attains 

 a thickness of several thousand feet in places. In some parts of 

 Colorado 3 (Leadville) the Mississippian limestone and dolomite 

 constitute one of the richest ore horizons of the state. 



In many parts of the west the Mississippian system is uncon- 

 formable beneath the Pennsylvanian, and in many places there 

 is an unconformity in the undifferentiated Carboniferous which 

 probably represents the division between the two systems. 



Igneous activity. According to present determinations, there 

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

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

 from Alaska on the north to California on the south. 4 West of the 

 Gold ranges in British Columbia, the early Carboniferous is made 

 up largely of igneous rock, with intercalated beds of clastic sedi- 

 ments. Dikes affect the system of Southern Illinois and adjacent 

 parts of Kentucky, but the date of their intrusion is not known. 



General Considerations 



Thickness and outcrops. In keeping with the variations in 

 the sediments, the thickness of the Mississippian system varies 

 greatly. In Pennsylvania, there is a thickness of 1,400 feet of 

 sandstone (Pocono) , with 3,000 feet of shale (Mauch Chunk) above 



1 Emmons, S. F., Bull. Geol. Soc. Am., Vol. I, pp. 263-267. 



2 The Mississippian is not differentiated from the Pennsylvanian on the 

 maps of most of the western folios of the U. S. Geol. Surv., though the two 

 are sometimes differentiated in the text, especially in the later folios. 



3 Eldridge, Anthracite-Crested Butte folio, U. S. Geol. Surv.; and Girty, 

 Prof. Paper No. 16, pp. 162, 163, and 217. 

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



